The purpose of this study is to examine the effect of habitual physical activity on the force control (steadiness) in children and postpubertal persons. Nine children (9-11 yr) were assigned to the group who had habitual physical activity over 12-h per week (C-ACTIVE), and nine subjects (9-11 yr) were assigned to the group who did not have habitual physical activity (C-CONT). Eight postpubertal persons (18-22 yr) were also assigned to the group who had habitual physical activity over 12-h per week (Y-ACTIVE) from children, and seven subjects (9-11 yr) were assigned to the group who did not have habitual physical activity (Y-CONT) from children. Subjects performed unilateral isometric contractions with the plantarflexor muscles to exert a steady force to match the target force at 2.5-20 % maximal voluntary contraction. Surface electromyogram (EMG) was recorded from the right gastrocnemius medialis (MG) and soleus muscles (SOL). The standard deviation (SD) of force and averaged EMG (aEMG) were calculated by the standard methods. The slope and y-intercept for the linear relation between the SD of force and mean force were compared between ACTIVE and CONT with an ANCOVA with mean force as a covariate using the individual data. The smaller SD of force in CACTIVE compared with C-CONT was confirmed by the smaller y-intercept in C-ACTIVE compared with C-CONT (p＜0.05, ANCOVA) and no differences in slope (P＞0.05, ANCOVA) in the linear relation between the SD and mean force. aEMG of MG in ACTIVE was significantly greater (p＜0.05) than CONT at 5-10% MVC. There was no significant difference in EMG amplitude of SOL between groups except for 10% MVC. In contrast, there were no differences in SD of force and aEMG between Y-ACTIVE and Y-CONT. These data suggest that habitual physical activity reduces the fluctuations in plantar flexion force during steady contractions with respect to children. In addition, force fluctuations during plantar flexion are attenuated with greater activation level of the medial gastrocnemius muscle as suggested by our prior study (Shinohara et al. 2005).

DESCENTE SPORTS SCIENCE Vol.27/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE