Muscle conduction velocity, strength, neural activity, and morphological changes after eccentric and concentric training

Torque, power and muscle activation of eccentric and concentric isokinetic cycling

This study aimed to establish the effect of cycling mode and cadence on torque, external power output, and lower limb muscle activation during maximal, recumbent, isokinetic cycling. After familiarisation, twelve healthy males completed 6 × 10 s of maximal eccentric (ECC) and concentric (CON) cycling at 20, 40, 60, 80, 100, and 120 rpm with five minutes recovery. Vastus lateralis, medial gastrocnemius, rectus femoris, and biceps femoris surface electromyography was recorded throughout. As cadence increased, peak torque linearly decreased during ECC (350-248 N·m) and CON (239-117 N·m) and peak power increased in a parabolic manner. Crank angle at peak torque increased with cadence in CON (+13°) and decreased in ECC (-9.0°). At all cadences, peak torque (mean +129 N·m, range 111-143 N·m), and power (mean +871 W, range 181-1406 W), were greater during ECC compared to CON. For all recorded muscles the crank angle at peak muscle activation was greater during ECC compared to CON. This difference increased with cadence in all muscles except the vastus lateralis. Additionally, peak vastus laterallis and biceps femoris activation was greater during CON compared to ECC. Eccentric cycling offers a greater mechanical stimulus compared to concentric cycling but the effect of cadence is similar between modalities. Markers of technique (muscle activation, crank angle at peak activation and torque) were different between eccentric and concentric cycling and respond differently to changes in cadence. Such data should be considered when comparing between, and selecting cadences for, recumbent, isokinetic, eccentric and concentric cycling.

Muscle fibers and their synapses differentially adapt to aging and endurance training

BACKGROUND

This project aimed to determine the adaptability of the neuromuscular system to the stimuli of exercise training, and aging.

METHODS

Young adult, and aged male rats were randomly assigned to either exercise training, or sedentary control groups. Exercise training featured an 8 week program of treadmill running. At the end of the intervention period, neuromuscular function was quantified with ex vivo stimulation procedures on isolated soleus muscles. Morphological adaptations were determined by quantifying myofiber profiles (fiber size and type) of soleus muscles.

RESULTS

Ex vivo procedures confirmed that rested (fresh) young muscles were significantly (P < 0.05) stronger than aged ones. By the end of the 5 min stimulation protocol, however, young and aged muscles displayed similar levels of strength. Neuromuscular transmission efficacy as assessed by comparing force produced during indirect (neural) and direct (muscle) stimulation was unaffected by aging, or training, but under both conditions significantly declined over the stimulation protocol mimicking declines in strength. Myofiber size was unaffected by age, but training caused reductions in young, but not aged myofibers. Aged solei displayed a higher percentage of Type I fibers, along with a lower percentage of Type II fibers than young muscles.

CONCLUSIONS

The greater strength of young muscles has a neural, rather than a muscular focal point. The loss of strength discerned over the 5 min stimulation protocol was linked to similar fatigue-related impairments in neuromuscular transmission. The two components of the neuromuscular system, i.e. nerves and muscles, do not respond in concert to the stimulus of either aging, or exercise training.

End-of-Fiber Signals Strongly Influence the First and Second Phases of the M Wave in the Vastus Lateralis: Implications for the Study of Muscle Excitability

It has been recurrently observed that, for compound muscle action potentials (M wave) recorded over the innervation zone of the vastus lateralis, the descending portion of the first phase generally shows an “inflection” or “shoulder.” We sought to clarify the electrical origin of this shoulder-like feature and examine its implications. M waves evoked by maximal single shocks to the femoral nerve were recorded in monopolar and bipolar configurations from 126 individuals using classical (10-mm recording diameter, 20-mm inter-electrode distance) electrodes and from eight individuals using small electrodes arranged in a linear array. The changes of the M-wave waveform at different positions along the muscle fibers' direction were examined. The shoulder was identified more frequently in monopolar (97%) than in bipolar (46%) M waves. The shoulder of M waves recorded at different distances from the innervation zone had the same latency. Furthermore, the shoulder of the M wave recorded over the innervation zone coincided in latency with the positive peak of that recorded beyond the muscle. The positive phase of the M wave detected 20 mm away from the innervation zone was essentially composed of non-propagating components. The shoulder-like feature in monopolar and bipolar M waves results from the termination of action potentials at the superficial aponeurosis of the vastus lateralis. We conclude that, only the amplitude of the first phase, and not the second, of M waves recorded monopolarly and/or bipolarly in close proximity to the innervation zone can be used reliably to monitor possible changes in muscle membrane excitability.

Rectus Femoris Echo Intensity Correlates with Muscle Strength, but Not Endurance, in Younger and Older Men

We examined correlations between echo intensity and muscle strength and endurance. Rectus femoris echo intensity, maximal voluntary contraction (MVC) force and time to task failure during a 50% MVC task were determined for 12 younger (mean age = 25 y) and 13 older (mean age = 74 y) men. Bivariate correlations between echo intensity and normalized MVC force were similar for younger and older men, but was only statistically significant for the latter (younger r = -0.559, p = 0.059; older r = -0.580, p = 0.038). When all patients were combined, the correlation was significant (r = -0.733, p < 0.001). Significant correlations were not observed for time to task failure (younger r = -0.382, p = 0.221; older r = -0.347, p = 0.246; all patients r = -0.229, p = 0.270). Rectus femoris echo intensity is associated with muscle strength, but not endurance, in younger and older men.

Skeletal Muscle Remodeling in Response to Eccentric vs. Concentric Loading: Morphological, Molecular, and Metabolic Adaptations

Skeletal muscle contracts either by shortening or lengthening (concentrically or eccentrically, respectively); however, the two contractions substantially differ from one another in terms of mechanisms of force generation, maximum force production and energy cost. It is generally known that eccentric actions generate greater force than isometric and concentric contractions and at a lower metabolic cost. Hence, by virtue of the greater mechanical loading involved in active lengthening, eccentric resistance training (ECC RT) is assumed to produce greater hypertrophy than concentric resistance training (CON RT). Nonetheless, prevalence of either ECC RT or CON RT in inducing gains in muscle mass is still an open issue, with some studies reporting greater hypertrophy with eccentric, some with concentric and some with similar hypertrophy within both training modes. Recent observations suggest that such hypertrophic responses to lengthening vs. shortening contractions are achieved by different adaptations in muscle architecture. Whilst the changes in muscle protein synthesis in response to acute and chronic concentric and eccentric exercise bouts seem very similar, the molecular mechanisms regulating the myogenic adaptations to the two distinct loading stimuli are still incompletely understood. Thus, the present review aims to, (a) critically discuss the literature on the contribution of eccentric vs. concentric loading to muscular hypertrophy and structural remodeling, and, (b) clarify the molecular mechanisms that may regulate such adaptations. We conclude that, when matched for either maximum load or work, similar increase in muscle size is found between ECC and CON RT. However, such hypertrophic changes appear to be achieved through distinct structural adaptations, which may be regulated by different myogenic and molecular responses observed between lengthening and shortening contractions.

The effects of high intensity interval training on muscle size and quality in overweight and obese adults

OBJECTIVES

Despite growing popularity of high intensity interval training (HIIT) for improving health and fitness, limited data exist identifying the effects of HIIT on muscle characteristics. The purpose of the current study was to investigate the effects of a 3-week HIIT intervention on muscle size and quality in overweight and obese men and women.

DESIGN

Randomized controlled trial.

METHODS

Forty-four overweight and obese men and women (mean±SD; age: 35.4±12.3years; height: 174.9±9.7cm; weight: 94.6±17.0kg; %fat: 32.7±6.5%) completed the current study. During baseline and post testing, muscle cross sectional area (mCSA) and echo intensity (EI) were determined from a panoramic scan of the vastus lateralis obtained by B-mode ultrasonography. Body composition variables were measured using dual energy X-ray absorptiometry. Participants were randomized into either a 1:1 work-to-rest ratio HIIT group (SIT; n=16), a 2:1 work-to-rest ratio HIIT group (LIT; n=19), or control (CON; n=9). HIIT participants performed five, 2-min bouts (LIT) or 10, 1-min bouts (SIT) at 85-100% VO_2peak for 9 sessions over three weeks.

RESULTS

Analysis of covariance demonstrated a significant increase in mCSA for SIT (p=0.038; change (Δ)=3.17±3.36cm²) compared to CON (Δ=-0.34±2.36cm²). There was no significant difference in EI across groups (p=0.672).

CONCLUSIONS

HIIT may be an effective exercise modality to influence muscle size in overweight and obese individuals. Future studies should investigate muscle characteristics and remodeling in an overweight population following interventions of longer duration and varying work-to-rest protocols.

Brain-derived neurotrophic factor (BDNF) serum basal levels is not affected by power training in mobility-limited older adults - A randomized controlled trial

Brain-derived neurotrophic factor (BDNF) is a potential important factor involved in neuroplasticity, and may be a mediator for eliciting adaptations in neuromuscular function and physical function in older individuals following physical training. As power training taxes the neural system to a very high extent, it may be particularly effective in terms of eliciting increases in systemic BDNF levels. We examined the effects of 12weeks of power training on mature BDNF (mBDNF) and total BDNF (tBDNF) in mobility-limited older adults from the Healthy Ageing Network of Competence (HANC) study. We included 47 older men and women: n=22 in the training group (TG: progressive high intensity power training, 2 sessions per week; age 82.7±5.4years, 55% women) and n=25 in the control group (CG: no interventions; age 82.2±4.5years, 76% women). Following overnight fasting, basal serum levels of mBDNF and tBDNF were assessed (human ELISA kits) at baseline and post-intervention. At baseline, mBDNF and tBDNF levels were comparable in the two groups, TG and CG. Post-intervention, no significant within-group or between-group changes were observed in mBDNF or tBDNF. Moreover, when divided into responder tertiles based upon changes in mBDNF and tBDNF (i.e. decliners, maintainers, improvers), respectively, comparable findings were observed for TG and CG. Altogether, basal systemic levels of serum mBDNF and tBDNF are not affected in mobility-limited older adults following 12-weeks of power training, and do not appear to be a major mechanistic factor mediating neuroplasticity in mobility-limited older adults.

Are the Responses to Resistance Training Different Between the Preferred and Nonpreferred Limbs?

Humans preferentially recruit limbs to functionally perform a range of daily tasks, which may lead to performance asymmetries. Because initial training status plays an important role in the rate of progression during resistance training, could asymmetries between the preferred and nonpreferred limbs lead to different magnitudes of strengthening during a resistance training program? This issue motivated this study, in which 12 healthy and physically active men completed a 4-week control period followed by a 12-week isokinetic resistance training program, performed twice a week, including 3-5 sets of 10 maximal eccentric contractions for each limb. Every 4 weeks, knee extensor peak torques at concentric, isometric, and eccentric tests were measured using an isokinetic dynamometer and the sum of quadriceps muscle thickness was determined by ultrasound images. Before training, concentric peak torque was similar between limbs but isometric and eccentric peak torques were significantly smaller in the nonpreferred compared with the preferred limb (4.9 and 5.8%, respectively). Bilateral strength symmetry remained constant throughout the training period for concentric tests. For eccentric and isometric tests, symmetry was reached at the fourth and eighth training weeks, respectively. After 12 weeks, between-limb percent nonsignificant differences were -0.62% for isometric and -1.93% for eccentric tests. The sum of knee extensor muscle thickness had similar values before training and presented similar changes throughout the study for both the preferred and the nonpreferred limbs. In conclusion, the nonpreferred limb presents higher strength gain than the preferred limb at the initial phase of an isokinetic resistance training program, and this increased strength gain is not associated with muscle hypertrophy.

Greater Strength Gains after Training with Accentuated Eccentric than Traditional Isoinertial Loads in Already Strength-Trained Men

As training experience increases it becomes more challenging to induce further neuromuscular adaptation. Consequently, strength trainers seek alternative training methods in order to further increase strength and muscle mass. One method is to utilize accentuated eccentric loading, which applies a greater external load during the eccentric phase of the lift as compared to the concentric phase. Based upon this practice, the purpose of this study was to determine the effects of 10 weeks of accentuated eccentric loading vs. traditional isoinertial resistance training in strength-trained men. Young (22 ± 3 years, 177 ± 6 cm, 76 ± 10 kg, n = 28) strength-trained men (2.6 ± 2.2 years experience) were allocated to concentric-eccentric resistance training in the form of accentuated eccentric load (eccentric load = concentric load + 40%) or traditional resistance training, while the control group continued their normal unsupervised training program. Both intervention groups performed three sets of 6-RM (session 1) and three sets of 10-RM (session 2) bilateral leg press and unilateral knee extension exercises per week. Maximum force production was measured by unilateral isometric (110° knee angle) and isokinetic (concentric and eccentric 30°.s⁻¹) knee extension tests, and work capacity was measured by a knee extension repetition-to-failure test. Muscle mass was assessed using panoramic ultrasonography and dual-energy x-ray absorptiometry. Surface electromyogram amplitude normalized to maximum M-wave and the twitch interpolation technique were used to examine maximal muscle activation. After training, maximum isometric torque increased significantly more in the accentuated eccentric load group than control (18 ± 10 vs. 1 ± 5%, p < 0.01), which was accompanied by an increase in voluntary activation (3.5 ± 5%, p < 0.05). Isokinetic eccentric torque increased significantly after accentuated eccentric load training only (10 ± 9%, p < 0.05), whereas concentric torque increased equally in both the accentuated eccentric load (10 ± 9%, p < 0.01) and traditional (9 ± 6%, p < 0.01) resistance training groups; however, the increase in the accentuated eccentric load group was significantly greater (p < 0.05) than control (1 ± 7%). Knee extension repetition-to-failure improved in the accentuated eccentric load group only (28%, p < 0.05). Similar increases in muscle mass occurred in both intervention groups. In summary, accentuated eccentric load training led to greater increases in maximum force production, work capacity and muscle activation, but not muscle hypertrophy, in strength-trained individuals.

Physiological and Neural Adaptations to Eccentric Exercise: Mechanisms and Considerations for Training

Eccentric exercise is characterized by initial unfavorable effects such as subcellular muscle damage, pain, reduced fiber excitability, and initial muscle weakness. However, stretch combined with overload, as in eccentric contractions, is an effective stimulus for inducing physiological and neural adaptations to training. Eccentric exercise-induced adaptations include muscle hypertrophy, increased cortical activity, and changes in motor unit behavior, all of which contribute to improved muscle function. In this brief review, neuromuscular adaptations to different forms of exercise are reviewed, the positive training effects of eccentric exercise are presented, and the implications for training are considered.

Progressive Resistance Exercise with Eccentric Loading for the Management of Knee Osteoarthritis

INTRODUCTION

The patient was a 58-year-old African-American male with radiographic evidence of bilateral knee osteoarthritis (OA). He participated in a standardized 12-week eccentric strengthening program within a Veterans Affairs (VA) medical center.

BACKGROUND

The use of an eccentric training paradigm may prove to be beneficial for older adults with knee OA since eccentric muscle actions are involved in the energy absorption at the knee joint during gait and controlled movement during stair descent. Furthermore, in comparison to standard muscle actions, eccentric muscle actions result in higher torque generation and a lower rate of oxygen consumption at a given level of perceived exertion. Therefore, this mode of progressive resistance exercise may be ideal for older adults.

DISCUSSION

The patient completed an eccentric strengthening regimen for the knee flexors and extensors twice per week without an exacerbation of knee pain. Muscle morphology measures of the rectus femoris were measured using diagnostic ultrasound. Isokinetic measures of muscle peak torque were obtained at 60°/s and 180°/s. Functional performance was assessed using a physical performance battery and stair-step performance was assessed from the linear displacement of the center of gravity trajectories obtained with a force plate. Visual analog scale pain ratings and self-reported global disease status were also documented. Post-exercise assessments revealed improvements in sonographic muscle size and tissue composition estimates, peak knee extensor torque (ranging from 60 to 253%), functional performance, and global disease status.

CONCLUDING REMARKS

The patient exhibited improvements in muscle morphology, muscle strength, functional performance, pain, and global disease status after 12 weeks of an eccentric strengthening regimen. The intervention and outcomes featured in this case were feasible to implement within a VA medical center and merit further investigation.

Test-Retest Reliability of Single Transverse versus Panoramic Ultrasound Imaging for Muscle Size and Echo Intensity of the Biceps Brachii

This study compared test-retest reliability and sensitivity to change for muscle size and echo intensity (EI) measurements from single transverse (EIST) versus panoramic (EIP) ultrasound (US) images of the biceps brachii. Forearm flexor muscle thickness and EIST and biceps brachii muscle cross-sectional area and EIP were quantified from single transverse and panoramic US images in 14 men (age = 21.8 ± 2.5 y [mean ± standard deviation]) on two separate days. The intra-class correlation coefficients, coefficients of variation, and minimum differences for muscle thickness, EIST, muscle cross-sectional area and EIP ranged from 0.78 to 0.99, from 2.26% to 3.29%, and from 6.26% to 9.12%, respectively. These findings suggested that single transverse imaging and panoramic US imaging are comparable, reliable techniques for quantifying muscle size and EI of the biceps brachii. Single transverse images may be simpler to obtain; thus, future studies may choose to quantify muscle size and EI from a single transverse US image in the biceps brachii.

Reliability of panoramic ultrasound imaging in simultaneously examining muscle size and quality of the hamstring muscles in young, healthy males and females

The purpose of this study was to examine the reliability of ultrasound (US) measures of cross-sectional area (CSA), muscle thickness (MT) and echo intensity (EI) of the hamstrings, with comparisons between males and females. In 20 healthy participants (10 males, 10 females), CSA, MT and EI were measured from panoramic US scans of the hamstrings on 2 separate days. The intra-class correlation coefficients and standard errors of measurement as a percentage of the mean for CSA, MT and EI ranged from 0.715 to 0.984 and from 3.145 to 12.541% in the males and from 0.724 to 0.977 and from 4.571 to 17.890% in the females, respectively. The males had greater CSAs and MTs and lower EIs than the females (p = 0.002-0.049), and significant relationships were observed between CSA and MT (r = 0.714-0.938, p ≤ 0.001-0.023). From an overall reliability standpoint, these findings suggest that panoramic US may be a reliable technique for examining muscle size and quality of the hamstrings in both males and females.