VOS, E. J., J. HARLAAR, and G. J. VAN INGEN SCHENAU. Electromechanical delay during knee extensor contractions. Med. Sci. Sports Exerc., Vol. 23, No. 10, pp. 1187–1193, 1991. The purpose of this study was to investigate the magnitude of electromechanical delay (EMD) and its possible dependence on muscle type, type of contraction, fatigue, level of force, initial muscle length, and muscle contraction velocity. This was achieved using an experiment that measured voluntary knee extensor torques and surface EMG activity for a variety of different contractile conditions in seven male subjects. EMD values were obtained using a cross-correlation technique in three experimental KIN-COM dynamometer conditions of vastus medialis, rectus femoris, and vastus lateralis. In the first condition, a series of 10 repetitive submaximal (50% and 70% MVC) isometric knee extensor contractions were performed at knee angles of 90° and 130° extension. In the second condition, 10 maximal isokinetic knee extensor contractions were performed during passive shortening and lengthening. As such, the dynamometer was used to passively move the knee joint at 30°.s-1 and 60°.s-1. Both during lengthening and shortening, the contractions occurred at an angular position of 110°. In the last condition, a repetitive submaximal isometric knee extensor fatigue test was performed for 100 s (150 contractions). At 10, 40, and 90 s during the time course of this fatigue test, a series of 10 contractions were recorded. To avoid a phase lag, which is introduced with one-way filtering, the EMG was processed with a bidirectional low-pass filter application. A significant main effect in EMD for the factor level of force was found. The EMD values obtained at a force level of 50% MVC were longer than at 70% MVC (107 vs 98 ms). However, no significant main effects in EMD for the factors initial muscle length, type of contraction, fatigue, and muscle contraction velocity were found. In addition, all the EMD values obtained in the three conditions lie in the same range. A mean EMD value (over conditions) of 106 ms in knee extensor muscles was found. Moreover, contraction and relaxation delays were computed and compared with the conditions 1 and 3. There was a significant difference between the contraction and relaxation delays; the mean (over condition 1 and 3) was 107 ms, and the mean (over condition 1 and 3) was 118 ms, respectively. Despite this difference, the computed contraction and relaxation delay values also lie in the same range as the total phase lag, calculated with the cross-correlation technique. Thus, our results suggest that the EMD values obtained under these three exercise conditions behave much like a constant latency. Additionally, the magnitude of EMD values found supports the need to account for this delay when interpreting temporal aspects of patterns of intermuscular coordination.