Copyright @ 200 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited. 9 Methodological Advances Comparison of Critical Force to EMG Fatigue Thresholds during Isometric Leg Extension C. RUSSELL HENDRIX, TERRY J. HOUSH, GLEN O. JOHNSON, MICHELLE MIELKE, CLAYTON L. CAMIC, JORGE M. ZUNIGA, and RICHARD J. SCHMIDT Human Performance Laboratory, Department of Nutrition and Health Sciences, University of Nebraska-Lincoln, Lincoln, NE ABSTRACT 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. 956–965, 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 G 0.05) mean difference between the CF (25.9 T 12.1 NIm) and the EMG FT value for the RF (41.1 T 20.7 NIm) 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. Key Words: ANAEROBIC WORK CAPACITY, ISOMETRIC WORK (W lim ), LIMIT TIME (T lim ), EMG, EMG AMPLITUDE T he critical force (CF) test (27), theoretically, provides approximations of two parameters: the CF and the anaerobic work capacity (AWC). CF is the maximal isometric force (or torque) that a muscle or a muscle group can maintain for an extended duration without fatigue and is related to the circulatory conditions of the muscle. AWC is defined as the total ‘‘isometric work’’ that can be achieved using only energy reserves within the muscle such as adeno- sine triphosphate (ATP), phosphocreatine, glycogen, and oxygen bound to myoglobin (7,27,29). Although, by definition, no mechanical work is accomplished during an isometric muscle action, Monod and Scherrer (27) suggested that an appreciation of the fatiguing nature of isometric muscle actions is found in the time to exhaustion for a given force or torque level. Thus, to derive the CF and the AWC from the CF test, Monod and Scherrer (27) used an isometric analogue of mechanical work called the limit work (W lim ) that is calculated as the force (F) of the isometric muscle action multiplied by the time to exhaustion (or limit time, T lim ) that the F can be maintained. That is, W lim ¼ FT lim ; ½1 where W lim is the total amount of ‘‘isometric work’’ that can be accomplished at a specific level of F; F is the level of isometric force of the muscle action; and T lim is the time to exhaustion at the specific F. On the basis of these definitions, the CF test involves a series of fatiguing, isometric muscle actions (three or four) at different F levels that are maintained continuously for as long as possible. The W lim (FT lim ) is then calculated for each F and plotted as a function of the T lim values for the muscle actions (Fig. 1). The linear relationship between W lim and T lim , W lim ¼ AWC þ CFðT lim Þ; ½2 is then used to estimate the CF and the AWC values. Theoretically, the slope of the W lim versus T lim relationship SPECIAL COMMUNICATIONS Address for correspondence: C. Russell Hendrix, M.S., Department of Nutrition and Health Sciences, University of Nebraska-Lincoln, 110 Ruth Leverton Hall, Lincoln, NE 68583-0806; E-mail: rhendri7@unlserve. unl.edu. Submitted for publication June 2008. Accepted for publication September 2008. 0195-9131/09/4104-0956/0 MEDICINE & SCIENCE IN SPORTS & EXERCISE Ò Copyright Ó 2009 by the American College of Sports Medicine DOI: 10.1249/MSS.0b013e318190bdf7 956