Meeting Abstract

LBS1.7  Thursday, Jan. 3  Interpreting muscle function from EMG: lessons learned from direct measurements of muscle force. ROBERTS, Thomas; Brown University roberts@brown.edu

Electromyographs are often used to infer the pattern of skeletal muscle force production. However, in vitro studies of muscle contractile behavior show that for a given muscle activation, both the timing and magnitude of force production can vary with the speed of muscle shortening. Simultaneous measurements of EMG, force and length in hindlimb muscles of wild turkeys allow us to probe the quantitative link between force and EMG. The force-velocity relationship of skeletal muscle provides one testable prediction: for a given muscle activation, the force produced should decrease in direct proportion to fascicle shortening velocity. Measurements of force and EMG during running demonstrate this effect in vivo; the integrated EMG per unit force increases in proportion to shortening velocity. In vitro studies have also demonstrated an increase in the rate of deactivation with increases in fascicle velocity. This shortening-dependent deactivation might be expected to decrease the electromechanical delay from EMG offset to the offset of force. This prediction can be tested by measuring the delay between EMG offset and force offset for incline running, when muscles shorten, and decline running, when muscles actively lengthen. In the turkey lateral gastrocnemius, the prediction is not supported; the electromechanical delays for uphill and downhill running are not significantly different. It is possible that the shortening velocities for these activities are not high enough to affect deactivation rates. Simultaneous measurements of muscle force and EMG provide both cautions and encouragements for the interpretation of EMG. Force-velocity effects should be considered when interpreting integrated EMGs, but electromechanical delays within a single muscle may be less variable than expected.