ANTICIPATORY CHANGES IN CENTER OF MASS AND CENTER OF PRESSURE IN FULL BODY REACHING TASKS

1. ANTICIPATORY CHANGES IN CENTER OF MASS AND CENTER OF PRESSURE IN FULL BODY REACHING TASKS James S. Thomas, Nikki J.Vander Wiele, and Stacey L Moenter 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 on 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 CNS may reduce the complexity of multi-joint tasks by constraints at the level of the dynamic joint torques (Gottlieb et al. 1996). Alternatively, the central nervous system may organize movements to constrain the displacement of the body’s center of mass (COM). The purpose of this study was determine if COM displacement is constrained in full body reaching tasks. Specifically, we examined anticipatory changes in COM and center of pressure (COP), as well as the peak to peak displacement of the COM and COP of healthy individuals performing 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 15 subjects (7 males and 8 females) performing whole-body reaching tasks. Subjects reached for the targets with both hands, paused at the target for 1 second and then returned to an upright posture. In this paradigm the targets were located around a clock face such that the subject could, in theory, reach them by flexing the hips 15º, 30º, and 60º with the shoulder flexed 90º and the elbow extended. The target locations were chosen to create a task that progressively challenges the subject with larger excursions of the trunk. Subjects stood with each leg on a force plate and reached for the targets at two speeds (self-selected and fast-paced). They were given no instructions on the limb segment geometry to use while performing these reaching tasks. Using the known orientations of each of the limb segments, whole body COM was determined by estimating the mass and location of the COM of each limb segment using regression equations of Dempster. The location of the COP in the anterior posterior directions was determined from vertical components of force and the moments about the frontal plane axis. Anticipatory changes in COM and COP were defined as the difference in the mean values from the first 100 ms of the movement trial from the mean values 100 ms prior to onset of the focal movement. Onset of the focal movement was defined as the point where shoulder angular acceleration exceeded 2.5% of peak acceleration. The peak to peak displacement (from initial posture to target contact) of both COM and COP were determined as well as the joint angle displacements. Data Analysis Repeated measures ANOVAs ,in which movement speed (2), target location(3), and movement trial were within subject variables, were used to analyze all dependent measures. A B A B Figure 2.(A) Change in COM from initial posture to target contact (B) Change in COP from initial posture to target contact. Figure 1.A) Plot of COM and COP data for a typical movement trial to Target 2 (30 degree) made at a comfortable pace. B) Plot of COM and COP data for a typical movement trial to Target 2 (30 degree) made at a fast paced movement speed. A B Results Time series data of COM and COP of a typical subject reaching for Target 2 at comfortable and fast paced movement speeds are illustrated in Figure 1A and B. The most striking aspect of these time series is the large increase in the posterior displacement of the COP for the fast paced movement trials. The aggregate effect of movement speed and target height on anticipatory changes in COM and COP are illustrated in Figure 2A and B. There was no significant effect of movement speed or target height on anticipatory COM (Figure 2A). However, as seen in Figure 2B, as movement speed increased, the posterior displacement of anticipatory COP decreased (F(1,14)= 8.54, p<.05). Although these values are quite small, it indicates small anterior changes in COP prior to the focal movement may be in preparation for the larger posterior displacement of COP needed to generate the external moment to rapidly forward bend the trunk. With respect to delta COM, as movement speed increased there was a decrease in the change in COM (F(1,14)= 4.82, p<.05) Figure 3 A, and a significant increase in the change in COM as the target height was lowered (F(1,14)= 28.2, p<.05). With respect to delta COP, the peak to peak change in COP increased at the fast pace movement (F(1,14)= 15.7, p<.05). Furthermore, as expected, as target height was lowered there were increases the change in COP (F(1,14)= 26.1, p<.05). Conclusions These findings suggest that while the displacement of COM is not a tightly controlled variable in multi-joint reaching tasks, COM decreased slightly as movement speed increased. This research was supported by The National Institutes of Health Grant R01-HD045512 to J.S. Thomas Figure 4.A) Anticipatory change in COM. The COM averaged over 100 ms prior to the focal arm movement is subtracted from the the COM averaged over the initial 100 ms data collected. Note that we collect data for 500 ms prior to the presentation of the “go” signal. B) Anticipatory change in COP. Uses the same algorithm described in for anticipatory COM.