Uniformity of performance during the collection of maximum voluntary contraction tasks for the muscles of the forearm

Electromyographic (EMG) signals are used to gain insight into muscle activation patterns and thus neuromuscular control. To allow for comparisons between studies and participants, the EMG signal is generally normalised, with the signal obtained during a task that is designed to elicit maximum voluntary contraction (MVC) frequently used as the basis. Recommendations for how to collect MVCs have been made; however, previous studies have not been able to determine if the EMG variation noted within a population was due to different muscle activation patterns, or the tasks being performed differently, or other variables such as skin impedance, or the stochastic nature of electromyography. EMG signals were recorded during hand-wrist tasks selected to elicit MVCs in the muscles of the forearm - pull up, push down, radial pull, ulnar pull, pull, pronation, finger flexion, finger extension, and grip - as well as two activities of daily living - pouring a glass of water from a jug and turning a key in a lock. A load cell mounted to a statically mounted handle was used to record contemporaneously the forces and moments exerted by participants in pull up, push down, radial pull, ulnar pull, pull, pronation, finger flexion, and finger extension tasks. Ninety percent of tasks yielded the expected load cell outputs for the directed tasks and thus were considered as having been performed correctly. The tasks performed incorrectly were not the same for all participants, nor were they all performed by the same participants. Of note was that there were instances when a task was performed incorrectly but still an expected MVC was achieved. The EMG signals showed similar variation to that seen in previous studies. However, the applied forces and moments did not appear to explain the variation seen in the tasks that elicited MVCs. The results of this study indicate that different muscle activation patterns may be used to exert the same force by the hand. Thus, it may not be possible for a given task to elicit MVC in the same muscle in all people. However, by using several activities, MVCs for the forearm muscles may be obtained for most of the population. Beyond designing EMG protocols, the results of this study suggest that people have unique muscle activation patterns and raise questions as to whether this is a result of physiology or conditioning.