Single Muscle Contractions

Subjects performed dynamic single contractions in the right leg as previously described at 20 and 40% of their maximum work rate (WR_max) determined on a previous study day (15, 16). Subjects were seated in a semirecumbent position on a modified adjustable bucket seat that accommodates variable body and leg lengths allowing the subject’s lower leg to move through a full range of movement (90° flexion to ~0° extension). Both knees were flexed at 90° with a form-fitting orthopedic boot attached to the right ankle. The boot was attached to a leg shaft located behind the right knee that had a one-way clutch bearing allowing for no resistance as the leg returns to 90° flexion (eccentric). Resistance (torque) was developed by contracting (concentric) against the leg shaft with the device electronically developing the torque via an alternating current motor turning at a constant revolution per minute transferred to the leg shaft. A dedicated computer monitored the elapsed time and the angle of the leg controlling the actual torque presented to the leg during contraction. In this way, the subject was required to develop enough power to extend the leg through a full range of motion against a given torque eliminating any inertial load. Subjects were instructed to contract and relax through a full range of movement (90° flexion to ~0° extension) within ~1 s, on a verbal command from laboratory personnel. WR_max intensities were randomized before the experimental protocol, and each contraction intensity was performed in duplicate to calculate the average response for each subject for a given condition. Each contraction was visually observed by the laboratory personnel to ensure proper timing of contraction. One minute of relaxation was given between each contraction to allow continuous measures of limb hemodynamics postcontraction.