Effects of high-intensity resistance training on untrained older men. II. Muscle fiber characteristics and nucleo-cytoplasmic relationships

The isolated muscle fibre as a model of disuse atrophy: characterization using PhAct, a method to quantify f-actin

Research into muscle atrophy and hypertrophy is hampered by limitations of the available experimental models. Interpretation of in vivo experiments is confounded by the complexity of the environment while in vitro models are subject to the marked disparities between cultured myotubes and the mature myofibres of living tissues. Here we develop a method (PhAct) based on ex vivo maintenance of the isolated myofibre as a model of disuse atrophy, using standard microscopy equipment and widely available analysis software, to measure f-actin content per myofibre and per nucleus over two weeks of ex vivo maintenance. We characterize the 35% per week atrophy of the isolated myofibre in terms of early changes in gene expression and investigate the effects on loss of muscle mass of modulatory agents, including Myostatin and Follistatin. By tracing the incorporation of a nucleotide analogue we show that the observed atrophy is not associated with loss or replacement of myonuclei. Such a completely controlled investigation can be conducted with the myofibres of a single muscle. With this novel method we can distinguish those features and mechanisms of atrophy and hypertrophy that are intrinsic to the muscle fibre from those that include activities of other tissues and systemic agents.

The anabolic response to resistance exercise and a protein-rich meal is not diminished by age

OBJECTIVES

The synergistic effect of resistance exercise and protein ingestion on muscle protein anabolism in young adults has been well described. However, it is unclear if this relationship is maintained in older adults who are at greater risk of sarcopenic muscle loss. To this end, we sought to determine if the synergistic response to a bout of resistance exercise and a protein-rich lean beef meal was altered by age.

SETTING

The University of Texas Medical Branch, Clinical Research Center, Galveston, Texas.

PARTICIPANTS

Healthy young (n=7, 29±3 y) and older (n=7, 67±2 y) adults.

DESIGN

Mixed muscle fractional synthesis rate (FSR) was calculated during a 3 h post-absorptive/rest period and again during a 5 h period following ingestion of a protein-rich meal (340 g lean beef) and bout of resistance exercise (6 sets of 8 repetitions of isotonic knee extension exercise at 80% one repetition maximum).

MEASUREMENTS

Venous blood samples and vastus lateralis muscle biopsy samples were obtained during a primed (2.0 µmol/kg) constant infusion (0.08 µmol∙kg(-1)min(-1)) of L- [ring-13C6] phenylalanine.

RESULTS

Mixed muscle FSR increased by approximately 108% in both young [pre: 0.073±0.008; post: 0.156±0.021(SE) %/h, p<0.001] and older adults (pre: 0.075±0.004; post: 0.152±0.017 %/h, p=0.003) following the meal and resistance exercise bout.

CONCLUSION

Aging does not diminish the increase in muscle protein synthesis following a high-quality protein rich meal and bout of resistance exercise.

Origin and hierarchy of basal lamina-forming and -non-forming myogenic cells in mouse skeletal muscle in relation to adhesive capacity and Pax7 expression in vitro

As a novel approach to distinguish skeletal myogenic cell populations, basal lamina (BL) formation of myogenic cells was examined in the mouse compensatory enlarged plantaris muscles in vivo and in fiber-bundle cultures in vitro. MyoD(+) myogenic cells located inside the regenerative muscle fiber BL were laminin(-) but interstitial MyoD(+) cells were laminin(+). This was also confirmed by electron microscopy as structural BL formation. Similar trends were observed in the fiber-bundle cultures including satellite cells and interstitial myogenic cells and laminin(+) myogenic cells predominantly showed non-adhesive (non-Ad) behavior with Pax7(-), whereas laminin(-) cells were adhesive (Ad) with Pax7(+). Moreover, non-Ad/laminin(+) and Ad/laminin(-) myotubes were also observed and the former type showed spontaneous contractions, while the latter type did not. The origin and hierarchy of Ad/Pax7(+)/laminin(-) and non-Ad/Pax7(-)/laminin(+) myogenic cells were also examined using skeletal muscle interstitium-derived CD34(+)/45(-) (Sk-34) and CD34(-)/45(-) (Sk-DN) multipotent stem cells, which were composed of non-committed myogenic cells with a few (<1%) Pax7(+) cells in the Sk-DN cells at fresh isolation. Both cell types were separated by Ad/non-Ad capacity in repetitive culture. As expected, both Ad/Pax7(+)/laminin(-) and non-Ad/Pax7(-)/laminin(+) myogenic cells consistently appeared in the Ad and non-Ad cell culture. However, Ad/Pax7(+)/laminin(-) cells were repeatedly detected in the non-Ad cell culture, while the opposite phenomenon did not occur. This indicates that the source of non-Ad/ Pax7(-)/laminin(+) myogenic cells was present in the Sk-34 and Sk-DN stem cells and they were able to produce Ad/ Pax7(+)/ laminin(-) myogenic cells during myogenesis as primary myoblasts and situated hierarchically upstream of the latter cells.

Heavy resistance exercise training and skeletal muscle androgen receptor expression in younger and older men

Effects of heavy resistance exercise on serum testosterone and skeletal muscle androgen receptor (AR) concentrations were examined before and after a 21-week resistance training period. Seven healthy untrained young adult men (YT) and ten controls (YC) as well as ten older men (OT) and eight controls (OC) volunteered as subjects. Heavy resistance exercise bouts (5 × 10 RM leg presses) were performed before and after the training period. Muscle biopsies were obtained before and 1h and 48 h after the resistance exercise bouts from m.vastus lateralis (VL) to determine cross-sectional area of muscle fibers (fCSA) and AR mRNA expression and protein concentrations. No changes were observed in YC and OC while resistance training led to significant increases in maximal strength of leg extensors (1 RM), fCSA and lean body mass in YT and OT. Acute increases occurred in serum testosterone concentrations due to resistance exercises but basal testosterone remained unaltered. Mean AR mRNA expression and protein concentration remained unchanged after heavy resistance exercise bouts compared to pre-values. The individual pre- to post-training changes in resting (pre-exercise) AR protein concentration correlated with the changes in fCSA and lean body mass in the combined group of YT and OT. Similarly, it correlated with the changes in 1 RM in YT. Although mean AR expression did not changed due to the resistance exercise training, the present findings suggest that the individual changes of AR protein concentration in skeletal muscle following resistance training may have an impact on training-induced muscular adaptations in both younger and older men.

Effects of aging and gender on the spatial organization of nuclei in single human skeletal muscle cells

The skeletal muscle fibre is a syncitium where each myonucleus regulates the gene products in a finite volume of the cytoplasm, i.e., the myonuclear domain (MND). We analysed aging- and gender-related effects on myonuclei organization and the MND size in single muscle fibres from six young (21-31 years) and nine old men (72-96 years), and from six young (24-32 years) and nine old women (65-96 years), using a novel image analysis algorithm applied to confocal images. Muscle fibres were classified according to myosin heavy chain (MyHC) isoform expression. Our image analysis algorithm was effective in determining the spatial organization of myonuclei and the distribution of individual MNDs along the single fibre segments. Significant linear relations were observed between MND size and fibre size, irrespective age, gender and MyHC isoform expression. The spatial organization of individual myonuclei, calculated as the distribution of nearest neighbour distances in 3D, and MND size were affected in old age, but changes were dependent on MyHC isoform expression. In type I muscle fibres, average NN-values were lower and showed an increased variability in old age, reflecting an aggregation of myonuclei in old age. Average MND size did not change in old age, but there was an increased MND size variability. In type IIa fibres, average NN-values and MND sizes were lower in old age, reflecting the smaller size of these muscle fibres in old age. It is suggested that these changes have a significant impact on protein synthesis and degradation during the aging process.

Role of the nervous system in sarcopenia and muscle atrophy with aging: strength training as a countermeasure

Aging is characterized by loss of spinal motor neurons (MNs) due to apoptosis, reduced insulin-like growth factor I signaling, elevated amounts of circulating cytokines, and increased cell oxidative stress. The age-related loss of spinal MNs is paralleled by a reduction in muscle fiber number and size (sarcopenia), resulting in impaired mechanical muscle performance that in turn leads to a reduced functional capacity during everyday tasks. Concurrently, maximum muscle strength, power, and rate of force development are decreased with aging, even in highly trained master athletes. The impairment in muscle mechanical function is accompanied and partly caused by an age-related loss in neuromuscular function that comprise changes in maximal MN firing frequency, agonist muscle activation, antagonist muscle coactivation, force steadiness, and spinal inhibitory circuitry. Strength training appears to elicit effective countermeasures in elderly individuals even at a very old age (>80 years) by evoking muscle hypertrophy along with substantial changes in neuromuscular function, respectively. Notably, the training-induced changes in muscle mass and nervous system function leads to an improved functional capacity during activities of daily living.

Muscle fiber size increases following resistance training in multiple sclerosis

Objective: To test the hypothesis that lower body progressive resistance training (PRT) leads to an increase of the muscle fiber cross-sectional area (CSA) and a shift in the proportion of fiber types in patients with multiple sclerosis (MS).

Methods: The present study was a two-arm, randomized controlled trial (RCT). Thirty-eight MS patients (Expanded Disability Status Scale (EDSS) 3—5.5) were randomized to a PRT group (Exercise, n = 19) or a control group (Control, n = 19). The Exercise group performed a biweekly 12-week lower body PRT program [five exercises progressing from 15RM (Repetition Maximum) towards 8RM], whereas the Control group maintained their usual daily activity level during the trial period. Muscle biopsies from vastus lateralis were taken before (pre) and after the trial (post). Thigh volume (TV) was estimated from anthropometric measurements. Isokinetic muscle strength of the knee extensors (KE) and flexors (KF) were evaluated at slow (90°/s) and fast (180°/s) angular velocities.

Results: In the Exercise group the mean CSA of all muscle fibers (7.9 ± 15.4% vs. -3.5 ± 9.0%, p = 0.03) and of type II muscle fibers (14.0 ± 19.4% vs. -2.6 ± 15.5%, p = 0.02) increased in comparison with the Control group. No changes occurred in the proportion of fiber types in the Exercise group. Neither was there any change in total TV. Isokinetic strength at KE180, KF90 and KF180 improved significantly after PRT when compared with the control group (10.2—21.3%, p ≤ 0.02).

Conclusions: We conclude that progressive resistance training induces a compensatory increase of muscle fiber size in patients with the central nervous system disorder, multiple sclerosis.

Efficacy of systematic endurance and resistance training on muscle strength and endurance performance in elderly adults--a randomized controlled trial

BACKGROUND

Aging is associated with loss in both muscle mass and the metabolic quality of skeletal muscle. A major part of these changes is associated with an age-related decrease in the level of physical activity and may be counteracted by endurance training (ET) and resistance training (RT).

OBJECTIVE

Since both muscle strength and aerobic power decrease with age, we investigated what form of training might be best for improvements in physical performance in the elderly. In detail, we wanted to know whether systematic ET can augment muscle strength and/or whether systematic RT can augment the aerobic power of healthy elderly adults.

METHODS

Forty-two volunteers (32 women, 10 men) were recruited for the study and randomized into three groups: 13 persons undertook a continuous 6-month ET program, 15 undertook a continuous 6-month RT program and 14 served as a control group. All persons performed a cycling test to measure aerobic power (VO(2max)) and maximum workload (W(max)) before and after the training period. Maximum strength was determined from one repetition maximum (1-RM).

RESULTS

After 6 months of RT, maximum strength increased by an average of 15% for leg press (P < 0.01), 25% for bench press (P < 0.01) and 30% for bench pull (P < 0.001); ET showed no effect on maximum strength except for the 1-RM in bench pull. Aerobic power improved by 6% in the ET group and by 2.5% in the RT group, neither of which was significant. Maximum workload improved significantly by 31% in the ET group (P < 0.001) and by 6% in the RT group (P = 0.05). ET resulted in a significant 5.3% reduction of body fat (P < 0.05), whereas only RT increased lean body mass by 1.0 +/- 0.5 kg.

CONCLUSION

RT leads to a genuine increase in lean body mass and muscle strength in healthy elderly adults and is therefore the best method for treatment of amyotrophia. ET appears to be the most efficacious training mode for maintaining and improving maximum aerobic power in the elderly and should be viewed as a complement to RT. The loading intensity to promote hypertrophy should approach 60-80% of 1-RM with an exercise volume ranging from 3 to 6 sets per muscle group per week of 10-15 repetitions per exercise. ET should be performed on two days per week controlled by a heart rate according to 60% of VO(2max) and an exercise volume ranging from 30 to 60 minutes per week.

No change in skeletal muscle satellite cells in young and aging rat soleus muscle

Satellite cells are muscle stem cells capable of replenishing or increasing myonuclear number. It is postulated that a reduction in satellite cells may contribute to age-related sarcopenia. Studies investigating an age-related decline in satellite cells have produced equivocal results. This study compared the satellite cell content of young and aging soleus muscle in rat, using four different methods: dystrophin-laminin immunohistochemistry, MyoD immunohistochemistry, electron microscopy, and light microscopy of semi-thin sections. The absolute quantity of satellite cells increase with age, but satellite cell percentages were similar in young and aging soleus muscles. There were no differences in satellite cell quantity among MyoD immunohistochemistry, electron microscopy, and semi-thin sections. All three methods had significantly more satellite cells than with dystrophin-laminin immunohistochemistry. We conclude that satellite cell number does not decrease with age and postulate that satellite cell functionality may be responsible for age-related sarcopenia.

The impact of sarcopenia and exercise training on skeletal muscle satellite cells

It has been well-established that the age-related loss of muscle mass and strength, or sarcopenia, impairs skeletal muscle function and reduces functional performance at a more advanced age. Skeletal muscle satellite cells (SC), as precursors of new myonuclei, have been suggested to be involved in the development of sarcopenia. In accordance with the type II muscle fiber atrophy observed in the elderly, recent studies report a concomitant fiber type specific reduction in SC content. Resistance type exercise interventions have proven effective to augment skeletal muscle mass and improve muscle function in the elderly. In accordance, recent work shows that resistance type exercise training can augment type II muscle fiber size and reverse the age-related decline in SC content. The latter is supported by an increase in SC activation and proliferation factors that generally appear following exercise training. Present findings strongly suggest that the skeletal muscle SC control myogenesis and have an important, but yet unresolved, function in the loss of muscle mass with aging. This review discusses the contribution of skeletal muscle SC in the age-related loss of muscle mass and the efficacy of exercise training as a means to attenuate and/or reverse this process.

Validated treatments and therapeutic perspectives regarding physical activities

Current knowledge on physical activity in regard to sarcopenia is reported in this manuscript. The consequences of inactivity on muscle mass and function are discussed. Impact of resistance training on muscle and mass and function as well as its interaction with other factors associated with sarcopenia such as denervation, hormones modification and protein intake will be discussed.

Ageing influences myonuclear domain size differently in fast and slow skeletal muscle of rats

AIM

In multinucleated skeletal muscle, a myonuclear domain is the region of cytoplasm governed by one nucleus, and myofibres are mosaics of overlapping myonuclear domains. Association of ageing and myonuclear domain is important in the understanding of sarcopenia and with prevention or combating age-related muscle declines. This study examined the effects of age, fibre type and muscle on nucleo-cytoplasmic (N/C) relationships as reflecting myonuclear domain size.

METHODS

The N/C was compared in fibre types of soleus and plantaris muscles from young (n = 6) and ageing (n = 8) male Fisher 344 rats.

RESULTS

There were no significant differences in fibre type composition or cross-sectional area of the soleus across ages. The old soleus had significantly more myonuclei, resulting in a significantly smaller myonuclear domain size. The plantaris muscle showed a higher percentage of slow fibres in old compared with young fibres. There were no differences in the number of myonuclei or in myonuclear domain size between young and older animals.

CONCLUSION

We found muscle-specific differences in the effects of ageing on myonuclear domain, possibly as a result of reduced efficiency of the myonuclei in the slow muscles.

Alterations of protein turnover underlying disuse atrophy in human skeletal muscle

Unloading-induced atrophy is a relatively uncomplicated form of muscle loss, dependent almost solely on the loss of mechanical input, whereas in disease states associated with inflammation (cancer cachexia, AIDS, burns, sepsis, and uremia), there is a procatabolic hormonal and cytokine environment. It is therefore predictable that muscle loss mainly due to disuse alone would be governed by mechanisms somewhat differently from those in inflammatory states. We suggest that in vivo measurements made in human subjects using arterial-venous balance, tracer dilution, and tracer incorporation are dynamic and thus robust by comparison with static measurements of mRNA abundance and protein expression and/or phosphorylation in human muscle. In addition, measurements made with cultured cells or in animal models, all of which have often been used to infer alterations of protein turnover, appear to be different from results obtained in immobilized human muscle in vivo. In vivo measurements of human muscle protein turnover in disuse show that the primary variable that changes facilitating the loss of muscle mass is protein synthesis, which is reduced in both the postabsorptive and postprandial states; muscle proteolysis itself appears not to be elevated. The depressed postprandial protein synthetic response (a phenomenon we term "anabolic resistance") may even be accompanied by a diminished suppression of proteolysis. We therefore propose that most of the loss of muscle mass during disuse atrophy can be accounted for by a depression in the rate of protein synthesis. Thus the normal diurnal fasted-to-fed cycle of protein balance is disrupted and, by default, proteolysis becomes dominant but is not enhanced.

Moderate exercise attenuates the loss of skeletal muscle mass that occurs with intentional caloric restriction-induced weight loss in older, overweight to obese adults

BACKGROUND

Aging is associated with a loss of muscle mass and increased body fat. The effects of diet-induced weight loss on muscle mass in older adults are not clear.

PURPOSE

This study examined the effects of diet-induced weight loss, alone and in combination with moderate aerobic exercise, on skeletal muscle mass in older adults.

METHODS

Twenty-nine overweight to obese (body mass index = 31.8 +/- 3.3 kg/m(2)) older (67.2 +/- 4.2 years) men (n = 13) and women (n = 16) completed a 4-month intervention consisting of diet-induced weight loss alone (WL; n = 11) or with exercise (WL/EX; n = 18). The WL intervention consisted of a low-fat, 500-1,000 kcal/d caloric restriction. The WL/EX intervention included the WL intervention with the addition of aerobic exercise, moderate-intensity walking, three to five times per week for 35-45 minutes per session. Whole-body dual-energy x-ray absorptiometry, thigh computed tomography (CT), and percutaneous muscle biopsy were performed to assess changes in skeletal muscle mass at the whole-body, regional, and cellular level, respectively.

RESULTS

Mixed analysis of variance demonstrated that both groups had similar decreases in bodyweight (WL, -9.2% +/- 1.0%; WL/EX, -9.1% +/- 1.0%) and whole-body fat mass (WL, -16.5%, WL/EX, -20.7%). However, whole-body fat-free mass decreased significantly (p < .05) in WL (-4.3% +/- 1.2%) but not in WL/EX (-1.1% +/- 1.0%). Thigh muscle cross-sectional area by CT decreased in both groups (WL, -5.2% +/- 1.1%; WL/EX, -3.0% +/- 1.0%) and was not statistically different between groups. Type I muscle fiber area decreased in WL (-19.2% +/- 7.9%, p = .01) but remained unchanged in WL/EX (3.4% +/- 7.5%). Similar patterns were observed in type II fibers (WL, -16.6% +/- 4.0%; WL/EX, -0.2% +/- 6.5%).

CONCLUSION

Diet-induced weight loss significantly decreased muscle mass in older adults. However, the addition of moderate aerobic exercise to intentional weight loss attenuated the loss of muscle mass.

Skeletal muscle hypertrophy following resistance training is accompanied by a fiber type-specific increase in satellite cell content in elderly men

We determined muscle fiber type-specific hypertrophy and changes in satellite cell (SC) content following a 12-week resistance training program in 13 healthy, elderly men (72 +/- 2 years). Leg strength and body composition (dual-energy X-ray absorptiometry and computed tomography) were assessed, and muscle biopsy samples were collected. Leg strength increased 25%-30% after training (p < .001). Leg lean mass and quadriceps cross-sectional area increased 6%-9% (p < .001). At baseline, mean fiber area and SC content were smaller in the Type II versus Type I muscle fibers (p < .01). Following training, Type II muscle fiber area increased from 5,438 +/- 319 to 6,982 +/- 503 microm(2) (p < .01). Type II muscle fiber SC content increased from 0.048 +/- 0.003 to 0.084 +/- 0.008 SCs per fiber (p < .001). No changes were observed in the Type I muscle fibers. In older adults, skeletal muscle tissue is still capable of inducing SC proliferation and differentiation, resulting in Type II muscle fiber hypertrophy.

Protein supplementation before and after exercise does not further augment skeletal muscle hypertrophy after resistance training in elderly men

BACKGROUND

Considerable discrepancy exists in the literature on the proposed benefits of protein supplementation on the adaptive response of skeletal muscle to resistance-type exercise training in the elderly.

OBJECTIVE

The objective was to assess the benefits of timed protein supplementation on the increase in muscle mass and strength during prolonged resistance-type exercise training in healthy elderly men who habitually consume adequate amounts of dietary protein.

DESIGN

Healthy elderly men (n = 26) aged 72 +/- 2 y were randomly assigned to a progressive, 12-wk resistance-type exercise training program with (protein group) or without (placebo group) protein provided before and immediately after each exercise session (3 sessions/wk, 20 g protein/session). One-repetition maximum (1RM) tests were performed regularly to ensure a progressive workload during the intervention. Muscle hypertrophy was assessed at the whole-body (dual-energy X-ray absorptiometry), limb (computed tomography), and muscle fiber (biopsy) level.

RESULTS

The 1RM strength increased approximately 25-35% in both groups (P < 0.001). Dual-energy X-ray absorptiometry and computed tomography scans showed similar increases in leg muscle mass (6 +/- 1% in both groups; P < 0.001) and in the quadriceps (9 +/- 1% in both groups), from 75.9 +/- 3.7 and 73.8 +/- 3.2 to 82.4 +/- 3.9 and 80.0 +/- 3.0 cm2 in the placebo and protein groups, respectively (P < 0.001). Muscle fiber hypertrophy was greater in type II (placebo: 28 +/- 6%; protein: 29 +/- 4%) than in type I (placebo: 5 +/- 4%; protein: 13 +/- 6%) fibers, but the difference between groups was not significant.

CONCLUSION

Timed protein supplementation immediately before and after exercise does not further augment the increase in skeletal muscle mass and strength after prolonged resistance-type exercise training in healthy elderly men who habitually consume adequate amounts of dietary protein. This trial was registered at clinicaltrials.gov as NCT00744094.

Resistance training in patients with single, large-scale deletions of mitochondrial DNA

Dramatic tissue variation in mitochondrial heteroplasmy has been found to exist in patients with sporadic mitochondrial DNA (mtDNA) mutations. Despite high abundance in mature skeletal muscle, levels of the causative mutation are low or undetectable in satellite cells. The activation of these typically quiescent mitotic cells and subsequent shifting of wild-type mtDNA templates to mature muscle have been proposed as a means of restoring a more normal mitochondrial genotype and function in these patients. Because resistance exercise is known to serve as a stimulus for satellite cell induction within active skeletal muscle, this study sought to assess the therapeutic potential of resistance training in eight patients with single, large-scale mtDNA deletions by assessing: physiological determinants of peak muscle strength and oxidative capacity and muscle biopsy-derived measures of damage, mtDNA mutation load, level of oxidative impairment and satellite cell numbers. Our results show that 12 weeks of progressive overload leg resistance training led to: (i) increased muscle strength; (ii) myofibre damage and regeneration; (iii) increased proportion of neural cell adhesion molecule (NCAM)-positive satellite cells; (iv) improved muscle oxidative capacity. Taken together, we believe these findings support the hypothesis of resistance exercise-induced mitochondrial gene-shifting in muscle containing satellite cells which have low or absent levels of deleted mtDNA. Further investigation is warranted to refine parameters of the exercise training protocol in order to maximize the training effect on mitochondrial genotype and treatment potential for patients with selected, sporadic mutations of mtDNA in skeletal muscle.

Target population for clinical trials on sarcopenia

The term "sarcopenia" describes the progressive decline of muscle mass, strength and function occurring with aging. It is not considered a disease, but the direct consequence of the aging process on the skeletal muscle. Multiple demographic (e.g. gender, race), biological (e.g. inflammatory status) and clinical (e.g. diabetes, metabolic syndrome, congestive heart failure, medications) factors are able to influence (positively or negatively) the skeletal muscle quality and quantity. The extreme paucity of clinical trials on sarcopenia in literature is mainly due to difficulties in designing studies able to isolate the aging process from its multiple and interconnected consequences. In the present review, we present the major factors to consider as potential sources of biased results when evaluating potential candidates for clinical trials on sarcopenia. The development of clinical trials exploring the nature of the sarcopenia process is urgent, but several controversial issues on this hallmark of aging still need clarification.

Sarcopenia: its assessment, etiology, pathogenesis, consequences and future perspectives

Sarcopenia is a loss of muscle protein mass and loss of muscle function. It occurs with increasing age, being a major component in the development of frailty. Current knowledge on its assessment, etiology, pathogenesis, consequences and future perspectives are reported in the present review. On-going and future clinical trials on sarcopenia may radically change our preventive and therapeutic approaches of mobility disability in older people.

Potent myofiber hypertrophy during resistance training in humans is associated with satellite cell-mediated myonuclear addition: a cluster analysis

A present debate in muscle biology is whether myonuclear addition is required during skeletal muscle hypertrophy. We utilized K-means cluster analysis to classify 66 humans after 16 wk of knee extensor resistance training as extreme (Xtr, n = 17), modest (Mod, n = 32), or nonresponders (Non, n = 17) based on myofiber hypertrophy, which averaged 58, 28, and 0%, respectively (Bamman MM, Petrella JK, Kim JS, Mayhew DL, Cross JM. J Appl Physiol 102: 2232-2239, 2007). We hypothesized that robust hypertrophy seen in Xtr was driven by superior satellite cell (SC) activation and myonuclear addition. Vastus lateralis biopsies were obtained at baseline and week 16. SCs were identified immunohistochemically by surface expression of neural cell adhesion molecule. At baseline, myofiber size did not differ among clusters; however, the SC population was greater in Xtr (P < 0.01) than both Mod and Non, suggesting superior basal myogenic potential. SC number increased robustly during training in Xtr only (117%; P < 0.001). Myonuclear addition occurred in Mod (9%; P < 0.05) and was most effectively accomplished in Xtr (26%; P < 0.001). After training, Xtr had more myonuclei per fiber than Non (23%; P < 0.05) and tended to have more than Mod (19%; P = 0.056). Both Xtr and Mod expanded the myonuclear domain to meet (Mod) or exceed (Xtr) 2,000 mum(2) per nucleus, possibly driving demand for myonuclear addition to support myofiber expansion. These findings strongly suggest myonuclear addition via SC recruitment may be required to achieve substantial myofiber hypertrophy in humans. Individuals with a greater basal presence of SCs demonstrated, with training, a remarkable ability to expand the SC pool, incorporate new nuclei, and achieve robust growth.

Resistance training, sarcopenia, and the mitochondrial theory of aging

Skeletal muscle aging is associated with a significant loss of muscle mass, strength, function, and quality of life. In addition, the healthcare cost of aging and age-related disease is growing, and will continue to grow as a larger proportion of our population reaches retirement age and beyond. The mitochondrial theory of aging has been identified as a leading explanation of the aging process and describes a path leading to cellular senescence that includes electron transport chain deficiency, reactive oxygen species production, and the accumulation of mitochondrial DNA deletions and mutations. It is also quite clear that regular resistance exercise is a potent and effective countermeasure for skeletal muscle aging. In this review, we discuss age-related sarcopenia, the mitochondrial theory of aging, and how resistance exercise may directly affect key components of the mitochondrial theory. It is clear from the data discussed that regular resistance training can effectively disturb processes that contribute to the progression of aging as it pertains to the mitochondrial theory.

Muscle-strengthening activity and its association with insulin sensitivity

OBJECTIVE

Muscle-strengthening activities (MSAs) may increase insulin sensitivity, thereby reducing the risk of diabetes. The purpose of this study was to assess the relationship between MSAs and insulin sensitivity among American adults.

RESEARCH DESIGN AND METHODS

We analyzed data on 4,504 adults without diabetes, aged 20-79 years, who participated in the National Health and Nutrition Examination Survey 1999-2004 and had information on MSAs. Self-reported frequency (times/week) of MSAs was grouped as low (<1), moderate (1-2.9), or high (>or=3). Insulin sensitivity was measured by the fasting quantitative insulin sensitivity check index x 100 (QUICKI).

RESULTS

After adjustment for age, race/ethnicity, physical activity other than MSAs, BMI, smoking, alcohol consumption, and daily total caloric intake, the mean values for QUICKI by low, moderate, and high MSA were 33.6, 33.9, and 34.2, respectively (P for linear trend = 0.008) for men and 34.2, 34.6, 34.6, respectively (P for linear trend = 0.009) for women. Mean fasting insulin (picomols per liter) concentrations were 75.0, 68.9, and 65.9, respectively (P for linear trend = 0.017) for men and 66.9, 63.3, 61.2, respectively (P for linear trend = 0.007) for women. There were no significant differences across MSA groups for fasting glucose among men or women.

CONCLUSIONS

MSA is independently associated with higher insulin sensitivity among U.S. adults. Efforts to increase MSA may be a realistic, feasible, and effective method of reducing insulin resistance among the U.S. population.

Resistance training in the oldest old: consequences for muscle strength, fiber types, fiber size, and MHC isoforms

Resistance training-related changes in muscle strength, muscle size, fiber type, and myosin heavy chain isoform composition in 11 elderly subjects (age range, 85-97 years) after 12 weeks of heavy resistance training (80% of 1 RM) were examined. Twelve subjects constituted a control group. Resistance training increased isometric knee extensor strength 37% (P<0.05) and isokinetic knee strength 41-47% (P<0.05). Lean-quadriceps cross-sectional area increased 9.8% (P<0.05). Muscle fiber hypertrophy occurred only in the type 2 fibers (22% (P<0.05)). Type 1 fiber-type area percentage decreased [4.0 % (P<0.05)] whereas fiber-type area percentage of type 2a fibers increased [5.9% (P<0.05)]. The relative amount of myosin heavy chain (MHC) I (P<0.05) decreased and the relative amount of MHC IIA increased (P<0.05). No effects in the overall number of capillaries per area was observed, but an increase in the number of capillary contacts in the type 2 fiber pool was observed. Heavy resistance training does have beneficial effects on muscle strength and muscle volume in very old frail humans. Furthermore, an increase in fiber size of the fast muscle fibers and an overall increase in the relative amount of fast MHC IIA can lead not only to a stronger, but maybe more importantly, to a more powerful skeletal muscle.

Circuit resistance training in chronic heart failure improves skeletal muscle mitochondrial ATP production rate--a randomized controlled trial

BACKGROUND

We aimed to determine the role of skeletal muscle mitochondrial ATP production rate (MAPR) in relation to exercise tolerance after resistance training (RT) in chronic heart failure (CHF).

METHODS AND RESULTS

Thirteen CHF patients (New York Heart Association functional class 2.3 +/- 0.5; Left ventricular ejection fraction 26 +/- 8%; age 70 +/- 8 years) underwent testing for peak total body oxygen consumption (VO(2peak)), and resting vastus lateralis muscle biopsy. Patients were then randomly allocated to 11 weeks of RT (n = 7), or continuance of usual care (C; n = 6), after which testing was repeated. Muscle samples were analyzed for MAPR, metabolic enzyme activity, and capillary density. VO(2peak) and MAPR in the presence of the pyruvate and malate (P+M) substrate combination, representing carbohydrate metabolism, increased in RT (P < .05) and decreased in C (P < .05), with a significant difference between groups (VO(2peak), P = .005; MAPR, P = .03). There was a strong correlation between the change in MAPR and the change in peak total body oxygen consumption (VO(2peak)) over the study (r = 0.875; P < .0001), the change in MAPR accounting for 70% of the change in VO(2peak).

CONCLUSIONS

These findings suggest that mitochondrial ATP production is a major determinant of aerobic capacity in CHF patients and can be favorably altered by muscle strengthening exercise.

Cluster analysis tests the importance of myogenic gene expression during myofiber hypertrophy in humans

We applied K-means cluster analysis to test the hypothesis that muscle-specific factors known to modulate protein synthesis and satellite cell activity would be differentially expressed during progressive resistance training (PRT, 16 wk) in 66 human subjects experiencing extreme, modest, and failed myofiber hypertrophy. Muscle mRNA expression of IGF-I isoform Ea (IGF-IEa), mechanogrowth factor (MGF, IGF-IEc), myogenin, and MyoD were assessed in muscle biopsies collected at baseline (T1) and 24 h after the first (T2) and last (T3) loading bouts from previously untrained subjects clustered as extreme responders (Xtr, n=17), modest responders (Mod, n=32), and nonresponders (Non, n=17) based on mean myofiber hypertrophy. Myofiber growth averaged 2,475 microm2 in Xtr, 1,111 microm2 in Mod, and -16 microm2 in Non. Main training effects revealed increases in all transcripts (46-83%, P<0.005). For the entire cohort, IGF-IEa, MGF, and myogenin mRNAs were upregulated by T2 (P<0.05), while MyoD did not increase significantly until T3 (P<0.001). Within clusters, MGF and myogenin upregulation was robust in Xtr (126% and 65%) and Mod (73% and 41%) vs. no changes in Non. While significant in all clusters by T3, IGF-IEa increased most in Xtr (105%) and least in Non (44%). Although MyoD expression increased overall, no changes within clusters were detected. We reveal for the first time that MGF and myogenin transcripts are differentially expressed in subjects experiencing varying degrees of PRT-mediated myofiber hypertrophy. The data strongly suggest the load-mediated induction of these genes may initiate important actions necessary to promote myofiber growth during PRT, while the role of MyoD is less clear.

The Adaptations to Strength Training

High-resistance strength training (HRST) is one of the most widely practiced forms of physical activity, which is used to enhance athletic performance, augment musculo-skeletal health and alter body aesthetics. Chronic exposure to this type of activity produces marked increases in muscular strength, which are attributed to a range of neurological and morphological adaptations. This review assesses the evidence for these adaptations, their interplay and contribution to enhanced strength and the methodologies employed. The primary morphological adaptations involve an increase in the cross-sectional area of the whole muscle and individual muscle fibres, which is due to an increase in myofibrillar size and number. Satellite cells are activated in the very early stages of training; their proliferation and later fusion with existing fibres appears to be intimately involved in the hypertrophy response. Other possible morphological adaptations include hyperplasia, changes in fibre type, muscle architecture, myofilament density and the structure of connective tissue and tendons. Indirect evidence for neurological adaptations, which encompasses learning and coordination, comes from the specificity of the training adaptation, transfer of unilateral training to the contralateral limb and imagined contractions. The apparent rise in whole-muscle specific tension has been primarily used as evidence for neurological adaptations; however, morphological factors (e.g. preferential hypertrophy of type 2 fibres, increased angle of fibre pennation, increase in radiological density) are also likely to contribute to this phenomenon. Changes in inter-muscular coordination appear critical. Adaptations in agonist muscle activation, as assessed by electromyography, tetanic stimulation and the twitch interpolation technique, suggest small, but significant increases. Enhanced firing frequency and spinal reflexes most likely explain this improvement, although there is contrary evidence suggesting no change in cortical or corticospinal excitability. The gains in strength with HRST are undoubtedly due to a wide combination of neurological and morphological factors. Whilst the neurological factors may make their greatest contribution during the early stages of a training programme, hypertrophic processes also commence at the onset of training.

Interactions of VEGF isoforms with VEGFR-1, VEGFR-2, and neuropilin in vivo: a computational model of human skeletal muscle

The vascular endothelial growth factor (VEGF) family of cytokines is involved in the maintenance of existing adult blood vessels as well as in angiogenesis, the sprouting of new vessels. To study the proangiogenic activation of VEGF receptors (VEGFRs) by VEGF family members in skeletal muscle, we develop a computational model of VEGF isoforms (VEGF121, VEGF165), their cell surface receptors, and the extracellular matrix in in vivo tissue. We build upon our validated model of the biochemical interactions between VEGF isoforms and receptor tyrosine kinases (VEGFR-1 and VEGFR-2) and nonsignaling neuropilin-1 coreceptors in vitro. The model is general and could be applied to any tissue; here we apply the model to simulate the transport of VEGF isoforms in human vastus lateralis muscle, which is extensively studied in physiological experiments. The simulations predict the distribution of VEGF isoforms in resting (nonexercising) muscle and the activation of VEGFR signaling. Little of the VEGF protein in muscle is present as free, unbound extracellular cytokine; the majority is bound to the cell surface receptors or to the extracellular matrix. However, interstitial sequestration of VEGF165does not affect steady-state receptor binding. In the absence of neuropilin, VEGF121and VEGF165behave similarly, but neuropilin enhances the binding of VEGF165to VEGFR-2. This model is the first to study VEGF tissue distribution and receptor activation in human muscle, and it provides a platform for the design and evaluation of therapeutic approaches.

Efficacy of myonuclear addition may explain differential myofiber growth among resistance-trained young and older men and women

Skeletal muscle stem (satellite) cells supporting growth/regeneration are thought to be activated and incorporated into growing myofibers by both endocrine and locally expressed autocrine/paracrine growth factors, the latter being load sensitive. We recently found that myofiber hypertrophy with resistance training is superior in young men (YM) vs. young women and older adults (Kosek DJ, Kim JS, Petrella JK, Cross JM, and Bamman MM. J Appl Physiol 101: 531-544, 2006). We hypothesized that the advanced myofiber hypertrophy in YM is facilitated by myonuclear addition in response to a milieu promoting stem cell activation. Twenty-six young (27.0 +/- 1 yr, 50% women) and 26 older (63.7 +/- 1 yr, 50% women) adults completed 16 wk of knee extensor resistance training. Vastus lateralis biopsies were obtained at baseline, 24 h after one bout, and after 16 wk. Muscle stem cells were identified immunohistochemically with anti-neural cell adhesion molecule (NCAM+). Muscle transcript levels of IGF-I and mechanogrowth factor (MGF) were determined by RT-PCR. Serum IGF-I, IGF-binding protein (IGFBP)-3, IGFBP-1, total and free testosterone, sex hormone-binding globulin (SHBG), and androstenedione were assessed by radioimmunoassay. Myofiber hypertrophy was twofold greater in YM vs. others, and only YM increased NCAM+ cells per 100 myofibers (49%) and myonuclei per fiber (19%) (P < 0.05). IGF-IEa mRNA was higher in young and increased acutely (29%) with summation by 16 wk (96%) (P < 0.05). MGF mRNA increased only in young after one bout (81%) and by 16 wk (85%) (P < 0.001). Circulating IGF-I was twofold higher in young, whereas IGFBP-1 was lowest in YM (P < 0.05). Among men, free testosterone was 59% higher in YM (P < 0.01). Myonuclear addition was most effectively accomplished in YM, which likely drove the superior growth.

Age and sex affect human muscle fibre adaptations to heavy-resistance strength training

This study assessed age and sex effects on muscle fibre adaptations to heavy-resistance strength training (ST). Twenty-two young men and women (20-30 years old) and 18 older men and women (65-75 years old) completed 9 weeks of heavy-resistance knee extension exercises with the dominant leg 3 days week(-1); the non-dominant leg served as a within-subject, untrained control. Bilateral vastus lateralis muscle biopsies were obtained before and after ST for analysis of type I, IIa and IIx muscle fibre cross-sectional area (CSA) and fibre type distribution. One-repetition maximum (1-RM) strength was also assessed before and after ST. ST resulted in increased CSA of type I, IIa and IIx muscle fibres in the trained leg of young men, type I and IIa fibres in young women, type IIa fibres in older men, and type IIx fibres in older women (all P<0.05). Analysis of fibre type distribution revealed a significant increase in the percentage of type I fibres (P<0.05) along with a decrease in type IIx fibres (P=0.054) after ST only in young women. There were no significant changes in muscle fibre CSA or fibre type distribution in the untrained leg for any group. All groups displayed significant increases in 1-RM (27-39%; all P<0.01). In summary, ST led to significant increases in 1-RM and type II fibre CSA in all groups; however, age and sex influence specific muscle fibre subtype responses to ST.

Exercise and Training in Mitochondrial Myopathies

The intriguing concept of exercise training as therapy for mitochondrial disease is currently unsettled: in the unique setting of mitochondrial heteroplasmy, what are the effects of chronic exercise on skeletal muscle containing a mixture of mutated and wild-type mitochondrial DNA (mtDNA)? Furthermore, what are the consequences of habitual physical inactivity on mitochondrial heteroplasmy? In patients with mtDNA defects, deleterious effects of limited physical activity likely magnify the mitochondrial oxidative impairment contributing to varying degrees of exercise intolerance. Normal adaptive responses to endurance training offer the potential to increase levels of functional mitochondria, improving exercise tolerance. The few clinical studies assessing such training effects in patients with mtDNA defects have unequivocally demonstrated physiologic and biochemical adaptations that improve exercise tolerance and quality of life. Uncertain, however, is the training effect on mitochondrial heteroplasmy. To determine therapeutic advisability of endurance training, it remains imperative to establish whether: reported increases in mutant mtDNA levels can be offset by increases in absolute wild-type mtDNA levels; and chronic physical inactivity leads to a selective down-regulation of wild-type mtDNA. Resistance exercise training offers an alternate, innovative therapeutic approach in patients with sporadic mtDNA mutations; exercise-induced transfer of normal mtDNA templates from muscle satellite cells to mature myofibers, thereby lowering mutation load (increasing functional mitochondrial load). Efficacy and safety of this approach needs to be replicated in a larger group of patients. Currently, appropriate recommendation (either in support or against) exercise training in mitochondrial disease is lacking, which is frustrating for physicians and disheartening for patients. Although considerable progress has been made, an immediate urgency exists to resolve the effects of chronic exercise on skeletal muscle in patients with heteroplasmic mtDNA mutations.

Angiotensin-converting enzyme inhibition attenuates myonuclear addition in overloaded slow-twitch skeletal muscle

Because optimal overload-induced skeletal muscle hypertrophy requires ANG II, we aimed to determine the effects of blocking ANG II production [via angiotensin-converting enzyme (ACE) inhibition] on potential mediators of hypertrophy in overloaded skeletal muscle, namely, myonuclear addition and fibroblast content. In a 2 × 2 design, adult (200–225 g) female Sprague-Dawley rats were placed into one of four groups ( n = 8/group): 7-day skeletal muscle overload, sham operation, 7-day skeletal muscle overload with ACE inhibition, or sham operation with ACE inhibition. Functional overloads of the plantaris and soleus muscles were produced via bilateral surgical ablation of the synergistic gastrocnemius muscle, and ACE inhibition was accomplished by the addition of the ACE inhibitor enalapril maleate to the animals' daily drinking water (0.3 mg/ml). Myonuclear addition and extrasarcolemmal nuclear proliferation, as measured by in vivo 5-bromo-2′-deoxyuridine labeling, were significantly ( P ≤ 0.05) increased by overload in both the slow-twitch soleus and fast-twitch plantaris muscles. Furthermore, ACE inhibition attenuated these overload-induced increases in the soleus muscle but not in the plantaris muscle. However, the effect of ACE inhibition on soleus extrasarcolemmal nuclei was not likely due to differences in fibroblast content because overload elicited significant increases in vimentin-positive areas in soleus and plantaris muscles, and these areas were unaffected by ACE inhibition in either muscle. There was no effect of ACE inhibition on any measure in sham-operated muscles. Collectively, these data indicate that ANG II may mediate the satellite cell response to overload in slow-twitch soleus but not in fast-twitch plantaris muscles and that this effect may occur independently of changes in fibroblast content.

The behaviour of satellite cells in response to exercise: what have we learned from human studies?

Understanding the complex role played by satellite cells in the adaptive response to exercise in human skeletal muscle has just begun. The development of reliable markers for the identification of satellite cell status (quiescence/activation/proliferation) is an important step towards the understanding of satellite cell behaviour in exercised human muscles. It is hypothesised currently that exercise in humans can induce (1) the activation of satellite cells without proliferation, (2) proliferation and withdrawal from differentiation, (3) proliferation and differentiation to provide myonuclei and (4) proliferation and differentiation to generate new muscle fibres or to repair segmental fibre injuries. In humans, the satellite cell pool can increase as early as 4 days following a single bout of exercise and is maintained at higher level following several weeks of training. Cessation of training is associated with a gradual reduction of the previously enhanced satellite cell pool. In the elderly, training counteracts the normal decline in satellite cell number seen with ageing. When the transcriptional activity of existing myonuclei reaches its maximum, daughter cells generated by satellite cell proliferation are involved in protein synthesis by enhancing the number of nuclear domains. Clearly, delineating the events and the mechanisms behind the activation of satellite cells both under physiological and pathological conditions in human skeletal muscles remains an important challenge.

Skeletal muscle morphology in power-lifters with and without anabolic steroids

The morphological appearance of the vastus lateralis (VL) muscle from high-level power-lifters on long-term anabolic steroid supplementation (PAS) and power-lifters never taking anabolic steroids (P) was compared. The effects of long- and short-term supplementation were compared. Enzyme-immunohistochemical investigations were performed to assess muscle fiber type composition, fiber area, number of myonuclei per fiber, internal myonuclei, myonuclear domains and proportion of satellite cells. The PAS group had larger type I, IIA, IIAB and IIC fiber areas (p<0.05). The number of myonuclei/fiber and the proportion of central nuclei were significantly higher in the PAS group (p<0.05). Similar results were seen in the trapezius muscle (T) but additionally, in T the proportion of fibers expressing developmental myosin isoforms was higher in the PAS group compared to the P group. Further, in VL, the PAS group had significantly larger nuclear domains in fibers containing > or = 5 myonuclei. The results of AS on VL morphology in this study were similar to previously reported short-term effects of AS on VL. The initial effects from AS appear to be maintained for several years.

Resistance training for chronic heart failure patients on beta blocker medications

BACKGROUND

Resistance training increases the skeletal muscle strength and functional ability of chronic heart failure patients. However, there is limited data regarding the effect of resistance training on the hemodynamic responses and peak oxygen consumption (peak VO(2)) of chronic heart failure patients treated with beta-blocker. This study examined the effect of resistance training on hemodynamics, peak aerobic capacity, muscle strength and quality of life of chronic heart failure patients on beta-blockers medication.

METHODS

Fifteen men diagnosed with chronic heart failure were matched to either a resistance training program or non-training control group. At baseline and after 8 weeks of resistance training patients performed both Balke incremental and maximal strength tests and completed quality of life questionnaires.

RESULTS

The resistance training group demonstrated a significant increase of walking time and peak VO(2) by 11.7% (p=0.002) and approximately 19% (p<0.05), respectively. Peak VO(2) was significantly correlated with both walking time (r=0.54, p=0.038) and change in total weight lifted (r=0.55, p=0.034). Quality of life significantly increased by 87% (p=0.030). The improvement in quality of life was correlated with post training peak VO(2) (r=0.58, p=0.025) and total weight lifted during the post maximal strength test (r=-0.52, p=0.047).

CONCLUSIONS

The benefits from resistance training for chronic heart failure patients on beta-blocker medication included an increased aerobic and exercise capacity, skeletal muscle strength and most importantly, an improvement in the quality of life, which is the main goal of cardiac rehabilitation programs. Furthermore, with appropriate supervision, it is recommended that resistance exercise be added to the exercise rehabilitation program of these patients when possible.

The role of resistance exercise intensity on muscle fibre adaptations

Although many training variables contribute to the performance, cellular and molecular adaptations to resistance exercise, relative intensity (% 1 repetition maximum [%1RM]) appears to be an important factor. This review summarises and analyses data from numerous resistance exercise training studies that have monitored percentage fibre type, fibre type cross-sectional areas, percentage cross-sectional areas, and myosin heavy chain (MHC) isoform expression. In general, relative intensity appears to account for 18-35% of the variance for the hypertrophy response to resistance exercise. On the other hand, fibre type and MHC transitions were not related to the relative intensity used for training. When competitive lifters were compared, those typically utilising the heaviest loads (> or =90% 1RM), that is weightlifters and powerlifters, exhibited a preferential hypertrophy of type II fibres when compared with body builders who appear to equally hypertrophy both type I and type II fibres. These data suggest that maximal hypertrophy occurs with loads from 80-95% 1RM.

Implications of exercise training in mtDNA defects—use it or lose it?

Whether regular exercise is beneficial or should be avoided is a question currently unsettled in patients with heteroplasmic mitochondrial DNA (mtDNA) disorders of skeletal muscle. Deleterious effects of habitual physical inactivity superimposed upon impaired mitochondrial oxidative phosphorylation may contribute to varying degrees of exercise intolerance in these patients. Endurance exercise training is widely known to improve exercise capacity in healthy subjects and various chronic-disease patient populations. Although we have shown that beneficial physiological and biochemical responses to training increase exercise tolerance in patients with mtDNA defects, knowledge of the muscle adaptive response to endurance training within the setting of mitochondrial heteroplasmy remains limited. In order to determine advisability of endurance training as therapy, it remains to be established whether potential endurance training-induced increases in mutant mtDNA levels may be offset by increases in absolute wild-type mtDNA levels, and whether chronic inactivity leads to a selective down-regulation of wild-type mtDNA. Resistance training utilizes a different adaptive exercise approach to induce the transfer of normal mitochondrial templates from satellite cells to mature muscle fibers of patients with sporadic mtDNA disorders. The efficacy and safety of this approach needs to be further established. Our current inability to clearly advise patients to "use it or lose it" underscores the immediate urgency of studying the effects of exercise on skeletal muscle of patients with heteroplasmic mtDNA defects.

Influence of corticosteroids on myonuclear domain size in the rat diaphragm muscle

Skeletal muscle fibers are multinucleated. Each myonucleus regulates gene products and protein expression in only a restricted portion of the muscle fiber, the myonuclear domain (MND). In the rat diaphragm muscle (DIAm), corticosteroid (CoS) treatment causes atrophy of fibers containing myosin heavy chain (MHC): MHC2X and/or MHC2B. We hypothesized that DIAm fiber MND size is maintained during CoS-induced atrophy. Adult male rats received methylprednisolone for 11 days at 1 (CoS-Low, n = 8) or 8 mg·kg−1·day−1 (CoS-High, n = 8). Age-matched (CTL-AgeM, n = 8), sham-operated (SHAM-AgeM, n = 8), and weight-matched (CTL-WtM, n = 8) animals served as controls. In single DIAm fibers, cross-sectional area (CSA), MND size, and MHC expression were determined. Fiber CSA and MND size were similar in CTL-AgeM and SHAM-AgeM groups. Only fibers containing MHCslow or MHC2A displayed smaller CSA in CTL-WtM than in CTL-AgeM and SHAM-AgeM groups, and MND size was reduced in all fibers. Thus fibers containing MHCslow and MHC2A maintain the number of myonuclei, whereas MHC2X or MHC2B fibers show loss of myonuclei during normal muscle growth. Both CoS groups displayed smaller CSA and MND size than CTL-AgeM and SHAM-AgeM groups. However, compared with CTL-WtM DIAm fibers, only fibers containing MHC2X or MHC2B displayed reduced CSA and MND size after CoS treatment. Thus little, if any, loss of myonuclei was associated with CoS-induced atrophy of MHC2X or MHC2B DIAm fibers. In summary, MND size does not appear to be regulated during CoS-induced DIAm atrophy.

Muscle tissue changes with aging

PURPOSE OF REVIEW

This review article focuses on the changes that occur in muscle with age, specifically the involuntary loss of muscle mass, strength and function, termed sarcopenia. Particular emphasis is given to the metabolic alterations that characterize sarcopenia, and to the potentially treatable causes of this condition, including age-related endocrine and nutritional changes, and inactivity.

RECENT FINDINGS

Recent data reported include those regarding the potential role of insulin resistance in the development of sarcopenia, the potential role of androgens and growth hormone in the treatment of this condition, the usefulness of exercise including both resistance and aerobic training to improve muscle growth and function, and, finally, the possible use of nutritional manipulations to improve muscle mass.

SUMMARY

Sarcopenia is likely a multifactorial condition that impairs physical function and predisposes to disability. It may be prevented or treated with lifestyle interventions and pharmacological treatment. Further long-term investigations are needed, however, to ascertain what type and combinations of interventions are the most efficacious in improving muscle mass and function in older people.

Once Weekly Combined Resistance and Cardiovascular Training in Healthy Older Men

PURPOSE

To compare the effects of the 16-wk training period (2 d.wk(-1)) of resistance training alone (S), endurance training alone (E), or combined resistance (once weekly) and endurance (once weekly) training (SE) on muscle mass, maximal strength and power of the leg and arm extensor muscles, and maximal workload (Wmax) by using a incremental cycling test in older men.

METHODS

Thirty-one healthy men (65-74 yr) were divided into three treatment groups to train 2x wk(-1) for 16 wk: S (N = 10), E (N = 11), or SE (N = 10; 1x wk(-1) S + 1x wk(-1) E). The subjects were tested at 8-wk intervals (i.e., weeks 8 and 16).

RESULTS

There were no significant differences between S- and SE-induced muscle hypertrophy (11% and 11%) and maximal strength (41% and 38%) gains of the legs as well as between E- and SE-induced Wmax (28% and 23%) gains. The increase in arm strength in S (36%) was greater than that recorded in SE (22%) and greater than that recorded in E (0%).

CONCLUSIONS

Prolonged combined resistance and endurance training in older men seemed to lead to similar gains in muscle mass, maximal strength, and power of the legs as resistance training alone and to similar gains in maximal peak power output measured in an incremental cycling test as endurance training alone. These findings may have an effect on how resistance exercise is prescribed to older adults.

Effects of Resistance Training on Older Adults

Using an integrative approach, this review highlights the benefits of resistance training toward improvements in functional status, health and quality of life among older adults. Sarcopenia (i.e. muscle atrophy) and loss of strength are known to occur with age. While its aetiology is poorly understood, the multifactorial sequelae of sarcopenia are well documented and present a major public health concern to our aging population, as both the quality of life and the likelihood of age-associated declines in health status are influenced. These age-related declines in health include decreased energy expenditure at rest and during exercise, and increased body fat and its accompanying increased dyslipidaemia and reduced insulin sensitivity. Quality of life is affected by reduced strength and endurance and increased difficulty in being physically active. Strength and muscle mass are increased following resistance training in older adults through a poorly understood series of events that appears to involve the recruitment of satellite cells to support hypertrophy of mature myofibres. Muscle quality (strength relative to muscle mass) also increases with resistance training in older adults possibly for a number of reasons, including increased ability to neurally activate motor units and increased high-energy phosphate availability. Resistance training in older adults also increases power, reduces the difficulty of performing daily tasks, enhances energy expenditure and body composition, and promotes participation in spontaneous physical activity. Impairment in strength development may result when aerobic training is added to resistance training but can be avoided with training limited to 3 days/week.

Quantitative measurement of muscle fiber composition in a normal population

To obtain normative muscle morphology data on a healthy population recruited from a population survey, we examined vastus lateralis biopsies from 58 men and 33 women, aged 26-67 years. Biopsies were measured with automated, computer-aided techniques. Data were analyzed according to gender and age, and the influence of blood pressure, body mass index (BMI), and smoking habits was also examined. Men had larger muscle fibers (fiber area approximately 5400 microm(2)) than women ( approximately 4000 microm(2), P = 0.003). No gender differences were seen in fiber composition, fiber roundness, percentage of connective tissue, or capillary density. Blood pressure did not influence fiber size or composition, but was correlated with fiber roundness in men. BMI was associated with fiber area in men, but not in women. Variations in age, smoking habits, and physical activity did not influence muscle morphology data substantially. Thus, in a normal population, men have larger muscle fibers than women, but similar fiber composition. Variation in gender, BMI, blood pressure, and physical activity may influence morphological features to a minor degree.

Molecular basis of skeletal muscle plasticity--from gene to form and function

Skeletal muscle shows an enormous plasticity to adapt to stimuli such as contractile activity (endurance exercise, electrical stimulation, denervation), loading conditions (resistance training, microgravity), substrate supply (nutritional interventions) or environmental factors (hypoxia). The presented data show that adaptive structural events occur in both muscle fibres (myofibrils, mitochondria) and associated structures (motoneurons and capillaries). Functional adaptations appear to involve alterations in regulatory mechanisms (neuronal, endocrine and intracellular signalling), contractile properties and metabolic capacities. With the appropriate molecular techniques it has been demonstrated over the past 10 years that rapid changes in skeletal muscle mRNA expression occur with exercise in human and rodent species. Recently, gene expression profiling analysis has demonstrated that transcriptional adaptations in skeletal muscle due to changes in loading involve a broad range of genes and that mRNA changes often run parallel for genes in the same functional categories. These changes can be matched to the structural/functional adaptations known to occur with corresponding stimuli. Several signalling pathways involving cytoplasmic protein kinases and nuclear-encoded transcription factors are recognized as potential master regulators that transduce physiological stress into transcriptional adaptations of batteries of metabolic and contractile genes. Nuclear reprogramming is recognized as an important event in muscle plasticity and may be related to the adaptations in the myosin type, protein turnover, and the cytoplasma-to-myonucleus ratio. The accessibility of muscle tissue to biopsies in conjunction with the advent of high-throughput gene expression analysis technology points to skeletal muscle plasticity as a particularly useful paradigm for studying gene regulatory phenomena in humans.