Excitation of Knee Extensor Muscles During Maximal Isometric Contraction at Different Hip and Knee Angles

Abstract

The quadriceps consists of monoarticular and biarticular muscles. Since changes in hip angle only influence the length of biarticular muscles, it can be expected that monoarticular muscle excitation is not influenced by hip angles. PURPOSE: To compare excitation of the monoarticular (vastus lateralis VL and vastus medialis VM) and biarticular (rectus femoris RF) knee extensor muscles during maximal isometric contraction at different hip and knee angles. METHODS: Nine subjects (4 male; 5 female; mean ± SD: age 23 ± 2.29 yrs, mass 64.2 ± 11.7 kg, height 1.69 ± 0.10 m) gave informed consent to participate in this study. Each subject performed a 3-s maximal isometric knee extension at 6 knee angles (90°, 100°, 110°, 120°, 130° and 140°, full knee extension = 180°) on an isovelocity dynamometer with a 1-min rest period between trials. The test was repeated at three hip positions: sitting (H1), inclined (H2) and lying (H3). Hip angles were measured using a goniometer during isometric contractions. Torque data were corrected for gravity effects and normalized to the peak value recorded in H1 (any knee angle). A surface electromyography (EMG) system was used to measure muscle excitation of the RF, VL and VM. Raw EMG data were full-wave rectified and then root mean squared (RMS) with a 0.5-s window. The highest RMS value averaged over a 1-s window was used for analysis. All EMG data were normalized to the RMS value at 90° knee angle in H1. Repeated measures ANOVA (hip × knee) was used to determine the effect of hip and knee angles on peak isometric torque and muscle excitation. RESULTS: The three hip angles were 109.6 ± 3.16° (H1), 138.6 ± 3.5° (H2) and 163.6 ± 3.28° (H3). Peak torques were higher in H2 than in H1 and H3 (P = 0.044). A quadratic torque-angle relationship was observed as knee angle changed (P < 0.0005) at all hip angles. Hip angle influenced VL excitation (P = 0.029, H1 > H2 > H3) but not RF or VM excitation. All knee extensor EMG decreased as knee angle increased (RF: P < 0.0005, VL: P = 0.001, VM: P = 0.002) and there were no differences in the pattern of response at different hip angles. CONCLUSION: Knee extensor EMG-angle relationship does not correspond well with the torque-angle relationship. The fact that changes in hip angle do not affect biarticular but monoarticular muscles excitation urges further research on neuromuscular control mechanism.