Regional regulation of focal adhesion kinase after concentric and eccentric loading is related to remodelling of human skeletal muscle

Eight weeks of eccentric training at long-muscle length increases fascicle length independently of adaptations in passive mechanical properties

Eccentric exercise training is believed to induce an increase in muscle fascicle length. However, the mechanisms underlying this adaptation are not fully understood. This study aimed to determine if an increase in gastrocnemius medialis fascicle length following an 8-wk eccentric training is linked to changes in muscle tissue and joint mechanical properties. Thirty-three physically active young adults were assigned to one of the two training protocols targeting the plantar flexors: eccentric exercise at 1) long-muscle length or 2) short-muscle length. Fascicle length and muscle shear modulus of the gastrocnemius medialis were assessed using ultrasound imaging during passive ankle rotations, alongside measurements of passive ankle torque. A total of 27 participants successfully completed the training program and data processing stage (long-muscle length, n = 15; short-muscle length, n = 12). Maximal voluntary isometric torque increased significantly following the training programs (9.5%), with no differences between training groups. An increase in fascicle length (mean 8.5%) was observed in the long-muscle length group, from 39.5 ± 0.7° to 36.8 ± 0.8° in plantar flexion, but not in the short-muscle length group. Notably, this macrostructural adaptation was detected only at muscle lengths shorter than the slack length (i.e., absence of any muscle passive tension). The eccentric training did not alter the muscle shear modulus or slack length. Collectively, these findings suggest that fascicle length adaptations in response to eccentric training were unrelated to changes in passive muscle-tendon mechanical properties. Consequently, the increase in fascicle length may be attributed to an increase in sarcomere length and/or an addition of sarcomeres in series.NEW & NOTEWORTHY We demonstrate that an 8-wk eccentric training program significantly increases gastrocnemius medialis fascicle length in humans, independent of any adaptions in passive muscle-tendon mechanical properties. Fascicle length adaptions were specific to the group that trained at long-muscle lengths, highlighting the importance of the muscle-tendon length range during eccentric exercise programs. This factor may be crucial for fine-tuning structural adaptations at the fascicle level, likely through the addition of sarcomeres in series.

Focal adhesion kinase promotes aerobic glycolysis in hepatic stellate cells via the cyclin D1/c-Myc/MCT-1 pathway to induce liver fibrosis

Hepatic stellate cells (HSCs) transdifferentiate into myofibroblasts during liver fibrosis and exhibit increased glycolysis. Phosphorylated focal adhesion kinase (FAK) (pY397-FAK) promotes monocarboxylate transporter 1 (MCT-1) expression in HSCs to increase aerobic glycolysis and cause liver fibrosis. A combined multiomics analysis of C57BL/6 mice with tetrachloromethane (CCl4)-induced liver fibrosis was performed to identify the downstream FAK signaling pathway. The effect of the FAK inhibitor PF562271 on CCl4-induced liver fibrosis was explored by immunofluorescence of liver tissues. The migration, proliferation and aerobic glycolysis of LX-2 cells after stimulation and activation by transforming growth factor beta-1 (TGF-β1) or suppression by PF562271 was assessed in vitro. Multiomics analysis of a successfully generated CCl4-induced liver fibrosis mouse model was performed. FAK and cyclin D1 were significantly enriched in mice with CCl4-induced liver fibrosis. In vivo, the MCT-1 and alpha smooth muscle actin (α-SMA) levels were increased in mice with CCl4-induced liver fibrosis, and MCT-1 and α-SMA expression decreased after PF562271 treatment. In vitro, PF562271 alleviated TGF-β1-induced LX-2 activation. LX-2 cells showed diminished migration, proliferation, and aerobic glycolysis after PF562271 intervention. FAK promotes aerobic glycolysis in LX-2 cells through the cyclin D1/c-Myc/MCT-1 pathway, thereby increasing liver fibrosis.

Previous short-term disuse dictates muscle gene expression and physiological adaptations to subsequent resistance exercise

Short-term unloading experienced following injury or hospitalisation induces muscle atrophy and weakness. The effects of exercise following unloading have been scarcely investigated. We investigated the functional and molecular adaptations to a resistance training (RT) programme following short-term unloading. Eleven males (22.09 ± 2.91 years) underwent 10 days of unilateral lower limb suspension (ULLS) followed by 21 days of knee extensor RT (three times/week). Data collection occurred at Baseline (LS0), after ULLS (LS10) and at active recovery (AR21). Knee extensor maximum voluntary contraction (MVC) was evaluated. Quadriceps volume was estimated by ultrasonography. Muscle fibre cross-sectional area, fibre type distribution, glycogen content and succinate dehydrogenase (SDH) activity were measured from vastus lateralis biopsies. Mitochondrial-related proteins were quantified by western blot and transcriptional responses were assessed by RNA sequencing. Following ULLS, quadriceps volume and MVC decreased significantly (3.7%, P < 0.05; 29.3%, P < 0.001). At AR21 (vs. LS10), MVC was fully restored (42%) and quadriceps volume increased markedly (18.6%, P < 0.001). Glycogen content and whole-body water increased at AR21 (14%, P < 0.001; 3.1%, P < 0.05). We observed a marked increase in fibre type I at AR21 (38%, P < 0.05). SDH immunoreactivity increased significantly after exercise (20%, P < 0.001). Mitochondrial fusion (MFN1, MFN2 and OPA1) and fission (DRP1) proteins were markedly increased by RT, and the most differentially expressed genes belonged to oxidative phosphorylation pathways. In contrast with what is usually observed after RT, oxidative metabolism, slow fibre type and mitochondrial dynamics were enhanced beyond expected. We propose that prior exposure to short-term muscle unloading may drive the nature of molecular adaptations to subsequent RT. KEY POINTS: Short-term unloading is often experienced during recovery from injuries and hospitalisation, leading to loss of muscle mass and strength. Although exercise can be beneficial in mitigating/reversing such alterations during disuse, only a few studies have focused on the effects of exercise following muscle unloading. With an integrative physiological approach, we aimed to elucidate the basic mechanisms of muscle function recovery in response to 21 days of resistance exercise that followed 10 days of unilateral lower limb suspension (ULLS), assessing whether the mechanisms underlying recovery are defined by a specific reversal of those that occurred during disuse. Resistance training was successful in recovering functional and structural muscle properties after 10 days of ULLS, but in contrast with what is usually observed in response to this training modality, oxidative metabolism and slow fibre type were mostly enhanced. We propose that prior exposure to short-term muscle unloading may drive the adaptations to subsequent exercise.

Genotypic Influences on Actuators of Aerobic Performance in Tactical Athletes

BACKGROUND

This study examines genetic variations in the systemic oxygen transport cascade during exhaustive exercise in physically trained tactical athletes. Research goal: To update the information on the distribution of influence of eleven polymorphisms in ten genes, namely ACE (rs1799752), AGT (rs699), MCT1 (rs1049434), HIF1A (rs11549465), COMT (rs4680), CKM (rs8111989), TNC (rs2104772), PTK2 (rs7460 and rs7843014), ACTN3 (rs1815739), and MSTN (rs1805086)-on the connected steps of oxygen transport during aerobic muscle work.

METHODS

251 young, healthy tactical athletes (including 12 females) with a systematic physical training history underwent exercise tests, including standardized endurance running with a 12.6 kg vest. Key endurance performance metrics were assessed using ergospirometry, blood sampling, and near-infrared spectroscopy of knee and ankle extensor muscles. The influence of gene polymorphisms on the above performance metrics was analyzed using Bayesian analysis of variance.

RESULTS

Subjects exhibited good aerobic fitness (maximal oxygen uptake (VO2max): 4.3 ± 0.6 L min-1, peak aerobic power: 3.6 W ± 0.7 W kg-1). Energy supply-related gene polymorphisms rs1799752, rs4680, rs1049434, rs7843014, rs11549465, and rs8111989 did not follow the Hardy-Weinberg equilibrium. Polymorphisms in genes that regulate metabolic and contractile features were strongly associated with variability in oxygen transport and metabolism, such as body mass-related VO2 (rs7843014, rs2104772), cardiac output (rs7460), total muscle hemoglobin content (rs7460, rs4680), oxygen saturation in exercised muscle (rs1049434), and respiration exchange ratio (rs7843014, rs11549465) at first or secondary ventilatory thresholds or VO2max. Moderate influences were found for mass-related power output.

CONCLUSIONS

The posterior distribution of effects from genetic modulators of aerobic metabolism and muscle contractility mostly confirmed prior opinions in the direction of association. The observed genetic effects of rs4680 and rs1049434 indicate a crucial role of dopamine- and lactate-modulated muscle perfusion and oxygen metabolism during running, suggesting self-selection in Swiss tactical athletes.

Hamstring Muscle-Tendon Geometric Adaptations to Resistance Training Using the Hip Extension and Nordic Hamstring Exercises

Targeted resistance training stimulates hamstring muscle hypertrophy, but its effect on tendon-aponeurosis geometry is unknown. This study examined changes in hamstring muscle, free tendon, and aponeurosis geometry following a 10 week Nordic or hip extension exercise intervention. Thirty recreationally active males were randomly allocated (n = 10 per group) to a Nordic, hip extension, or control group. Magnetic resonance imaging of both thighs was acquired pre- and post-intervention. Changes in free tendon and aponeurosis volume for each hamstring muscle, biceps femoris long head (BFlh) aponeurosis interface area and muscle volume-to-interface area ratio were compared between groups. Regional changes in muscle CSA were examined via statistical parametric mapping. The change in semimembranosus free tendon volume was greater for the Nordic than control group (mean difference = 0.06 cm3, 95% CI = 0.02-0.11 cm3). No significant between-group differences existed for other hamstring free tendons or aponeuroses. There were no between-group differences in change in BFlh interface area. Change in BFlh muscle volume-to-interface area ratio was greater in the hip extension than Nordic (mean difference = 0.10, 95% CI = 0.007-0.19, p = 0.03) and control (mean difference = 0.12, 95% CI = 0.03-0.22, p = 0.009) groups. Change in muscle CSA following training was greatest in the mid-portion of semitendinosus for both intervention groups, and the mid-portion of BFlh for the hip extension group. There was limited evidence for tendon-aponeurosis hypertrophy after 10 weeks of training with the Nordic or hip extension exercises. For the BFlh, neither intervention altered the interface area although hip extension training stimulated an increase in the muscle volume-to-interface area ratio, which may have implications for localized tissue strains. Alternative muscle-tendon loading strategies appear necessary to stimulate hamstring tendon adaptations.

Acute effects of Nordic hamstring exercise on hip and knee joints proprioception

BACKGROUND AND OBJECTIVES

Nordic Hamstring Exercise (NHE) is one of the best exercises proposed for injury prevention of hamstring muscles. However, its effects on lower extremity proprioception are unclear. The aim of this study was to investigate the immediate effects of a single bout of NHE on hip and knee joints' proprioception.

METHODS

Forty collegiate male soccer players participated in this study with a mean age of 22.85 ± 1.82 years and were randomized into either control (n = 20) or experimental (n = 20) groups. Each subject participated in pre-test measurements in which hip and knee active joints position sense (JPS) were assessed in standing and lying tasks using the image-capturing method. The experimental group then performed three sets of NHE with 10 repetitions in each set, while the control group rested for 10 min. Paired and independent t-tests were used for calculating the differences within and between groups on SPSS software, respectively. The level of significance was P ≤ 0.05.

RESULTS

Hip JPS in the lying task and knee JPS in both of the standing and lying tasks were impaired significantly after performing a single bout of NHE (P ≤ 0.05). However, the effects of this exercise on hip JPS in the standing task were not significant (P ≥ 0.05).

CONCLUSIONS

NHE performing with three sets of 10 repetitions can significantly impair hip and knee JPS immediately after exercise and reduce the proprioception acuity of the lower limbs. It is recommended to perform this exercise at a time rather than before training or match sessions.

Age-related blunting of serial sarcomerogenesis and mechanical adaptations following 4 wk of maximal eccentric resistance training

During aging, muscles undergo atrophy, which is partly accounted for by a loss of sarcomeres in series. Serial sarcomere number (SSN) is associated with aspects of muscle mechanical function including the force-length and force-velocity-power relationships; hence, the age-related loss of SSN contributes to declining performance. Training emphasizing eccentric contractions increases SSN in young healthy rodents; however, the ability for eccentric training to increase SSN in old age is unknown. Ten young (8 mo) and 11 old (32 mo) male Fisher344/BN rats completed 4 wk of unilateral eccentric plantar flexion training. Pre- and posttraining, the plantar flexors were assessed for the torque-frequency, passive torque-angle, and torque-velocity-power relationships. The soleus, lateral gastrocnemius (LG), and medial gastrocnemius (MG) were harvested for SSN assessment via laser diffraction, with the untrained leg used as a control. In the untrained leg/pretraining, old rats had lower SSN in the soleus, LG, and MG, lower maximum torque, power, and shortening velocity, and greater passive torque than young. Young showed increased soleus and MG SSN following training. In contrast, old had no change in soleus SSN and experienced SSN loss in the LG. Pre- to posttraining, young experienced an increase in maximum isometric torque, whereas old had reductions in maximum torque, shortening velocity, and power, and increased passive torque. Our results show that although young muscle has the ability to add sarcomeres in response to maximal eccentric training, this stimulus could be not only ineffective, but also detrimental to aged muscle leading to dysfunctional remodeling.NEW & NOTEWORTHY The loss of sarcomeres in series with age contributes to declining muscle performance. The present study investigated whether eccentric training could improve performance via serial sarcomere addition in old muscle, like in young muscle. Four weeks of maximal eccentric training induced serial sarcomere addition in the young rat plantar flexors and improved in vivo performance, however, led to dysfunctional remodeling accompanied by further impaired performance in old rats.

Determining Changes in Muscle Size and Architecture After Exercise Training: One Site Does Not Fit all

ABSTRACT

Nunes, JP, Blazevich, AJ, Schoenfeld, BJ, Kassiano, W, Costa, BDV, Ribeiro, AS, Nakamura, M, Nosaka, K, and Cyrino, ES. Determining changes in muscle size and architecture after exercise training: One site does not fit all. J Strength Cond Res 38(4): 787-790, 2024-Different methods can be used to assess muscle hypertrophy, but the effects of training on regional changes in muscle size can be detected only using direct muscle measurements such as muscle thickness, cross-sectional area, or volume. Importantly, muscle size increases vary across regions within and between muscles after resistance training programs (i.e., heterogeneous, or nonuniform, muscle hypertrophy). Muscle architectural changes, including fascicle length and pennation angle, after resistance and stretch training programs are also region-specific. In this paper, we show that the literature indicates that a single-site measure of muscle shape does not properly capture the effects achieved after exercise training interventions and that conclusions concerning the magnitude of muscle adaptations can vary substantially depending on the muscle site to be examined. Thus, we propose that measurements of muscle size and architecture should be completed at multiple sites across regions between the agonist muscles within a muscle group and along the length of the muscles to provide an adequate picture of training effects.

The effects of regional quadriceps architecture on angle-specific rapid force expression

Evaluating anatomical contributions to performance can increase understanding of muscle mechanics and guide physical preparation. While the impact of anatomy on muscular performance is well studied, the effects of regional quadriceps architecture on rapid torque or force expression are less clear. Regional (proximal, middle, and distal) quadriceps (vastus lateralis, rectus femoris, and vastus intermedius) thickness (MT), pennation angle (PA), and fascicle length (FL) of 24 males (48 limbs) were assessed via ultrasonography. Participants performed maximal isometric knee extensions at 40°, 70°, and 100° of knee flexion to evaluate rate of force development from 0 to 200 ms (RFD0-200). Measurements were repeated on three occasions with the greatest RFD0-200 and mean muscle architecture measures used for analysis. Linear regression models predicting angle-specific RFD0-200 from regional anatomy provided adjusted correlations (√adjR2) with bootstrapped compatibility limits. Mid-rectus femoris MT (√adjR2 = 0.41-0.51) and proximal vastus lateralis FL (√adjR2 = 0.42-0.48) were the best single predictors of RFD0-200, and the only measures to reach precision with 99% compatibility limits. Small simple correlations were found across all regions and joint angles between RFD0-200 and vastus lateralis MT (√adjR2 = 0.28 ± 0.13; mean ± SD), vastus lateralis FL (√adjR2 = 0.33 ± 0.10), rectus femoris MT (√adjR2 = 0.38 ± 0.10), and lateral vastus intermedius MT (√adjR2 = 0.24 ± 0.10). Between-correlation comparisons are reported within the article. Researchers should measure mid-region rectus femoris MT and vastus lateralis FL to efficiently and robustly evaluate potential anatomical contributions to rapid knee extension force changes, with distal and proximal measurements providing little additional value. However, correlations were generally small to moderate, suggesting that neurological factors may be critical in rapid force expression.

Biceps Femoris Fascicle Behavior during Submaximal and Maximal Slow Speed Contractions

PURPOSE

The present study compared the effects of contraction intensity (submaximal vs maximal) and mode (concentric vs eccentric) on biceps femoris long head (BFlh) fascicle lengthening, rotation, and architectural gear ratio at long and short muscle lengths.

METHODS

Data were captured from 18 healthy adults (10 men and 8 women) without history of right hamstring strain injury. BFlh fascicle length ( Lf ), fascicle angle (FA), and muscle thickness (MT) were assessed in real time using two serially aligned ultrasound devices while submaximal and maximal concentric and eccentric isokinetic knee flexions were performed at 30°·s -1 . Ultrasound videos were exported and edited to create a single, synchronized video, and three fascicles were analyzed through the range of motion (10° to 80°). Changes (Δ) in Lf , FA, MT, and muscle gear at long (60° to 80° knee angle; 0° = full knee extension) and short (10° to 30°) muscle lengths and across the full knee flexion range were measured and compared.

RESULTS

Greater Δ Lf was observed at long muscle length ( P < 0.001) during both submaximal and maximal eccentric and concentric contractions. When the full length range was analyzed, a slightly greater ΔMT was observed in concentric contractions ( P = 0.03). No significant differences between submaximal and maximal contractions were observed for Δ Lf , ΔFA, or ΔMT. No changes were detected in the calculated muscle gear between muscle lengths, intensities, or conditions ( P > 0.05).

CONCLUSIONS

Although gear ratio ranged ~1.0 to 1.1 under most conditions, the increased fascicle lengthening observed at long muscle lengths might influence acute myofiber damage risk but also speculatively play a role in chronic hypertrophic responses to training.

Mechanisms of mechanical overload-induced skeletal muscle hypertrophy: current understanding and future directions

Mechanisms underlying mechanical overload-induced skeletal muscle hypertrophy have been extensively researched since the landmark report by Morpurgo (1897) of "work-induced hypertrophy" in dogs that were treadmill trained. Much of the preclinical rodent and human resistance training research to date supports that involved mechanisms include enhanced mammalian/mechanistic target of rapamycin complex 1 (mTORC1) signaling, an expansion in translational capacity through ribosome biogenesis, increased satellite cell abundance and myonuclear accretion, and postexercise elevations in muscle protein synthesis rates. However, several lines of past and emerging evidence suggest that additional mechanisms that feed into or are independent of these processes are also involved. This review first provides a historical account of how mechanistic research into skeletal muscle hypertrophy has progressed. A comprehensive list of mechanisms associated with skeletal muscle hypertrophy is then outlined, and areas of disagreement involving these mechanisms are presented. Finally, future research directions involving many of the discussed mechanisms are proposed.

Regional Hypertrophy: The Effect of Exercises at Long and Short Muscle Lengths in Recreationally Trained Women

The aim of the present study was to analyse the role of exercises' resistance profile in regional hypertrophy. Thirty-eight healthy women completed a 9-week resistance training program consisting of either 4 sets of 12 repetitions to volitional failure of inclined bicep curls (INC group) or preacher curls (PREA group), three times per week. Pre- and post-intervention muscle thickness was measured using B-mode ultrasound imaging with a linear-array transducer. Scan acquisition sites were determined by measuring 50%, 60% and 70% of the distance between the posterior crest of the acromion and the olecranon. Statistical significance was set at p < 0.05. No region of the INC group grew when comparing pre- to post-intervention. The 70% region of the PREA group grew significantly (muscle thickness increased from 2.7 ± 0.43 cm to 2.94 ± 0.44 cm). We found no growth differences between regions when analysing per group (p = 0.274), region (p = 0.571) or group*region (p = 0.367). Our results show that the distal region of the arm grows in response to the preacher curl that places the highest amount of strain in the range of motion in which the arm muscles are more elongated.

Effects of submaximal and supramaximal accentuated eccentric loading on mass and function

Introduction: Eccentric-overload (EO) resistance training emerges as an alternative to more optimally prescribe intensity relative to the force generation capabilities of the eccentric muscle contraction. Given the difficulties to individually prescribe absolute eccentric loads relative to each person's eccentric ability, setting the load relative to the concentric one-repetition maximum (1-RM) is the most used EO training approach. Therefore, we investigated the effects of submaximal and supramaximal (i.e., eccentric loads above 100% of 1-RM) accentuated eccentric training on changes in lean mass, anabolic hormonal responses and muscle function. Methods: Physically active university students (n = 27) were randomly assigned to two training groups. Participants in the training groups performed dominant leg isotonic training twice a week for 10 weeks (four sets of eight repetitions). Isotonic resistance was generated by an electric-motor device at two different percentages of 1-RM for the eccentric phase; 90% submaximal load, SUB group) and 120% (supramaximal load, SUPRA group). Concentric load was the same for both groups (30% of 1-RM). Changes in total thigh lean mass (TTLM), anabolic hormonal responses (growth hormone, IGF-1, IL-6, and total testosterone), unilateral leg-press 1-RM, maximal voluntary isometric contractions (MVIC), local muscle endurance (XRM), muscle power at 40 (PP40), 60 (PP60) and 80% (PP80) of the 1-RM, and unilateral vertical jump height before and after training were compared between groups. Results: After training, both SUB and SUPRA groups showed similar increases (p < 0.05) in MVIC (19.2% and 19.6%), XRM (53.8% and 23.8%), PP40 (16.2% and 15.7%), TTLM (2.5% and 4.2%), IGF-1 (10.0% and 14.1%) and IL-6 (58.6% and 28.6%). However, increases in 1-RM strength (16.3%) and unilateral vertical jump height (10.0%-13.4%) were observed for SUPRA only. Indeed, SUPRA was shown to be more favorable than SUB training for increasing 1-RM [ES = 0.77 (1.49-0.05)]. Unilateral muscle power at medium and high intensity (10.2% and 10.5%) also increased in SUB but without significant differences between groups. Discussion: Similar functional and structural effects were demonstrated after 10 weeks EO training with submaximal and supramaximal eccentric loads. Although supramaximal loading might be superior for increasing 1-RM, the use of this approach does not appear to be necessary in healthy, active individuals.

Older postmenopausal women with lower lean mass have hypermethylated sites in the PI3K-Akt pathway

Introduction: The decrease in lean mass is directly related to the loss of independence, muscle strength, and worse quality of life over the years. Although the genetic determinants of muscle mass were well recognized, recent literature has been uncovering new epigenetic factors affecting the state of muscular tissue. This study aimed to verify differences in the DNA methylation profile among Brazilian postmenopausal women aged 50-70 years according to the lean mass evaluation. Methods: A cross-sectional study comprised 40 women aged 50-70 years. After K-means cluster analysis the 40 participants were divided into two groups, the Lower Lean Mass group with 20 participants (61.1 ± 4.6 years) and the Higher Lean Mass group with 20 participants (60.7 ± 3.2 years). Lean mass was measured by dual-energy X-ray emission densitometry (DEXA). The participants' DNA was extracted using the Salting Out technique and subsequently, the Illumina 850k EPIC Infinium Methylation BeadChip was performed to obtain methylation data. Results: We obtained 1,913 differentially methylated sites (p ≤ 0.005 of β > 5% and β < -5%) in a total of 979 genes between groups (p ≤ 0.005; -5% > β > 5%). In addition, the PI3K-Akt pathway had the greatest power of significance with an FDR of 4.6 × 10^-3. Conclusion: Our results demonstrate a differentiation between specific sites of different genes, which have essential functions in body composition and energy metabolism, supporting future studies that aim to relate lean mass with epigenetics.

Changes in vastus lateralis fibre cross-sectional area, pennation angle and fascicle length do not predict changes in muscle cross-sectional area

NEW FINDINGS

What is the central question of this study? Do changes in myofibre cross-sectional area, pennation angle and fascicle length predict vastus lateralis whole-muscle cross-sectional area changes following resistance training? What is the main finding and its importance? Changes in vastus lateralis mean myofibre cross-sectional area, fascicle length and pennation angle following a period of resistance training did not collectively predict changes in whole-muscle cross-sectional area. Despite the limited sample size in this study, these data reiterate that it remains difficult to generalize the morphological adaptations that predominantly drive tissue-level vastus lateralis muscle hypertrophy.

ABSTRACT

Myofibre hypertrophy during resistance training (RT) poorly associates with tissue-level surrogates of hypertrophy. However, it is underappreciated that, in pennate muscle, changes in myofibre cross-sectional area (fCSA), fascicle length (Lf ) and pennation angle (PA) likely coordinate changes in whole-muscle cross-sectional area (mCSA). Therefore, we determined if changes in fCSA, PA and Lf predicted vastus lateralis (VL) mCSA changes following RT. Thirteen untrained college-aged males (23 ± 4 years old, 25.4 ± 5.2 kg/m2 ) completed 7 weeks of full-body RT (twice weekly). Right leg VL ultrasound images and biopsies were obtained prior to (PRE) and 72 h following (POST) the last training bout. Regression was used to assess if training-induced changes in mean fCSA, PA and Lf predicted VL mCSA changes. Correlations were also performed between PRE-to-POST changes in obtained variables. Mean fCSA (+18%), PA (+8%) and mCSA (+22%) increased following RT (P < 0.05), but not Lf (0.1%, P = 0.772). Changes in fCSA, Lf and PA did not collectively predict changes in mCSA (R2 = 0.282, adjusted R2 = 0.013, F3,8  = 1.050, P = 0.422). Moderate negative correlations existed for percentage changes in PA and Lf (r = -0.548, P = 0.052) and changes in fCSA and Lf (r = -0.649, P = 0.022), and all other associations were weak (|r| < 0.500). Although increases in mean fCSA, PA and VL mCSA were observed, inter-individual responses for each variable and limitations for each technique make it difficult to generalize the morphological adaptations that predominantly drive tissue-level VL muscle hypertrophy. However, the small subject pool is a significant limitation, and more research in this area is needed.

Identification of the Exercise and Time Effects on Human Skeletal Muscle through Bioinformatics Methods

Background

The human body has more than 600 kinds of skeletal muscles, which accounts for about 40% of the whole weight. Most skeletal muscles can make bones move, and their strength and endurance directly affect their performance during exercise.

Methods

To determine the effects of exercise and time on human skeletal muscle, we downloaded the microarray expression profile of GSE1832 and analyzed it to select differentially expressed genes (DEGs). Then, a protein-protein interaction (PPI) network was established, and the hub genes were identified. Afterwards, DEGs were applied to perform Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis. Finally, with the help of Gene Set Enrichment Analysis (GSEA), the gene sets in the 7 samples were enriched in the KEGG pathway.

Results

Through a series of bioinformatics analyses, we obtained a total of 271 DEGs. After that, four hub genes were determined through the PPI network, namely, EP300, STAT1, CDKN1A, and RAC2. In addition, we got that these DEGs were enriched in GO, such as regulation of cell population proliferation, cellular water homeostasis, and so on, and in KEGG, namely, hepatitis B, Epstein-Barr virus infection, small cell lung cancer, pathways in cancer, and others. Finally, the gene set in the samples obtained by GSEA was enriched in the cell cycle, chemokine signaling pathway, DNA replication, cytokine receptor interaction, ECM receptor interaction, and focal adhesion in KEGG.

Conclusion

The findings obtained in this study will provide new clues for elucidating the mechanism of exercise and time on human skeletal muscles.

Metabolic and molecular responses of human patellar tendon to concentric- and eccentric-type exercise in youth and older age

Exercise training can induce adaptive changes to tendon tissue both structurally and mechanically; however, the underlying compositional changes that contribute to these alterations remain uncertain in humans, particularly in the context of the ageing tendon. The aims of the present study were to determine the molecular changes with ageing in patellar tendons in humans, as well as the responses to exercise and exercise type (eccentric (ECC) and concentric (CON)) in young and old patellar tendon. Healthy younger males (age 23.5 ± 6.1 years; n = 27) and older males (age 68.5 ± 1.9 years; n = 27) undertook 8 weeks of CON or ECC training (3 times per week; at 60% of 1 repetition maximum (1RM)) or no training. Subjects consumed D₂O throughout the protocol and tendon biopsies were collected after 4 and 8 weeks for measurement of fractional synthetic rates (FSR) of tendon protein synthesis and gene expression. There were increases in tendon protein synthesis following 4 weeks of CON and ECC training (P < 0.01; main effect by ANOVA), with no differences observed between young and old males, or training type. At the transcriptional level however, ECC in young adults generally induced greater responses of collagen and extracellular matrix-related genes than CON, while older individuals had reduced gene expression responses to training. Different training types did not appear to induce differential tendon responses in terms of protein synthesis, and while tendons from older adults exhibited different transcriptional responses to younger individuals, protein turnover changes with training were similar for both age groups.

Exercise-induced signaling pathways to counteracting cardiac apoptotic processes

Cardiovascular diseases are the most common cause of death in the world. One of the major causes of cardiac death is excessive apoptosis. However, multiple pathways through moderate exercise can reduce myocardial apoptosis. After moderate exercise, the expression of anti-apoptotic proteins such as IGF-1, IGF-1R, p-PI3K, p-Akt, ERK-1/2, SIRT3, PGC-1α, and Bcl-2 increases in the heart. While apoptotic proteins such as PTEN, PHLPP-1, GSK-3, JNK, P38MAPK, and FOXO are reduced in the heart. Exercise-induced mechanical stress activates the β and α5 integrins and subsequently, focal adhesion kinase phosphorylation activates the Akt/mTORC1 and ERK-1/2 pathways, leading to an anti-apoptotic response. One of the reasons for the decrease in exercise-induced apoptosis is the decrease in Fas-ligand protein, Fas-death receptor, TNF-α receptor, Fas-associated death domain (FADD), caspase-8, and caspase-3. In addition, after exercise mitochondrial-dependent apoptotic factors such as Bid, t-Bid, Bad, p-Bad, Bak, cytochrome c, and caspase-9 are reduced. These changes lead to a reduction in oxidative damage, a reduction in infarct size, a reduction in cardiac apoptosis, and an increase in myocardial function. After exercising in the heart, the levels of RhoA, ROCK1, Rac1, and ROCK2 decrease, while the levels of PKCε, PKCδ, and PKCɑ are activated to regulate calcium and prevent mPTP perforation. Exercise has an anti-apoptotic effect on heart failure by increasing the PKA-Akt-eNOS and FSTL1-USP10-Notch1 pathways, reducing the negative effects of CaMKIIδ, and increasing the calcineurin/NFAT pathway. Exercise plays a protective role in the heart by increasing HSP20, HSP27, HSP40, HSP70, HSP72, and HSP90 along with increasing JAK2 and STAT3 phosphorylation. However, research on exercise and factors such as Pim-1, Notch, and FAK in cardiac apoptosis is scarce, so further research is needed. Future research is recommended to discover more anti-apoptotic pathways. It is also recommended to study the synergistic effect of exercise with gene therapy, dietary supplements, and cell therapy for future research.

Eccentric exercise improves joint flexibility in adults: A systematic review update and meta-analysis

BACKGROUND

Eccentric exercise is thought to improve joint flexibility, but the size of the effect is not known. We aimed to quantify the overall effect of eccentric exercise on joint flexibility in adults.

DESIGN

Systematic review, meta-analysis.

DATA SOURCES

AMED, CINAHL, MEDLINE, EMBASE, SportDiscus.

PARTICIPANTS

Adults.

INTERVENTION

Eccentric exercise compared to no intervention or to a different intervention.

OUTCOME MEASURES

Joint range of motion or muscle fascicle length.

DATA EXTRACTION AND SYNTHESIS

Descriptive data of included trials and estimates of effect sizes were extracted. Standardised mean differences (SMD) of range of motion or fascicle length outcomes were meta-analysed using random effects models. Overall quality of evidence was assessed using the GRADE scale.

RESULTS

32 trials (1122 participants, 108 lost to follow-up) were included in the systematic review. The mean (SD) PEDro score was 5.2 (1.3). Four trials reported insufficient data for meta-analysis. Data from 27 trials (911 participants, 82 lost to follow-up) were meta-analysed. Eccentric exercise improved joint flexibility in adults (pooled random effects Hedges' g SMD = 0.54, 95% CI 0.34 to 0.74). The true effect size is different across studies and 50% of the variance in observed effects is estimated to reflect variance in true effects rather than sampling error (I² = 50%, Q = 67.6, d.f. = 34, p = 0.001). Overall quality of evidence ranged from 'low' to 'high'.

CONCLUSION

Eccentric exercise improves joint flexibility in adults. The overall standardised mean effect of eccentric exercise was moderately large, and the narrow width of the 95% confidence interval indicates the effect was estimated with good precision.

REGISTRATION

Open Science Foundation (https://osf.io/mkdqr); PROSPERO registration CRD42020151303.

Influence of weighted downhill running training on serial sarcomere number and work loop performance in the rat soleus

Increased serial sarcomere number (SSN) has been observed in rats following downhill running training due to the emphasis on active lengthening contractions; however, little is known about the influence on dynamic contractile function. Therefore, we employed 4 weeks of weighted downhill running training in rats, then assessed soleus SSN and work loop performance. We hypothesised trained rats would produce greater net work output during work loops due to a greater SSN. Thirty-one Sprague-Dawley rats were assigned to a training or sedentary control group. Weight was added during downhill running via a custom-made vest, progressing from 5–15% body mass. Following sacrifice, the soleus was dissected, and a force-length relationship was constructed. Work loops (cyclic muscle length changes) were then performed about optimal muscle length (L_O) at 1.5–3-Hz cycle frequencies and 1–7-mm length changes. Muscles were then fixed in formalin at L_O. Fascicle lengths and sarcomere lengths were measured to calculate SSN. Intramuscular collagen content and crosslinking were quantified via a hydroxyproline content and pepsin-solubility assay. Trained rats had longer fascicle lengths (+13%), greater SSN (+8%), and a less steep passive force-length curve than controls (P<0.05). There were no differences in collagen parameters (P>0.05). Net work output was greater (+78–209%) in trained than control rats for the 1.5-Hz work loops at 1 and 3-mm length changes (P<0.05), however, net work output was more related to maximum specific force (R²=0.17-0.48, P<0.05) than SSN (R²=0.03-0.07, P=0.17-0.86). Therefore, contrary to our hypothesis, training-induced sarcomerogenesis likely contributed little to the improvements in work loop performance.

This article has an associated First Person interview with the first author of the paper.

Summary: An investigation of adaptations in mechanical function induced by a novel method of weighted downhill running training in rats, and the connections to adaptations in muscle architecture.

The influence of longitudinal muscle fascicle growth on mechanical function

Skeletal muscle has the remarkable ability to remodel and adapt, such as the increase in serial sarcomere number (SSN) or fascicle length (FL) observed after overstretching a muscle. This type of remodelling is termed longitudinal muscle fascicle growth, and its impact on biomechanical function has been of interest since the 1960s due to its clinical applications in muscle strain injury, muscle spasticity, and sarcopenia. Despite simplified hypotheses on how longitudinal muscle fascicle growth might influence mechanical function, existing literature presents conflicting results partly due to a breadth of methodologies. The purpose of this review is to outline what is currently known about the influence of longitudinal muscle fascicle growth on mechanical function and suggest future directions to address current knowledge gaps and methodological limitations. Various interventions indicate longitudinal muscle fascicle growth can increase the optimal muscle length for active force, but whether the whole force-length relationship widens has been less investigated. Future research should also explore the ability for longitudinal fascicle growth to broaden the torque-angle relationship's plateau region, and the relation to increased force during shortening. Without a concurrent increase in intramuscular collagen, longitudinal muscle fascicle growth also reduces passive tension at long muscle lengths; further research is required to understand whether this translates to increased joint range of motion. Lastly, some evidence suggests longitudinal fascicle growth can increase maximum shortening velocity and peak isotonic power, however, there has yet to be direct assessment of these measures in a neurologically intact model of longitudinal muscle fascicle growth.

Biceps femoris long head sarcomere and fascicle length adaptations after 3 weeks of eccentric exercise training

BACKGROUND

Eccentric exercise increases muscle fascicle lengths; however, the mechanisms behind this adaptation are still unknown. This study aimed to determine whether biceps femoris long head (BFlh) fascicle length increases in response to 3 weeks of eccentric exercise training are the result of an in-series addition of sarcomeres within the muscle fibers.

METHODS

Ten recreationally active participants (age = 27 ± 3 years; mass = 70 ± 14 kg; height = 174 ± 9 cm; mean ± SD) completed 3 weeks of Nordic hamstring exercise (NHE) training on a custom exercise device that was instrumented with load cells. We collected in vivo sarcomere and muscle fascicle images of the BFlh in 2 regions (central and distal) by using microendoscopy and 3 dimension ultrasonography. We then estimated sarcomere length, sarcomere number, and fascicle length before and after the training intervention.

RESULTS

Eccentric knee flexion strength increased after the training (15%; p < 0.001; η_p² = 0.75). Further, we found a significant increase in fascicle length (21%; p < 0.001; η_p² = 0.81) and sarcomere length (17%; p < 0.001; η_p² = 0.90) in the distal but not in the central portion of the muscle. The estimated number of sarcomeres in series did not change in either region.

CONCLUSION

Fascicle length adaptations appear to be heterogeneous in the BFlh in response to 3 weeks of NHE training. An increase in sarcomere length, rather than the addition of sarcomeres in series, appears to underlie increases in fascicle length in the distal region of the BFlh. The mechanism driving regional increases in fascicle and sarcomere length remains unknown, but we speculate that it may be driven by regional changes in the passive tension of muscle or connective tissue adaptations.

Gastrocnemius Muscle Architecture in Elite Basketballers and Cyclists: A Cross-Sectional Cohort Study

Eccentric and concentric actions produce distinct mechanical stimuli and result in different adaptations in skeletal muscle architecture. Cycling predominantly involves concentric activity of the gastrocnemius muscles, while playing basketball requires both concentric and eccentric actions to support running, jumping, and landing. The aim of this study was to examine differences in the architecture of gastrocnemius medialis (GM) and gastrocnemius lateralis (GL) between elite basketballers and cyclists. A trained sonographer obtained three B-mode ultrasound images from GM and GL muscles in 44 athletes (25 basketballers and 19 cyclists; 24 ± 5 years of age). The images were digitized and average fascicle length (FL), pennation angle (θ), and muscle thickness were calculated from three images per muscle. The ratio of FL to tibial length (FL/TL) and muscle thickness to tibial length (MT/TL) was also calculated to account for the potential scaling effect of stature. In males, no significant differences were identified between the athletic groups in all parameters in the GM, but a significant difference existed in muscle thickness in the GL. In basketballers, GL was 2.5 mm thicker (95% CI: 0.7-4.3 mm, p = 0.011) on the left side and 2.6 mm thicker (95% CI: 0.6-5.7 mm, p = 0.012) on the right side; however, these differences were not significant when stature was accounted for (MT/TL). In females, significant differences existed in the GM for all parameters including FL/TL and MT/TL. Female cyclists had longer FL in both limbs (MD: 11.2 and 11.3 mm), narrower θ (MD: 2.1 and 1.8°), and thicker muscles (MD: 2.1 and 2.5 mm). For the GL, female cyclists had significantly longer FL (MD: 5.2 and 5.8 mm) and narrower θ (MD: 1.7 and 2.3°) in both limbs; no differences were observed in absolute muscle thickness or MT/TL ratio. Differences in gastrocnemius muscle architecture were observed between female cyclists and basketballers, but not between males. These findings suggest that participation in sport-specific training might influence gastrocnemius muscle architecture in elite female athletes; however, it remains unclear as to whether gastrocnemius architecture is systematically influenced by the different modes of muscle activation between these respective sports.

Frequent Manipulation of Resistance Training Variables Promotes Myofibrillar Spacing Changes in Resistance-Trained Individuals

We sought to determine if manipulating resistance training (RT) variables differentially altered the expression of select sarcoplasmic and myofibril proteins as well as myofibrillar spacing in myofibers. Resistance-trained men (n = 20; 26 ± 3 years old) trained for 8 weeks where a randomized leg performed either a standard (CON) or variable RT protocol (VAR: manipulation of load, volume, muscle action, and rest intervals at each RT session). A pre-training (PRE) vastus lateralis biopsy was obtained from a randomized single leg, and biopsies were obtained from both legs 96 h following the last training bout. The sarcoplasmic protein pool was assayed for proteins involved in energy metabolism, and the myofibril protein pool was assayed for relative myosin heavy chain (MHC) and actin protein abundances. Sections were also histologically analyzed to obtain myofibril spacing characteristics. VAR resulted in ~12% greater volume load (VL) compared to CON (p < 0.001). The mean fiber cross-sectional area increased following both RT protocols [CON: 14.6% (775.5 μm²), p = 0.006; VAR: 13.9% (743.2 μm²), p = 0.01 vs. PRE for both], but without significant differences between protocols (p = 0.79). Neither RT protocol affected a majority of assayed proteins related to energy metabolism, but both training protocols increased hexokinase 2 protein levels and decreased a mitochondrial beta-oxidation marker (VLCAD protein; p < 0.05). Citrate synthase activity levels increased with CON RT (p < 0.05), but not VAR RT. The relative abundance of MHC (summed isoforms) decreased with both training protocols (p < 0.05). However, the relative abundance of actin protein (summed isoforms) decreased with VAR only (13.5 and 9.0%, respectively; p < 0.05). A decrease in percent area occupied by myofibrils was observed from PRE to VAR (−4.87%; p = 0.048), but not for the CON (4.53%; p = 0.979). In contrast, there was an increase in percent area occupied by non-contractile space from PRE to VAR (10.14%; p = 0.048), but not PRE to CON (0.72%; p = 0.979). In conclusion, while both RT protocols increased muscle fiber hypertrophy, a higher volume-load where RT variables were frequently manipulated increased non-contractile spacing in resistance-trained individuals.

The role of exercise selection in regional Muscle Hypertrophy: A randomized controlled trial

There is emerging evidence suggesting that muscle growth is not homogeneous through the muscle. The aim of the present study was to analyse the role of exercise selection in regional hypertrophy. Two randomly allocated groups with equal training volume and intensity performed squats in the smith machine (SMTH group) or the leg extension exercise (LEG group). Growth in proximal, central and distal regions of the rectus femoris (RF) and vastus lateralis (VL) muscles, jump height and body composition were analysed. Results show that the three regions of RF grew significantly in the participants of the LEG group (p < 0.05), while only the central region of VL grew significantly in the SMTH group (p < 0.05). In summary, this study confirms that exercise selection plays a role in regional hypertrophy. Whilst there may be still other factors that determine how muscles grow, it seems that the chosen exercises may be responsible of the differences observed in this study.

JNK activation in TA and EDL muscle is load-dependent in rats receiving identical excitation patterns

As the excitation-contraction coupling is inseparable during voluntary exercise, the relative contribution of the mechanical and neural input on hypertrophy-related molecular signalling is still poorly understood. Herein, we use a rat in-vivo strength exercise model with an electrically-induced standardized excitation pattern, previously shown to induce a load-dependent increase in myonuclear number and hypertrophy, to study acute effects of load on molecular signalling. We assessed protein abundance and specific phosphorylation of the four protein kinases FAK, mTOR, p70S6K and JNK after 2, 10 and 28 min of a low- or high-load contraction, in order to assess the effects of load, exercise duration and muscle-type on their response to exercise. Specific phosphorylation of mTOR, p70S6K and JNK was increased after 28 min of exercise under the low- and high-load protocol. Elevated phosphorylation of mTOR and JNK was detectable already after 2 and 10 min of exercise, respectively, but greatest after 28 min of exercise, and JNK phosphorylation was highly load-dependent. The abundance of all four kinases was higher in TA compared to EDL muscle, p70S6K abundance was increased after exercise in a load-independent manner, and FAK and JNK abundance was reduced after 28 min of exercise in both the exercised and control muscles. In conclusion, the current study shows that JNK activation after a single resistance exercise is load-specific, resembling the previously reported degree of myonuclear accrual and muscle hypertrophy with repetition of the exercise stimulus.

Muscle and tendon adaptations to moderate load eccentric vs. concentric resistance exercise in young and older males

Resistance exercise training (RET) is well-known to counteract negative age-related changes in both muscle and tendon tissue. Traditional RET consists of both concentric (CON) and eccentric (ECC) contractions; nevertheless, isolated ECC contractions are metabolically less demanding and, thus, may be more suitable for older populations. However, whether submaximal (60% 1RM) CON or ECC contractions differ in their effectiveness is relatively unknown. Further, whether the time course of muscle and tendon adaptations differs to the above is also unknown. Therefore, this study aimed to establish the time course of muscle and tendon adaptations to submaximal CON and ECC RET. Twenty healthy young (24.5 ± 5.1 years) and 17 older males (68.1 ± 2.4 years) were randomly allocated to either isolated CON or ECC RET which took place 3/week for 8 weeks. Tendon biomechanical properties, muscle architecture and maximal voluntary contraction were assessed every 2 weeks and quadriceps muscle volume every 4 weeks. Positive changes in tendon Young's modulus were observed after 4 weeks in all groups after which adaptations in young males plateaued but continued to increase in older males, suggesting a dampened rate of adaptation with age. However, both CON and ECC resulted in similar overall changes in tendon Young's modulus, in all groups. Muscle hypertrophy and strength increases were similar between CON and ECC in all groups. However, pennation angle increases were greater in CON, and fascicle length changes were greater in ECC. Notably, muscle and tendon adaptations appeared to occur in synergy, presumably to maintain the efficacy of the muscle-tendon unit.

Active older dancers have lower C-terminal Agrin fragment concentration, better balance and gait performance than sedentary peers

Motor neuron degeneration, denervation, neuromuscular junction (NMJ) fragmentation and loss of motor units (MUs), play a key-role in the development of sarcopenia. The aim of the present study was to investigate the beneficial effects of regular practice of dancing in physically active elders on concentration of C-terminal Agrin fragment (CAF), a marker of NMJ instability, muscle mass, strength, and physical performance in a group of 16 recreationally active older dancers (AOD; 70.1 ± 3.4 yr) compared to 15 age-matched sedentary peers (OS; 70.9 ± 6.2 yr). Circulating concentration of CAF was measured in serum, while morphology of the vastus lateralis and multifidus muscles was assessed by ultrasound imaging. In addition, the participants underwent two functional performance tests, the Timed Up and Go (TUG) and the 10-meter walk test (10-MWT), a lower and upper limb isometric strength test, a static and a dynamic balance test. Although no statistically significant differences were detected for both muscle morphology and isometric strength, higher CAF concentration (20%, p < 0.01) was found in OS. AOD showed a better performance in TUG (22%, p < 0.001), 10-MWT (17%, p < 0.001) and dynamic balance (25%, p < 0.01) than OS. Notably, CAF concentration correlated with dynamic balance performance (r = 0.3711, p < 0.05). Our results provide evidence that the regular practice of dancing in older age, together with non-structured light aerobic physical activities, is associated to lower CAF concentration and improved walking and balance performance. Our findings also suggest that NMJ instability, as indicated by elevated CAF serum concentration, seems to precede the loss of muscle size and alterations in muscle architecture normally associated with sarcopenia.

New Insights on Fak and Fak Inhibitors

BACKGROUND

Focal adhesion kinase (Fak) is a cytoplasmic protein tyrosine kinase overexpressed and activated in different solid cancers; it has shown an important role in metastasis formation, cell migration, invasion and angiogenesis and consequently it has been proposed as a potential target in cancer therapy, particularly in a metastatic phase. In recent years, different investigations have highlighted the importance of new Fak inhibitors as potential anti-cancer drugs, but other studies evidenced its role in different pathologies related to the cardiac function or viral infection.

METHODS

An extensive bibliographic research (104 references) has been done concerning the structure of Fak, its importance in tumor development, but also in other pathologies currently under study. The compounds currently subjected to clinical studies were therefore treated using the appropriate databases. Finally, the main chemical scaffolds currently under preclinical investigation were analyzed, focusing on their molecular structures and on the activity structure relationships (SAR).

RESULTS

At the moment, only a few reversible ATP-competitive inhibitors are under investigation in pre-clinical studies and clinical trials. Other compounds, with different chemical scaffolds, are investigated to obtain more active and selective Fak inhibitors. This mini-review is a summary of different Fak functions in cancer and other pathologies; the compounds today in clinical trials and the recent chemical scaffolds (also included in patents) giving the most interesting results are investigated. In addition, PROTAC molecules are reported.

CONCLUSION

All reported results evidenced that additional studies are necessary to design and synthesize new selective and more active compounds, although promising information has been obtained from associations between Fak inhibitors and other different anti- cancer drugs. In addition, the other important roles evidenced, both at the nuclear level and in non-cancerous cells, make this protein an increasingly important target in pharmaceutical chemistry.

Implementing Ultrasound Imaging for the Assessment of Muscle and Tendon Properties in Elite Sports: Practical Aspects, Methodological Considerations and Future Directions

Ultrasound (US) imaging has been widely used in both research and clinical settings to evaluate the morphological and mechanical properties of muscle and tendon. In elite sports scenarios, a regular assessment of such properties has great potential, namely for testing the response to training, detecting athletes at higher risks of injury, screening athletes for structural abnormalities related to current or future musculoskeletal complaints, and monitoring their return to sport after a musculoskeletal injury. However, several practical and methodological aspects of US techniques should be considered when applying this technology in the elite sports context. Therefore, this narrative review aims to (1) present the principal US measures and field of applications in the context of elite sports; (2) to discuss, from a methodological perspective, the strengths and shortcomings of US imaging for the assessment of muscle and tendon properties; and (3) to provide future directions for research and application.

Down stair walking: A simple method to increase muscle mass and performance in 65+ year healthy people

Exercise is important for prevention of sarcopenia in the elderly population. We tested two training modalities, ascending or descending stair walking, representing concentric (CON) and eccentric (ECC) exercise, respectively. We also tested the effects of additional weight during eccentric exercise (ECC+). Thirty-two healthy men and women (70 ± 3 [mean ± SE] yrs.) were randomly assigned to CON, ECC, or ECC+ (carrying +15% of body weight in a vest) in a 3 (n = 32) or 6 (n = 21) week intervention (3 sessions/week). Data was analysed by mixed models approach. Rate of perceived exertion (RPE; Borg scale 6-20; mean values from 3 and 6 weeks) during training did not differ between CON (12.3 ± 0.4), ECC (11.5 ± 0.3), and ECC+ (11.7 ± 0.4). After 6 weeks, leg muscle mass increased more in ECC+ (+0.29 ± 0.09 kg) vs CON (+0.08 ± 0.05 kg) (P<0.05) but not different from ECC (+0.16 ± 0.06 kg). 6-minute walk test (6MWT) increased after 6 weeks more (P<0.05) in ECC+ (+85 ± 23 m) compared with ECC (+37 ± 13 m) and CON (+27 ± 12 m). Intramyocellular glycogen content increased from 359 ± 19 nmol/mg d.w. in CON (to 511 ± 65 and 471 ± 44 after 3 and 6 wks, respectfully (P<0.05)), but not in ECC (to 344 ± 28 after 6 weeks) or in ECC+ (to 389 ± 20 after 6 weeks). Conclusion: carrying extra weight while descending stair walking do not increase RPE, but the ECC+ training resulted in greater muscle responses compared with CON, but glycogen synthesis was stimulated only in CON. Descending stairs is a simple model for prevention and treatment of sarcopenia and the stimulus is enhanced by carrying extra weights.

Are muscle fibres of body builders intrinsically weaker? A comparison with single fibres of aged-matched controls

AIM

Skeletal muscles of Body Builders (BB) represent an interesting model to study muscle mass gains in response to high volume resistance training. It is debated whether muscle contractile performance improves in proportion to mass. Here, we aim to assess whether muscle hypertrophy does not occur at the expense of performance.

METHODS

Six BB and Six untrained controls (CTRL) were recruited. Cross-sectional area (CSA) and maximum voluntary contraction (MVC) of quadriceps femoris muscle (QF) and CSA and architecture of vastus lateralis (VL) were determined. Moreover, a biopsy was taken from VL mid-portion and single fibres were analysed.

RESULTS

QF CSA and MVC were 32% (n.s., P = .052) and 58% (P = .009) higher in BB than in CTRL, respectively. VL CSA was 37% higher in BB (P = .030). Fast 2A fibres CSA was 24% (P = .048) greater in BB than in CTRL, when determined in immunostained sections of biopsy samples. Single permeabilized fast fibres CSA was 37% (n.s., P = .052) higher in BB than in CTRL, and their force was slightly higher in BB (n.s.), while specific tension (P₀ ) was 19% (P = .024) lower. The lower P₀ was not explained either by lower myosin content or by impaired calcium diffusion. Conversely, the swelling caused by skinning-induced permeabilization was different and, when used to correct P₀ , differences between populations disappeared.

CONCLUSIONS

The results show that high degree of muscle hypertrophy is not detrimental for force generation capacity, as increases in fibre size and force are strictly proportional once the differential swelling response is accounted for.

Sex-Related Differences After a Single Bout of Maximal Eccentric Exercise in Response to Acute Effects: A Systematic Review and Meta-analysis

Morawetz, D, Blank, C, Koller, A, Arvandi, M, Siebert, U, and Schobersberger, W. Sex-related differences after a single bout of maximal eccentric exercise in response to acute effects: a systematic review and meta-analysis. J Strength Cond Res 34(9): 2697-2707, 2020-The most prominent effects after unaccustomed eccentric exercise are muscle damage, muscle soreness, strength loss, and higher concentrations of muscle proteins in the plasma. The aim of this systematic review is to evaluate sex-related differences in these acute effects. A systematic literature search in MEDLINE following the PRISMA guidelines was performed. Inclusion criteria were the difference in absolute outcomes between sexes in eccentric muscle strength, strength loss after eccentric exercise, blood concentrations of creatine kinase (CK), and delayed onset muscle soreness (DOMS). Results for maximal eccentric torque and CK data were pooled using a random-effect meta-analysis. A meta-regression was conducted to explain heterogeneity. Based on the 23 included trials, men showed significantly higher absolute eccentric strength. No sex-related differences were detected when normalizing strength for body mass, cross-sectional area of the muscle, or fat-free mass. Women displayed a tendency toward greater relative strength loss immediately after exercise. The absolute CK concentrations of men were significantly higher after exercise-induced muscle damage. No significant difference was found between sexes in DOMS. Untrained men and women display similar responses in all measures of relative muscle strength and DOMS. Apart from the enzymatic activity after exercise and the levels of absolute eccentric torque, there is no evidence for sex-related differences immediately after eccentric exercise. Therefore, eccentric training might have the same impact on men and women. One potential sex difference with practical relevance would be the possible difference in fatigue pattern immediately after eccentric exercise.

Eccentric exercise per se does not affect muscle damage biomarkers: early and late phase adaptations

PURPOSE

Acute high-intensity unaccustomed eccentric exercise performed by naive subjects is accompanied by disturbances in muscle damage biomarkers. The aim of the study was to investigate whether a causal relationship indeed exists between eccentric exercise and muscle damage.

METHODS

Twenty-four men randomly assigned into a concentric only or an eccentric-only training group and performed 10 weeks of isokinetic resistance exercise (one session/week of 75 maximal knee extensors actions). Physiological markers of muscle function and damage (i.e., range of motion, delayed onset muscle soreness, isometric, concentric and eccentric peak torque) were assessed prior to and 1-3 and 5 days post each session. Biochemical markers of muscle damage (creatine kinase) and inflammation (C-reactive protein) were measured prior and 2 days post each session.

RESULTS

After the first bout, eccentric exercise induced greater muscle damage compared to concentric exercise; however, during the nine following sessions, this effect progressively diminished, while after the 10th week of training, no alterations in muscle damage biomarkers were observed after either exercise protocol. Additionally, strength gains at the end of the training period were comparable between the two groups and were mode-specific.

CONCLUSION

(1) eccentric exercise per se does not affect muscle damage biomarkers; (2) muscle damage occurs as a result of muscle unaccustomedness to this action type; (3) exercise-induced muscle damage is not a prerequisite for increased muscle strength. Collectively, we believe that muscle unaccustomedness to high-intensity eccentric exercise, and not eccentric exercise per se, is the trigger for muscle damage as indicated by muscle damage biomarkers.

Modulation of physiological cross-sectional area and fascicle length of vastus lateralis muscle in response to eccentric exercise

In the current study, we investigated the effect of lengthening velocity during eccentric exercise on the modulation of the physiological cross-sectional area (PCSA) and fascicle length of the vastus lateralis (VL) muscle. We hypothesized a greater increase in muscle PCSA after training with lower lengthening velocities and a greater increase in fascicle length after higher lengthening velocities. Forty-seven young men were randomly assigned to either a control (n = 14) or an intervention group (n = 33). The participants of the intervention group were randomly allocated to one of four isokinetic eccentric training protocols of the knee extensors, with four different knee angular velocities (45°/s, 120°/s, 210°/s and 300°/s), yet similar range of motion (25-100° knee joint angle), load magnitude (100% of isometric maximum) and load volume (i.e. similar time under tension for one training set). Before and after an 11-week training period with 3 times per week exercise, muscle volume, pennation angle, fascicle length and PCSA of the VL muscle were measured using magnetic resonance imaging and ultrasonography. After the training, the VL muscle volume and fascicle length increased similarly and approximately 5% in all investigated protocols. The PCSA and pennation angles of the VL did not change after any exercise protocol, indicating negligible radial muscle adaptation after the training. The reason for the found hypertrophy of VL muscle after eccentric training in a wide range of lengthening velocities was mainly a longitudinal muscle growth. Further, the longitudinal muscle growth was independent of the lengthening velocity.

Short-term neuromuscular, morphological, and architectural responses to eccentric quasi-isometric muscle actions

PURPOSE

Eccentric quasi-isometric (EQI) contractions have been proposed as a novel training method for safely exposing the musculotendinous system to a large mechanical load/impulse, with few repetitions. However, understanding of this contraction type is rudimentary. We aimed to compare the acute effects of a single session of isotonic EQIs with isokinetic eccentric (ECC) contractions.

METHODS

Fifteen well-trained men performed a session of impulse-equated EQI and ECC knee extensions, with each limb randomly allocated to one contraction type. Immediately PRE, POST, 24/48/72 h, and 7 days post-exercise, regional soreness, quadriceps swelling, architecture, and echo intensity were evaluated. Peak concentric and isometric torque, rate of torque development (RTD), and angle-specific impulse were evaluated at each time point.

RESULTS

There were substantial differences in the number of contractions (ECC: 100.8 ± 54; EQI: 3.85 ± 1.1) and peak torque (mean: ECC: 215 ± 54 Nm; EQI: 179 ± 28.5 Nm). Both conditions elicited similar responses in 21/53 evaluated variables. EQIs resulted in greater vastus intermedius swelling (7.1-8.8%, ES = 0.20-0.29), whereas ECC resulted in greater soreness at the distal and middle vastus lateralis and distal rectus femoris (16.5-30.4%, ES = 0.32-0.54) and larger echogenicity increases at the distal rectus femoris and lateral vastus intermedius (11.9-15.1%, ES = 0.26--0.54). Furthermore, ECC led to larger reductions in concentric (8.3-19.7%, ES = 0.45-0.62) and isometric (6.3-32.3%, ES = 0.18-0.70) torque and RTD at medium-long muscle lengths.

CONCLUSION

A single session of EQIs resulted in less soreness and smaller reductions in peak torque and RTD versus impulse-equated ECC contractions, yet morphological shifts were largely similar. Long-term morphological, architectural, and neuromuscular adaptations to EQI training requires investigation.

Longitudinal hypertrophic and transcriptional responses to high-load eccentric-concentric vs concentric training in males

High-load eccentric training reputedly produces greater muscle hypertrophy than concentric training, possibly due to greater loading and/or inflammation. We quantified the temporal impact of combined maximal concentric-eccentric training vs maximal concentric training on muscle cross-sectional area (CSA), volume, and targeted mRNA expression (93 transcripts). Eight recreationally active males (24 ± 5 years, BMI 23.5 ± 2.5 kg/m² ) performed 3 x 30 maximal eccentric isokinetic knee extensions and 2 x 30 maximal concentric knee extensions in dominant limb (ECC + CON) and 5 x 30 maximal concentric contractions (CON) in the non-dominant limb for 12 weeks (all 90°/s, 3x/wk). Quadriceps muscle CSA and volume were measured at baseline, 28 days (d), and 84 d in both limbs (3T MRI). Resting vastus lateralis biopsies were obtained from both limbs at baseline, 24 hours (h), 7, 28, and 84 d for mRNA abundance measurements (RT-PCR microfluidic cards). Work output was greater throughout training in ECC + CON vs CON (20.8 ± 9.7%, P < .001). Muscle CSA increased from baseline in both limbs at 28 d (CON 4.3 ± 2.6%, ECC + CON 4.0 ± 1.9%, both P < .001) and 84d (CON 3.9 ± 2.3%, ECC + CON 4.0 ± 3.1%, both P < .001), and muscle volume and isometric strength at 84 d (CON 44.8 ± 40.0%, P < .001; ECC + CON 36.9 ± 40.0%, P < .01), but no between-limb differences existed in any parameter. Ingenuity Pathway Analysis identified several cellular functions associated with regulation of muscle mass and metabolism as altered by both modalities at 24 h and 7 d, but particularly with ECC + CON. However, mRNA responses waned thereafter, regardless of modality. Initial muscle mRNA responses to training did not reflect chronic training-induced hypertrophy. Moreover, ECC + CON did not produce greater hypertrophy than CON, despite greater loading throughout and a differential mRNA response during the initial training week.

The Time-Course of Changes in Muscle Mass, Architecture and Power During 6 Weeks of Plyometric Training

Purpose

To investigate the time-course of changes in knee-extensors muscle mass, architecture and function in response to plyometric training (PLT) performed on a novel training device, the Tramp-Trainer. This machine consists in a trampoline connected to an inclined sledge which allows the performance of repeated jumps while the subject is sitting on a chair.

Methods

Eight healthy males (173.6 ± 4.7 cm, 69.7 ± 13.5 kg, 25.3 ± 4.6 years) underwent 6 weeks of bilateral PLT on the tramp-trainer machine. Training was performed three times per week (between 120 and 150 bounces per session). Knee-extensor maximum voluntary torque (MVT) and power, quadriceps femoris (QF) volume (VOL), cross-sectional area from the 20% to the 60% of femur length and CSA_mean, together with vastus lateralis (VL) architecture (fascicle length, Lf, and pennation angle, PA) were assessed after 2, 4, and 6 weeks of PLT.

Results

All results are presented as changes versus baseline values. MVT increased by 17.8% (week 2, p < 0.001) and 22.2% (week 4, p < 0.01), respectively, and declined to 13.3% (p < 0.05) at week 6 of PLT. Power increased by 18.2% (week 4, p < 0.05) and 19.7% (week 6, p < 0.05). QF VOL increased by 4.7% (week 4, p < 0.05) and 5.8% (week 6, p < 0.01); VL VOL increased by 5.2%, (p < 0.05), 8.2%, (p < 0.01), and 9.6% (p < 0.05) at weeks 2, 4, and 6, respectively. An increase in Lf was detected already at wk 2 (2.2%, p < 0.05), with further increase at 4 and 6 weeks of PLT (4 and 4.4%, respectively, p < 0.01). PA increased by 5.8% (p < 0.05) at week 6. Significant positive correlations were found between CSA_mean and Power (R² = 0.46, p < 0.001) and between QF VOL and Power (R² = 0.44, p < 0.024).

Conclusions

PLT induced rapid increases in muscle volume, fascicle length, pennation angle, torque and power in healthy younger adults. Notably, changes in VL VOL and Lf were detectable already after 2 weeks, followed by increases in knee extensors VOL and power from week 4 of PLT. Since the increase in CSA_mean and QF VOL cannot fully explain the increment in muscle power, it is likely that other factors (such as adaptations in neural drive or tendon mechanical properties) may have contributed to such fucntional changes.

Serum extracellular vesicle miR-203a-3p content is associated with skeletal muscle mass and protein turnover during disuse atrophy and regrowth

Small noncoding microRNAs (miRNAs) are important regulators of skeletal muscle size, and circulating miRNAs within extracellular vesicles (EVs) may contribute to atrophy and its associated systemic effects. The purpose of this study was to understand how muscle atrophy and regrowth alter in vivo serum EV miRNA content. We also associated changes in serum EV miRNA with protein synthesis, protein degradation, and miRNA within muscle, kidney, and liver. We subjected adult (10 mo) F344/BN rats to three conditions: weight bearing (WB), hindlimb suspension (HS) for 7 days to induce muscle atrophy, and HS for 7 days followed by 7 days of reloading (HSR). Microarray analysis of EV miRNA content showed that the overall changes in serum EV miRNA were predicted to target major anabolic, catabolic, and mechanosensitive pathways. MiR-203a-3p was the only miRNA demonstrating substantial differences in HS EVs compared with WB. There was a limited association of EV miRNA content to the corresponding miRNA content within the muscle, kidney, or liver. Stepwise linear regression demonstrated that EV miR-203a-3p was correlated with muscle mass and muscle protein synthesis and degradation across all conditions. Finally, EV miR-203a-3p expression was significantly decreased in human subjects who underwent unilateral lower limb suspension (ULLS) to induce muscle atrophy. Altogether, we show that serum EV miR-203a-3p expression is related to skeletal muscle protein turnover and atrophy. We suggest that serum EV miR-203a-3p content may be a useful biomarker and future work should investigate whether serum EV miR-203a-3p content is mechanistically linked to protein synthesis and degradation.

Novel Essential Amino Acid Supplements Following Resistance Exercise Induce Aminoacidemia and Enhance Anabolic Signaling Irrespective of Age: A Proof-of-Concept Trial

We investigated the effects of ingesting a leucine-enriched essential amino acid (EAA) gel alone or combined with resistance exercise (RE) versus RE alone (control) on plasma aminoacidemia and intramyocellular anabolic signaling in healthy younger (28 ± 4 years) and older (71 ± 3 years) adults. Blood samples were obtained throughout the three trials, while muscle biopsies were collected in the postabsorptive state and 2 h following RE, following the consumption of two 50 mL EAA gels (40% leucine, 15 g total EAA), and following RE with EAA (combination (COM)). Protein content and the phosphorylation status of key anabolic signaling proteins were determined via immunoblotting. Irrespective of age, during EAA and COM peak leucinemia (younger: 454 ± 32 µM and 537 ± 111 µM; older: 417 ± 99 µM and 553 ± 136 µM) occurred ~60–120 min post-ingestion (younger: 66 ± 6 min and 120 ± 60 min; older: 90 ± 13 min and 78 ± 12 min). In the pooled sample, the area under the curve for plasma leucine and the sum of branched-chain amino acids was significantly greater in EAA and COM compared with RE. For intramyocellular signaling, significant main effects were found for condition (mTOR (Ser2481), rpS6 (Ser235/236)) and age (S6K1 (Thr421/Ser424), 4E-BP1 (Thr37/46)) in age group analyses. The phosphorylation of rpS6 was of similar magnitude (~8-fold) in pooled and age group data 2 h following COM. Our findings suggest that a gel-based, leucine-enriched EAA supplement is associated with aminoacidemia and a muscle anabolic signaling response, thus representing an effective means of stimulating muscle protein anabolism in younger and older adults following EAA and COM.

Reduced Active Muscle Stiffness after Intermittent Submaximal Isometric Contractions

PURPOSE

Whether muscle stiffness is influenced by fatigue remains unclear. Classical methods used to assess muscle stiffness provide a global measure at the joint level. As fatigue may selectively affect specific muscles, a joint-level approach may not be sensitive enough to detect potential changes in muscle stiffness. Taking advantage of ultrasound shear wave elastography, this study aimed to determine the influence of a fatiguing protocol involving intermittent submaximal isometric contractions on muscle shear modulus (an index of stiffness).

METHODS

Shear modulus was measured on either the vastus lateralis (n = 9) or the abductor digiti minimi (n = 10) before and after 15 min of intermittent submaximal isometric contractions at 60% of maximal voluntary contraction (MVC) (4 s ON, 4 s OFF). An index of active muscle stiffness was estimated PRE- and POST-fatigue as the slope of the linear regression established between shear modulus and absolute joint force up to 60% MVC.

RESULTS

After the fatiguing exercise, MVC was significantly decreased by 22% ± 7% and 32% ± 15% for knee extension and little finger abduction, respectively (P < 0.001). When compared to PRE-fatigue, the index of active muscle stiffness was 12% ± 15% lower for the vastus lateralis (P < 0.031) and 44% ± 19% lower for the abductor digiti minimi (P < 0.001) POST-fatigue.

CONCLUSIONS

Although the present results cannot clearly determine the involved mechanisms, they demonstrate a decreased active muscle stiffness after a fatiguing task involving intermittent submaximal isometric contractions. Further studies should now determine whether this change in stiffness affects performance and risk of injury.

Muscle activation during leg-press exercise with or without eccentric overload

PURPOSE

This study sought to investigate the electromyographic activity of the vastus lateralis (VL) muscle during concentric-eccentric exercise using a new concept leg press machine enabling a preset overloading in the eccentric phase.

METHODS

Ten young males familiar with resistive exercise were recruited for this study. Tests were performed on a Leg-press Biostrength® (Technogym S.p.A., Italy). The load was set to 70% and 80% of one-repetition maximum (1-RM). The participants performed 2 sets of 6 repetitions at each relative load with (ECC +) and without (ISOW) an eccentric overload equivalent to 150% of the concentric load. A metronome was employed to maintain the selected cadence. Sets were separated by a 5-min rest. Surface electromyography (EMG) of VL was recorded and integrated (iEMG).

RESULTS

Results showed a higher iEMG in ECC + with respect to ISOW at both intensities (+ 29% for 70% 1-RM, p < 0.01 and + 31% for 80% 1-RM, p < 0.001). No statistically significant differences were detected between concentric and eccentric phase in both ECC + conditions.

CONCLUSIONS

Training with a 150% eccentric overload provides a ~ 30% greater motor unit recruitment of the VL muscle in leg press exercise. Moreover, the results show that the eccentric overloading provided by the Biostrength® machine enables training at the same level of neural activation of the concentric phase. Hence, the derecruitment of motor units, normally observed during the eccentric phase when using conventional training machines, was overcome using the Biostrength® machine; this observation seems particularly important for maximizing neuromuscular responses to strength training.

Variability of regional quadriceps echo intensity in active young men with and without subcutaneous fat correction

Quantifying echo intensity (EI), a proposed measure of muscle quality, is becoming increasingly popular. Additionally, much attention has been paid to regional differences in other ultrasonically evaluated measures of muscle morphology and architecture. However, the variability of regional (proximal, middle, distal) EI of the vastus lateralis, rectus femoris, and lateral and anterior vastus intermedius has yet to be determined. Twenty participants (40 limbs), were evaluated on 3 occasions, separated by 7 days. Intersession variability of EI with and without subcutaneous fat correction was quantified. Furthermore, the interchangeability of corrected EI across regions was evaluated. Variability of regional quadriceps EI was substantially lower with subcutaneous fat correction (intraclass correlation coefficient (ICC) = 0.81-0.98, coefficient of variation (CV) = 4.5%-16.8%, typical error of measure (TEM) = 0.13-0.49) versus raw values (ICC = 0.69-0.98, CV = 7.7%-42.7%, TEM = 0.14-0.68), especially when examining the vastus intermedius (ICC = 0.81-0.95, CV = 7.1%-16.8%, TEM = 0.23-0.49 vs. ICC = 0.69-0.92, CV = 22.9%-42.7%, TEM = 0.31-0.68). With the exception of the rectus femoris and vastus intermedius (p ≥ 0.143, effect size (ES) ≤ 0.18), corrected EI was greater for proximal and distal regions when compared with the midpoint (p ≤ 0.038, ES = 0.38-0.82). Researchers and practitioners should utilize subcutaneous fat thickness correction to confidently evaluate EI at all regions of the quadriceps. Regional EI cannot be used interchangeably for the vastus muscles, likely because of an increase in fibrous content towards the myotendinous junctions. Novelty Regional quadriceps echo intensity was reliable with and without correction for subcutaneous fat thickness. Intersession variability of regional quadriceps echo intensity was substantially improved following subcutaneous fat correction. Quadriceps echo intensity increased towards myotendinous junctions in the vastus muscles.

What makes long-term resistance-trained individuals so strong? A comparison of skeletal muscle morphology, architecture, and joint mechanics

The greater muscular strength of long-term resistance-trained (LTT) individuals is often attributed to hypertrophy, but the role of other factors, notably maximum voluntary specific tension (ST), muscle architecture, and any differences in joint mechanics (moment arm), have not been documented. The aim of the present study was to examine the musculoskeletal factors that might explain the greater quadriceps strength and size of LTT vs. untrained (UT) individuals. LTT (n = 16, age 21.6 ± 2.0 yr) had 4.0 ± 0.8 yr of systematic knee extensor heavy-resistance training experience, whereas UT (n = 52; age 25.1 ± 2.3 yr) had no lower-body resistance training experience for >18 mo. Knee extension dynamometry, T1-weighted magnetic resonance images of the thigh and knee, and ultrasonography of the quadriceps muscle group at 10 locations were used to determine quadriceps: isometric maximal voluntary torque (MVT), muscle volume (Q_VOL), patella tendon moment arm (PTMA), pennation angle (QΘ_P) and fascicle length (QF_L), physiological cross-sectional area (QPCSA), and ST. LTT had substantially greater MVT (+60% vs. UT, P < 0.001) and Q_VOL (+56%, P < 0.001) and QPCSA (+41%, P < 0.001) but smaller differences in ST (+9%, P < 0.05) and moment arm (+4%, P < 0.05), and thus muscle size was the primary explanation for the greater strength of LTT. The greater muscle size (volume) of LTT was primarily attributable to the greater QPCSA (+41%; indicating more sarcomeres in parallel) rather than the more modest difference in F_L (+11%; indicating more sarcomeres in series). There was no evidence in the present study for regional hypertrophy after LTT.

NEW & NOTEWORTHY Here we demonstrate that the larger muscle strength (+60%) of a long-term (4+ yr) resistance-trained group compared with untrained controls was due to their similarly larger muscle volume (+56%), primarily due to a larger physiological cross-sectional area and modest differences in fascicle length, as well as modest differences in maximum voluntary specific tension and patella tendon moment arm. In addition, the present study refutes the possibility of regional hypertrophy, despite large differences in muscle volume.

Activation of protein synthesis, regeneration, and MAPK signaling pathways following repeated bouts of eccentric cycling

The aim of this study was to examine the activation of skeletal muscle signaling pathways related to protein synthesis and the gene expression of regeneration/degradation markers following repeated bouts of eccentric cycling. Nine untrained men (25.4 ± 1.9 yr) performed two 30-min eccentric cycling bouts (ECC1, ECC2) at 85% of maximal concentric workload, separated by 2 wk. Muscle biopsies were taken from the vastus lateralis before and 2 h after each bout. Indirect markers of muscle damage were assessed before and 24-48 h after exercise. Changes in the Akt/mammalian target of rapamycin (mTOR)/rbosomal protein S6 kinase 1 (S6K1)/ribosomal protein S6 (rpS6) and MAPK signaling pathways were measured by Western blot and changes in mRNA expression of IL-6 and IL-1β, and myogenic regulatory factors (MRFs) were measured by real-time PCR. ECC1 induced greater increases in indirect markers of muscle damage compared with ECC2. Phosphorylation of S6K1 and rpS6 increased after both exercise bouts (P < 0.05), whereas phosphorylation of mTOR increased after ECC2 only (P = 0.03). Atrogin-1 mRNA expression decreased after ECC1 and ECC2 (P < 0.05) without changes in muscle RING-finger protein-1 mRNA. Basal mRNA levels of myoblast determination protein-1 (MyoD), MRF4, and myogenin were higher 2 wk after ECC1 (P < 0.05). MRF4 mRNA increased after ECC1 and ECC2 (P < 0.05), whereas MyoD mRNA expression increased only after ECC1 (P = 0.03). Phosphorylation of JNK and p38 MAPK increased after both exercise bouts (P < 0.05), similar to IL-6 and IL-1β mRNA expression. All together, these results suggest that differential regulation of the mTOR pathway and MRF expression could mediate the repeated bout effect observed between an initial and secondary bout of eccentric exercise.