Quadriceps EMG/force relationship in knee extension and leg press

Purpose: This study compared the relationship between surface electromyogra phic (EMG) activity and isometric force of m. quadriceps femoris (QF) in th e single-joint knee extension (KE) and the multi-joint leg press (LP) exerc ises. Methods: Nine healthy men performed unilateral actions at a knee angl e of 90 degrees at 20, 40, 60, 80, and 100% of maximal voluntary contractio n (MVC). EMG was measured from m. vastus lateralis (VL), m. vastus medialis (VM), m. rectus femoris (RF), and m, biceps femoris (BF). Results: There w ere no differences in maximum EMG activity of individual muscles between KE and LP. The QF EMG/force relationship was nonlinear in each exercise modal ity. VL showed no deviation from linearity in neither exercise, whereas VM and RF did. BF activity increased linearly with increased loads. Conclusion s: The EMG/force relationship of all quadricep muscles studied appears to b e similar in isometric multi-joint LP and single-joint KE actions at a knee angle of 90 degrees. This would indicate the strategy of reciprocal force increment among muscles involved is comparable:in the two models. Furthermo re, these data suggest a nonuniform recruitment pattern among the three sup erficial QF muscles and surface EMG recordings from VL to be most reliable in predicting force output.