Force Steadiness Training Reduces Force Fluctuations during Isometric Plantar Flexion in Young Adults

Abstract

The purpose of this study was to investigate the effects of low frequency force steadiness training on force fluctuations during isometric plantar flexion in young adults. Sixteen young healthy males (21±1 yrs) were randomly assigned to two training groups or a non-exercising control group (n=5). Training groups were divided by the frequency of training; six subjects trained once a week (1TR) and five subjects trained twice a week (2TR) for 4 wk. Subjects performed unilateral (non-preferred leg) force matching tasks and strength test in plantar flexor before and after 4 wk training. Strength was assessed with a maximal voluntary contraction (MVC). Force matching tasks were performed to maintain isometric plantar flexion for 15s at levels corresponding to 10% and 20% MVC with the visual feedback of force. Force fluctuation during force matching tasks was quantified as the standard deviation of force and the coefficient of variability. Force fluctuation was reduced by the force steadiness training in training groups. Further, force fluctuation was greater in 20% MVC than in 10% MVC. Relative change in force fluctuation was greater in the TR2 than in the TR1. Further, it was greater in 20% MVC than in 10% MVC. However, the interaction between training frequency and contraction intensity was not found to be statistically significant. Force fluctuation of < 4 Hz power (contribution of afferent input from Ia circuit) was reduced by force steadiness training. However, force fluctuation of 8-12 Hz power (contribution of physiological tremor) was not changed by force steadiness training. These results suggest that (1) force steadiness training reduces force fluctuation in plantar flexor, (2) the effects of low frequency training on fluctuation are affected by training frequency, and (3) reduction of fluctuation might be influenced by the afferent input from Ia circuit.