Fig. 9. Muscles classified as motors, springs, brakes and struts with respect to contraction type have different qualitative effects on multi-joint limb behavior. Top row, the dot product between the ankle force vector produced by muscle contraction and the instantaneous velocity vector of the ankle during four different behaviors (A, swimming; B, hindlimb wiping; C, defensive kicking; D, jumping). For D, the dot product is calculated between the force vector produced by semitendinosus (ST) contraction (at the tip of the astragalus segment) and the total force vector applied to the ground (see text). Dot products are calculated during periods of muscle activation and shown as circles. Dot products were calculated between the unit vectors (normalized to a magnitude of 1.0). The light gray box represents regions where dot products were greater than 0.5 or the angle between vectors was less than 45°. The dark gray box represents regions where dot products were less than -0.5 or the angle between vectors was greater than 135°. Bottom row, kinematics of the thigh, calf and astragalus segments during the different behaviors. Small arrows represent the direction of ankle movement; the small arrow in D is the direction of body movement. Larger arrows represent the direction of force produced by muscle contraction (gray) and ankle velocity (black) at a time point during the kinematic cycle. In A—C, kinematic parameters are shown at 16.67 ms intervals. In D, kinematic parameters are shown at 5 ms intervals. (A) The ankle forces produced by semimembranosus (SM) contraction during the swimming cycle act to support ankle motion (dot products greater than 0.5). (B) The ankle forces produced by cruralis (CR) contraction during the hindlimb cycle act briefly to oppose and then to support ankle motion (dot products initially less than -0.5 quickly shift to values greater than 0.5). (C) The ankle forces produced by sartorius (SA) contraction during the kicking cycle oppose the entire extension phase (dot products during the 250 ms extension phase were less than -0.5). (D) The forces applied to the ground by semitendinosus (ST) contraction do not clearly support or oppose body motion (dot products between ST forces and the total forces applied to the ground were less than 0.5 but greater than -0.5)