End-state comfort and joint configuration variance during reaching

Abstract

This study joined two approaches to motor control. The first approach comes from cognitive psychology and is based on the idea that goal postures and movements are chosen to satisfy task-specific constraints. The second approach comes from the principle of motor abundance and is based on the idea that control of apparently redundant systems is associated with the creation of multi-element synergies stabilizing important performance variables. The first approach has been tested by relying on psychophysical ratings of comfort. The second approach has been tested by estimating variance along different directions in the space of elemental variables such as joint postures. The two approaches were joined here. Standing subjects performed series of movements in which they brought a hand-held pointer to each of four targets oriented within a frontal plane, close to or far from the body. The subjects were asked to rate the comfort of the final postures, and the variance of their joint configurations during the steady state following pointing was quantified with respect to pointer endpoint position and pointer orientation. The subjects showed consistent patterns of comfort ratings among the targets, and all movements were characterized by multi-joint synergies stabilizing both pointer endpoint position and orientation. Contrary to what was expected, less comfortable postures had higher joint configuration variance than did more comfortable postures without major changes in the synergy indices. Multi-joint synergies stabilized the pointer position and orientation similarly across a range of comfortable/uncomfortable postures. The results are interpreted in terms conducive to the two theoretical frameworks underlying this work, one focusing on comfort ratings reflecting mean postures adopted for different targets and the other focusing on indices of joint configuration variance.

Figure 1

Experimental setup showing the plastic hoop at the two relative distances: 40% and 80% of the participant arm length. The center of the hoop was aligned with the vertical midline and adjusted so that it was at shoulder height. A reflective target marker was placed one the inner surface of the hoop either at 3, 6, 9, and 12 o’clock positions. X, Y, and Z-axes represent global coordinate system with origin at suprasternal notch marker.

Figure 2

An illustration of the calibration position for the computation of joint angles, including depiction of the rigid bodies on each segment and additional markers used to determine the joint locations and segment lengths. All angles in the configuration position were zero by default.

Figure 3

Median comfort ratings of pointing postures for the four targets (3, 6, 9, and 12 o’clock positions) placed at 40% and 80% of arm length. Integer comfort ratings ranged from 1 (least comfortable) to 5 (most comfortable). Error bars show the 25%–75% ranges, Stars show statistically significant differences (p < 0.05).

Figure 4

Mean joint excursions (with standard error bars), averaged across all participants for each pointing posture. Each graph represents excursion (range of motion) of individual joints at different target marker position (3, 6, 9, and 12 o’clock), for two relative hoop distance (40% or 80% of the arm length). Local coordinate systems of each segment originated at proximal joint centers. During the anatomical calibration, local coordinate systems were aligned with global coordinate system in such a way that x-axes pointed to the right, y-axes pointed posterior-anterior, and z-axes pointed upwards. DOFs were defined as follow: For clavicular rotation relative to the trunk, θ₁, θ₂, θ₃ are rotations about z-axis, x-axis, and y-axis, respectively; for the relative rotation of the upper arm, θ₄, θ₅, θ₆ are rotations about z-axis, x-axis, and y-axis, respectively; for the relative rotation of the forearm, θ₇ is a rotation about x-axis estimated from markers placed on medial and lateral epicondyles, and θ₈ is a rotation about y-axis; for the relative rotation of the hand, θ₉ and θ₁₀ are rotations about z-axis and x-axis, respectively.

Figure 5

Average indices of the UCM analysis with standard error bars for different targets. The top panels show variance within the uncontrolled manifold, UCM (V_UCM-POS and V_UCM-ORI); the middle panels show variance orthogonal to the UCM (V_ORT-POS and V_ORT-ORI); and the lower panels show z-transformed indices of synergy (ΔV_Z-POS and ΔV_Z-ORI). The left columns present results of analysis with respect to pointer position, and the right columns – with respect to pointer orientation.

Figure 6

Averaged variance indices (with standard error bars) grouped by level of comfort rating that ranged from 1 (least comfortable) to 5 (most comfortable). The left columns show results of the pointer position-related analysis; the right columns show results of the pointer orientation-related parameters. A, D - variance within the uncontrolled manifold, UCM (V_UCM-POS and V_UCM-ORI), B, E - variance orthogonal to the UCM (V_ORT-POS and V_ORT-ORI), and C – z-transformed index of synergy (ΔV_Z-POS and ΔV_Z-ORI). The trend of the variance indices with higher comfort ratings is represented by linear regression line (solid, black). Polyserial coefficients (r) and significance levels (p, N = 5) for correlations between comfort ratings and variance indices for pointer position-related and pointer orientation-related analyses. Variance indices were averaged across comfort rates. * Statistically significant at the 0.05 level.