In an attempt to explore the relationship between force production during voluntary contractions at different speeds of isokinetic movement and the myofibrillar protein isoform expression in humans, an improved isokinetic dynamometer that corrects for gravitation, controls for acceleration and deceleration, and identifies a maximum voluntary activation was used. Muscle torque recordings were compared at the same muscle length (knee angle) and the torque was calculated as the average torque at each angle over a large knee angle range (75degrees-25degrees) to reduce the influence of small torque oscillation on the calculated torque. Muscle torque at fast (240degrees s(-1)) versus slow (30degrees s(-1)) speeds of movement, torque normalized to muscle cross-sectional area (specific tension), and absolute torque at fast speeds of movement were measured in 34 young healthy male and female short-, middle-, and long-distance runners. The relationship between the different measures of muscle function and the expression of myosin heavy chain (MyHC) isoforms using enzyme-histochemical and electrophoretic protein separation techniques were investigated. A significant correlation between the 240degrees s(-1) vs 30degrees s(-1) torque ratio and the relative area of the type 11 fibers and type 11 MyHC isoforms were observed in both the men (r = 0.74; P < 0.001) and the women (r = 0.81; P < 0.05). Thus, the present results confirm a significant relationship between in vivo human muscle function and the MyHC isoform expression in the contracting muscle.