A simple joint control pattern dominates performance of unconstrained arm movements of daily living tasks

Abstract

A trailing joint control pattern, during which a single joint is rotated actively and the mechanical effect of this motion is used to move the other joints, was previously observed during simplified, laboratory-based tasks. We examined whether this simple pattern also underlies control of complex, unconstrained arm movements of daily activities. Six tasks were analyzed. Using kinematic data, we estimated motion of 7 degrees of freedom (DOF) of the shoulder, elbow, and wrist, and the contribution of muscle and passive interaction and gravitational torques to net torque at each joint. Despite task variety, the hand was transported predominantly by shoulder and elbow flexion/extension, although shoulder external/internal rotation also contributed in some tasks. The other DOF were used to orient the hand in space. The trailing pattern represented by production of net torque by passive torques at the shoulder or elbow or both was observed during the biggest portion of each movement. Net torque generation by muscle torque at both joints simultaneously was mainly limited to movement initiation toward the targets and movement termination when returning to the initial position, and associated with needing to overcome gravity. The results support the interpretation of previous studies that prevalence of the trailing pattern is a feature of skillful, coordinated movements. The simplicity of the trailing pattern is promising for quantification of dyscoordination caused by motor disorders and formulation of straightforward instructions to facilitate rehabilitation and motor learning.

Fig 1. A schematic representation of the initial and final position of the arm during the 6 tasks.

The top panels show the starting arm posture and target postures for the forward reach, shoulder-level reach, and drinking. The bottom panels show the starting and target arm postures for hair combing, pledge, and page turning.

Fig 2. Experimental procedures.

A: Locations of the reflective markers. C7: 7^th cervical vertebra; RA: Right acromion; LA: Left acromion; RC: Right clavicle; Xi: Xiphoid process; RB: Right biceps; RLE: Right lateral epicondyle; RME: Right medial epicondyle; RF: Right forearm; RR: Right radial styloid; RU: Right ulnar styloid; RCMP2: Right 2^nd metacarpophalangeal joint; RCMP5: Right 5^th metacarpophalangeal joint. B: Definition of the 3 shoulder DOF (internal/external rotation, abduction/adduction, and flexion/extension) and 2 elbow DOF (elbow flexion/extension and forearm pronation/supination). C: The global coordinate system was centered in the shoulder joint. The X-axis was directed laterally to the right, the Y-axis was directed anteriorly, and the Z-axis was directed vertically upward.

Fig 3. Means and SD of normalized hand displacements along the X, Y, Z coordinates of the global coordinate system.

The normalization was applied to each movement by shifting the system’s origin to the initial hand position, re-scaling movement time to 100%, and dividing the displacement along each axis at each moment of time by the distance between the initial and final hand location. The x-values increased when the hand moved away from the body, the y-values increased when the hand moved to the right from the body, and the z-values increased when the hand moved up.

Fig 4. Individual displacements of the 7 DOF during all pledge movements.

Each color shows 3 curves representing DOF displacements during the 3 trials performed by each subject. The 3 curves of the same color often had the same shape and amplitude, suggesting that at least some subjects used the same pattern of DOF motions during different trials of the task. The patterns of DOF motions differed across subjects.

Fig 5. Means and SD of the hand velocity and the contributions of each DOF to hand velocity in each task.

The data were averaged across all movements performed by all subjects for each task.

Fig 6. Individual examples of projections of MT, GT, and IT on NT at the shoulder and elbow during performance of the 6 ADL tasks by a representative participant.

The blue curves show the magnitude of NT. See the definition of the movement phases in the text.

Fig 7. The sequence and relative durations of the 4 joint control phases during each trial for the 6 ADL tasks.

The horizontal white lines separate trials performed by each subject. While each of the 14 subjects performed 3 trials in the majority of the tasks, data from 12 subjects (3 trials from each subject) were available during forward reaching and only 2 trials were available from subject 4 during drinking.