INFLUENCE OF SHORT-TERM IMMOBILIZATION ON THE OUTPUT FUNCTION AT HIGH AND LOW CONTRACTION FORCE WITH ABDUCTION OF INDEX FINGER

Although a large number of studies have been conducted into negative influences on motor function after long-term muscle disuse, little is known about the effect of short-term muscle disuse and its mechanism. Moreover, the aspect of functional decrease during short-term muscle disuse may be different at high and low levels of muscle output. Knowledge of this will be useful for the development of appropriate exercise prescriptions and health-welfare devices that can prevent creeping functional decrease. This study examined the influence of seven days of immobilization on motor function at high- and low-level contraction force with abduction of the index finger.Healthy subjects (12 males, mean 25.9 years) participated in the present study. Their left hands were immobilized with plaster casts. Changes in the cross-sectional area (CSA) of the first dorsal interosseous (FDI) were evaluated with a magnetic resonance imaging system. Subjects performed maximum voluntary contraction (MVC) with isometric abduction of the index finger and they were regulated at the constant force levels of 500 g, 1000 g, 1500 g, and 2000 g. Standard deviation of the force trajectory (F-S.D.) was calculated at each force level as an index of stability of the force regulation. EMG activities were recorded with surface electrodes placed over the FDI. The root mean square (RMS) of the EMG amplitude was calculated at each force level and was normalized with the amplitude of supra-maximum M-wave in order to compare the degree of change by muscle immobilization. Twitch force evoked by supramaximum electrical stimulation at rest was measured to consider factors of the peripheral neuromuscular system. Twitch force at MVC was measured by the twitch interpolation method to consider central factors. Firing rate of motor units at super 80% MVC was then measured. These measurements were taken before and after the immobilization and again in the recovery period.The CSA of the FDI showed no significant changes by short-term immobilization. In contrast, MVC force and muscle activity (RMS) were lost after short-term immobilization. The decline of twitch force at rest suggests that several troubles occurred in neuromuscular function. In addition, the increased twitch force at MVC and the decreased firing rate suggest that the central drive was reduced by short-term immobilization. In the recovery period, these values almost returned to those before immobilization. Therefore, both the peripheral factor and central factor affect the decline of high-level contraction force during short-term disuse before muscular atrophy.The fluctuation (F-S.D.) during force regulation at the same force in the low level increased after immobilization. This result shows that it became more difficult to regulate constant force levels after short-term disuse. The RMS at the same force level also increased after immobilization. In the recovery period, these values almost returned to those before immobilization. Therefore, the decline of neuromuscular efficiency affects the functional decrease of low-level contraction force during short-term disuse.