Physical activity as a metabolic stressor

Both physical activity and diet stimulate processes that, over time, alter the morphologic composition and biochemical function of the body. physical activity provides stimuli that promote very specific and varied adaptations according to the type, intensity, and duration of exercise performed. Ther e is further interest in the extent to which diet or supplementation can en hance the positive stimuli. Prolonged walking at low intensity presents lit tle metabolic, hormonal, or cardiovascular stress, and the greatest perturb ation from rest appears to be from increased fat oxidation and plasma free fatty acid mobilization resulting from a combination of increased lipolysis and decreased reesterification. More intense jogging or running largely st imulates increased oxidation of glycogen and triacylglycerol, both of which are stored directly within the muscle fibers. Furthermore, these intramusc ular stores of carbohydrate and fat appear to be the primary substrates for the enhanced oxidative and performance ability derived from endurance trai ning-induced increases in muscle mitochondrial density. Weightlifting that produces fatigue in brief periods (ie, in 15-90 s and after 15 repetitive c ontractions) elicits a high degree of motor unit recruitment and muscle fib er stimulation. This is a remarkably potent stimulus for altering protein s ynthesis in muscle and increasing neuromuscular function. The metabolic str ess of physical activity can be measured by substrate turnover and depletio n, cardiovascular response, hormonal perturbation, accumulation of metaboli tes, or even the extent to which the synthesis and degradation of specific proteins are altered, either acutely or by chronic exercise training.