ALKNER, B. A., P. A. TESCH, and H. E. BERG. Quadriceps EMG/force relationship in knee extension and leg press. Med. Sci. Sports Exerc., Vol. 32, No. 2, pp. 459–463, 2000.
Purpose: This study compared the relationship between surface electromyographic (EMG) activity and isometric force of m. quadriceps femoris (QF) in the single-joint knee extension (KE) and the multi-joint leg press (LP) exercises.
Methods: Nine healthy men performed unilateral actions at a knee angle of 90° at 20, 40, 60, 80, and 100% of maximal voluntary contraction (MVC). EMG was measured from m. vastus lateralis (VL), m. vastus medialis (VM), m. rectus femoris (RF), and m. biceps femoris (BF).
Results: There were no differences in maximum EMG activity of individual muscles between KE and LP. The QF EMG/force relationship was nonlinear in each exercise modality. VL showed no deviation from linearity in neither exercise, whereas VM and RF did. BF activity increased linearly with increased loads.
Conclusions: The EMG/force relationship of all quadricep muscles studied appears to be similar in isometric multi-joint LP and single-joint KE actions at a knee angle of 90°. This would indicate the strategy of reciprocal force increment among muscles involved is comparable in the two models. Furthermore, these data suggest a nonuniform recruitment pattern among the three superficial QF muscles and surface EMG recordings from VL to be most reliable in predicting force output.
The relationship between EMG activity and the generated force has been examined in several studies. While early reports (7,20) disclosed a linear relationship for the calf muscle, the results of recent studies of different muscles are equivocal (3). Although not a consistent finding (26), the EMG/force relationship for individual muscles of the quadriceps femoris muscle group (QF) in an isometric knee extension has been described as nonlinear, such that EMG amplitude increases out of proportion to force (4,10,14,17,18,24). Reported differences may reflect that individual QF muscles are not necessarily uniformly activated at increased loads in a specific action. Thus, the EMG/force-relationship may differ among QF muscles. Discrepancies in interpretation may arise from the fact that either the two-joint muscle rectus femoris or the single-joint muscles vastus lateralis or medialis have been chosen to reflect over-all QF activity. Moreover, some reports suggest that EMG activity of individual muscles is not uniform in multi-joint leg press compared with single-joint knee extension exercises (9,10,27). Thus, the EMG/force relationship might be different in multi-joint and single-joint actions. To our knowledge, no study has compared the EMG/force relationship for QF muscles during both multi-joint and single-joint exercise.
It has been put forth that antagonistic activity increases progressively at heavier loads to stabilize the knee joint and hence reduces the net force output of the QF muscle in the knee extension exercise (3). It follows that EMG signals would appear greater relative to external force at heavier loads.
Thus, the purpose of this study was to compare the EMG/force relationship of individual QF muscles in the single-joint knee extension and the multi-joint leg press. In an effort to assess the potential influence of antagonist activity on the EMG/force relationship of the knee extensor muscle group, EMG activity of m. biceps femoris was measured as well.