Trunk muscle activity during the simultaneous performance of two motor tasks

A unique feature of trunk muscles is that they can be activated to meet fun ctional requirements for combined behaviors, including those related to pos ture and breathing. Trunk muscles therefore: may have developed mechanisms for dealing with simultaneous inputs for different task requirements. This study was designed to test the hypothesis that a linear addition in trunk m uscle activities would occur when an isometric trunk task and a pulsed expi ration task was performed simultaneously. Surface electromyograms (EMG) wer e recorded from four trunk regions (medial and lateral back, upper and lowe r lateral abdomen) in sitting during the performance of the individual isom etric trunk task, the individual pressure task, and the combined task (isom etric trunk and pressure task). The direction of static holding for the iso metric trunk task was varied between flexion and extension positions. For t he pressure task subjects produced two consecutive pressure pulses (2/s) to a target oral pressure. For each muscle recording, a linear prediction was calculated from the mathematical addition of the EMG recorded from the ind ividual trunk and pressure tasks. This linear prediction was compared to th e actual muscle activity recorded during the combined task. Typically the E MG from two muscles showed linear addition, such that the relative contribu tion of muscle activity did not change for the combined task. This suggests that the motor commands for Each task reached these motor neuron pools ess entially unmodified. The other two muscles showed nonlinear combination of two EMG patterns. That is, qualitatively both EMG patterns, specific to eac h command, were evident in the measured EMG traces for the combined task, b ut quantitatively the muscle did not meet all criteria for linear addition. Linear addition may provide a simple mechanism for combining breathing-rel ated behaviors (expiratory efforts) with other trunk behaviors (holding aga inst gravity). This suggests that some muscles can be shared for two differ ent voluntary tasks without changing their contribution to either component task. At the same time. nonlinear combination suggests that some muscles a re shared, but their contribution to either component task may be modulated , thus avoiding the construction of a third new and unique plan.