Enhancement of Skeletal Muscle in Aged Rats Following High-Intensity Stretch-Shortening Contraction Training

Elevated muscle mass accompanied by transcriptional and nuclear alterations several months following cessation of resistance-type training in rats

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.

The Relationship between Resistance Training Frequency and Muscle Quality in Adolescents

Previous research has established the role of resistance training (RT) on muscle function in adolescents, but a lack of evidence to optimize RT for enhancing muscle quality (MQ) exists. This study examined whether RT frequency is associated with MQ in a nationally representative adolescent cohort. A total of 605 adolescents (12−15 year) in NHANES were stratified based on RT frequency. MQ was calculated as combined handgrip strength divided by arm lean mass (via dual-energy X-ray absorptiometry). Analysis of covariance was adjusted for sex, race/ethnicity, and arm fat percentage; p < 0.05 was considered significant. RT frequency was associated with MQ for 2−7 day/week but not 1 day/week. When no RT was compared to 1−2 and 3−7 day/week, associations were present for 3−7 day/week but not 1−2 day/week. When comparing no RT to 1−4 and 5−7 day/week, associations existed for 5−7 day/week but not 1−4 day/week. Next, no RT was compared to 1, 2−3, and 4−7 day/week; associations were found for 4−7 day/week, while 2−3 day/week had a borderline association (p = 0.06); there were no associations for 1 day/week. Finally, no RT was compared to 1, 2, 3, 4, and 5−7 day/week; associations were present for all except 1 and 3 day/week. These prospective data suggest a minimum RT frequency of 2 day/week is associated with MQ in adolescents as indicated by the lack of differences in MQ between 1 day/week RT versus no RT.

Improved impedance to maladaptation and enhanced VCAM-1 upregulation with resistance-type training in the long-lived Snell dwarf (Pit1dw/dw) mouse

Snell dwarf mice with the Pit1^dw/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 Pit1^dw/dw mutation of long-lived Snell dwarf mice.

Skeletal Muscle Quality: A Biomarker for Assessing Physical Performance Capabilities in Young Populations

Muscle quality (MQ), defined as the amount of strength and/or power per unit of muscle mass, is a novel index of functional capacity that is increasingly relied upon as a critical biomarker of muscle health in low functioning aging and pathophysiological adult populations. Understanding the phenotypical attributes of MQ and how to use it as an assessment tool to explore the efficacy of resistance exercise training interventions that prioritize functional enhancement over increases in muscle size may have implications for populations beyond compromised adults, including healthy young adults who routinely perform physically demanding tasks for competitive or occupational purposes. However, MQ has received far less attention in healthy young populations than it has in compromised adults. Researchers and practitioners continue to rely upon static measures of lean mass or isolated measures of strength and power, rather than using MQ, to assess integrated functional responses to resistance exercise training and physical stress. Therefore, this review will critically examine MQ and the evidence base to establish this metric as a practical and important biomarker for functional capacity and performance in healthy, young populations. Interventions that enhance MQ, such as high-intensity stretch shortening contraction resistance exercise training, will be highlighted. Finally, we will explore the potential to leverage MQ as a practical assessment tool to evaluate function and enhance performance in young populations in non-traditional research settings.

Age-dependent stress response DNA demethylation and gene upregulation accompany nuclear and skeletal muscle remodeling following acute resistance-type exercise in rats

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.

Commentary on Some Recent Theses Relevant to Combating Aging: June 2019

Theses reviewed in this issue include "Age-Associated Lamin-B1 Reduction in the Thymic Epithelial Cells and Its Role in Thymic Involution," "Brainstem Structural Integrity in the Progression of Alzheimer's Disease," "Challenging Current Paradigms: Increasing the Efficacy of Radiation Therapy with Novel Radiation Schemes," "Cross-Talk Between the Skeletal Muscle Stem Cells and Endothelial Cells," "Design and Synthesis of Enzyme-MOF (Metal-Organic Framework) Composites for Long-Persistent Biomedical Applications," and "Megakaryocyte-Membrane Wrapped Nanoparticles for Targeted Delivery to Hematopoietic Stem Cells."

Reduced frequency of resistance-type exercise training promotes adaptation of the aged skeletal muscle microenvironment

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.

Resistance Training Frequency Confers Greater Muscle Quality in Aged Individuals: A Brief NHANES Report

Background: Sarcopenia, the age-related decline in skeletal muscle mass, results in a loss of strength and functional capacity, which subsequently increases the risk of disease, disability frailty, and all-cause mortality. Skeletal muscle quality (MQ), i.e., strength per unit muscle mass, is the ability of muscle to perform its functions, and evidence indicates it is a more influential variable underlying age-related declines in muscle function than losses in muscle mass. Resistance training (RT) is known for enhancing skeletal MQ, improving health span, and reducing mortality; however, to the best of our knowledge, no studies have examined the relationship between RT frequency and MQ in an aged population. Thus, this study was designed to test the hypothesis that greater MQ in older individuals is associated with RT frequency. Methods: Utilizing data from 2,391 older adults in the National Health and Nutrition Survey (NHANES; 1999-2002), a secondary analysis of data was performed to see if an association existed between RT frequency and MQ in persons aged 55 years and older. Data were analyzed using analysis of covariance (ANCOVA) with three different models. Individuals were stratified into two groups based on how many days per week they performed RT: Insufficient (i.e., < two days per week) or sufficient (≥ two days per week). Muscle quality was calculated by taking the average peak force (Newtons) obtained from an isokinetic dynamometer and dividing it by lean mass, excluding bone mineral content (grams), obtained from dual-energy X-ray absorptiometry. The alpha level was set at <0.05. Results: For persons aged 55 and over, a statistically significant association was found between sufficient RT and greater MQ in both unadjusted as well as adjusted models that accounted for various demographic, behavioral, and clinical characteristics (p<0.05 for all). However, when limited to those 65 and older, no statistically significant associations were observed between sufficient RT and greater MQ (p>0.05 for all). When partitioned according to those 55 to 64 years of age and those 55 to 79 years, a statistically significant association was again observed (p<0.05 for all). No statistically significant associations were observed for individuals 65-79 years of age or those 80 years of age and older (p>0.05 for all). Conclusions: Sufficient amounts of RT are associated with greater MQ in selected older individuals. A need exists for future randomized controlled trials that examine the dose-response relationship between resistance training and MQ in older adults.

VCAM-1 upregulation accompanies muscle remodeling following resistance-type exercise in Snell dwarf (Pit1dw/dw ) mice

Snell dwarf mice (Pit1^dw/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.

Role of exercise in age-related sarcopenia

Sarcopenia is an age-associated decline of skeletal muscle mass and function and is known to lead to frailty, cachexia, osteoporosis, metabolic syndromes, and death. Notwithstanding the increasing incidence of sarcopenia, the molecular and cellular mechanisms driving age-related sarcopenia are not completely understood. This article reviews current definitions of sarcopenia, its potential mechanisms, and effects of exercise on sarcopenia. The pathogenesis of age-related sarcopenia is multifactorial and includes myostatin, inflammatory cytokines, and mitochondria-derived problems. Especially, age-induced mitochondrial dysfunction triggers the production of reactive oxygen species (ROS) by mitochondria, impedes mitochondrial dynamics, interrupts mitophagy, and leads to mitochondria-mediated apoptosis. Aerobic exercise provides at least a partial solution to sarcopenia as it ameliorates mitochondria-derived problems, and resistance exercise strengthens muscle mass and function. Furthermore, combinations of these exercise types provide the benefits of both. Collectively, this review summarizes potential mechanisms of age-related sarcopenia and emphasizes the use of exercise as a therapeutic strategy, suggesting that combined exercise provides the most beneficial means of combating age-related sarcopenia.

High-intensity stretch-shortening contraction training modifies responsivity of skeletal muscle in old male rats

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.

Efficacy of Age-Specific High-Intensity Stretch-Shortening Contractions in Reversing Dynapenia, Sarcopenia, and Loss of Skeletal Muscle Quality

During the aging process, skeletal muscle performance and physiology undergoes alterations leading to decrements in functional capacity, health-span, and independence. Background: The utility and implementation of age-specific exercise is a paramount research agenda focusing on ameliorating the loss of both skeletal muscle performance and physiology; yet, to date, no consensus exists as to the most appropriate mechanical loading protocol design or overall exercise prescription that best meets this need. Thus, the purpose of this review is to highlight the most optimal type of exercise presently available and provide the most current, evidence-based findings for its efficacy. The hypothesis that high-intensity, stretch-shortening contractions (SSCs)-a form of "resistance-type exercise" training-present as the preferred exercise mode for serving as an intervention-based modality to attenuate dynapenia, sarcopenia, and decreased muscle quality with aging, even restoring the overall youthful phenotype, will be demonstrated. Conclusions: Appreciating the fundamental evidence supporting the use of high-intensity SSCs in positively impacting aging skeletal muscle's responsivity and their use as a specific and sensitive countermeasure is crucial. Moreover, from an applied perspective, SSCs may improve skeletal muscle quality and rejuvenate health-span and, ultimately, lead to augmented functional capacity, independence, and quality of life concomitant with decreased morbidity.

Inflammaging and the Age-Specific Responsiveness to Stretch-Shortening Contractions

With aging, muscle injury from rapid, continuous stretch-shortening contractions (SSC) is prolonged, and maladaptation to moderate-velocity, intermittent SSC is more common. We hypothesize that high baseline levels of inflammatory signaling and oxidative stress may underlie these outcomes, whereas careful modulation of high-intensity SSC training design resets basal conditions and permits muscle adaptation to SSC.

Agonist muscle adaptation accompanied by antagonist muscle atrophy in the hindlimb of mice following stretch-shortening contraction training

Background

The vast majority of dynamometer-based animal models for investigation of the response to chronic muscle contraction exposure has been limited to analysis of isometric, lengthening, or shortening contractions in isolation. An exception to this has been the utilization of a rat model to study stretch-shortening contractions (SSCs), a sequence of consecutive isometric, lengthening, and shortening contractions common during daily activity and resistance-type exercise. However, the availability of diverse genetic strains of rats is limited. Therefore, the purpose of the present study was to develop a dynamometer-based SSC training protocol to induce increased muscle mass and performance in plantarflexor muscles of mice.

Methods

Young (3 months old) C57BL/6 mice were subjected to 1 month of plantarflexion SSC training. Hindlimb muscles were analyzed for muscle mass, quantitative morphology, myogenesis/myopathy relevant gene expression, and fiber type distribution.

Results

The main aim of the research was achieved when training induced a 2-fold increase in plantarflexion peak torque output and a 19% increase in muscle mass for the agonist plantaris (PLT) muscle. In establishing this model, several outcomes emerged which raised the value of the model past that of being a mere recapitulation of the rat model. An increase in the number of muscle fibers per transverse muscle section accounted for the PLT muscle mass gain while the antagonist tibialis anterior (TA) muscle atrophied by 30% with preferential atrophy of type IIb and IIx fibers. These alterations were accompanied by distinct gene expression profiles.

Conclusions

The findings confirm the development of a stretch-shortening contraction training model for the PLT muscle of mice and demonstrate that increased cross-sectional fiber number can occur following high-intensity SSC training. Furthermore, the TA muscle atrophy provides direct evidence for the concept of muscle imbalance in phasic non-weight bearing muscles, a concept largely characterized based on clinical observation of patients. The susceptibility to this imbalance is demonstrated to be selective for the type IIb and IIx muscle fiber types. Overall, the study highlights the importance of considering muscle fiber number modulation and the effect of training on surrounding muscles in exercise comprised of SSCs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12891-017-1397-4) contains supplementary material, which is available to authorized users.