The effect of isometric contraction on the regulation of force tremor in the contralateral limb

Abstract

AIM: When performing isometric voluntary contractions, the force exerted by the muscle is not entirely steady. Instead, small fluctuations exist that are dependent on the intensity and duration of contraction. Although it is clear that variability in force production in a single limb is enhanced during submaximal isometric contractions, little is known about how force tremor is regulated in the opposite limb during this task. This study determined how force tremor in a single limb is altered when the opposite limb is engaged in a force generating task. METHODS: Index finger abduction force and first dorsal interosseous (FDI) activity were assessed in thirteen healthy (23 ± 4 yrs) at target forces of 5, 10, 15, 20 and 60% MVC for the non-dominant limb (unilateral task). Force tremor was again quantified in the non-dominant limb when the dominant limb generated a sustained submaximal abduction force at 60% MVC (bilateral task). Force tremor was quantified as the coefficient of variation of abduction force, and frequency parameters of force and EMG data were examined using power spectral analysis. RESULTS: When the non-dominant limb generated force at 20% MVC, tremor was greater during the bilateral task compared with the unilateral task; a finding reflected in the amplitude of peak power of force (Figure 1). In contrast to the task differences observed for abduction force tremor, no FDI EMG differences were detected between the unilateral and bilateral tasks. CONCLUSION: The process of performing a bilateral isometric task invoked tremor-related changes in the non-dominant limb at selective force targets. It is possible that some feature of common drive to the motoneuron pool, or the proportion of the pool that is activated at 20% MVC, may have contributed to the task differences observed at this moderate force output during a bilateral isometric contraction.