The Median Nerve Displays Adaptive Characteristics When Exposed to Repeated Pinch Grip Efforts of Varying Rates of Force Development: An Ultrasonic Investigation

Abstract

Objectives: Repeated gripping with high grip forces and high rates of grip force development are risk factors for carpal tunnel syndrome. As the nerve's adaptive ability is crucial to prevent disease progression, we investigated how these risk factors influence median nerve deformation and displacement over the time course of a repeated pinch grip task.

Methods: Seventeen healthy participants performed a repeated grip task against a load cell while their carpal tunnel was scanned with ultrasound. The grip task involved pulp-pinching three consecutive times from 0 to 40% maximal voluntary exertion (MVE), performed at three different rates of force development (RFD): 40% MVE/1 second; 2 seconds; and 5 seconds. Ultrasound images were analyzed at 10% MVE intervals. Nerve circularity, width, height, and cross-sectional area were measured to assess deformation. Median nerve displacement was assessed by its change in position relative to the flexor digitorum superficialis tendon of the third digit (FD) in both radioulnar and palmodorsal axes.

Results: Linear mixed modeling indicated that median nerve deformation increased, becoming more circular, with each repeated pulp-pinch (P < .01) and with grip force magnitude (P < .01). However, a faster RFD decreased nerve deformation (P < .01). Furthermore, the nerve displaced ulnarly during pulp-pinching, with greater displacement during the fastest (ie, 40% MVE/1 second) RFD (P < .01).

Conclusions: The median nerve deformed and displaced in response to pulp-pinching; however, faster rates of force development hindered this adaptive response. This likely reflects the viscoelastic properties of the healthy nerve and subsynovial connective tissue, highlighting the importance of tissue compliance in preventing nerve compression.

Keywords: carpal tunnel; connective tissue; ultrasound; viscoelastic substances.

Figure 1

Custom built testing apparatus supporting the right arm of a participant. Set‐up includes: A, Monitor displaying force profiles; B, 80/20 aluminum extrusion and linear motion components; C, dynamometer; D, probe holder; E, hinge allowing for wrist flexion; F, metal brackets supporting forearm; G, Velcro strap holding forearm in supination.

Figure 2

Real‐time display of repeated pinch grip tasks (A) 40% MVE/1 second rate of force development (RFD), (B) 40% MVE/2 seconds RFD, and (C) 40% MVE/5 seconds RFD. The participant traced trapezoidal force profiles (red) while receiving visual feedback of their force magnitude (white).

Figure 3

Static images A and B correspond to 0% MVE and images C and D correspond to 40% MVE during ramp 1 of a 2‐second rate of force development (RFD) grip task. The median nerve (m) and tendon (t) of the flexor digitorum superficialis for the third digit (FDS3) are traced with the ImageJ polygon tool in B and D, depicting the increase in nerve circularity and dorsoulnar displacement relative to the FDS3.

Figure 4

Mean (±95% confidence interval) median nerve circularity as a function of (A) pinch force magnitude, (B) ramp number, and (C) rate of force development (RFD). Asterisk indicates a significant linear trend or a significant pairwise difference (P < 0.05).

Figure 5

Mean (±95% confidence interval) median nerve cross‐sectional area (CSA) as a function of ramp number and rate of force development (RFD). Asterisk indicates a significant pairwise difference (P < 0.05).

Figure 6

Mean (±95% confidence interval) median nerve radioulnar relative displacement as a function of (A) pinch force magnitude, and (B) ramp number. Asterisk indicates a significant linear trend or a significant pairwise difference (P < 0.05). Positive indicates ulnar displacement, negative indicates radial displacement.

Figure 7

Mean (±95% confidence interval) median nerve palmodorsal relative displacement as a function of (A) ramp number, and (B) rate of force development (RFD). Asterisk indicates a significant pairwise difference (P < 0.05). Positive indicates palmar displacement, negative indicates dorsal displacement.