Dose-response effect of isometric force production on the perception of pain

Abstract

Isometric contractions can influence the way that we perceive pain, but conclusions on the dose-response effect of force amplitude on pain perception are limited because previous studies have not held the duration of force contractions constant while varying force amplitude. To address this issue we designed an experiment that allowed us to accurately guide the amplitude of an isometric pinch grip force contraction on a trial-by-trial basis, while a thermal pain eliciting stimulus was simultaneously delivered for the same duration to the non-contracting hand. Our results show that an increase in the amplitude of force produced by one hand corresponded with a decrease in pain perception in the opposite hand. Our observations provide novel evidence that the centralized inhibitory response that underlies analgesia is sensitive to and enhanced by stronger isometric contractions.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1. Experimental paradigm.

A. The white bar represented the target force level which was set at the beginning of each trial to 5%, 25%, or 50% of each subjects’ MVC. A second red/green force bar was controlled by the subject and provided real-time visual feedback of their force production during active conditions, or moved according to a pre-recorded force trace during passive conditions. The bar was red during rest periods and green when the subject had to produce force or view the pre-recorded force trace. When the force bar turned green, the subject’s goal was to match the green bar with the white bar. Each trial was 15s long, and included 5 force pulses. Each pulse was 1.8s (bar turns green) and pulses were separated by a 1.2s rest period (bar turns red). Trial timing was the same during passive conditions but subjects were instructed to view the green bar and not produce any force. The pre-recorded force trace was collected during each participant’s practice session. Each 15s trial was followed by a 7.5s rating period. Subjects rated the level of stimulation they experienced during the previous 15s trial. Ratings were made using a button box to control a cursor on a visual analog scale (VAS) presented on the screen. B. Example force traces (black lines – left y-axis) and temperature traces (red lines – right y-axis) from active trials from one subject for the hot condition at 5%, 25%, and 50% of MVC.

Figure 2. Analgesic score (passive condition – active condition) are shown for each target force level for the hot condition.

Analgesic scores at 50% were greater than at 5% of MVC. Error bars represent SE.

Figure 3. Mean amplitude and variability of force production during active trials.

Figures A-C show mean force amplitude, mean standard deviation, and mean coefficient of variation of force production during active trials for each temperature and each target MVC level. Error bars represent SE.