QUANTITATION OF PROGRESSIVE MUSCLE FATIGUE DURING DYNAMIC LEG EXERCISE IN HUMANS

There is virtually no published information on muscle fatigue, defined as a gradual decline in force-generating capacity, during conventiona l dynamic (D) leg exercise. To quantitate progression of fatigue, we d eveloped 1) a model featuring integration of maximal voluntary static contraction (MVC) of knee extension (KE) muscles with ongoing DKE and 2) a device that allows frequent rapid transfer between DKE isolated t o the quadriceps femoris muscles and measurement of KE MVC. Eight heal thy men performed graded and submaximal constant work rate one-leg DKE to exhaustion while seated. Work rate, a product of a contraction rat e (1 Hz), force measured at the ankle, and distance of ankle movement from 90 degrees to 150 degrees of KE, was precisely controlled. Lack o f rise in myoelectric activity in biceps femoris of the active leg dur ing DKE and MVC was consistent with restriction of muscle action to qu adriceps femoris. The slope of the linear relationship between O-2 upt ake and work rate was 13.7 mi O-2/W (r = 0.93). This slope and the inc rease of heart rate relative to increasing work intensity agreed with published values for D leg exercise. Test-retest values for O-2 uptake were similar (P > 0.05) for matched DKE work rates. To track fatigue, MVC (90 degrees knee angle) was performed every 2 min of DKE. After 4 min of DKE at work rates corresponding to (mean +/- SE) 66 +/- 2, 78 +/- 2, and 100% of peak DKE O-2 uptake, MVC fell to 95 +/- 3, 90 +/- 5 , and 65 +/- 7% of MVC of rested muscle, respectively (* P < 0.01 fro m previous work rates). Virtually identical declines in MVC were obser ved by the end of graded work rate DKE and submaximal constant work ra te DKE tests. Quantitation of progressive muscle fatigue during D leg exercise provides a framework to study the effects of a variety of int erventions on the fatigue process and may permit unique insights into the involved mechanisms.