Concurrent Strength and Endurance Training: A Systematic Review and Meta-Analysis on the Impact of Sex and Training Status

BACKGROUND

Many sports require maximal strength and endurance performance. Concurrent strength and endurance training can lead to suboptimal training adaptations. However, how adaptations differ between males and females is currently unknown. Additionally, current training status may affect training adaptations.

OBJECTIVE

We aimed to assess sex-specific differences in adaptations in strength, power, muscle hypertrophy, and maximal oxygen consumption ( V ˙ O2max) to concurrent strength and endurance training in healthy adults. Second, we investigated how training adaptations are influenced by strength and endurance training status.

METHODS

A systematic review and meta-analysis was conducted according to PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines, and a Cochrane risk of bias was evaluated. ISI Web of science, PubMed/MEDLINE, and SPORTDiscus databases were searched using the following inclusion criteria: healthy adults aged 18-50 years, intervention period of ≥ 4 weeks, and outcome measures were defined as upper- and lower-body strength, power, hypertrophy, and/or V ˙ O2max. A meta-analysis was performed using a random-effects model and reported in standardized mean differences.

RESULTS

In total, 59 studies with 1346 participants were included. Concurrent training showed blunted lower-body strength adaptations in males, but not in females (male: - 0.43, 95% confidence interval [- 0.64 to - 0.22], female: 0.08 [- 0.34 to 0.49], group difference: P = 0.03). No sex differences were observed for changes in upper-body strength (P = 0.67), power (P = 0.37), or V ˙ O2max (P = 0.13). Data on muscle hypertrophy were insufficient to draw any conclusions. For training status, untrained but not trained or highly trained endurance athletes displayed lower V ˙ O2max gains with concurrent training (P = 0.04). For other outcomes, no differences were found between untrained and trained individuals, both for strength and endurance training status.

CONCLUSIONS

Concurrent training results in small interference for lower-body strength adaptations in males, but not in females. Untrained, but not trained or highly trained endurance athletes demonstrated impaired improvements in V ˙ O2max following concurrent training. More studies on females and highly strength-trained and endurance-trained athletes are warranted.

CLINICAL TRIAL REGISTRATION

PROSPERO: CRD42022370894.

Research trends and hotspots of exercise for people with sarcopenic: A bibliometric analysis

This study aimed to analyze the trends and themes in exercise and sarcopenia research using a bibliometric approach. The Web of Science citation database was used to identify papers published on exercise and sarcopenia. The retrieved data on institutions, journals, countries, authors, journal distribution, and keywords were analyzed scientometric ally using CiteSpace and VOSviewer. 2895 papers were included according to our specified inclusion criteria eventually. The data showed an upward trend in the number of published articles on exercise and sarcopenia. The countries with the highest number of publications were the United States, Japan, and England; research institutions were mainly composed of universities in Europe and the United States, and high-producing authors formed major collaborative teams, but cross-geographical and cross-institutional collaboration was not apparent; research was closely focused on 3 aspects: resistance exercise, resistance combined with other forms of exercise, and exercise combined with nutritional supplementation, of which resistance exercise was a particular focus; and recently, the research hotspots were mainly the effects of exercise on grip strength. The most cited articles were consensus guidelines published by the working group on sarcopenia in the elderly from different continents. The prevention and rehabilitation of sarcopenia in the elderly are gaining attention. Current primary exercise therapies for sarcopenia and exercise combined with nutritional supplementation have significant advantages and the potential to delay muscle decay. This suggests a promising area for future research that could benefit from further advances.

Effects of Concurrent Resistance and Endurance Training Using Continuous or Intermittent Protocols on Muscle Hypertrophy: Systematic Review With Meta-Analysis

ABSTRACT

Monserdà-Vilaró, A, Balsalobre-Fernández, C, Hoffman, JR, Alix-Fages, C, and Jiménez, SL. Effects of concurrent resistance and endurance training using continuous or intermittent protocols on muscle hypertrophy: Systematic review with meta-analysis. J Strength Cond Res 37(3): 688-709, 2023-The purpose of this systematic review with meta-analysis was to explore the effects of concurrent resistance and endurance training (CT) incorporating continuous or intermittent endurance training (ET) on whole-muscle and type I and II muscle fiber hypertrophy compared with resistance training (RT) alone. Randomized and nonrandomized studies reporting changes in cross-sectional area at muscle fiber and whole-muscle levels after RT compared with CT were included. Searches for such studies were performed in Web of Science, PubMed, Scopus, SPORTDiscus, and CINAHL electronic databases. The data reported in the included studies were pooled in a random-effects meta-analysis of standardized mean differences (SMDs). Twenty-five studies were included. At the whole-muscle level, there were no significant differences for any comparison (SMD < 0.03). By contrast, RT induced greater type I and type II muscle fiber hypertrophy than CT when high-intensity interval training (HIIT) was incorporated alone (SMD > 0.33) or combined with continuous ET (SMD > 0.27), but not compared with CT incorporating only continuous ET (SMD < 0.16). The subgroup analyses of this systematic review and meta-analysis showed that RT induces greater muscle fiber hypertrophy than CT when HIIT is included. However, no CT affected whole-muscle hypertrophy compared with RT.

High-intensity circuit training change serum myostatin but not myogenin in adolescents' soccer players: a quasi-experimental study

BACKGROUND

Skeletal muscle contractions due to exercise lead to the secretion of many proteins and proteoglycan peptides called myokines. Myostatin (MSTN) and Myogenin (MyoG) are two of the most important skeletal muscle growth regulatory factors related to myoblast differentiation and muscle hypertrophy. The present study aims at investigating the effects over eight weeks of high-intensity circuit training (HICT) on serum MyoG and MSTN in male soccer players.

METHOD

The present study is a quasi-experimental study on 21 male soccer players (Experimental group: n = 11, Control group: n = 10) (ages 15.0 ± 3.4 years, body mass 55.7 ± 7.8 kg, height 173.3 ± 8.0 cm, Body mass index 18.4 ± 1.9 kg m^-2, maximum oxygen uptake 61.89 ± 3.01 ml kg^-1 and the peak height velocity 14.5 ± 0.3 years). Participants were randomly divided into two groups: training group and a control group. The first resting blood samples were obtained in the morning-fasting state, and the second blood samples were obtained after the maximum aerobic test at pre- and post-HICT.

RESULTS

There were non-significant differences in resting serum values of MyoG (p = 0.309, p > 0.05) but significant differences in resting serum values of MSTN between the training and control groups after eight weeks of HICT (p = 0.003, p < 0.05). No significant differences were observed between groups in the acute response of serum values of MyoG (p = 0.413, p < 0.05) and MSTN (p = 0.465, p < 0.05) to the maximum aerobic test after eight weeks of HICT.

CONCLUSION

These results suggest that eight weeks of HICT can decrease the resting serum values of MSTN but not change the resting serum values of MyoG in male adolescent soccer players. Also, eight weeks of HICT does not affect the acute response of MSTN and MyoG after a maximum aerobic test.

The Effects of Concurrent Aerobic and Strength Training on Muscle Fiber Hypertrophy: A Systematic Review and Meta-Analysis

Background

Whole muscle hypertrophy does not appear to be negatively affected by concurrent aerobic and strength training compared to strength training alone. However, there are contradictions in the literature regarding the effects of concurrent training on hypertrophy at the myofiber level.

Objective

The current study aimed to systematically examine the extent to which concurrent aerobic and strength training, compared with strength training alone, influences type I and type II muscle fiber size adaptations. We also conducted subgroup analyses to examine the effects of the type of aerobic training, training modality, exercise order, training frequency, age, and training status.

Design

A systematic literature search was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) [PROSPERO: CRD42020203777]. The registered protocol was modified to include only muscle fiber hypertrophy as an outcome.

Data Sources

PubMed/MEDLINE, ISI Web of Science, Embase, CINAHL, SPORTDiscus, and Scopus were systematically searched on 12 August, 2020, and updated on 15 March, 2021.

Eligibility Criteria

Population: healthy adults of any sex and age; intervention: supervised, concurrent aerobic and strength training of at least 4 weeks; comparison: identical strength training prescription, with no aerobic training; and outcome: muscle fiber hypertrophy.

Results

A total of 15 studies were included. The estimated standardized mean difference based on the random-effects model was − 0.23 (95% confidence interval [CI] − 0.46 to − 0.00, p = 0.050) for overall muscle fiber hypertrophy. The standardized mean differences were − 0.34 (95% CI − 0.72 to 0.04, p = 0.078) and − 0.13 (95% CI − 0.39 to 0.12, p = 0.315) for type I and type II fiber hypertrophy, respectively. A negative effect of concurrent training was observed for type I fibers when aerobic training was performed by running but not cycling (standardized mean difference − 0.81, 95% CI − 1.26 to − 0.36). None of the other subgroup analyses (i.e., based on concurrent training frequency, training status, training modality, and training order of same-session training) revealed any differences between groups.

Conclusions

In contrast to previous findings on whole muscle hypertrophy, the present results suggest that concurrent aerobic and strength training may have a small negative effect on fiber hypertrophy compared with strength training alone. Preliminary evidence suggests that this interference effect may be more pronounced when aerobic training is performed by running compared with cycling, at least for type I fibers.

The Effects of Concurrent Aerobic and Strength Training on Muscle Fiber Hypertrophy: A Systematic Review and Meta-Analysis

Background

Whole muscle hypertrophy does not appear to be negatively affected by concurrent aerobic and strength training compared to strength training alone. However, there are contradictions in the literature regarding the effects of concurrent training on hypertrophy at the myofiber level.

Objective

The current study aimed to systematically examine the extent to which concurrent aerobic and strength training, compared with strength training alone, influences type I and type II muscle fiber size adaptations. We also conducted subgroup analyses to examine the effects of the type of aerobic training, training modality, exercise order, training frequency, age, and training status.

Design

A systematic literature search was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) [PROSPERO: CRD42020203777]. The registered protocol was modified to include only muscle fiber hypertrophy as an outcome.

Data Sources

PubMed/MEDLINE, ISI Web of Science, Embase, CINAHL, SPORTDiscus, and Scopus were systematically searched on 12 August, 2020, and updated on 15 March, 2021.

Eligibility Criteria

Population: healthy adults of any sex and age; intervention: supervised, concurrent aerobic and strength training of at least 4 weeks; comparison: identical strength training prescription, with no aerobic training; and outcome: muscle fiber hypertrophy.

Results

A total of 15 studies were included. The estimated standardized mean difference based on the random-effects model was − 0.23 (95% confidence interval [CI] − 0.46 to − 0.00, p = 0.050) for overall muscle fiber hypertrophy. The standardized mean differences were − 0.34 (95% CI − 0.72 to 0.04, p = 0.078) and − 0.13 (95% CI − 0.39 to 0.12, p = 0.315) for type I and type II fiber hypertrophy, respectively. A negative effect of concurrent training was observed for type I fibers when aerobic training was performed by running but not cycling (standardized mean difference − 0.81, 95% CI − 1.26 to − 0.36). None of the other subgroup analyses (i.e., based on concurrent training frequency, training status, training modality, and training order of same-session training) revealed any differences between groups.

Conclusions

In contrast to previous findings on whole muscle hypertrophy, the present results suggest that concurrent aerobic and strength training may have a small negative effect on fiber hypertrophy compared with strength training alone. Preliminary evidence suggests that this interference effect may be more pronounced when aerobic training is performed by running compared with cycling, at least for type I fibers.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40279-022-01688-x.

Skeletal Muscle Adaptive Responses to Different Types of Short-Term Exercise Training and Detraining in Middle-Age Men

INTRODUCTION

Whether short-term, single-mode exercise training can improve physical fitness prior to a period of reduced physical activity (e.g. post-surgery recovery) is not well characterized in clinical populations nor middle-age adults. We investigated skeletal muscle adaptive responses following endurance exercise training (ENT), high-intensity interval training (HIIT) or resistance exercise training (RET), and a subsequent period of detraining, in sedentary, middle-age men.

METHODS

Thirty-five sedentary, males (39±3 yr) were randomized to parallel groups and undertook six weeks of either ENT (n=12), HIIT (n=12) or RET (n=11) followed by 2.5 weeks of detraining. Skeletal muscle fiber characteristics, body composition, muscle thickness, muscle strength, aerobic capacity, resting energy expenditure and glucose homeostasis were assessed at baseline, and after exercise training and detraining.

RESULTS

Lean mass increased after RET and HIIT (+3.2±1.6% and +1.6±2.1%, P<0.05). Muscle strength (sum of leg press, leg extension and bench press 1RMs) increased after all training interventions (RET: +25±5%; HIIT: +10±5%; ENT: +7±7%, P<0.05). Aerobic capacity increased only after HIIT and ENT (+14±7% and +11±11%, P<0.05). Type I and II muscle fiber size increased for all groups post-training (main effect of time, P<0.05). Following a period of detraining, the gains in lean mass and maximal muscle strength were maintained in RET and HIIT groups, but maximal aerobic capacity declined below post-training levels in HIIT and ENT (P<0.05).

CONCLUSION

Six weeks of HIIT induced widespread adaptations prior to detraining in middle-age men. Exercise training-induced increases in aerobic capacity declined during 2.5 weeks of detraining but gains in lean mass and muscle strength were maintained.

Interference Phenomenon with Concurrent Strength and High-Intensity Interval Training-Based Aerobic Training: An Updated Model

Previous research has suggested that concurrent training (CT) may attenuate resistance training (RT)-induced gains in muscle strength and mass, i.e.' the interference effect. In 2000, a seminal theoretical model indicated that the interference effect should occur when high-intensity interval training (HIIT) (repeated bouts at 95-100% of the aerobic power) and RT (multiple sets at ~ 10 repetition maximum;10 RM) were performed in the same training routine. However, there was a paucity of data regarding the likelihood of other HIIT-based CT protocols to induce the interference effect at the time. Thus, based on current HIIT-based CT literature and HIIT nomenclature and framework, the present manuscript updates the theoretical model of the interference phenomenon previously proposed. We suggest that very intense HIIT protocols [i.e., resisted sprint training (RST), and sprint interval training (SIT)] can greatly minimize the odds of occurring the interference effect on muscle strength and mass. Thus, very intensive HIIT protocols should be implemented when performing CT to avoid the interference effect. Long and short HIIT-based CT protocols may induce the interference effect on muscle strength when HIIT bout is performed before RT with no rest interval between them.

Concentrations of Circulating Irisin and Myostatin in Race and Endurace Purebred Arabian Horses-Preliminary Study

Simple Summary

Irisin and myostatin are regulatory proteins produced by muscle cells. The aim of the study was to evaluate the effect of exercise on plasma irisin and myostatin concentrations in horses in different types of training (speed versus endurance). To find out, we tested 20 Arabian horses, submitted to the two different equestrian disciplines, and consequently different training regimes. The first group of horses realized a short-term, high-speed bout of exercise whereas the second group of horses were submitted to long-lasting, endurance effort. The obtained results showed that the single bout of exercise induced an increase in plasma myostatin concentration. Plasma irisin level decreased during the race season in racehorses. This means that irisin and myostatin may play a regulatory role in the maintenance of the energy balance processes.

Abstract

Skeletal muscle is considered to be the largest endocrine organ determining the maintenance of energy homeostasis. Adaptive changes in skeletal muscles in response to physical exercise influence the production as well as secretion of myokines, which are bioactive factors that play a crucial role in energy expenditure processes. The aim of the study was to investigate the impact of two different types of exercise on the circulating level of two of these, myostatin and irisin, in trained horses. Twenty purebred Arabian horses were involved in the study: 10 three-year-old horses trained on the racetrack and 10 endurance horses aged 7.4 ± 1.9 years. The horses from both groups were regularly trained throughout the entire season, during which they also participated in Polish National competitions. To assess the influence of the training sessions on plasma myostatin and irisin concentrations, blood samples taken at rest and 30 min after the end of exercise were analyzed. In the studied horses, the single bout of exercise did not influence plasma irisin but induced an increase in plasma myostatin concentration. In racehorses, plasma irisin concentration decreased with the length of the training season. Plasma myostatin was higher in endurance horses than in three-year-old racehorses. Lack of exercise-induced fluctuation in circulating irisin in studied horses suggests that myostatin released in response to exercise provides a negative feedback signal to irisin release.

The effect of 12-week resistance exercise training on serum levels of cellular aging process parameters in elderly men

BACKGROUND

Regular physical activity has a positive effect on the prevention of cellular aging. The present study investigated the effect of 12-week resistance training (RT) on serum levels of Sirtuin-1 (SIRT1), Sirtuin-3 (SIRT3), Sirtuin-6 (SIRT6), Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1-α), and telomerase enzyme in elderly men.

METHODS

For this purpose, 30 elderly men (age 66.23 ± 0.57 years) were randomly divided into two groups: resistance training group (RET, n = 15) and control group (CTR, n = 15). Participants in RET performed RT protocols with intensity of 60% one-repetition maximum (3×/week, 4 sets of the six exercise circuits). Body composition, physical functioning and, blood samples were assessed before (pre-test) and after (post-test) a 12-week intervention.

RESULTS

The results showed that there was a significant increase in serum levels of SIRT1 (P = 0.001), SIRT3 (P = 0.001), SIRT6 (P = 0.02), PGC1-α (P = 0.001), and telomerase enzyme (P = 0.001) in RET. Also, we found a significant difference between the RET and CTR in serum levels of SIRT1 (P = 0.001), SIRT3 (P = 0.001), SIRT6 (P = 0.037), PGC1-α (P = 0.007), and telomerase enzyme (P = 0.001).

CONCLUSIONS

12-Week RT increased the levels of proteins associated with the biological aging process in elderly men. It seems that the RT may have beneficial effects on cellular senescence and also improved impaired mitochondrial protein and enzymatic functional induced aging.

Myostatin as a potential biomarker to monitor sarcopenia in hip fracture patients undergoing a multidisciplinary rehabilitation and nutritional treatment: a preliminary study

Hip fractures are the most common osteoporotic fractures related to disability in older adults, requiring surgery and a subsequent rehabilitation treatment. Sarcopenia is currently considered as a predictive of worse outcome in hip fracture patients and myostatin has been recently proposed a potential biomarker of this condition. Twenty hip fracture patients after total hip replacement (mean aged 75.9 ± 2.4 years) were randomly divided into two groups of ten subjects (groups A and B). Both groups performed a rehabilitation program (5 sessions of 40 min/week for 2 weeks, followed by home-based exercise protocol). Group A received also 2-month amino acid supplementation. Serum myostatin levels significantly decreased after 2 months in both group A (p = 0.01) and group B (p = 0.03) in sarcopenic patients only in group A (p = 0.04). These results suggest that myostatin might be considered a promising biomarker of sarcopenia in hip fracture older adults' patients undergoing rehabilitation and amino acid supplementation.

The Effects of Concurrent Training Order on Satellite Cell-Related Markers, Body Composition, Muscular and Cardiorespiratory Fitness in Older Men with Sarcopenia

OBJECTIVES

Concurrent Training (CT) is described as a combination of resistance training (RT) and endurance training (ET) in a periodized program to maximize all aspects of physical performance. To date, effects of CT order on muscular and cardiorespiratory fitness adaptations are controversial. Owing to the age-related decrement in satellite cells (SC) which are critical for fiber repair, conservation, muscle hypertrophy as well as cardiorespiratory fitness, the present study examined the response of SC related markers to CT order in older sarcopenic men.

PARTICIPANTS

Thirty older men (age= 64.3 ± 3.5 years) were randomly assigned into one of 3 groups, ET followed by RT (E+R; n=10), RT followed by ET (R+E; n= 10) or a control (C; n=10).

INTERVENTION

The training protocol consisted of 3 exercise sessions per week for 8 weeks. Blood samples were obtained at baseline and 48 hours after the final training session.

RESULTS

Weight, skeletal muscle mass, lower and upper body power, maximal oxygen consumption (VO2max), Paired Box 7 (Pax7), and Myogenic factor 5 (Myf5) significantly increased, while were percent body fat significantly decreased following E+R and R+E compared to C. Importantly, the improvement in skeletal muscle mass, lower and upper body power, Myf5 and Pax7 in the E+R was significantly greater than the R+E group. Myogenin (Myog) and Paired Box 3 (Pax3) significantly increased (P < 0.01) in both training groups compared to no changes in C.

CONCLUSION

An 8-week CT intervention improves SC related markers, body composition and enhances power and VO2max in older sarcopenic participants, regardless of the order of RT and ET. However, performing ET before RT may be more effective at enhancing skeletal muscle mass, Myf5 and Pax7, in addition to both lower and upper body power. While both CT programs produced notable physiological and performance benefits, performing ET before RT during CT may provide the greatest therapeutic benefits for aging individuals.

Effect of eccentric action velocity on expression of genes related to myostatin signaling pathway in human skeletal muscle

The aim of this study was to investigate the effects of an acute bout of eccentric actions, performed at fast velocity (210º^.s^-1) and at slow velocity (20º^.s^-1), on the gene expression of regulatory components of the myostatin (MSTN) signalling pathway. Participants performed an acute bout of eccentric actions at either a slow or a fast velocity. Muscle biopsy samples were taken before, immediately after, and 2 h after the exercise bout. The gene expression of the components of the MSTN pathway was assessed by real-time PCR. No change was observed in MSTN, ACTRIIB, GASP-1 or FOXO-3a gene expression after either slow or fast eccentric actions (p > 0.05). However, the MSTN inhibitors follistatin (FST), FST-like-3 (FSTL3) and SMAD-7 were significantly increased 2 h after both eccentric actions (p < 0.05). No significant difference between bouts was found before, immediately after, or 2 h after the eccentric actions (slow and fast velocities, p > 0.05). The current findings indicate that a bout of eccentric actions activates the expression of MSTN inhibitors. However, no difference was observed in MSTN inhibitors’ gene expression when comparing slow and fast eccentric actions. It is possible that the greater time under tension induced by slow eccentric (SE) actions might compensate the effect of the greater velocity of fast eccentric (FE) actions. Additional studies are required to address the effect of eccentric action (EA) velocities on the pathways related to muscle hypertrophy.

Low-Intensity Pulsed Ultrasound Promotes Exercise-Induced Muscle Hypertrophy

The purpose of this study was to investigate whether low-intensity pulsed ultrasound (LIPUS) promotes exercise-induced muscle hypertrophy. Twenty-four adult Sprague-Dawley (SD) rats were randomly assigned to three groups (n = 8 per group): normal control group (NC), treadmill exercise group (TE) and treadmill exercise + LIPUS group (TE + LIPUS). The TE + LIPUS group received a LIPUS treatment (1 MHz, 30 mW/cm²) at the gastrocnemius for 20 min/d after treadmill exercise. The TE group was sham-treated. Eight weeks of treadmill training successfully established the exercise-induced muscle hypertrophy model. Muscle strength, muscle mass and muscle fiber cross-sectional area were significantly increased in the TE + LIPUS group compared with the TE group. Moreover, LIPUS treatment significantly upregulated the expression of Akt, mTOR, p-Akt and p-mTOR and significantly downregulated the expression of MSTN, ActRIIB, FoxO1 and its phosphorylation. The results indicated that LIPUS promotes exercise-induced muscle hypertrophy by facilitating protein synthesis and inhibiting the protein catabolism pathway.

Fructose Consumption in the Development of Obesity and the Effects of Different Protocols of Physical Exercise on the Hepatic Metabolism

Fructose consumption has been growing exponentially and, concomitant with this, the increase in the incidence of obesity and associated complications has followed the same behavior. Studies indicate that fructose may be a carbohydrate with greater obesogenic potential than other sugars. In this context, the liver seems to be a key organ for understanding the deleterious health effects promoted by fructose consumption. Fructose promotes complications in glucose metabolism, accumulation of triacylglycerol in the hepatocytes, and alterations in the lipid profile, which, associated with an inflammatory response and alterations in the redox state, will imply a systemic picture of insulin resistance. However, physical exercise has been indicated for the treatment of several chronic diseases. In this review, we show how each exercise protocol (aerobic, strength, or a combination of both) promote improvements in the obesogenic state created by fructose consumption as an improvement in the serum and liver lipid profile (high-density lipoprotein (HDL) increase and decrease triglyceride (TG) and low-density lipoprotein (LDL) levels) and a reduction of markers of inflammation caused by an excess of fructose. Therefore, it is concluded that the practice of aerobic physical exercise, strength training, or a combination of both is essential for attenuating the complications developed by the consumption of fructose.

Biomechanical implications of skeletal muscle hypertrophy and atrophy: a musculoskeletal model

Muscle hypertrophy and atrophy occur frequently as a result of mechanical loading or unloading, with implications for clinical, general, and athletic populations. The effects of muscle hypertrophy and atrophy on force production and joint moments have been previously described. However, there is a paucity of research showing how hypertrophy and atrophy may affect moment arm (MA) lengths. The purpose of this model was to describe the mathematical relationship between the anatomical cross-sectional area (ACSA) of a muscle and its MA length. In the model, the ACSAs of the biceps brachii and brachialis were altered to hypertrophy up to twice their original size and to atrophy to one-half of their original size. The change in MA length was found to be proportional to the arcsine of the square root of the change in ACSA. This change in MA length may be a small but important contributor to strength, especially in sports that require large joint moments at slow joint angular velocities, such as powerlifting. The paradoxical implications of the increase in MA are discussed, as physiological factors influencing muscle contraction velocity appear to favor a smaller MA length for high velocity movements but a larger muscle MA length for low velocity, high force movements.