Fig. 1. Testing equipment and experiment set up. (A) Testing equipment. (B) Schematic diagram of the testing equipment and experimental set up. The subject lies prone, strapped to the lying board, with the assumed axes of rotation of the knee and ankle aligned with potentiometers by laser pointers. The right foot, strapped to the footplate, is manually rotated through the ankle's range of movement. The footplate is controlled by a `floating' balance mechanism to accommodate slight changes in the location of the ankle axis during rotation as well as the subjects' differing foot shapes. The lying board can be moved up and down by a motor to change the knee angle without moving the lower leg. An ultrasound transducer was stabilized over the midbelly of the medial gastrocnemius to generate images of muscle fascicles during ankle rotation. Passive ankle torque, ankle and knee angles and ultrasound images were recorded simultaneously.