INFLUENCE OF MUSCLE LENGTH AND FORCE ON ENDURANCE AND PRESSER RESPONSES TO ISOMETRIC-EXERCISE

Experiments were performed to determine whether endurance time, mean a rterial pressure, or heart rate was related to either muscle length or external torque production in humans during isometric knee extension. Eight men and nine women performed isometric knee extension to the en durance limit at each of three muscle lengths, determined by knee angl es of 40 degrees (0.698 rad, shortest), 60 degrees (1.047 rad, interme diate), and 90 degrees (1.571 rad, longest), and at intensities of 30 and 50% maximal voluntary contraction (MVC). Knee extension forms an a scending-descending length-torque curve, and lengths were chosen to re sult in different external torques. MVC was greatest at a knee angle o f 60 degrees (P < 0.05 vs. 40 degrees, 90 degrees), with no significan t difference between 90 degrees and 40 degrees. Endurance time was inv ersely related to muscle length, independent of torque production, at 30% MVC [40 degrees, 395 +/- 139 (SE); 60 degrees, 237 +/- 60; 90 degr ees, 165 +/- 51 s; P < 0.05 vs. each other] and 50% MVC (40 degrees, 1 76 +/- 64; 60 degrees, 137 +/- 40; 90 degrees, 85 +/- 23 s; P < 0.05 v s. each other). Evidence is presented that endurance is a function of internal muscle force and not resultant external torque. The experimen tal design allowed the relationship of external torque and cardiovascu lar responses to be examined independent of exercise intensity. Muscle mass was also controlled in that the same muscle group was involved i n all contractions. There were no differences in mean arterial pressur e, heart rate, or rating of perceived exertion at any percentage of en durance time under any condition. Although cardiovascular responses we re related to indexes of fatigue, they were not related to external to rque when intensity or muscle mass was controlled.