Sustained Isometric Wrist Flexion and Extension Maximal Voluntary Contractions Similarly Impair Hand-Tracking Accuracy in Young Adults Using a Wrist Robot

Abstract

Due to their stabilizing role, the wrist extensor muscles demonstrate an earlier onset of performance fatigability and may impair movement accuracy more than the wrist flexors. However, minimal fatigue research has been conducted at the wrist. Thus, the purpose of this study was to examine how sustained isometric contractions of the wrist extensors/flexors influence hand-tracking accuracy. While gripping the handle of a three-degrees-of-freedom wrist manipulandum, 12 male participants tracked a 2:3 Lissajous curve (±32° wrist flexion/extension; ±18° radial/ulnar deviation). A blue, circular target moved about the trajectory and participants tracked the target with a yellow circle (corresponding to the handle's position). Five baseline tracking trials were performed prior to the fatiguing task. Participants then exerted either maximal wrist extension or flexion force (performed on separate days) against a force transducer until they were unable to maintain 25% of their pre-fatigue maximal voluntary contraction (MVC). Participants then performed 7 tracking trials from immediately post-fatigue to 10 min after. Performance fatigability was assessed using various metrics to account for errors in position-tracking, error tendencies, and movement smoothness. While there were no differences in tracking error between flexion/extension sessions, tracking error significantly increased immediately post-fatigue (Baseline: 1.40 ± 0.54°, Post-fatigue: 2.02 ± 0.51°, P < 0.05). However, error rapidly recovered, with no differences in error from baseline after 1-min post-fatigue. These findings demonstrate that sustained isometric extension/flexion contractions similarly impair tracking accuracy of the hand. This work serves as an important step to future research into workplace health and preventing injuries of the distal upper-limb.

Keywords: extension; flexion; forearm; isometric; kinematics; performance fatigability; robotics; tracking.

Figure 1

(A) Experimental setup for the tracking trials. Participant's forearm is positioned atop the WristBot support, and their hand is gripping the handle of the device. (B) Example of the 2:3 Lissajous curve. The white circle represents the target as it moves around the curve, while the gray circle represents the real-time position of the handle. Data collected during the initial dotted-line portion was not analyzed. (C) Experimental setup for MVCs and the sustained isometric fatigue trial. In this example, the participant was setup for wrist flexion MVCs/wrist flexion fatigue.

Figure 2

Schematic of the experimental protocol. This protocol was repeated for both the wrist flexion fatigue session and the wrist extension fatigue session (sessions separated by 7 days). Gray bars represent MVCs while white bars represent a single tracking completion of the Lissajous curve.

Figure 3

(A) Group averages of relative (% of baseline) MVC force between the wrist flexion and wrist extension sessions. Gray lines depict wrist flexion MVC force collected on the wrist flexion fatigue day, while black lines represent wrist extension MVC force collected on the wrist extension fatigue day. The x-axis denotes the time of collection; the numbers refer to time of collection after fatigue. The dotted-line represents pre-fatigue (or baseline) MVC force. * denotes a significant difference of MVC force at one time point to all other time points. (B) Time to exhaustion for all 12 participants on both testing sessions. Black lines represent the 6 participants who took longer to fatigue on the extension day; light gray lines denote the 6 participants who took longer to fatigue on the flexion day. Lastly, the thick, dark gray line shows the group average.

Figure 4

Group averages of mean tracking error calculated over the entire Lissajous curve (excluding the dotted-line portion). Tracking error is normalized to baseline (shown by the dashed horizontal line), and data points are shown in minutes after fatigue (0–10). Black lines represent tracking error from the wrist extension fatigue session, while gray lines represent tracking error from the wrist flexion fatigue session. * denotes a significant difference of both extension and flexion compared to baseline.

Figure 5

Group averages of mean tracking error (A) only when the wrist was flexing, (B) only when the wrist was extending, (C) only when the wrist was moving in radial deviation, and (D) only when the wrist was moving in ulnar deviation. Tracking error is normalized to baseline (shown by the dashed horizontal lines in each graph), and data points are shown in minutes after fatigue (0–10). Black lines represent tracking error from the wrist extension fatigue session, while gray lines represent tracking error from the wrist flexion fatigue session. * denotes a significant difference of both extension and flexion compared to baseline.

Figure 6

Group averages of the mean (A) longitudinal component of the tracking error, and (B) the normal component of the tracking error. For both metrics, error is shown in degrees (°) and data points are shown from pre-fatigue to 10 min-post. Black lines represent tracking error from the wrist extension fatigue session, while gray lines represent tracking error from the wrist flexion fatigue session. * denotes a significant difference of both extension and flexion compared to baseline.

Figure 7

(A) Group averages of figural error, and (B) group averages of the jerk ratio. Error is relative to baseline (shown by the dashed horizontal lines in each graph), and data points are shown in minutes after fatigue (0–10). Black lines represent tracking error from the wrist extension fatigue session, while gray lines represent tracking error from the wrist flexion fatigue session. * denotes a significant difference of both extension and flexion compared to baseline.