The influence of movement speed and handedness on the expenditure of potential and kinetic energy in full body reaching movements Nicole J. Vander Wiele, Stacey L. Moenter, Daohang Sha, Christopher R. France, and James S. Thomas School of Physical Therapy, Ohio University, Athens, OH Introduction Starting from an upright standing posture and reaching for a target that requires some forward bending of the trunk can be accomplished in many different configurations of the trunk and limb segments due to the large number of joints involved in these reaching tasks. That is, there are more mechanical degrees of freedom than are strictly required to complete this task. In a previous paper we showed that the rotational excursion of each segment depends not only on target location, but also on speed and subject preference (Thomas et al. 2003)-the latter dependences are made possible by kinematic redundancy. The resolution of kinematic redundancy, when it does not entail freezing some of the degrees of freedom, calls for constraining relationships amongst them. It has been proposed that the central nervous system (CNS) may reduce the complexity of multi-joint tasks by constraints at the level of the dynamic joint torques (Gottlieb et al. 1996). It has also been suggested that movements are planned such that kinetic energy is minimized (Nishikawa et al., 1999; Rosenbaum et al., 1999; Soechting et al., 1995). However, in reaching tasks that involve the trunk and lower extremity, peak KE costs may be small when compared to the potential energy (PE) costs. The purpose of this study was to determine the influence of movement speed and handedness on the contribution of KE and PE to the total energy costs associated with full body reaching tasks. Methods The time-series joint angles of the elbow, shoulder, thoracic spine, lumbar spine, hip, knee and ankle were measured in fifteen healthy subjects (7 males, 8 females) performing full body reaching tasks to three targets located in the mid- sagittal plane. The targets were placed in positions calculated such that the subject (with the elbow fully extended and shoulder flexed to 90 degrees) could, in theory, reach each target by flexing the hips 15, 30, and 60 degrees, respectively, relative to an upright, vertical posture. The target locations were chosen to create a task that progressively challenges the subject with larger excursions of the trunk. While standing on two force plates, subjects performed reaching movements first with their right hand and then with their left hand. Subjects reached for the targets at two speeds (comfortable and fast paced) and were given no instructions on the limb segment geometry to be used. Motion Monitor software (Innovation Sports, Chicago, IL) was used to derive time series 3D joint angle data using an Euler angle sequence of flexion, rotation, and abduction. We used the kinematic data and the location of the COM of each segment to determine the potential and kinetic energy of each segment for each instant in time. The total kinetic and potential energy was derived from the following equations: PE Segment (i) = m i g i h i Total PE = ∑ PE Segment (i) Data Analysis Peak KE and the change in peak PE from initial posture to target contact were analyzed using repeated measures ANOVAs in which the within subject factors were movement speed, target height, hand, and trial. Results There were significant interactions of movement speed and target height (F=10.6, p<.05) as well as target height and movement hand (F=7.16, p<.05) on the change in PE (see fig 2). Analysis of the simple effects reveals that there was a significant effect of movement speed for the middle (F=11.6, p<.05), and low targets (F=15.1, p<.05) (see fig 3). Additionally, subjects had larger changes in PE for reaches with the left hand compared to the right hand for the high (F=15.9, p<.05), middle (F=21.3, p<.05), and low targets (F=5.8, p<.05) (see fig 4). While there would obviously have to be an effect of movement speed on peak KE, there was no effect of reaching hand on peak KE. Conclusions In movement tasks limited to the upper extremity, KE costs dominate the total energy costs as movement speed increases. However, for full body reaching tasks the potential energy costs are greater than the peak kinetic energy costs for both comfortable and fast paced movements. Furthermore this difference increases dramatically for reaches to the low target. Increases in PE costs for fast paced reaches indicate that stored PE is used to accelerate the trunk and is not minimized in these tasks. This research was supported by The National Institutes of Health Grant R01-HD to J.S. Thomas Figure 4. There was a significant difference in the change in Potential Energy at High, Middle, and Low targets for each speed and hand. Figure 1. a.) stick figures derived from time series data illustrating a reaching trial to the middle target. b.)Time series display of change in PE during a reaching trial to high, middle, and low targets Figure 3. Change in PE at each target height for the two movement speeds. Significant differences were found at the middle and low targets only. Figure. Total KE Figure 2.