Maximal Voluntary Co-Contraction Training may not Always be Effective for Some Leg Muscles.

Simultaneous voluntary contractions of antagonistic pairs (co-contraction) produce resistive forces that act against each other (Maeo et al., 2013c). It has been reported that a training program in which subjects performed voluntary co-contractions of elbow flexors and extensors with maximal effort produced significant increases in the strength capability of both muscle groups, without the use of an external load as resistance (Maeo et al., 2013c). In addition, the level of their muscular activities during the task, expressed as the value relative to those during maximal voluntary contraction (MVC) of corresponding muscles (% EMGmax), has been shown to be a sufficient training stimulus (~40% for elbow flexors and ~60% for elbow extensors) for improving the strength capability of both muscle groups (Maeo et al., 2013c). Thus, it is considered that maximal voluntary co-contraction can be an effective form of resistance raining. However, the muscle groups examined previously are limited to the elbow flexors and extensors. It is known that the influence of aging on muscle mass is more apparent in the lower body than in the upper body (Janssen et al., 2000), and the antigravity muscles located in the lower extremity have a greater influence of decreased mechanical loading conditions such as bed rest, limb immobilization, and space flight (Kanehisa, 2005). To establish the efficacy of the co-contraction training, therefore, it is essential to clarify the muscle activation levels of lower limb muscles during the co-contraction tasks. In the task which requires maximal voluntary co-contractions of the upper arm muscles, stronger elbow flexors would need only to be activated to the extent to which they cancel the torque produced by weaker elbow extensors, resulting in a lower activation level in the elbow flexors than in the extensors during the task (Maeo et al., 2013a, c). On the other hand, in leg muscles, it is often reported that there are large differences in maximal strength between muscles in the antagonistic pairs (e.g. the knee extensors/flexors pair as well as the plantar flexors/dorsi flexors pair) (Billot et al., 2010; Macaluso et al., 2002). Thus, theoretically, it is assumed that during maximal voluntary co-contraction in the leg muscles, the stronger side of the muscle groups (i.e. knee extensors and plantar flexors) result in much lower level of the muscular activity than the weaker side (i.e. knee flexors and dorsi flexors). The present study aimed to clarify this by comparing the muscular activity levels of …