Everyone knows it is easier to walk downhill than uphill, but the mechanisms underlying this commonplace phenomenon are complex. Muscle forces are identical in the two situations. However, moving the body uphill against gravity involves muscle shortening, or concentric contractions. In contrast, walking downhill primarily involves muscle tension development that resists muscle lengthening, or eccentric contractions. All routine forms of physical activity, in fact, involve combinations of concentric, eccentric, and isometric contractions. Because less ATP is required for force development during a contraction when external forces lengthen the muscle, the number of active motor units is reduced and energy demands are less for eccentric work. However, perhaps because the force per active motor unit is greater in eccentric exercise, eccentric contractions can readily cause muscle damage. These include weakness (apparent the first day), soreness and edema (delayed 1 to 3 days in peak magnitude), and elevated plasma levels of intramuscular enzymes (delayed 2 to 6 days). Histological evidence of damage may persist for 2 weeks. Damage is ac companied by an acute phase reaction that includes complement activation, increases in circulating cytokines, neu-trophil mobilization, and increased monocyte cell adhesion capacity. Training adaptation to the eccentric components of exercise is efficient, soreness after a second episode is minimal if it occurs within two weeks of a first episode.
Eccentric contraction-induced muscle damage and its subsequent response may be the essential stimulus for muscle hypertrophy. While standard resistance exercise involves a mixture of contraction types, careful studies show that when one limb works purely concentrically and the other purely eccentrically at equivalent force, only the eccentric limb hypertrophies. The immediate changes in actin and myosin production that lead to hypertrophy are mediated at the posttranslational level, after a week of loading, mRNA for these proteins is altered. Although its precise role remains unclear, the activity of the 70-kDa S6 protein kinase is tightly linked with long-term changes in muscle mass. The cellular mechanisms for hypertrophy include the induction of insulin-like growth factor I, and up-regulation of several members of the fibroblast growth factor family.
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