Myosin heavy-chain isoform distribution, fibre-type composition and fibre size in skeletal muscle of patients on haemodialysis

Skeletal muscle atrophy in clinical and preclinical models of chronic kidney disease: A systematic review and meta-analysis

Patients with chronic kidney disease (CKD) are often regarded as experiencing wasting of muscle mass and declining muscle strength and function, collectively termed sarcopenia. The extent of skeletal muscle wasting in clinical and preclinical CKD populations is unclear. We evaluated skeletal muscle atrophy in preclinical and clinical models of CKD, with multiple sub-analyses for muscle mass assessment methods, CKD severity, sex and across the different preclinical models of CKD. We performed a systematic literature review of clinical and preclinical studies that measured muscle mass/size using the following databases: Ovid Medline, Embase and Scopus. A random effects meta-analysis was utilized to determine standard mean difference (SMD; Hedges' g) between healthy and CKD. Heterogeneity was evaluated using the I2 statistic. Preclinical study quality was assessed via the Systematic Review Centre for Laboratory Animal Experimentation and clinical studies quality was assessed via the Newcastle-Ottawa Scale. This study was registered in PROSPERO (CRD42020180737) prior to initiation of the search. A total of 111 studies were included in this analysis using the following subgroups: 106 studies in the primary CKD analysis, 18 studies that accounted for diabetes and 7 kidney transplant studies. Significant atrophy was demonstrated in 78% of the preclinical studies and 49% of the clinical studies. The random effects model demonstrated a medium overall SMD (SMD = 0.58, 95% CI = 0.52-0.64) when combining clinical and preclinical studies, a medium SMD for the clinical population (SMD = 0.48, 95% CI = 0.42-0.55; all stages) and a large SMD for preclinical CKD (SMD = 0.95, 95% CI = 0.76-1.14). Further sub-analyses were performed based upon assessment methods, disease status and animal model. Muscle atrophy was reported in 49% of the clinical studies, paired with small mean differences. Preclinical studies reported significant atrophy in 78% of studies, with large mean differences. Across multiple clinical sub-analyses such as severity of CKD, dialysis modality and diabetes, a medium mean difference was found. Sub-analyses in both clinical and preclinical studies found a large mean difference for males and medium for females suggesting sex-specific implications. Muscle atrophy differences varied based upon assessment method for clinical and preclinical studies. Limitations in study design prevented conclusions to be made about the extent of muscle loss with disease progression, or the impact of dialysis. Future work would benefit from the use of standardized measurement methods and consistent clinical staging to improve our understanding of atrophy changes in CKD progression, and analysis of biological sex differences.

Nurses' and medical doctors' attitudes towards exercise for people with chronic kidney disease in Denmark

BACKGROUND

Implementation of exercise training in people with kidney failure may be affected by clinicians' attitudes.

OBJECTIVES

To investigate Danish nephrology nurses' and medical doctors' attitudes towards: exercise for people undergoing dialysis; use of physical activity interventions in chronic kidney disease; and to compare Danish and previously reported Australian nurse attitudes.

DESIGN

Cross-sectional survey.

PARTICIPANTS

Nurses and medical doctors from the nephrology field in Denmark.

MEASUREMENTS

The questionnaire attitudes towards exercise in dialysis, and questions about exercise advice, counselling and interventions.

RESULTS

Nephrology nurses (n = 167) and 17 medical doctors (women 92%, age 47 ± 11 years) from 19 dialysis units participated. There were no differences between nurses' and medical doctors attitudes about training. Ninety-five % and 88% of nurses and medical doctors, respectively, agreed that most people undergoing dialysis could benefit from exercise. Exercise training was offered to people undergoing haemodialyses in 88% of 17 departments. Danish nurses reported more positive attitudes than Australian towards exercise (p < 0.05). Ninety-five % and 86% of the Danish and Australian nurses, respectively, agreed/strongly agreed that most people undergoing dialysis could benefit from exercise. Six % and 35% of the Danish and Australian nurses, respectively, agreed/strongly agreed that most people with dialysis were too sick to exercise.

CONCLUSION

Danish nephrology nurses and medical doctors had mostly positive attitudes to exercise training to people undergoing dialysis, and exercise to people with dialysis was offered frequently. Danish and Australian nurses had positive attitudes to exercise to people undergoing dialysis, it was however more positive in Danish nurses.

Muscle plasticity is influenced by renal function and caloric intake through the FGF23-vitamin D axis

Skeletal muscle, the main metabolic engine in the body of vertebrates, is endowed with great plasticity. The association between skeletal muscle plasticity and two highly prevalent health problems: renal dysfunction and obesity, which share etiologic links as well as many comorbidities, is a subject of great relevance. It is important to know how these alterations impact on the structure and function of skeletal muscle because the changes in muscle phenotype have a major influence on the quality of life of the patients. This literature review aims to discuss the influence of a nontraditional axis involving kidney, bone, and muscle on skeletal muscle plasticity. In this axis, the kidneys play a role as the main site for vitamin D activation. Renal disease leads to a direct decrease in 1,25(OH)₂-vitamin D, secondary to reduction in renal functional mass, and has an indirect effect, through phosphate retention, that contributes to stimulate fibroblast growth factor 23 (FGF23) secretion by bone cells. FGF23 downregulates the renal synthesis of 1,25(OH)₂-vitamin D and upregulates its metabolism. Skeletal production of FGF23 is also regulated by caloric intake: it is increased in obesity and decreased by caloric restriction, and these changes impact on 1,25(OH)₂-vitamin D concentrations, which are decreased in obesity and increased after caloric restriction. Thus, both phosphate retention, that develops secondary to renal failure, and caloric intake influence 1,25(OH)₂-vitamin D that in turn plays a key role in muscle anabolism.

[What is currently available for sport medicine therapy?]

After 30 years of age physical capacity decreases with increasing age by 5-20% per decade. High physical activity in daily life as well as exercise training of endurance, strength, coordination and mobility can delay the functional and anatomical loss of muscle, bone, cartilage and connective tissue by more than 10 years. In recent years, numerous concepts have scientifically been proven in the exercise therapy of internal diseases; therefore, similar to drug treatment, cellular mechanisms of exercise training adaptation are known in detail. With this knowledge the type, dose and intensity of exercise training can be defined in such a way that the targeted use of physical training can achieve health benefits similar to the effects achieved by drugs. This applies to the cardiovascular system, lungs, cancer, metabolic diseases and the immune system. In exercise training therapy of patients, individual exercise programs should be defined in a way that the contents of endurance, strength, coordination and mobility address all health and personal concerns of the patient. For sustained effects and high motivation, the individual and disease-specific definition of exercise programs as well as regular monitoring are necessary. The prescription for movement as well as the prescriptions for sports rehabilitation and functional training incorporate important assistance in this context.

High-phosphorus diet maximizes and low-dose calcitriol attenuates skeletal muscle changes in long-term uremic rats

Although disorders of mineral metabolism and skeletal muscle are common in chronic kidney disease (CKD), their potential relationship remains unexplored. Elevations in plasma phosphate, parathyroid hormone, and fibroblastic growth factor 23 together with decreased calcitriol levels are common features of CKD. High-phosphate intake is a major contributor to progression of CKD. This study was primarily aimed to determine the influence of high-phosphate intake on muscle and to investigate whether calcitriol supplementation counteracts negative skeletal muscle changes associated with long-term uremia. Proportions and metabolic and morphological features of myosin-based muscle fiber types were assessed in the slow-twitch soleus and the fast-twitch tibialis cranialis muscles of uremic rats (5/6 nephrectomy, Nx) and compared with sham-operated (So) controls. Three groups of Nx rats received either a standard diet (0.6% phosphorus, Nx-Sd), or a high-phosphorus diet (0.9% phosphorus, Nx-Pho), or a high-phosphorus diet plus calcitriol (10 ng/kg 3 day/wk ip, Nx-Pho + Cal) for 12 wk. Two groups of So rats received either a standard diet or a high-phosphorus diet (So-Pho) over the same period. A multivariate analysis encompassing all fiber-type characteristics indicated that Nx-Pho + Cal rats displayed skeletal muscle phenotypes intermediate between Nx-Pho and So-Pho rats and that uremia-induced skeletal muscle changes were of greater magnitude in Nx-Pho than in Nx-Sd rats. In uremic rats, treatment with calcitriol preserved fiber-type composition, cross-sectional size, myonuclear domain size, oxidative capacity, and capillarity of muscle fibers. These data demonstrate that a high-phosphorus diet potentiates and low-dose calcitriol attenuates adverse skeletal muscle changes in long-term uremic rats.

Reduced skeletal muscle function is associated with decreased fiber cross-sectional area in the Cy/+ rat model of progressive kidney disease

BACKGROUND

The combination of skeletal muscle wasting and compromised function plays a role in the health decline commonly observed in chronic kidney disease (CKD) patients, but the pathophysiology of muscle mass/strength changes remains unclear. The purpose of this study was to characterize muscle properties in the Cy/+ rat model of spontaneously progressive CKD.

METHODS

Leg muscle function and serum biochemistry of male Cy/+ (CKD) rats and their nonaffected littermates (NLs) were assessed in vivo at 25, 30 and 35 weeks of age. Architecture and histology of extensor digitorum longus (EDL) and soleus (SOL) muscles were assessed ex vivo at the conclusion of the experiment. We tested the hypothesis that animals with CKD have progressive loss of muscle function, and that this functional deficit is associated with loss of muscle mass and quality.

RESULTS

Thirty-five-week-old CKD rats produced significantly lower maximum torque in ankle dorsiflexion and shorter time to maximum torque, and longer half relaxation time in dorsiflexion and plantarflexion compared with NL rats. Peak dorsiflexion torque (but not plantarflexion torque) in CKD remained steady from 25 to 35 weeks, while in NL rats, peak torque increased. Mass, physiologic cross-sectional area (CSA) and fiber-type (myosin heavy chain isoform) proportions of EDL and SOL were not different between CKD and NL. However, the EDL of CKD rats showed reduced CSAs in all fiber types, while only MyHC-1 fibers were decreased in area in the SOL.

CONCLUSIONS

The results of this study demonstrate that muscle function progressively declines in the Cy/+ rat model of CKD. Because whole muscle mass and architecture do not vary between CKD and NL, but CKD muscles show reduction in individual fiber CSA, our data suggest that the functional decline is related to increased muscle fiber atrophy.

Blood flow-restricted strength training displays high functional and biological efficacy in women: a within-subject comparison with high-load strength training

Limited data exist on the efficacy of low-load blood flow-restricted strength training (BFR), as compared directly to heavy-load strength training (HST). Here, we show that 12 wk of twice-a-week unilateral BFR [30% of one repetition maximum (1RM) to exhaustion] and HST (6-10RM) of knee extensors provide similar increases in 1RM knee extension and cross-sectional area of distal parts of musculus quadriceps femoris in nine untrained women (age 22 ± 1 yr). The two protocols resulted in similar acute increases in serum levels of human growth hormone. On the cellular level, 12 wk of BFR and HST resulted in similar shifts in muscle fiber composition in musculus vastus lateralis, evident as increased MyHC2A proportions and decreased MyHC2X proportions. They also resulted in similar changes of the expression of 29 genes involved in skeletal muscle function, measured both in a rested state following 12 wk of training and subsequent to singular training sessions. Training had no effect on myonuclei proportions. Of particular interest, 1) gross adaptations to BFR and HST were greater in individuals with higher proportions of type 2 fibers, 2) both BFR and HST resulted in approximately four-fold increases in the expression of the novel exercise-responsive gene Syndecan-4, and 3) BFR provided lesser hypertrophy than HST in the proximal half of musculus quadriceps femoris and also in CSApeak, potentially being a consequence of pressure from the tourniquet utilized to achieve blood flow restriction. In conclusion, BFR and HST of knee extensors resulted in similar adaptations in functional, physiological, and cell biological parameters in untrained women.

Fiber type-specific response of skeletal muscle satellite cells to high-intensity resistance training in dialysis patients

INTRODUCTION

The aim of this study was to assess the effect of high-intensity resistance training on satellite cell (SC) and myonuclear number in the muscle of patients undergoing dialysis.

METHODS

Patients (n = 21) underwent a 16-week control period, followed by 16 weeks of resistance training 3 times weekly. SC and myonuclear number were determined by immunohistochemistry of vastus lateralis muscle biopsy cross-sections. Knee extension torque was tested in a dynamometer.

RESULTS

During training, SCs/type I fibers increased by 15%, whereas SCs/type II fibers remained unchanged. Myonuclear content of type II, but not type I, fibers increased with training. Before the control period, the SC content of type II fibers was lower than that of type I fibers, whereas contents were comparable when normalized to fiber area. Torque increased after training.

CONCLUSIONS

Increased myonuclear content of type II muscle fibers of dialysis patients who perform resistance training suggests that SC dysfunction is not the limiting factor for muscle growth.

Slow- and fast-twitch hindlimb skeletal muscle phenotypes 12 wk after ⅚ nephrectomy in Wistar rats of both sexes

This study describes fiber-type adaptations in hindlimb muscles, the interaction of sex, and the role of hypoxia on this response in 12-wk ⅚ nephrectomized rats (Nx). Contractile, metabolic, and morphological features of muscle fiber types were assessed in the slow-twitch soleus and the fast-twitch tibialis cranialis muscles of Nx rats, and compared with sham-operated controls. Rats of both sexes were considered in both groups. A slow-to-fast fiber-type transformation occurred in the tibialis cranialis of Nx rats, particularly in males. This adaptation was accomplished by impaired oxidative capacity and capillarity, increased glycolytic capacity, and no changes in size and nuclear density of muscle fiber types. An oxidative-to-glycolytic metabolic transformation was also found in the soleus muscle of Nx rats. However, a modest fast-to-slow fiber-type transformation, fiber hypertrophy, and nuclear proliferation were observed in soleus muscle fibers of male, but not of female, Nx rats. Serum testosterone levels decreased by 50% in male but not in female Nx rats. Hypoxia-inducible factor-1α protein level decreased by 42% in the tibialis cranialis muscle of male Nx rats. These data demonstrate that 12 wk of Nx induces a muscle-specific adaptive response in which myofibers do not change (or enlarge minimally) in size and nuclear density, but acquire markedly different contractile and metabolic characteristics, which are accompanied by capillary rarefaction. Muscle function and sex play relevant roles in these adaptations.

Mitochondrial protein synthesis is increased in oxidative skeletal muscles of rats with cardiac cachexia

Since cardiac cachexia could be associated with alterations in muscular mitochondrial metabolism, we hypothesized that the expected alterations in the activities of mitochondrial oxidative enzymes could be associated with changes in mitochondrial protein synthesis in oxidative skeletal muscles. Cardiac cachexia was provoked in male rats by the ligation of the left coronary artery. Six cachectic and 6 control rats were age-paired, and their food intake was observed. The synthesis of mitochondrial proteins was measured by [1-13C]-valine infusion in soleus, tibilais, myocardium, and liver. Muscles (soleus, gastrocnemius, and tibialis anterior), heart, kidneys, liver, and visceral adipose tissue were weighed. Mitochondrial cytochrome c oxydase IV as well as citrate synthase and myosin ATPase activities were measured. As expected, decreased food intake was observed in the cachectic group. Heart, kidney, and liver weights were higher in the cachectic group, while the visceral adipose tissue weight was lower (P < .01). No changes in muscle weights were observed. Soleus mitochondrial proteins fractional synthesis rate was higher in the cachectic group (P = .054). Cytochrome c oxydase IV activity was reduced (P = .009) and increased (P = .038) in the soleus and liver of the cachectic rats, respectively. No change in citrate synthase activity was observed. Myosin ATPase activity was reduced in the gastrocnemius of the cachectic group (P < .01). Mitochondrial protein synthesis is increased in the soleus of rats with cardiac cachexia, suggesting a compensatory mechanism of the impaired oxidative mitochondrial function. Further work should assess whether the mitochondrial protein synthesis is altered in chronic heart failure patients with cardiac cachexia, and whether this is the cause or the consequence of cachexia.

Improved glucose tolerance after high-load strength training in patients undergoing dialysis

BACKGROUND/AIMS

The aim of this controlled study was to investigate the effect of high-load strength training on glucose tolerance in patients undergoing dialysis.

METHODS

23 patients treated by dialysis underwent a 16-week control period followed by 16 weeks of strength training three times a week. Muscle fiber size, composition and capillary density were analyzed in biopsies obtained in the vastus lateralis muscle. Glucose tolerance and the insulin response were measured by a 2-hour oral glucose tolerance test.

RESULTS

All outcome measures remained unchanged during the control period. After strength training the relative area of type 2X fibers was decreased. Muscle fiber size and capillary density remained unchanged. After the strength training, insulin concentrations were significantly lower in patients with impaired glucose tolerance or type 2 diabetes (n = 14) (fasting insulin from 68 ± 12 (46-96) to 54 ± 10 (37-77) pmol/l, p < 0.05, 2-hour insulin from 533 ± 104 (356-776) to 344 ± 68 (226-510) pmol/l, p < 0.05, total insulin area under the curve from 1,868 ± 334 (1,268-2,536) to 1,465 ± 222 (1,094-1,913), p < 0.05). Insulin concentrations were unchanged in patients with normal glucose tolerance (n = 9).

CONCLUSION

The conducted strength training was associated with a significant improvement in glucose tolerance in patients with impaired glucose tolerance or type 2 diabetes undergoing dialysis. The effect was apparently not associated with muscle hypertrophy, whereas the muscle fiber type composition was changed.

The effects of high-load strength training with protein- or nonprotein-containing nutritional supplementation in patients undergoing dialysis

OBJECTIVE

The aim of this study was to investigate the effects of high-load strength training and protein intake in patients undergoing dialysis with a focus on muscle strength, physical performance, and muscle morphology.

DESIGN

This was a randomized controlled study conducted in three dialysis centers.

SUBJECTS

Subjects for the study included 29 patients undergoing dialysis.

INTERVENTION

The participants went through a control period of 16 weeks before completing 16 weeks of strength training. Before the training period, the participants were randomly assigned to receive a protein or a nonprotein drink after every training session.

MAIN OUTCOME MEASURE

Muscle strength and power were tested using the good strength equipment and the leg extensor power rig. Physical performance and function were assessed using a chair stand test and the Short Form 36 questionnaire. Muscle fiber type size and composition were analyzed in biopsies obtained from the m. vastus lateralis.

RESULTS

All variables remained unchanged during the control period. After training, muscle strength and power, physical performance, and physical function increased significantly. Muscle fiber composition was changed by a relative decrease in type 2x muscle fiber number whereas muscle size at the fiber level was unchanged. There were no effects of combining the training with protein intake.

CONCLUSIONS

High-load strength training is associated with improvements in muscle strength and power, physical performance, and quality of life. The effects were surprisingly not associated with muscle hypertrophy, and the results did not reveal any additional benefit of combining the training with protein intake. The positive results in muscle strength and physical performance have clinically relevant implications in the treatment of patients undergoing dialysis.

Metabolic and morphometric profile of muscle fibers in chronic hemodialysis patients

Muscle weakness and effort intolerance are common in maintenance hemodialysis (MHD) patients. This study characterized morphometric, histochemical, and biochemical properties of limb muscle in MHD patients compared with controls (CTL) with similar age, gender, and ethnicity. Vastus lateralis muscle biopsies were obtained from 60 MHD patients, 1 day after dialysis, and from 21 CTL. Muscle fiber types and capillaries were identified immunohistochemically. Individual muscle fiber cross-sectional areas (CSA) were quantified. Individual fiber oxidative capacities were determined (microdensitometric assay) to measure succinate dehydrogenase (SDH) activity. Mean CSAs of type I, IIA, and IIX fibers were 33, 26, and 28% larger in MHD patients compared with CTL. SDH activities for type I, IIA, and IIX fibers were reduced by 29, 40, and 47%, respectively, in MHD. Capillary to fiber ratio was increased by 11% in MHD. The number of capillaries surrounding individual fiber types were also increased (type I: 9%; IIA: 10%; IIX: 23%) in MHD patients. However, capillary density (capillaries per unit muscle fiber area) was reduced by 34% in MHD patients, compared with CTL. Ultrastuctural analysis revealed swollen mitochondria with dense matrix in MHD patients. These results highlight impaired oxidative capacity and capillarity in MHD patients. This would be expected to impair energy production as well as substrate and oxygen delivery and exchange and contribute to exercise intolerance. The enlarged CSA of muscle fibers may, in part, be accounted for by edema. We speculate that these changes contribute to reduce limb strength in MHD patients by reducing specific force.

Morphological, electrophysiological, and metabolic characteristics of skeletal muscle in people with end-stage renal disease: a critical review

PURPOSE

Fatigue is one of the most frequent debilitating symptoms reported by people with end-stage renal disease (ESRD) on haemodialysis (HD) therapy. A wide range of underlying abnormalities, including skeletal muscle weakness, have been implicated as causes of this fatigue. Skeletal muscle weakness is well established in this population, and such muscle weakness is amenable to physical therapy treatment. The purpose of this review was to identify morphological, electrophysiological, and metabolic characteristics of skeletal muscles in people with ESRD/HD that may cause skeletal muscle weakness.

METHOD

Electronic databases were searched for relevant literature from inception to March 2010. Inclusion criteria were English language; adult subjects with ESRD/HD; and the use of muscle biopsy, electromyography, and nuclear magnetic spectroscopy ((31)P-NMRS) techniques to evaluate muscle characteristics.

RESULTS

In total, 38 studies were included. All studies of morphological characteristics reported type II fibre atrophy. Electrophysiological characteristics included both neuropathic and myopathic skeletal muscle changes. Studies of metabolic characteristics revealed higher cytosolic inorganic phosphate levels and reduced effective muscle mass.

CONCLUSION

The results indicate an array of changes in the morphological, electrophysiological, and metabolic characteristics of skeletal muscle structure in people with ESRD/HD that may lead to muscle weakness.

Quantification of lean tissue losses during cancer and HIV infection/AIDS

PURPOSE OF REVIEW

Cancer and HIV infection/AIDS are associated with an increased risk of undernutrition and cachexia. During the past decade, patients became older, frequently overweight or obese and sedentary, conditions which are likely to result in fat-free mass (FFM) loss. This review sustains the hypothesis that FFM measurement should be implemented in routine clinical practice, to optimize the management of cancer and AIDS, as well as disease-related undernutrition.

RECENT FINDINGS

Undernutrition and FFM loss are associated with worse clinical outcome and increased therapy toxicity in cancer and AIDS patients. The emergence of the concept of sarcopenic obesity in cancer patients, a condition associated with decreased survival, demonstrates the necessity to assess their body composition with easily available methods, such as dual energy X-ray absorptiometry, computerized tomography and bioelectrical impedance analysis. FFM measurement could be helpful for guiding the choice of both disease-specific and nutritional therapies and for evaluating their efficacy and putative toxicity.

SUMMARY

FFM measurement at different steps of disease course could allow improving the guidance and efficacy of both cancer and HIV/AIDS-specific and nutritional therapies. The repeated measurement of FFM could allow reducing undernutrition-related morbidity, mortality and global healthcare costs, and could improve response and tolerance towards therapy, and quality of life.

Health-related quality of life and exercise tolerance in recipients of heart transplants and left ventricular assist devices: a prospective, comparative study

BACKGROUND

The aim of this study was to evaluate and compare health-related quality of life (HRQoL) and physical exercise tolerance in patients after heart transplantation (HTx) or implantation of a left ventricular assist device (LVAD).

METHODS

A prospective, comparative design was used to characterize changes over time in HRQoL (SF-36) and exercise tolerance in patients after HTx (n = 54) and during LVAD support (n = 36). Nine LVAD patients were lost for follow-up. The majority of patients in both groups were male (97%); the LVAD cohort tended to be younger (p = 0.06).

RESULTS

HRQoL improved significantly in HTx patients in the SF-36 physical (p = 0.02), but not in the psychosocial (p = 0.27) component score during follow-up. In the LVAD group, HRQoL showed improvements for both the SF-36 physical and psychosocial component scores (both p = 0.04). Between-group comparisons revealed better HRQoL for the HTx cohort than the LVAD cohort for 2 of 8 SF-36 subscales. Age-, gender- and body mass index (BMI)-adjusted exercise tolerance (workload; VO(2max)) showed significant improvements for both HTx (p = 0.01) and LVAD (p = 0.01) patients. Adjusted maximum oxygen consumption was higher for HTx patients (p = 0.05) relative to LVAD patients at 8 ± 1 months after implant.

CONCLUSION

HRQoL and exercise capacity increased in both groups over the time-course of the study. After adjusting for relevant variables, HTx patients showed a higher exercise tolerance compared with the LVAD group during follow-up. Thus, future large-scale intervention studies should emphasize the specific needs of these patient cohorts.

[Exercise and cellular adaptation of muscle]

Resistance training and to a lesser extent endurance training are capable of enhancing protein synthesis in skeletal muscle via various signaling pathways. Additionally, the expression of muscle fiber types responds to different regimes of training stimuli and immobilization as characterized by changes in myosin heavy chain isoforms (I<-->IIA<-->IIX). Eccentric resistance training has been shown to be highly efficient in inducing sarcomeric protein assembly in the longitudinal orientation of muscle cells. However, concentric contractions lead to a hypertrophic response (increased fiber diameter) in muscle which can still be activated in old age. The central signaling pathway to mediate the elevation of protein synthesis in response to training is the mTOR pathway, which is also stimulated by free amino acids. Moreover, adaptation to endurance training is mediated by the calcium-calcineurin-NFATc1 pathway which is strongly activated by the calcium transients involved in the muscle contraction process. High contraction frequency and long duration of training sessions are essential for activation and maintenance of fiber type I expression as well as for induction of transformation of type II into type I fibers. Endurance training sessions should therefore be longer than 30 min and dominated by periods of high frequency contractions. A further factor in the muscular response to training includes the recruitment and integration of satellite cells into muscle fibers. Satellite cells can respond to muscular stretch, activity and injury with increased proliferation and can later be integrated into muscle fibers. Therefore, new myonuclei are available to enhance mRNA synthesis and protein expression in muscle cells. New understanding of the cellular mechanisms of signal transduction in muscle in response to training, bed rest and ageing will help to optimize training and interventions in an ageing population.