Most muscle fiber damage occurs during the execution of the eccentric portion of an isotonic exercise (i.e.., the extension of the arm during a bicep curl), so it makes sense to emphasize slow extension of movements when weight training. It is the eccentric component of muscle movement that primarily contributes to delayed onset muscle soreness (DOMS). Muscular soreness is an indication of fiber damage and is a prelude to the anabolic process of increased muscle hypertrophy and strength.
Since it is known that the eccentric movement of exercises primarily contributes to muscle fiber damage and soreness, it follows that in order to get stronger and bigger it would be better to s-l-o-w down the extension of movements. There is some evidence that performing slow extended movements during exercise may help chronic tendon injuries heal. For example, a study in the Journal of Shoulder and Elbow Surgery discovered that six weeks of slow eccentric exercise improved strength and reduced pain and tenderness in people with chronic tennis elbow injuries better than conventional treatment (e.g., massage, ultrasound, and heat/ice application). Be sure to be careful when performing slow eccentric movements when lifting relatively heavy loads as the risk of injury increases.
Here are some examples of the eccentric movement of several exercises:
Brian Danley, CFT
"Motivation is what gets you started. Habit is what keeps you going."
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I'm a personal trainer who loves to help others fulfill their health and fitness goals. I consider myself a bodybuilder in that I live the lifestyle of eating healthy food, working out regularly, and sculpting my body.