Abdominal Exercises After Pregnancy


Shows that when an unloaded muscle contracts, it always shortens; this is called a concentric muscular contraction. In contrast, during an eccentric contraction, the muscle length increases. This occurs when a force applied to the muscle exceeds the force that the muscle can produce during contraction. In this type of contraction, the myosin heads are pulled out to angles of up to 90° or beyond during contraction, rather than flexing to 45° (see Exercises 1.3). Eccentric muscle contractions use less oxygen and ATP and recruit fewer muscle fibers than do equivalent concentric contractions (Friden, 1984). Maximal power production is also greatest in eccentric contractions.

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In running, eccentric muscle contractions occur, especially in the upper thigh muscles (quadriceps) during downhill running when the forces through these muscles become very large, particularly as the foot lands on the ground, and can equal 3 times the body weight. The initial contraction of the quadriceps is not quite strong enough to overcome this force; thus this muscle is stretched in an eccentric contraction for a brief instant every time either foot hits the ground. Muscles are not designed for repetitive eccentric contractions and are susceptible to damage when forced to contract this way (Friden, 1984). This explains why downhill running is especially painful and why it takes so much longer for the postrace muscle stiffness to disappear after downhill races than it does after uphill races. posts 10 and 14 discuss this concept of exercise-induced muscle damage in greater detail.

The final point about eccentric contractions is that during such contractions, the muscles act as brakes. During downhill running, muscles heat up just as brakes do when continually used. High-altitude mountaineers have always known this; they know that when descending a mountain they need to wear less clothing than they do when ascending, even though they are exercising at the same rate.

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