AbstractThe Snell dwarf mouse (Pit1dw/dw), an animal model of congenital combined pituitary hormone deficiency, displays skeletal muscle weakness. While enhanced responsivity to repeated exposures of muscle contractions have been documented for Snell dwarf mice, the response following single exposure to distinct contraction protocols remained uncharacterized. The purpose of this study was to investigate the muscle recovery of Snell dwarf and control littermate mice following a single exposure to two separate protocols—an intermittent slow velocity (30°/s) contraction protocol or a continuous rapid velocity (500°/s) contraction protocol. Following both protocols for control mice, torque values were 30% and 80% of pre‐protocol values at 5 min and 3 days, respectively. At 10 days, performance returned to baseline for the 30°/s protocol and were depressed for the 500°/s protocol. For Snell dwarf mice following both protocols, torques were depressed to 5% of pre‐protocol values at 5 min and returned to baseline by 3 days. Recovery following the 30°/s protocol for control mice and both protocols for Snell dwarf mice coincided with increased transcriptional output, upregulation of cytokine‐mediated signaling genes, and a distribution shift to smaller muscle fibers with reduced area per nucleus. These features represent efficacious remodeling ubiquitous across distinct contraction paradigms in the context of the Pit1 mutation.

Previously, our lab published age-related phenotypical outcomes following chronic resistance-type exercise training (RTET). In these rodent studies no performance enhancement was reported for either 6- or 27-mont-old rats, even though a hypertrophic response of 15.9% and 8.2%, respectively, was present. PURPOSE: To determine whether cellular and molecular outcomes may explain altered adaptation at the phenotype level for 6- and 27-month-old rats following RTET. METHODS: Fischer Brown Norway hybrid rats (N = 5 per group) were exposed to 8 sets of 10 intermittent stretch-shortening contractions at a velocity of 60°/s per day, 3 days per week for 1-month. Following the final exposure dorsiflexor muscles were harvested, weighed, allocated for use, and stored at -80 °C. PI3K-AKT pathway gene expression was evaluated for significantly differentially expressed genes (SDEGs) with a combination of +/- 1.3-Fold change and p < 0.05 utilized. RESULTS: With aging 37 SDEGs were downregulated in the PI3K-AKT pathway, including key signaling molecules involved in mechanotransduction and hypertrophy (e.g., Itgb1 and Irs1). RTET resulted in 27 and 8 SDEGs in the 6- and 27-month old’s, respectively; with limited hypertrophic SDEGs in the latter. Following training an observed interaction with increased age revealed 23 SDEGs (+1/-22) present, as genes contributing to mechanotransduction and hypertrophy were decreased (e.g., Itgb1, Igf1, Igf1r, Irs1, Pabpc1, and Rps6ka1). CONCLUSIONS: These findings indicate that transcriptional output representative of the PI3K-AKT pathway, although able to contribute to RTET hypertrophic outcomes, may be limiting with increased age. This indicates a disconnect between molecular signaling pathways contributing to muscle mass and performance outcomes during advancing adulthood.

Background:Previous research has documented the proportion of "tall and fall" (TF) and "drop and drive" (DD) pitching styles among Major League Baseball (MLB) pitchers who underwent ulnar collateral ligament reconstruction (UCLR). The proportion of these 2 styles among all MLB pitchers remains unknown. Purpose:To determine the proportion of the TF and DD pitching styles in all rostered MLB pitchers during a single season as well as the proportion of TF and DD pitchers who sustained an upper extremity (UE) injury and those who underwent UCLR. Study Design:Cross-sectional study; Level of evidence, 3. Methods:Pitcher demographic characteristics from the 2019 MLB season and pitching information were obtained via open-access sources. Two-dimensional video analysis was used to categorize the included pitchers into TF and DD groups. Statistical comparisons and contrasts were made using 2-tailed t tests, chi-square tests, and Pearson correlation analyses as appropriate. Results:Of the 660 MLB rostered pitchers in 2019 (age, 27.39 ± 3.51 years; body mass index, 26.34 ± 2.47 kg/m2; fastball velocity, 150.49 ± 3.99 kph [93.51 ± 2.48 mph]), 412 (62.4%) pitchers used the TF style and 248 (37.6%) pitchers used the DD style. Significantly more UE injuries were seen in the TF group compared with the DD group (112 vs 38 injuries, respectively; P < .001). Twelve pitchers underwent UCLR (TF, 10; DD, 2), representing a 1.8% UCLR rate among all pitchers. This was a second surgery for 2 pitchers, both of whom used the TF pitching style. Significantly more pitchers in the TF group than the DD group had undergone UCLR before 2019 (135 vs 56 pitchers, respectively; P = .005). Conclusion:The results of the present study demonstrated a higher prevalence of both UE injury and prior UCLR in TF pitchers. Further research is needed to explore the potential association between pitching style and UE injury.

Muscle swelling and performance loss several days following a physical task is often used as an indicator of long-term muscle deficits thereby influencing prescription. We were interested in testing this concept at young age (3 months old) in two mouse models - wild-type and long-lived Snell dwarf mice. PURPOSE: To determine whether changes in performance and muscle mass up to 3 days following two distinct muscle contraction protocols were indicative of the response at 10 days for Snell dwarf mice and wild-type littermates. METHODS: Mice were anesthetized, the knee secured, needle electrodes placed subcutaneously, and the foot taped to a servomotor footplate. Plantarflexor muscles (N = 9 to 10 per group) were exposed to either 8 sets of 10 intermittent stretch-shortening contractions (SSCs) at a velocity of 30°/s or 8 sets of 10 continuous SSCs at 500°/s. RESULTS: During the 500°/s protocol for both genotypes, torque, work, and power values were greater (by 2-fold, 2.5-fold, and 40-fold, respectively, P < 0.05) relative to values for the 30°/s protocol. Due to time under tension, the 30°/s protocol elicited 10-fold greater torque integral values. At 5 minutes post-protocol, torques were 33% ± 5% (SEM) of pre values for wild-type mice and 6% ± 2% for Snell dwarf mice independent of protocol utilized. For wild-type mice at 3 days, muscle masses were 108% ± 1% and 105% ± 2% of contralateral muscles (for 30°/s and 500°/s protocols, respectively) and torques were 79% ± 6% and 87 ± 5% of pre values with no differences between protocols. However, at 10 days, wild-type muscle mass and performance returned to baseline for the 30°/s protocol while losses in muscle mass and performance were observed for the 500°/s protocol. For Snell mice, measures were at baseline already by 3 days following both protocols. CONCLUSIONS: Muscle mass swelling and decreased performance capacity at early time points (minutes to a few days) following muscle contraction exposure are insufficient predictors of later muscle deficits. Additional outcomes should be considered during early assessment of physical activity. Disclaimer: "The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention.”

NIOSH researchers are pioneering the study of musculoskeletal health as professional ergonomists. We examine physical and social components of work environments to mitigate musculoskeletal injury risks. Part of our mission is to reduce the burden of work-related musculoskeletal disorders (MSDs) through a focused program of research and prevention that protects workers from MSDs, helps management mitigate related risks and liabilities, and helps practitioners improve the efficacy of workplace interventions. The purpose of this discussion panel is to disseminate research findings and recommendations (1) to practitioners to interpret and apply the results of research to real-world problems, and (2) to inspire researchers to continue their efforts to protect the millions of workers at risk.

In most industrialized countries, work-related musculoskeletal disorders (WMSDs) are a major occupational health problem resulting in productivity loss, employee absenteeism, and high workers’ compensation and healthcare costs. Understanding the etiology and control of WMSDs and associated risk factors is imperative for reducing the burden of this problem. This chapter is organized by five topics on WMSDs: (1) the problem and surveillance of WMSDs; (2) the etiology of WMSDs and their risk factors; (3) risk assessment methods for job-related physical risk factors; (4) risk intervention effectiveness; and (5) ergonomic guidelines and standards for the prevention of WMSDs. The authors focus on the breadth of the scientific knowledge and literature pertaining to WMSDs for occupational safety and health professionals interested in learning about the field of ergonomics. This chapter also provides anticipated future challenges in the areas of surveillance, risk interactions, risk assessments, and intervention evaluations. The research agenda for WMSDs published by the National Occupational Research Agenda (NORA) Musculoskeletal Health Cross-Sector (MUS) Council in 2018 is recommended as supplementary reading for the future direction of WMSD research.

Rodent studies investigating long-term effects following termination of hypertrophy-inducing loading have predominantly involved exposures such as synergist ablation and weighted wheel running or ladder climbing. This research yielded a spectrum of results regarding the extent of detraining in terms of muscle mass and myonuclei number. The studies were also limited in their lack of sensitive performance measures and indirect relatedness to resistance training. Our research group developed and validated a relevant rat model of resistance-type training that induces increased muscle mass and performance. The aim of the present study was to determine to what extent these features persist 3 months following the termination of this training. While performance returned to baseline, muscle mass remained elevated by 17% and a shift in distribution to larger muscle fibers persisted. A 16% greater total RNA and heightened mRNA levels of ribosomal protein S6 kinases implicated preserved transcriptional output and ribosomal content. Remodeling of muscle fiber nuclei was consistent with these findings - increased nuclear number and a distribution shift to a more circular nuclear shape. These findings indicate that muscle mass detrains at a slower rate than performance and implicates multiple forms of myonuclear remodeling in muscle memory.

Snell dwarf mice with the Pit1dw/dw mutation are deficient in growth hormone, prolactin, and thyroid stimulating hormone and exhibit >40% lifespan extension. This longevity is accompanied by compromised muscular performance. However, research regarding young (3-month-old) Snell dwarf mice demonstrate exceptional responsivity to resistance-type training especially in terms of a shifted fiber type distribution and increased protein levels of vascular cell adhesion molecule-1 (VCAM-1), a possible mediator of such remodeling. In the present study, we investigated whether this responsiveness persists at 12 months of age. Unlike 12-month-old control mice, age-matched Snell dwarf mice remained resistant to training-induced maladaptive decreases in performance and muscle mass. This was accompanied by retainment of the remodeling capacity in muscles of Snell dwarf mice to increase VCAM-1 protein levels and a shift in myosin heavy chain (MHC) isoform distribution with training. Even decreasing training frequency for control mice, an alteration which protected muscles from maladaptation at 12 months of age, did not result in the overt remodeling observed for Snell dwarf mice. The results demonstrate a distinct remodeling response to resistance-type exercise operative in the context of the Pit1dw/dw mutation of long-lived Snell dwarf mice.

Heat stress is associated with workplace injuries, likely through a combination of fatigue, reduced cognitive function, and thermal discomfort. The purpose of this study was to evaluate four cognitive tasks for sensitivity to heat stress. Eight participants performed treadmill exercise followed by assessments of serial reaction time (RT), Stroop effect, verbal delayed memory, and continuous performance working memory in an environmental chamber. A control (21.1 °C) trial, and "Hot 1" and "Hot 2" (both 37.8 °C) trials were run sequentially on two separate days to evaluate the four cognitive tasks. Heat strain (comparing Hot 1 and Hot 2 with the control trial) resulted in impairments in the serial RT test response and Stroop accuracy. Delayed memory was impacted only in the Hot 2 trial compared with the control trial. Given the demonstrated impact of heat on cognitive processes relevant to workers' real-world functioning in the workplace, understanding how to assess and monitor vigilant attention in the workplace is essential.

Background: Previous pilot research has investigated differences in elbow valgus torque between the “tall and fall” (TF) and “drop and drive” (DD) pitching styles. Whether one of these pitching styles is associated with a greater rate of ulnar collateral ligament reconstruction (UCLR) is currently unknown. Purpose: To determine the proportion of Major League Baseball (MLB) pitchers using the TF and DD pitching styles who underwent UCLR over a 10-year period. Study Design: Cross-sectional study; Level of evidence, 3. Methods: The demographic characteristics of pitchers who underwent UCLR between 2007 and 2017 were obtained via the open-source database MLB Player Analysis Tommy John Surgery List. Other information, such as previous UCLR and pitching videos and graphics, was obtained from other open-source databases. A comprehensive, 2-dimensional, kinesiology-based multicomponent definition of each pitching style was formulated and used to categorize the included pitchers into the TF and DD groups. Statistical comparisons and contrasts were made using chi-square and Pearson correlation tests. Results: Included were 223 MLB pitchers (mean ± SD age, 27.5 ± 3.6 years; body mass index [BMI], 27.6 ± 2.2; throwing velocity, 92.9 ± 2.6 mph [149.5 ± 4.2 km/h]) who underwent UCLR between 2007 and 2017. Of these pitchers, 162 were categorized as TF pitchers (72.6%) and 61 as DD pitchers (27.4%). Pitching velocity for injured pitchers was significantly correlated to BMI ( P < .001). We found no significant associations of pitching style with year of UCLR ( P = .941), BMI ( P = .549), age ( P = .647), handedness ( P = .501), or average pitch velocity ( P = .921). Conclusion: The study findings demonstrated that a higher proportion of UCL-injured MLB pitchers (72.6%) used the TF pitching style. Further research is needed to explore the potential association between pitching style and UCL injury.

PURPOSE: Two tenets of exercise programming/training are injury prevention and performance enhancement. The purpose of this study was to determine whether a validated model of resistance-type exercise training (RTET) utilizing stretch-shortening contractions (SSCs) could alter susceptibility to the mechanical induction of skeletal muscle strain injury with aging. METHODS: F344xBN rats’ dorsiflexor muscles were SSC RTET in vivo for 1 month on a custom-built isokinetic rodent dynamometer utilizing age-specific RTET protocols. Performance for dorsiflexor muscles were analyzed temporally, and immediately following skeletal muscle strain injury. ANOVA was used for statistical analysis; α was set at p < 0.05. RESULTS: Rodents receiving no SSC RTET prior to injury had significant static (-48.6% and -54.5%, respectively) and dynamic (-40.9% and -49.8%, respectively) peak force deficits. Age-specific, SSC RTET improved muscle performance in young and old rodents by 15% and 18%, respectively (p < 0.05). Interestingly, young and old rodents undergoing SSC RTET still incurred significant static (-48.8% and -55.7%, respectively) and dynamic (-47.5% and -48.7%, respectively) peak force deficits, which were similar deficits compared to untrained rodents. CONCLUSIONS: Although age-specific SSC RTET increases skeletal muscle adaptation, these results suggest that skeletal muscle strain induction susceptibility is unaltered following SSC RTET, irrespective of age.

Delayed aging in various tissues has been observed for Snell dwarf mice (Pit1dw/dw) yet muscular performance has not been characterized for this model. PURPOSE: The purpose of the present study was to characterize muscle mass and performance for 3 months old and 12 months old Snell dwarf mice in non-trained and resistance-type trained states. METHODS: Muscles of Snell dwarf mice and their wild-type littermates were exposed to 1 month of stretch-shortening contraction training. RESULTS: For non-trained muscles at both ages, muscles of Snell dwarf mice exhibited 70% less mass and 85% less isometric force relative to those of control mice. At young age, training 3 days per week had no effect regardless of mouse strain. With aging, 3 days per week training decreased muscle mass and isometric force by 12% and 25%, respectively, for control mice while no such decreases were observed for Snell dwarf mice. For control mice, training 2 days per week increased isometric force by 20% at young age with no training-induced decrements with aging. CONCLUSIONS: While Snell dwarf mice exhibit a trade-off between longevity and muscular performance, the Pit1 mutation counters age-related maladaptation to training. For wild-type muscle, modulation of frequency is a means for offsetting the maladaptive training response.

We recently demonstrated body fat %, fast ball velocity, and RPE were significant predictors of valgus torque in NCAA baseball pitchers. Even though changes in hyperemic-induced limb volume are associated with acute, repetitive pitching performance, possibly indicating approaching dominant elbow soft-tissue risk, no investigation, to our knowledge, has examined relationships between performance, biometric, and throwing-limb volume in collegiate pitchers. PURPOSE: To quantify the relationship between pitching performance, subject biometrics, and hyperemic-induced changes in collegiate baseball pitchers during game-simulated pitching sessions of 40, 80, or 120 pitches. METHODS: Following informed consent, 5 male subjects (x̄ age = 18.8 ± 0.8 years; x̄ BMI = 27.6 ± 1.8; x̄ body fat % = 22.9 ± 6.2; x̄ throwing velocity = 80.4 ± 1.8 mph) were block-assigned to groups of 40, 80, or 120 pitches. Bouts consisted of 10 pitches (~20s between pitches) delivered from an artificial mound with 1-2 mins rest between bouts. HR and RPE were recorded immediately following each 10-pitch bout. A MOTUS sensor and compression sleeve measured elbow valgus torque. A Stalker Sport II Radar Gun measured fastball velocity. Pre-test and post-test upper- and lower-extremity limb girths were measured, signifying reactive hyperemia. Wilcoxon non-parametric testing determined pre- to post-test differences. Pearson correlation identified relationships between variables. Alpha was set at p ≤ 0.05. RESULTS: No group differences were found on any performance, biometric, demographic, or hemodynamic variable. HR (72.6 ± 8.3bpm vs. 97.6 ± 10.0bpm, p = 0.02) and dominant forearm limb girth (29.4 ± 1.5cm vs. 30.9 ± 1.5cm, p = 0.04) increased from pre-test to post-test for subjects combined. Significant correlations were found for: pitching volume & post-test HR (r = 0.90, p = 0.039); post-test dominant upper arm circumference & RPE (r = 0.89, p = 0.042), and; valgus torque % change & pitching volume (r = 0.91, p = 0.031). CONCLUSIONS: Forearm limb girth increased for subjects combined, and; given this metric’s indication of reactive hyperemia, future research focused on elucidating and quantifying the biological components of the tissue (compartments), as well as their contribution to performance- and/or injury-specific outcomes, is warranted.

Efficacy of high-intensity resistance exercise becomes progressively compromised with aging. Previously, to investigate this, we developed a rodent model of high-intensity training consisting of stretch-shortening contractions (SSCs) and determined that following one month of training, young rats exhibit a robust stress response and 20% performance increase, whereas old rats display a muted stress response and 30% performance decrease. Whether these age-specific responses occur early in training and constitute primary factors in adaptation/maladaptation was not addressed. The aim of the present study was to characterize performance, remodeling, and stress response transcriptional profile 6–120 h following acute SSC exposure. For young rats, the stress response pathway was highly regulated (≥20 differentially expressed genes at each time point) and was accompanied by robust DNA demethylation, tissue remodeling, and isometric torque recovery. For old rats, a muted transcriptional profile (13 and 2 differentially expressed genes at 6 and 120 h, respectively) coincided with deficiencies in demethylation, muscle remodeling, and torque recovery. These findings occurred in the context of heightened chronic levels of stress response gene expression with aging. This demonstrates that age-related constitutive elevations in stress response gene expression was accompanied by diminished SSC-induced responsiveness in epigenomic regulation and tissue remodeling.

PURPOSE: Recently, a training-retraining (TRT) paradigm in which 3 month old rodents underwent an initial cycle of SSC RTET followed by another bout at 6 months led to increases in isometric/dynamic peak force and muscle mass relative to naïve 6 month old rats, thus augmenting the trainability of muscle into adulthood. However, the molecular underpinnings of this response is unknown. Therefore, we sought to determine whether this TRT paradigm has positive effects on transcription factor (TF) methylation and expression in adult skeletal muscle. METHODS: F344xBN hybrid rats were SSC RTET on an in vivo dynamometer 3 days/week for 1 month at 3 months and again at 6 months of age (TRT), or just at 6 months (T). Gene expression and DNA methylation were quantified via PCR Arrays (Qiagen®). RESULTS: TRT group had 17 significantly differentially expressed genes (SDEGs) in the TF pathway, including Myf5; T expressed only 3 SDEGs. TRT had decreased TF methylation compared to T (4.1±0.01 vs. 2.6±0.01%; p<0.05). CONCLUSIONS: Adaptive benefits at adulthood following an initial SSC RTET stimulus are supported by altered TF methylation and gene expression. These results advocate RTET at early life to preserve muscle as one ages through an epigenomic muscle memory.

The purpose of this study was to characterize the growth and remodeling molecular signaling response in aged skeletal muscle following 1 mo of "resistance-type exercise" training. Male Fischer 344 × Brown Norway hybrid rats aged 3 (young) and 30 mo (old) underwent stretch-shortening contraction (SSC) loading 2 or 3 days/wk; muscles were removed 72 h posttraining. Young rats SSC loaded 3 (Y3x) or 2 days/wk (Y2x) adapted via increased work performance. Old rats SSC loaded 3 days/wk (O3x) maladapted via decreased negative work; however, old rats SSC loaded 2 days/wk (O2x) adapted through improved negative and positive work. Y3x, Y2x, and O2x, but not O3x, displayed hypertrophy via larger fiber area and myonuclear domains. Y3x, Y2x, and O2x differentially expressed 19, 30, and 8 phosphatidylinositol 3-kinase-Akt genes, respectively, whereas O3x only expressed 2. Bioinformatics analysis revealed that rats in the adapting groups presented growth and remodeling processes (i.e., increased protein synthesis), whereas O3x demonstrated inflammatory signaling. In conclusion, reducing SSC-loading frequency in aged rodents positively influences the molecular signaling microenvironment, promoting muscle adaptation. NEW & NOTEWORTHY Decreasing resistance-type exercise training frequency in old rodents led to adaptation through enhancements in performance, fiber areas, and myonuclear domains. Modifying frequency influenced the molecular environment through improvements in phosphatidylinositol 3-kinase-Akt pathway-specific expression and bioinformatics indicating increased protein synthesis. Reducing training frequency may be appropriate in older individuals who respond unfavorably to higher frequencies (i.e., maladaptation); overall, modifying the parameters of the exercise prescription can affect the cellular environment, ultimately leading to adaptive or maladaptive outcomes.

Transposable elements (TEs) are mobile DNA and constitute approximately half of the human genome. LINE-1 (L1) is the only active autonomous TE in the mammalian genome and has been implicated in a number of diseases as well as aging. We have previously reported that skeletal muscle L1 expression is lower following acute and chronic exercise training in humans. Herein, we used a rodent model of voluntary wheel running to determine whether long-term exercise training affects markers of skeletal muscle L1 regulation. Selectively bred high-running female Wistar rats ( n = 11 per group) were either given access to a running wheel (EX) or not (SED) at 5 wk of age, and these conditions were maintained until 27 wk of age. Thereafter, mixed gastrocnemius tissue was harvested and analyzed for L1 mRNA expression and DNA content along with other L1 regulation markers. We observed significantly ( P < 0.05) lower L1 mRNA expression, higher L1 DNA methylation, and less L1 DNA in accessible chromatin regions in EX versus SED rats. We followed these experiments with 3-h in vitro drug treatments in L6 myotubes to mimic transient exercise-specific signaling events. The AMP-activated protein kinase (AMPK) agonist 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR; 4 mM) significantly decreased L1 mRNA expression in L6 myotubes. However, this effect was not facilitated through increased L1 DNA methylation. Collectively, these data suggest that long-term voluntary wheel running downregulates skeletal muscle L1 mRNA, and this may occur through chromatin modifications. Enhanced AMPK signaling with repetitive exercise bouts may also decrease L1 mRNA expression, although the mechanism of action remains unknown.

Snell dwarf mice (Pit1dw/dw ) exhibit deficiencies in growth hormone, prolactin, and thyroid stimulating hormone. Besides being an experimental model of hypopituitarism, these mice are long-lived (>40% lifespan extension) and utilized as a model of slowed/delayed aging. Whether this longevity is accompanied by a compromised quality of life in terms of muscular performance has not yet been characterized. In this study, we investigated nontrained and trained muscles 1 month following a general validated resistance-type exercise protocol in 3-month-old Snell dwarf mice and control littermates. Nontrained Snell dwarf gastrocnemius muscles exhibited a 1.3-fold greater muscle mass to body weight ratio than control values although muscle quality, maximum isometric torque normalized to muscle mass, and fatigue recovery were compromised. For control mice, training increased isometric torque (17%) without altering muscle mass. For Snell dwarf mice, isometric torque was unaltered by training despite decreased muscle mass that rendered muscle mass to body weight ratio comparable to control values. Muscle quality and fatigue recovery improved twofold and threefold, respectively, for Snell dwarf mice. This accompanied a fourfold increase in levels of vascular cell adhesion molecule-1 (VCAM-1), a mediator of progenitor cell recruitment, and muscle remodeling in the form of increased number of central nuclei, additional muscle fibers per unit area, and altered fiber type distribution. These results reveal a trade-off between muscle quality and longevity in the context of anterior pituitary hormone deficiency and that resistance-type training can diminish this trade-off by improving muscle quality concomitant with VCAM-1 upregulation and muscle remodeling.

Utilization of high-intensity resistance training to counter age-related sarcopenia is currently debated because of the potential for maladaptation when training design is inappropriate. Training design is problematic because the influence of various loading variables (e.g. contraction mode, repetition number, and training frequency) is still not well characterized at old age. To address this in a precisely controlled manner, we developed a rodent model of high-intensity training consisting of maximally-activated stretch-shortening contractions (SSCs), contractions typical during resistance training. With this model, we determined that at old age, high-repetition SSC training (80 SSCs: 8 sets of 10 repetitions) performed frequently (i.e. 3 days per week) for 4.5 weeks induced strength deficits with no muscle mass gain while decreasing frequency to 2 days per week promoted increases in muscle mass and muscle quality (i.e. performance normalized to muscle mass). This finding confirmed the popular notion that decreasing training frequency has a robust effect with age. Meanwhile, the influence of other loading variables remains contentious. The aim of the present study was to assess muscle adaptation following modulation of contraction mode and repetition number during high-intensity SSC training. Muscles of young (3 month old) and old (30 month old) male rats were exposed to 4.5 weeks of low-repetition static training of 4 (i.e. 4 sets of one repetition) isometric (ISO) contractions 3 days per week or a more moderate-repetition dynamic training of 40 SSCs (i.e. 4 sets of 10 repetitions) 3 days per week. For young rats, performance and muscle mass increased regardless of training protocol. For old rats, no muscle mass adaptation was observed for 4 ISO training while 40 SSC training induced muscle mass gain without improvement in muscle quality, an outcome distinct from modulating training frequency. Muscle mass gain for old rats was accompanied by decreased protein levels of tumor necrosis factor alpha, a mediator of age-related chronic inflammatory signaling, to young levels. These findings suggest that while dynamic high-intensity training with a moderate number of repetitions has a limited capacity for altering muscle quality, such training is a viable strategy for countering age-related inflammatory signaling and modifying muscle mass.