Journal of Motor Learning and Development, 2013, 1, 89-95 © 2013 Human Kinetics, Inc. 89 www.JMLD-Journal.com ORIGINAL RESEARCH Urbin is with the Dept. of Kinesiology, Auburn University, Auburn, AL. Stodden is with the Dept. of Physical Education and Athletic Training, University of South Carolina, Columbia, SC. Fleisig is with the American Sports Medicine Institute, Birmingham, AL. Overarm Throwing Variability as a Function of Trunk Action M. A. Urbin, David Stodden, and Glenn Fleisig Individual body segment actions evolve during throwing skill development. Maximal trunk involvement is typically the last feature of the movement pattern to fully develop. The current study examined developmental levels of trunk action and the associated variability in the throwing motion. The throwing motions of children and adolescents were analyzed via motion capture and trunk actions were classied as exhibiting no rotation (n = 7), blocked rotation (n = 6), or differentiated rotation (n = 11). Results indicated nonrotators exhibited greater variability than blocked-rotators in maximum humeral external rotation and humeral horizontal adduc- tion angles at ball release; nonrotators also demonstrated greater variability than differentiated-rotators on these parameters, in addition to forward trunk tilt and elbow extension angle at ball release. Nonrotators produced more variable peak upper torso and humeral horizontal adduction angular velocities, as well as peak upper torso linear velocity, relative to differentiated-rotators. Blocked-rotators produced more variable peak pelvis, upper torso, and humeral horizontal adduction angular velocities, as well peak pelvis linear velocity, relative to differentiated-rotators. Nonrotators were less consistent relative to blocked- and differentiated-rotators in the time that elapsed from peak pelvis angular velocity to ball release. These results indicate that greater trunk involvement is associated with more consistent movement production. Keywords: coordination, motion capture, motor development, motor control The overarm throw is a kinetic chain of events that complies with the summation of speed principle (Bunn, 1972; Putnam, 1991). This principle holds that each body segment makes a contribution to the throwing motion that is not independent of other segments (Neal, Snyder, & Kroonenberg, 1991). In essence, angular momentum can be transferred through the skeletal linkage to the projectile when the timing of body segment rotations is effective. Though kinetic chain principles can be exploited when only two body segments are involved (Alexander, 1991; Chowdhary & Challis, 2001), a thrower’s ability to exploit these principles with all of the appropriate body segments is intimately tied to motor skill development (Alexander, 1991; Langendorfer & Roberton, 2002; Southard, 2002, 2009; Stodden, Langendorfer, Fleisig, & Andrews, 2006a,b). Researchers have examined trends in throwing skill development using the component approach (Roberton & Halverson, 1984), which categorizes the action of indi- vidual segments in the overall spatiotemporal patterning of the throw. For example, the action of the trunk can be categorized as having no rotation or anterior-posterior sway, indicating a less developmentally advanced action. More developmentally advanced trunk actions can be characterized as having blocked or differentiated rotation of the pelvis and upper torso. Accordingly, these catego- rizations reect a continuum of kinetic chain exploita- tion, providing insight into throwing skill development. Given the various combinations of segmental actions that can potentially appear during skill development, the component approach captures variations that may not be accounted for using a whole-body approach (Roberton, 1978). This approach has been used in analyses of both children and adult throwers (Roberton, Halverson, Lan- gendorfer, & Williams, 1979; Williams, Haywood, & van Sant, 1998) and accounts for 85% of the variance in ball speed (Roberton & Konczak, 2001). Kinematic analyses have indicated that individual body segment actions classied with the component approach discriminate quantitative differences in the associated action (Stod- den et al., 2006a, 2006b), suggesting that the component approach is a valid method for evaluating throwing skill development. Longitudinal inquiring into throwing skill develop- ment using the component approach indicates there is considerable interindividual variation in the acquisition of a developmentally advanced movement pattern (Lan- gendorfer & Roberton, 2002). Stated another way, there is no one common pathway observed in the progression of skill development. Nevertheless, the most develop- mentally advanced component trunk action tends to be