The impact of voluntary exercise on structure, function and stem cell activation in aging skeletal muscle

The impact of sarcopenia led to this research to re‐examine the effects of age and voluntary exercise on factors correlated with maintenance of muscle. Baseline variables (body mass, muscle mass, fiber cross‐sectional area (CSA), muscle strength, proliferation, muscle precursor population and stretch‐activation in vitro) were obtained from normal C57BL6 mice at 3, 6, 8, 12 and 18 months‐of‐age. At 8 and 18 mo., mice were randomized to control and exercise groups to study changes in muscle over 3 weeks training on an exercise wheel. Running distance was recorded over 24 hr and grip strength measured. 2hr after an injection of 3H‐thymidine, mice were euthanised and extensor digitorum longus and gastrocnemius muscles were prepared for analysis of DNA synthesis and fiber CSA, respectively. Precursor activation by 1/2hr stretch was examined in vitro. 2‐way ANOVAs and post hoc means tests (p<0.05) were used. Body mass and stretch activation were highest at 8 mo. Muscle mass and CSA rose from 3 to 6 mo of age and then declined while grip strength peaked at 3 mo. In vivo DNA synthesis was greater at 8 than at 18 mo, and did not respond to 24 hr of exercise. At 18 mo., running distance increased over a 3 weeks and body mass decreased. Cells expressing pax7, MyoD1 or myogenin showed expansion of the precursor population with age. Therefore, complex interactions of precursor activation, proliferation, gene expression and activity responses change with age.