From molecular to physical function: The aging trajectory.

Aging is accompanied by a decline in muscle mass, strength, and physical function, a condition known as sarcopenia. Muscle disuse attributed to decreased physical activity, hospitalization, or illness (e.g. sarcopenia) results in a rapid decline in muscle mass in aging individuals and effectively accelerates sarcopenia. Consuming protein at levels above (at least 50-100% higher) the current recommended intakes of ∼0.8 g protein/kg bodyweight/d, along with participating in both resistance and aerobic exercise, will aid in the preservation of muscle mass. Physiological muscle adaptations often accompany the observable changes in physical independence an older adult undergoes. Muscle fibre adaptations include a reduction in type 2 fibre size and number, a loss of motor units, reduced sensitivity to calcium, reduced elasticity, and weak cross-bridges. Mitochondrial function and structure are impaired in relation to aging and are worsened with inactivity and disease states but could be overcome by engaging in exercise. Intramuscular connective tissue adaptations with age are evident in animal models; however, the adaptations in collagenous tissue within human aging are less clear. We know that the satellite muscle cell pool decreases with age, and there is a reduced capacity for muscle repair/regeneration. Finally, a pro-inflammatory state associated with age has detrimental impacts on the muscle. The purpose of this review is to highlight the physiological adaptations driving muscle aging and their potential mitigation with exercise/physical activity and nutrition.