Influence of Visual Feedback Removal on the Neural Control Strategies During Isometric Force Production

Variability in the force production is known to be influenced by intrinsic and extrinsic properties of the neuromuscular system. Here, we aim at investigating the role of visual feedback (presence or absence) on the neural control of muscle force during a broad range of isometric contractions of a hand’s muscle. Eight participants performed isometric abductions of the index finger in six submaximal contraction intensities (5–75% of the maximum voluntary contraction, MVC). In each trial, participants performed the task with and without visual feedback of the performed force. High-density surface electromyogram (EMG) was recorded from the first dorsal interosseous muscle. Spike trains of motor units were obtained after semi-automatic decomposition of surface EMG signals. The interspike intervals (ISI) of motor units and the effective neural drive to the muscle (smoothed cumulative spike train, sCST) were estimated. The mean value, standard deviation (SD) and coefficient of variation (CoV) were computed for the force, sCST, and ISI. No interaction between visual feedback and contraction conditions were found, neither an effect of visual feedback on the dependent variables. An increase in the contraction intensity was followed by an increase in the mean sCST and a decrease in the mean ISI. Contraction intensity increased the force SD and the sCST SD. CoV of force and sCST both only decreased from 5%MVC to 30%MVC. On the other hand, contraction intensity did not change ISI SD, but increased ISI CoV. These results revealed that visual feedback did not influence motor control strategies in a large range of force levels. Moreover, we provided evidence that force variability is mostly influenced by fluctuations in the activity of a population of motor units rather than the discharge properties of individual motor units.