Effects of experimental muscle pain on force variability during task-related and three directional isometric force task

Experimentally muscle pain induces changes in the distribution of muscle activity and affects the muscle coordination. The force steadiness is impaired during muscle pain in the task-related force direction as well as in the tangential directions. In addition, pain lead to a mismatch between the sense of effort and motor output during contractions. However, little is known about the pain effects on the force components when task-related or three-dimensional force matching task are required. The aim of this study was to quantify changes in the force variability during task-related and three-dimensional force task during acute muscle pain. Twelve right-handed healthy volunteers participated in the experiment. Three-dimensional force signals were acquired during isometric elbow flexion at 5%, 15%, and 30% of the maximum voluntary contraction (MVC). The force components were represented by a circle on a computer screen, and a moving square was used for the visual target. Subjects were asked to match the main direction of the contraction during the task-related (1D) or all the force components during the three-dimensional (3D) force matching tasks. Isotonic and hypertonic saline injections were randomly injected into the biceps
brachii muscle. The coefficient of variation (CV) was used to analyze the variability on the task-related force direction. The total excursion of the center of pressure (CoP) was used to quantify the variability on the tangential force directions. Complexity of the force was measured using sample entropy (SEn). Three-way repeated measures ANOVA with factors level of contraction, pain/control, and time were performed for the CV, the CoP, and the SEn of each component of the force. In the tangential forces, no significant effects were found for the 3D matching tasks. The ANOVA of parameters from the 1D task showed a significant interaction between level of contraction, pain/control, and time (P<0.05). Post-hoc analysis of the interaction showed that the total excursion of the center of pressure for the 1D task was higher during pain compared with the control session (P<0.05). In the task-related force, no significant effects were found for the CV during the three-dimensional task or for the task-related task. Finally, the ANOVA analysis of sample entropy showed a significant interaction between pain/control and time (P<0.05) in the main direction of the contraction, showing that the sample entropy was higher compare with the control session during muscle pain in the 1D tasks (P<0.001). The variability of the tangential forces and the complexity of the task-related force were increased during experimental muscle pain only for the task-related contractions. This could be associated with a lack of visual information that yields to an undesirable motor control situation increasing the force variability.