Comparison of Critical Force to EMG Fatigue Thresholds during Isometric Leg Extension

HENDRIX, C. R., T. J. HOUSH, G. O. JOHNSON, M. MIELKE, C. L. CAMIC, J. M. ZUNIGA, and R. J. SCHMIDT. Comparison of Critical Force to EMG Fatigue Thresholds during Isometric Leg Extension. Med. Sci. Sports Exerc., Vol. 41, No. 4, pp. 956-964, 2009. Theoretically, the critical force (CF) and the EMG fatigue threshold (EMG(FT)) tests demarcate fatiguing from nonfatiguing isometric torque levels. Purpose: The purpose of this study was twofold: 1) to determine whether the mathematical model for estimating the EMG(FT) during cycle ergometry was applicable to isometric leg extension muscle actions and 2) to compare the mean torque level from the CF test to those of EMG(FT) tests for the vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) muscles during isometric muscle actions. Methods: The slope coefficient of the linear relationship between total "isometric work" (W(lim) in newton meters per second) and time to exhaustion (T(lim) in seconds) was defined as the CF. The EMG(FT) was defined as the y-intercept of the isometric torque versus EMG fatigue Curve slope coefficient relationship. Results: There was a significant (P < 0.05) mean difference between the CF (25.9 +/- 12.1 N.m) and the EMG(FT) value for the RE (41.1 +/- 20.7 N.m) muscle. There were no significant differences, however, in EMG(FT) values among the three superficial muscles of the quadriceps femoris. In addition, the mean CF (17.6% maximum voluntary isometric contraction [MVIC]) occurred at a percentage of MVIC that is typically not affected by circulatory occlusion (20% MVIC), whereas the mean EMG(FT) Values for the VL (25.9% MVIC), VM (22.9% MVIC), and RF (27.8% MVIC) exceeded this threshold.