Muscles controlled by the brain provide the force and strength to move the body. With aging muscles are change.
Muscle mass decreases with aging. This seems to be most prevalent in the seventh decade and beyond. Research has shown that a 0.5% to 1.0% loss of muscle mass per year after 70 years of age and a 4.7% loss compared with peak mass in men and 3.7% decrease for women per decade.
Muscle protein synthesis is deceased without much change in degradation. This suggests that muscle turnover and repair capacity is likely decreased with age.
Muscle basal metabolic rate is deceased, insulin resistance is increased, and higher percentage body fat mass is increased. All these contribute to the decrease in lean body muscle mass. Other contributing factors include decreased physical activity, lower hormone excretion, nutritional deficits, and chronic inflammation.
With aging, muscle endurance capacity declines 10% per decade (as measured by maximal oxygen consumption) and aerobic energy production is decreased with age. Decrease in endurance can be due to the reduced number of mitochondria (energy provider) and the subsequent reduction in mitochondrial-based aerobic enzymes
With aging muscle fibers also change with increased percentage from type I muscle fibers and decreased type II muscle fibers. Type I fibers are small, slow-contracting, low-tension output fibers with many mitochondria and aerobic enzymes for energy production. These fibers are highly resistant to fatigue and are capable of metabolizing fat for energy expenditure. Type II fibers are much larger and faster contracting fibers that produce large tension output but fatigue quickly. with age, there is less of a contribution to tension output from the higher tension type II fibers because the lower tension–output type I fibers are now more predominant. Overall, the muscle mass of the elderly is smaller and weaker because of the loss of type II fibers.
References
Patrick N. Siparsky et al https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3874224/
