Influence of muscle length and force on endurance and pressor responses to isometric exercise

Experiments were performed to determine whether endurance time, mean arterial 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 endurance limit at each of three muscle lengths, determined by knee angles of 40° (0.698 rad, shortest), 60° (1.047 rad, intermediate), and 90° (1.571 rad, longest), and at intensities of 30 and 50% maximal voluntary contraction (MVC). Knee extension forms an ascending-descending length-torque curve, and lengths were chosen to result in different external torques. MVC was greatest at a knee angle of 60° (P < 0.05 vs. 40°, 90°), with no significant difference between 90° and 40°. Endurance time was inversely related to muscle length, independent of torque production, at 30% MVC [40°, 395 ± 139 (SE); 60°, 237 ± 60; 90°, 165 ± 51 s; P < 0.05 vs. each other] and 50% MVC (40°, 176 ± 64; 60°, 137 ± 40; 90°, 85 ± 23 s; P < 0.05 vs. each other). Evidence is presented that endurance is a function of internal muscle force and not resultant external torque. The experimental design allowed the relationship of external torque and cardiovascular responses to be examined independent of exercise intensity. Muscle mass was also controlled in that the same muscle group was involved in all contractions. There were no differences in mean arterial pressure, heart rate, or rating of perceived exertion at any percentage of endurance time under any condition. Although cardiovascular responses were related to indexes of fatigue, they were not related to external torque when intensity or muscle mass was controlled.