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 functional requirements for combined behaviors, including those related to posture 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 muscle activities would occur when an isometric trunk task and a pulsed expiration task was performed simultaneously. Surface electromyograms (EMG) were recorded from four trunk regions (medial and lateral back, upper and lower lateral abdomen) in sitting during the performance of the individual isometric trunk task, the individual pressure task, and the combined task (isometric trunk and pressure task). The direction of static holding for the isometric trunk task was varied between flexion and extension positions. For the 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 individual trunk and pressure tasks. This linear prediction was compared to the actual muscle activity recorded during the combined task. Typically the EMG from two muscles showed linear addition, such that the relative contribution of muscle activity did not change for the combined task. This suggests that the motor commands for each task reached these motor neuron pools essentially unmodified. The other two muscles showed nonlinear combination of two EMG patterns. That is, qualitatively both EMG patterns, specific to each command, were evident in the measured EMG traces for the combined task, but quantitatively the muscle did not meet all criteria for linear addition. Linear addition may provide a simple mechanism for combining breathing-related behaviors (expiratory efforts) with other trunk behaviors (holding against gravity). This suggests that some muscles can be shared for two different voluntary tasks without changing their contribution to either component task. At the same time, nonlinear combination suggests that some muscles are shared, but their contribution to either component task may be modulated, thus avoiding the construction of a third new and unique plan.
Farley
B G
BG
Department of Physiology, Arizona Health Sciences Center, University of Arizona, Tucson 85724, USA. bfarley@u.arizona.edu
Koshland
G F
GF
D000009:Abdominal Muscles / Q000502:physiology*; D000328:Adult; D001415:Back / Q000502:physiology; D004576:Electromyography; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D016014:Linear Models; D008297:Male; D017711:Nonlinear Dynamics; D011187:Posture / Q000502:physiology; D011597:Psychomotor Performance / Q000502:physiology; D015656:Respiratory Mechanics / Q000502:physiology; D012132:Respiratory Muscles / Q000502:physiology*