Scientists have argued about the relative merits of concentric, eccentric and static exercise for more than 100 years. In these contractions muscles shorten, lengthen or remain the same length as they exert tension.

Most studies measured strength changes resulting from the various kinds of training, but biochemical and genetic studies hold the key to the effects of each type of contraction on muscle growth.

Muscle physiologists- including Ken Baldwin from the University of California, Irvine- found that eccentric contractions (i.e., negatives) triggered the greatest increases in IGF-1 and mechano-growth factor (MGF) and the greatest decreases in myostatin. IGF-1 and MGF are essential to muscle hypertrophy (muscle growth), while myostatin prevents growth.

Growth factors also increased in tendons, which suggest that tendon changes are important in boosting strength. This study showed that eccentric contractions cause greater chemical changes in muscle and tendon than concentric or static exercise. [sources: Journal of Applied Physiology, Fitness RX]