A technique for quantifying and determining the site of isometric muscle fatigue in the clinical setting

We investigated a clinically applicable technique for quantifying and determining the site of fatigue during isometric contraction. We measured torque in the right knee extensors of 17 able-bodied subjects during an isometric contraction for 60sec at 100%, 75%, 50%, and 25% of maximal effort. During contractions, a 0.5-sec train of 50-Hz stimulation was applied at 15, 30, and 45sec. We measured the percent decline in force over 60 sec (fatigue index) and average augmentation in torque during stimulation (AVAUGTORQ). The mean fatigue index at full effort was 25.1%. The mean AVAUGTORQ at 100%, 75%, 50%, and 25% of maximal effort was 12.1%, 26.3%, 40.5%, and 69.7% of values at complete relaxation, respectively. These methods should be clinically useful for quantifying the level of isometric fatigue, determining the site of fatigue, and measuring the level of effort in subjects without neuromuscular disease. The results agree with previous studies reporting a peripheral site of fatigue in healthy, well-motivated subjects.