Alterations in habitual levels of p

Alterations in habitual levels of physical activity can influence the three contractile properties of motor• units (contraction speed, maximal force, and fatigability).• A decrease in muscle activity, such as occurs with aging, bed rest, limb immobilization, or space 1ight,• reduces the maximal capabilities of all three properties • The effects of increased physical activity depend on the intensity and duration of the activity.• Brief sets of high-intensity contractions performed a few times each week can increase contraction speed and motor unit force whereas prolonged periods of low-intensity contractions can reduce motor unit fatigability. • Physical activity regimens that involve such differences are often described as strength training and endurance training, respectively