Wright, K. D., Graham, S. M., Moir, G. L., & Connaboy, C. (2012). The effects of additional load on the occurrence of bilateral-deficit: mechanical or neural factors? Presentation 2178 at the 59th Annual Meeting of the American College of Sports Medicine, San Francisco, California; May 29-June 2, 2012.

"Bilateral deficit is the reduction in the maximal voluntary force output from a synchronous bilateral contraction, when compared to the combined force outputs in biomechanically similar unilateral contractions. A number of theories for the occurrence of bilateral deficit have been suggested; (1) an inhibition in neural drive or (2) differences in the mechanics between bilateral and unilateral jumping. If bilateral deficit is principally a neural phenomenon, then a change in load should not affect the relative magnitude of bilateral deficit observed. However, if there is a mechanical basis for bilateral deficit then a difference will present in the observed bilateral deficit between unloaded and loaded conditions."

This study examined the effects of an additional load of 10% body-weight (BW) on the occurrence of bilateral deficit in physically active students (M = 22; F = 4). Ss performed a series of counter-movement jumps under unloaded (BW) and loaded (BW+10%) conditions: 3 x unilateral left leg jumps (UL), 3 x unilateral right leg jumps (UR), and 3 x bilateral jumps (BL). A period of 30-seconds rest was taken between jumps, with five minutes between sets of jumps. Ss were randomly assigned to 1 of 2 groups. Group 1 performed unloaded followed by loaded jumps. Group 2 performed loaded followed by unloaded jumps. Vertical jump height was determined using a jump mat with the percentage difference (%diff) in jump height between the combined unilateral jumps (UL + UR) and bilateral deficit for the unloaded and loaded conditions being analyzed.

No effect for jump order was observed. A significant reduction in bilateral deficit was observed in the jump heights recorded during the unloaded and loaded conditions.

Implication. A significant reduction in the difference between unilateral and bilateral vertical jumps resulting from the addition of extra load suggests that mechanical factors may predominate in bilateral deficit, with the additional load causing a shift in the force-speed continuum of the active musculature. This study shows that an alteration in an activity results in different mechanics.

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