Three-dimensional hand movements during the execution of ball juggling : effect of expertise in street performers

To determine expertise-related differences in performance and movement variability during the execution of closed skill codified tasks, we quantitatively assessed the 3D hand movements of two groups of jugglers with different levels of expertise: six advanced (who could juggle up to 7 balls) and six intermediate jugglers (who could juggle at most with 5 balls). All participants performed three trials for each 3-, 4- and 5-ball juggling schemes. The coordinates of the middle fingers were recorded by an ptoelectronic motion analyzer (sampling rate 120 Hz), and were analyzed and compared between groups, number of juggled balls and the spatial decomposition of hand trajectories. The higher the level of expertise, the more stable the hand movements, as the number of juggled balls increased. Advanced jugglers also exhibited lower execution frequencies than intermediate jugglers in each scheme. When the level of difficulty rises, a slower play may be one of the factors accounting for the capability of the advanced jugglers to limit movement variability at the end-effector, and juggle a higher number of balls.