Effect of contraction form and contraction velocity on the differences between resultant and measured ankle joint moments

During a maximal isometric plantar flexion effort the moment measured at the dynamometer differs from the resultant ankle joint moment. The present study investigated the effects of contraction form and contraction velocity during isokinetic plantar/dorsal flexion efforts on the differences between resultant and measured moments due to the misalignment between ankle and dynamometer axes. Eleven male subjects (age: 31+/-6 years, mass: 80.6+/-9.6 kg, height: 178.4+/-7.4 cm) participated in this study. All subjects performed isometric-shortening-stretch-isometric contractions induced by electrical stimulation at three different angular velocities (25 degrees /s, 50 degrees /s and 100 degrees /s) on a customised dynamometer. The kinematics of the leg were recorded using the vicon 624 system with eight cameras operating at 250 Hz. The resultant moments at the ankle joint were calculated through inverse dynamics. The relative differences between resultant and measured ankle joint moments due to axis misalignment were fairly similar in all phases of the isometric-shortening-stretch-isometric contraction (in average 5-9% of the measured moment). Furthermore these findings were independent of the contraction velocity. During dynamic plantar/dorsal flexion contractions the differences between measured and resultant joint moment are high enough to influence conclusions regarding the mechanical response of ankle extensor muscles. However the relative differences were not increased during dynamic contractions as compared to isometric contractions.