Influence of hip and knee joint angles on excitation of knee extensor muscles.

The purpose of this study was to test if (and how) maximal excitation of the knee extensor muscles rectus femoris (RF), vastus lateralis (VL) and vastus medialis (VM) is influenced by knee and/or hip joint angles. Excitation was quantified using surface electromyography. Isometric knee extensions were performed at systematically varying knee and hip joint configurations using a strength testing machine. The results indicate that excitation of the one-joint knee extensor muscles (VL and VM) depends systematically on hip joint angles. In particular, excitation levels are higher at hip joint angles of 90 degrees (sitting) and 180 degrees (lying) compared to intermediate hip joint angles (112 degrees, 135 degrees, 157 degrees). Furthermore, it was found that excitation of all knee extensor muscles tested is higher near full knee extension (170 degrees) compared to an intermediate knee joint angle (130 degrees). Since knee extensor moments are much smaller at knee joint angles of 170 degrees compared to those at 130 degrees, it is speculated that the high excitation observed near full knee extension constitutes a neurophysiological compensation mechanism for the reduced force production ability of the muscles at this joint configuration.