Torque-EMG relationships in normal and spastic muscles

The linearity of the relationship between torque and electromyographic (EMG) activation has been widely debated for years, yet remains unresolved. Despite limitations and possible inaccuracies, an assumption of linearity is often made to simplify the relationship between these variables for computational and descriptive purposes. Although typically derived from isometric test conditions, these relationships have also been extrapolated, perhaps invalidly, to conditions where joint velocity and length are changing. The purpose of this study was to examine the degree of linearity between hamstring and quadriceps torques and their respective EMG signals, and to compare the slopes of these relationships in normal and spastic muscles at varying muscle lengths and conditions. We hypothesized that relationships would be linear for all muscles tested in both subject groups; however, slopes would differ across the two muscles, the three muscle lengths tested, and the two subject groups. We further hypothesized that the degree of linearity during an isotonic task would be less than for the isometric one, particularly for patients with spasticity who may demonstrate abnormal responses to changes in muscle length. Results indicated that torque--EMG relationships were linear for all subjects during isometric contractions, regardless of group, muscle, or knee angle. However, the degree of linearity was significantly less in CP in both conditions; and within the CP group, was less during isotonic compared to isometric conditions. Slope values differed between muscles at some lengths, across muscle lengths in the quadriceps, and subjects with CP showed consistently lower slopes for all quadriceps values. These data in general support the robustness of the linear assumption in isometric conditions for the knee musculature, caution against extrapolation to isotonic conditions particularly for those with movement abnormalities, and suggest that slope differences may provide valuable insights into pathology and warrant further investigation.