Boost power output by lifting faster...

When contracting muscle is first stretched (prestretch) and then immediately allowed to shorten, it generally exhibits higher contractile performance than it does without prestretch, e.g., enhanced productions of mechanical work and power in isokinetic and isotonic conditions, respectively.

Such a phenomenon is seen most frequently in the form of a countermovement. The countermovement consists of an initial lengthening of muscle (eccentric action) and a subsequent, rapid reversal of the movement to produce shortening of muscle (concentric action).

This rapid deceleration of the eccentric action gives rise to the generation of large force and thereby makes the following concentric action stronger.

Researchers from the University of Tokyo studied a group of five men who performed squatting exercises with a countermovement at varied deceleration rates before lifting the load [1].

The experiments were carried out with well-trained athletes because the generation of large eccentric force was thought to be dangerous for untrained subjects. In addition, some preliminary measurements with untrained subjects exhibited no reproducible enhancement in power output with the increase in peak eccentric force, suggesting that a certain level of skill for resistance-exercise training is required for obtaining consistent results.

The men performed squatting exercises on a force plate, using a barbell weighing 50% of the mens 1-repetition maximum. In the exercise without a countermovement, the subjects squatted at a knee angle of 90°, stayed still for 2-3 s, and then stood up to the upright position at their maximal effort.

The lowest hip position was recognized by the subjects correctly and reproducibly with the aid of a height-adjustable stool placed behind them. In the exercise with the countermovement, they squatted until their hip touched the stool, and at this moment, they reversed the motion as rapidly as possible to return to the upright position.

When an intense deceleration was taken at the end of downward movement, large eccentric force was developed, and the mechanical power subsequently produced during the lifting movement was consistently larger than that produced without the countermovement.

Numerous structural abnormalities are evident in muscle after exercise, especially exercise that involves eccentric contractions. These abnormalities include sarcolemmal disruption, dilation of the transverse tubule system, distortion of myofibrillar components, fragmentation of the sarcoplasmic reticulum, lesions of the plasma membrane, cytoskeletal damage, changes in the extracellular myofiber matrix, and swollen mitochondria.

Accompanying these changes, there can be a gradual increase in the soreness of the involved muscles that peaks 24-48 hours after the exercise. This effect is known as delayed-onset muscle soreness. It occurs frequently after the performance of unfamiliar exercises that include eccentric contractions and is attenuated as the exercises are repeated in subsequent sessions.

The short- and long-term consequences of including eccentric contractions in an exercise program can be to induce structural adaptations in muscle, to activate an inflammatory response, and to modify the neural commands used to control the movement. Whereas these adaptations can also be induced by other types of contractions, they seem to be maximized by eccentric contractions.

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Reference
1. Takarada Y, Hirano Y, Ishige Y, Ishii N. (1997). Stretch-induced enhancement of mechanical power output in human multijoint exercise with countermovement. Journal of Applied Physiology, 83, 1749-1755
2. Enoka, R. M. (1996). Eccentric contractions require unique activation strategies by the nervous system. Journal of Applied Physiology, 81, 2339-2346
3. Nardone, A., Romano, C., & Schieppati, M. (1988). Selective recruitment of high-threshold human motor units during voluntary isotonic lengthening of active muscles. Journal of Physiology, 395, 363-381