Medial gastrocnemius is a key muscle for involuntary alternate muscle activity of plantar flexor synergists.

Redundant and/or complicated muscle activations between synergist muscles have been demonstrated during low-level sustained contractions. Identification of a key muscle for this phenomenon allows for the simplification of motor control during prolonged contraction. In this study, we have identified a key muscle for involuntary alternate muscle activity of plantar flexor muscles based on a physiological tremor sequence that was recorded over 120 min. Two epochs where the muscle activity of medial gastrocnemius abruptly increased with decrease in other synergists (case ON) and vise verse (case OFF) were analyzed. Our results indicated that involuntary alternate muscle activity was associated with changes in physiological tremor of ankle angular acceleration when the muscle activity of medial gastrocnemius decreased in case OFF. In particular, the activity of the medial gastrocnemius muscle, but not the activity of other synergists, was accompanied by physiological tremor, demonstrating that the medial gastrocnemius is a key muscle for involuntary alternate muscle activity in plantar flexor synergists. In addition, weaker correlations between muscle activities and physiological tremor were found in case ON than case OFF. We suggest that, if the central nervous system can employ this unique muscle strategy, redundant and/or complicated neuromuscular activities will be reduced because of the existence of the key muscle.

Daytime physical activity patterns and physical fitness in institutionalized elderly women: an exploratory study.

This study investigated the relationship between daytime physical activity patterns and physical fitness in elderly women. The subjects comprised 19 elderly women who resided in a nursing home. Time spent lying, sitting, standing and walking and the number of steps taken during the daytime from 10:00 to 16:00 were measured to determine physical activity patterns. Physical fitness measures included muscle strength, balance, flexibility and physical performance. The elderly women spent 18.3% of their daytime walking, 7.31% in a standing position, 56.9% sitting and 17.4% lying down. Our results showed that the time spent in walking or standing positions was significantly associated with balance and physical performance such as walking speed, while the time spent in a sitting position was inversely associated with muscle strength, balance and physical performance. The results of this study suggest that the time the elderly spend on weight-bearing activities and in sedentary behavior are associated with physical fitness.