The dynamical stability of visual coupling and knee flexibility in skilled kendama players

We examined the role of postural control in a skilled task that requires perceptualmotor coordination, the kendama trick "Swing-in," which uses a moving ball on a string. Four expert and 4 novice players participated, each performing 200 trials of Swing-in. The head and knee movements of the experts were more extensive than those of the novices, and the movements of the head and the ball, as well as the knee and the ball, maintained the same patterns longer in the experts than in the novices. At the end of each trial, distances between the head and the ball in the expert group were more consistent than those in the novice group, but the 2 skill groups showed similar variability in distance between the knee and the ball. These findings suggest that kendama experts controlled their body movement to dynamically coordinate them with the moving ball, and the stability of the strong coupling of the head and the ball in the experts seemed to be supported by the flexibility of knee movement.