Interactions between naturally occurring muscle pain, externally induced thermal pain and effort during a self-regulated handgrip task

Citation

Mangin T, O’Malley CA, Bergevin M, Debray J, Monti I, Fullerton CL, Mauger AR, Rainville P & Pageaux B (2023) Interactions between naturally occurring muscle pain, externally induced thermal pain and effort during a self-regulated handgrip task. The 20th congress of ACAPS (Association of Researchers in Sport and Physical Activity), Reims, France, 31.10.2023-02.11.2023.

Abstract
Introduction During exercise, pain can originate from the muscles due to repeated contractions (naturally occurring muscle pain, NOMP; O’Connor & Cook, 1999), or outside the muscles. The former can be manipulated with the exercise intensity, and the latter via the use of thermal stimuli inducing thermal pain (TP). Theoretical Framework and Methodology Pain, regardless of its origin, is proposed to alter the motor command (Bank et al., 2013) and increase the task cognitive load (Torta et al., 2017). Effort is defined as the voluntary engagement of physical and mental resources to perform in a task (Richter et al., 2016). The motivational intensity theory (Richter et al., 2016) postulates that during a fixed effort task, participants would reduce the force produced due to the pain-increased physical and cognitive demand. We aimed to explore this possibility for NOMP and TP. We hypothesised that both sources of pain would lead to a reduction in force during fixed effort tasks. Forty young participants (50% female) completed either a low or hard fixed effort (13 or 50 on the CR100 scale, respectively) intermittent isometric handgrip contraction task, in two separate sessions. The task consisted of two blocks of ten trials, interspaced by 5 min rest. Each trial involved five contractions (3s:ON-OFF). Blocks were performed in the presence of warm/non-painful stimuli (control condition), or in the presence of TP (TP condition). Thermal stimuli were applied on the non-exercising forearm. Sessions and blocks order were randomized. After each trial, participants reported the intensity of NOMP in the exercising forearm and the TP. Results and Discussion The intensity × trial interaction revealed a larger increase in NOMP in the hard effort task (p<.001), as well as a greater reduction in peak force (p<.001) and force-time integral (FTI, p< .001) during the hard effort task, compared to the low effort task. This result suggests that in the presence of NOMP, participants decreased the force produced to maintain a fixed effort; thus, the higher the NOMP is, the more the force decreases. In the TP condition, contrary to one of our hypotheses, participants produced higher peak force (p=.034) and FTI (p=.017) compared to the control condition. During task completion, participants rated lower TP compared to before task completion (p<.001), confirming the hypoalgesic effect of effort allocation. Conclusions and Perspectives Altogether, our results suggests that during a self-regulated physical task performed at fixed effort, NOMP and TP have opposite effects on participants’ behaviour. While NOMP is associated with a decreased force production, TP is associated with increased force production. These observations could be due to: i) NOMP is exercise-intensity dependent, therefore, lowering the force reduces the intensity and associated NOMP; ii) TP is externally imposed, therefore, increasing effort and consequently the force exerted can reduce pain perception (hypoalgesia). While the behaviour observed in the presence of TP does not validate one of our hypotheses, the increased force to minimise TP concurs with recent literature in psychology suggesting that participants prioritise effort over pain in a decision-making task (Vogel et al., 2020).