Proximal and distal muscle fatigue differentially affect movement coordination

Abstract

Muscle fatigue can cause people to change their movement patterns and these changes could contribute to acute or overuse injuries. However, these effects depend on which muscles are fatigued. The purpose of this study was to determine the differential effects of proximal and distal upper extremity muscle fatigue on repetitive movements. Fourteen subjects completed a repetitive ratcheting task before and after a fatigue protocol on separate days. The fatigue protocol either fatigued the proximal (shoulder flexor) or distal (finger flexor) muscles. Pre/Post changes in trunk, shoulder, elbow, and wrist kinematics were compared to determine how proximal and distal fatigue affected multi-joint movement patterns and variability. Proximal fatigue caused a significant increase (7°, p < 0.005) in trunk lean and velocity, reduced humeral elevation (11°, p < 0.005), and increased elbow flexion (4°, p < 0.01). In contrast, distal fatigue caused small but significant changes in trunk angles (2°, p < 0.05), increased velocity of wrench movement relative to the hand (17°/s, p < 0.001), and earlier wrist extension (4%, p < 0.005). Movement variability increased at proximal joints but not distal joints after both fatigue protocols (p < 0.05). Varying movements at proximal joints may help people adapt to fatigue at either proximal or distal joints. The identified differences between proximal and distal muscle fatigue adaptations could facilitate risk assessment of occupational tasks.

Fig 1. Experimental design.

(A) Ratcheting Task. Subjects stood in front of a board placed at 60% of arm length in front of the toes, and rotated a bolt placed through the board at eye level using a ratcheting socket wrench. The torque required to rotate the bolt clockwise was ~ 4 Nm. (B) Experimental Session. Subjects performed three trials of a repetitive ratcheting task pre and post fatigue. Two different fatigue protocols (Proximal/Distal) were performed on separate days at least one week apart. Subjects performed the fatigue task during minutes 2 and 4 of the post-test to prevent recovery in the targeted muscle group. The order of test days was randomized. (C) Illustration of the wrench coordinate system.

Fig 2. Perceived exertion and strength.

(A) Average ratings of perceived exertion (RPE), (B) shoulder flexion maximum voluntary contraction (MVC), and (C) grip MVC for the proximal (squares) and distal (circles) fatigue sessions after the pre-test, fatigue, and post-test on each day. MVCs are reported as a percentage of the initial MVC. Error bars represent 95% confidence intervals. * indicates a difference from baseline strength.

Fig 3. Ratcheting task execution.

(A) Average position and angular velocity of the wrench pre (blue) and post (red) fatigue across all subjects are shown for proximal and distal fatigue. Data are normalized to 100% of the movement cycle (top position to top position). (B) The average range of motion (top) and movement duration (middle) of the wrench cycles did not change, but wrench rotation variability (MeanSD) increased after both fatigue protocols (bottom). Error bars show 95% confidence intervals.

Fig 4. Fatigue and joint angle.

Average joint angles across subjects for the ratcheting motion pre (blue) and post (red) two different fatigue protocols. Angles are normalized to 100% of the movement cycle (top position to top position). The angles shown represent those most affected by fatigue.

Fig 5. Changes in wrench-hand coordination post fatigue.

The change (post—pre fatigue) in the peak angular velocity of the wrench relative to the hand about the wrench Z axis and the timing of peak wrist extension velocity for proximal and distal fatigue. Positive values indicate an increase after fatigue.

Fig 6. Changes in movement variability post fatigue.

Change in MeanSD (post—pre fatigue) at the trunk (A), shoulder (B), elbow and wrist (C), and hand and wrench (D) after proximal (squares) and distal (circles) fatigue. Positive values indicate that variability increased after fatigue. Error bars represent the 95% confidence interval. * indicates PROX/DIST × PRE/POST interaction effect.