Trunk muscle activity during the simultaneous performance of two voluntary tasks: A trunk task and a pulsed expiration task

Abstract

Trunk control is intriguing because trunk muscles must meet multiple requirements during the performance of everyday tasks (e.g., balancing, locomotion, musical instrument playing, reaching, trunk bending, vocalizing). A unique feature of trunk muscles is that they often participate in simultaneous trunk movement and breathing-related behaviors. This study was designed to test the hypothesis that two voluntary task-specific muscle activity patterns could combine linearly when both tasks activate the same muscles. Surface electromyograms (EMG) were recorded from four trunk sites (upper and lower lateral abdominal, medial and lateral back) during the performance of a trunk task, pulsed expiration task, and combined task (hunk + pulsed expiration task). The trunk task varied across four experiments, and included a static holding task or a uni-directional movement task in both flexion and extension directions. The expiration task was constant. Selected task variables (lung volume, movement amplitude and duration, expiratory target pressure) were consistent across all tasks. For each EMG site, a linear prediction (predicted EMG trace) was calculated from the mathematical addition of the task-specific EMG recorded during the individual trunk and expiration tasks. This linear prediction was compared to the actual muscle activity recorded during the combined task (measured EMG trace) and a point-to-point correlation was performed on the two traces to determine how closely they matched. Findings showed that in just over half the comparisons, the combined muscle activity pattern (measured EMG trace) was the same as a linear addition of each individual muscle activity pattern (predicted EMG trace). Such linear addition implies that two sets of descending command signals reach motoneuron pools essentially unmodified, and that motoneurons supplying trunk muscles may be specially organized to receive dual and simultaneous voluntary neural drive. In the remaining comparisons, the EMG activity for the two individual tasks, were superimposed, but not linearly. This finding suggests that although individual muscles are activated as a unit, selected muscles may be modified by sensory feedback. This flexibility allows the nervous system to take advantage of a muscles mechanical effectiveness and to adapt to environmental constraints without having to reconfigure or construct a new set of instructions.