Electromechanical delay in skeletal muscle under normal movement conditions

Electromechanical delays (EMD), the time from onset of EMG activity to change in acceleration or deceleration of the forearm, were studied in concentric and eccentric contractions of biceps and triceps brachii muscles. Horizontal flexion and extension movements were performed at varying speeds by 10 subjects. EMD time in concentric contractions for biceps was 41 +/- 13 ms and for triceps was 26 +/- 11 ms and was not influenced by the velocity of the movement. In eccentric contractions at the slow velocity the biceps EMD time was 38 +/- 13 ms and shortened to 28 +/- 10 ms at the faster velocity. The eccentric triceps EMD, however, was not significantly altered by movement velocity and averaged 30 +/- 7 ms. The data provided support for the hypothesis that stretching of the series elastic component, to a point where muscle force can be detected, is the primary determinant of the EMD phenomenon. However, there are complex interactions of the effects on EMD of muscle fiber type composition, whether the contraction is concentric or eccentric, and the velocity of the movement as well as possible gamma system influence. These complications require that consideration of electromechanical delay be made when phasic relationships between muscle force or joint torque generation from different muscles are inferred from EMGs.