Coordination of degrees of freedom and stabilization of task variables in a complex motor skill: expertise-related differences in cello bowing.

Stringed instrument bowing is a complex sensorimotor skill, involving fine regulation of bow orientation and motion relative to the string. In this study, we characterize this skill in terms of stabilization of specific bow parameters as well as the underlying use and coordination of the degrees of freedom (DOF) of the right bowing arm. Age-matched samples of 10 advanced cellists and 10 cello novices took part in the study. Kinematic bow movement data were analyzed with respect to task variables suggested by the cello teaching literature: position and orientation of the bow relative to the string, bow velocity, and timing. Joint motion of the bowing arm was analyzed in terms of movement amplitude and inter-joint coordination (principal component analysis). As expected, novices showed poorer control of bowing parameters. In addition, novices differed markedly from advanced players in the use and coordination of the DOF of the bowing arm, with the elbow and wrist showing less overall movement and a reduced proportion of variance explained by the first principal component (PC1). In contrast, larger amounts of shoulder variance were explained by PC1 in novices compared to experts. Our findings support Bernstein's theory of graded skill acquisition, according to which early stages of motor skill learning are characterized by a "freezing" of movement DOF, while later learning stages exploit the DOF, possibly following a proximal-to-distal sequence, for improved task performance.