Comparison of critical force to EMG fatigue thresholds during isometric leg extension.
Theoretically, the critical force (CF) and the EMG fatigue threshold (EMGFT) tests demarcate fatiguing from nonfatiguing isometric torque levels.


OBJECTIVE
The purpose of this study was twofold: 1) to determine whether the mathematical model for estimating the EMGFT 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 EMGFT 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" (Wlim in newton-meters per second) and time to exhaustion (Tlim in seconds) was defined as the CF. The EMGFT 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 EMGFT value for the RF (41.1 +/- 20.7 N.m) muscle. There were no significant differences, however, in EMGFT 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 EMGFT values for the VL (25.9% MVIC), VM (22.9% MVIC), and RF (27.8% MVIC) exceeded this threshold.
Hendrix
C Russell
CR
Department of Nutrition and Health Sciences, Human Performance Laboratory, University of Nebraska-Lincoln, Lincoln, NE 68583-0806, USA. rhendri7@unlserve.unl.edu
Housh
Terry J
TJ
Johnson
Glen O
GO
Mielke
Michelle
M
Camic
Clayton L
CL
Zuniga
Jorge M
JM
Schmidt
Richard J
RJ
D000465:Algorithms; D001642:Bicycling / Q000502:physiology; D004576:Electromyography / Q000379:methods; D005080:Exercise Test; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D007866:Leg / Q000502:physiology*; D008297:Male; D018763:Muscle Fatigue / Q000502:physiology; D053580:Muscle Strength / Q000502:physiology*; D010807:Physical Endurance / Q000502:physiology; D005082:Physical Exertion; D013848:Thigh / Q000502:physiology; D019415:Torque; D055815:Young Adult