Up to this point, the ankle mortise joint axis has been considered as a fixed axis of motion. This has been done for the sake of simplicity and convenience using the approximate center of movement as the joint axis. But, as mentioned in Chapter 3, Arthrology, no joint moves about a stationary joint axis. As indicated by instant center analysis of knee joint motion, this is true of the ankle mortise joint as well. The surface velocities determined from the instant centers of movement show that when moving from full plantar flexion to full dorsiflexion there is initially a momentary distraction of the tibiotalar joint surfaces, followed by a movement of combined rolling and sliding throughout most of the range, and terminating with an approximation of joint surfaces at the position of extreme dorsiflexion. These findings are consistent with the fact that the close-packed position of the ankle mortise joint is dorsiflexion; the tightening of the joint capsule that occurs with movement of any joint into its close-packed position produces an approximation of the joint surfaces.