Christian Denis

Satellite cells and myonuclei in young and elderly women and men

The overall aim of this study was to assess the effects of aging on the satellite cell population. Muscle biopsies were taken from the tibialis anterior muscle of healthy, moderately active young (age range, 20–32 years; n = 31) and elderly (age range, 70–83 years; n = 27) women and men with comparable physical activity pattern. Satellite cells and myonuclei were visualized using a monoclonal antibody against neural cell adhesion molecule and counterstained with Mayers hematoxylin. An average of 211 (range, 192–241) muscle fibers were examined for each individual. Compared with the young women and men, the elderly subjects had a significantly lower (P < 0.011) number of satellite cells per muscle fiber but a significantly higher (P < 0.004) number of myonuclei per muscle fiber. The number of satellite cells relative to the total number of nuclei [satellite cells/(myonuclei + satellite cells)] was significantly lower in the elderly than in the young women and men. These results imply that a reduction in the satellite cell population occurs as a result of increasing age in healthy men and women. Muscle Nerve 29: 120–127, 2004

Leg power and hopping stiffness: relationship with sprint running performance.

PURPOSE Although sprint performance undoubtedly involves muscle power, the stiffness of the leg also determines sprint performance while running at maximal velocity. Results that include both of these characteristics have not been directly obtained in previous studies on human runners. We have therefore studied the link between leg power, leg stiffness, and sprint performance. METHODS The acceleration and maximal running velocity developed by 11 subjects (age 16 +/- 1) during a 40-m sprint were measured by radar. Their leg muscle volumes were estimated anthropometrically. Leg power was measured by an ergometric treadmill test and by a hopping test. Each subject executed a maximal sprint acceleration on the treadmill equipped with force and speed transducers, from which forward power was calculated. A hopping jump test was executed at 2 Hz on a force platform. Leg stiffness was calculated using the flight and contact times of the hopping test. RESULTS The treadmill forward leg power was correlated with both the initial acceleration (r = 0.80, P < 0.01) and the maximal running velocity (r = 0.73, P < 0.05) during track sprinting. The leg stiffness calculated from hopping was significantly correlated with the maximal velocity but not with acceleration. CONCLUSION Although muscle power is needed for acceleration and maintaining a maximal velocity in sprint performance, high leg stiffness may be needed for high running speed. The ability to produce a stiff rebound during the maximal running velocity could be explored by measuring the stiffness of a rebound during a vertical jump.

Molecular adaptations of neuromuscular disease-associated proteins in response to eccentric exercise in human skeletal muscle

The molecular events by which eccentric muscle contractions induce muscle damage and remodelling remain largely unknown. We assessed whether eccentric exercise modulates the expression of proteinases (calpains 1, 2 and 3, proteasome, cathepsin B+L), muscle structural proteins (α‐sarcoglycan and desmin), and the expression of the heat shock proteins Hsp27 and αB‐crystallin. Vastus lateralis muscle biopsies from twelve healthy male volunteers were obtained before, immediately after, and 1 and 14 days after a 30 min downhill treadmill running exercise. Eccentric exercise induced muscle damage as evidenced by the analysis of muscle pain and weakness, creatine kinase serum activity, myoglobinaemia and ultrastructural analysis of muscle biopsies. The calpain 3 mRNA level was decreased immediately after exercise whereas calpain 2 mRNA level was increased at day 1. Both mRNA levels returned to control values by day 14. By contrast, cathepsin B+L and proteasome enzyme activities were increased at day 14. The α‐sarcoglycan protein level was decreased immediately after exercise and at day 1, whereas the desmin level peaked at day 14. αB‐crystallin and Hsp27 protein levels were increased at days 1 and 14. Our results suggest that the differential expression of calpain 2 and 3 mRNA levels may be important in the process of exercise‐induced muscle damage, whereas expression of α‐sarcoglycan, desmin, αB‐crystallin and Hsp27 may be essentially involved in the subsequent remodelling of myofibrillar structure. This remodelling response may limit the extent of muscle damage upon a subsequent mechanical stress.

The interplay of central and peripheral factors in limiting maximal O2 consumption in man after prolonged bed rest

1 The effects of bed rest on the cardiovascular and muscular parameters which affect maximal O2 consumption (VO2,max) were studied. The fractional limitation of VO2,max imposed by these parameters after bed rest was analysed. 2 The VO2,max, by standard procedure, and the maximal cardiac output (Q̇max), by the pulse contour method, were measured during graded cyclo‐ergometric exercise on seven subjects before and after a 42‐day head‐down tilt bed rest. Blood haemoglobin concentration ([Hb]) and arterialized blood gas analysis were determined at the highest work load. 3 Muscle fibre types, oxidative enzyme activities, and capillary and mitochondrial densities were measured on biopsy samples from the vastus lateralis muscle before and at the end of bed rest. The measure of muscle cross‐sectional area (CSA) by NMR imaging at the level of biopsy site allowed computation of muscle oxidative capacity and capillary length. 4 The VO2max was reduced after bed rest (−16.6%). The concomitant decreases in Q̇max (−30.8%), essentially due to a change in stroke volume, and in [Hb] led to a huge decrease in O2 delivery (−39.7%). 5 Fibre type distribution was unaffected by bed rest. The decrease in fibre area corresponded to the significant reduction in muscle CSA (−17%). The volume density of mitochondria was reduced after bed rest (−16.6%), as were the oxidative enzyme activities (−11%). The total mitochondrial volume was reduced by 28.5%. Capillary density was unchanged. Total capillary length was 22.2% lower after bed rest, due to muscle atrophy. 6 The interaction between these muscular and cardiovascular changes led to a smaller reduction in VO2max than in cardiovascular O2 transport. Yet the latter appears to play the greatest role in limiting VO2max after bed rest (>70% of overall limitation), the remaining fraction being shared between peripheral O2 diffusion and utilization.

Effects of endurance training on satellite cell frequency in skeletal muscle of old men.

We investigated the effects of endurance training on satellite cells, which are a major component of the regenerative capacity of muscles. Muscle biopsies were obtained from the vastus lateralis of 11 men aged between 70 and 80 years who trained for 14 weeks (work load corresponding to 65–95% of peak oxygen consumption, V̇O2 peak). Satellite cells were identified by immunohistochemistry. There was a significant increase in satellite cell number. Additionally, V̇O2 peak, citrate synthase activity, and the area of type IIA fibers were significantly increased. Fiber type distribution and the myonuclear number were not significantly affected. The enhancement of satellite cell frequency and fiber area indicate that endurance training is an efficient strategy to improve muscle function in the elderly. Muscle Nerve 28: 87–92, 2003

Effects of training in normoxia and normobaric hypoxia on human muscle ultrastructure.

AbstractThe adaptive response of skeletal muscle to training in normoxia and in severe normobaric hypoxia was studied. The first group of five male subjects trained for 3 weeks on a bicycle (2 h/day, 6 days/week) in normoxia (Control training, Con T). A second group of five subjects trained in an ambient FIO2 decreasing progressively from 12.7% to a final level of 10.0% (hypoxic training, Hyp T). Fourteen months later, these subjects trained in normoxia at the same absolute power (normoxic training, Nor T). Peak oxygen consumption (

Ergometric and metabolic adaptation to a 5-s sprint training programme

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Interval training in elderly men increases both heart rate variability and baroreflex activity.

O2 max) was measured in normoxic and hypoxic conditions. Biopsies from the vastus lateralis muscle were analysed for fibre size, capillary and ultrastructural composition. Nor T had no effect on muscle tissue or