Fatiguing a Single Limb leads to Bilateral Changes in Postural Tremor and Forearm Muscle Activity

Abstract

For healthy adults, the physiological tremor within a single limb segment is uncorrelated to that in the contralateral limb. However, it has been suggested that some bilateral relation for tremor does exist, but that this would only emerge under conditions where the neuromuscular system is perturbed such as during unilateral fatigue. PURPOSE: To assess the effect of externally loading the index finger on bilateral tremor dynamics and forearm muscle activity. METHODS: Eighteen young adults participated in the study (mean age 22.4+4.2 yrs.). For all tasks, bilateral hand and finger tremor and forearm surface EMG activity (extensor digitorum) were collected. Subjects were seated with both forearms resting on a flat surface and their hands and index fingers extended (unsupported). Three 30 s trials were collected under this condition. For the fatigue conditions, a 400 gram weight was suspended from the index finger of one limb. Subjects were required to hold both fingers in an extended (horizontal) position with the weight attached for two, 2-min trials (rests were given between trials). The fatigue protocol was then repeated for the opposite limb. RESULTS: As expected, fatiguing a single limb led to significant increases in tremor amplitude (pre-fatigue: 0.18+0.04 ms-2; post: 0.37+0.08 ms-2, p<0.02), extensor muscle activity (pre: 0.11+0.04 mV; Post: 0.24+0.06 mV, p<0.05). In addition, fatiguing a single limb led to changes in the contralateral (un-weighted) limb, significant increases in the amount of muscle activity (pre: 0.10+0.03 mV; Post: 0.17+0.03 mV, p<0.05) and the amplitude of finger tremor (pre: 0.16+0.05 ms-2; post: 0.27+0.04 ms-2, p<0.05). However, these contralateral effects were observed even though there was no evidence of inter-limb coupling of tremor (r’s=0.08-0.18, p=0.64). CONCLUSION: Overall, the results demonstrated that the effects of fatiguing a single, distal limb segment are not restricted to that effector unit. Loading the index finger of one limb led to bilateral increases in both tremor amplitude and related forearm muscle activity. However, these changes were present even though there was no evidence of any tremor coupling between limbs. The findings illustrate that these bilateral effects may be mediated by an overflow effect caused by increased neural drive to the exercising limb.