Opposite effects of isometric exercise on pain sensitivity of healthy individuals: the role of pain modulation

Abstract

Introduction: Exercise-induced hypoalgesia (EIHypo) among healthy individuals is well documented; however, the opposite effect of exercise, ie, exercise-induced hyperalgesia (EIHyper), has mainly been described in patients with chronic pain or after intense/painful exercise.

Objectives: We investigated the extent to which EIHypo and/or EIHyper occur among healthy participants and whether these responses are associated with individuals' pain modulation capacity.

Methods: Fifty-seven participants (mean age 29.20 ± 5.21 years) underwent testing of pressure pain threshold as an index of EIHypo/EIHyper: pain adaptation, offset analgesia (OA), and conditioned pain modulation as indices of pain modulation, prior to and immediately postsubmaximal isometric exercise (n = 40) or rest (n = 17, control group). Body awareness and exercise-evoked stress were also evaluated. Test-retest repeatability of the pain modulation indices was performed as well.

Results: Twenty-four participants (60%) exhibited EIHypo, whereas 16 (40%) exhibited EIHyper. Pressure pain threshold did not change in the control group. Baseline (preexercise) OA efficacy predicted EIHypo/EIHyper. Furthermore, OA significantly decreased postexercise in the EIHyper subgroup and slightly increased in the EIHypo subgroup. Exercise-induced hypoalgesia was associated with magnitude of daily exercise while EIHyper was associated with increased exercise-evoked stress and body awareness.

Conclusion: Submaximal isometric exercise can induce opposite effects on pain sensitivity among healthy participants-EIHypo or EIHyper. Descending pain inhibition pathways, and top-down influences over these pathways, seem to be involved in EIHypo/EIHyper effects. As such isometric exercise is often preferred in early stages of rehabilitation, preliminary screening individuals' vulnerability to this exercise is important; OA test may be used for this purpose.

Keywords: Descending modulation; Exercise; Interoceptive; Pain perception; Stress.

Figure 1.

Experimental protocol. Preparations included explanations on the study and equipment, an interview regarding demographics and general health status, and a training session in quantitative sensory testing (QST). The exercise group performed T1, T2, and T3 QST evaluations, and the control group performed T1 and T2 QST evaluations. Each QST evaluation included the measurement of CPM, conditioned pain modulation; MVC, maximal voluntary contraction; OA, offset analgesia; PA, pain adaptation; PPT, pressure pain threshold; S-R, stimulus response function for heat pain; effort/stress, self-report scales (0–10). The break between the different phases lasted 2 to 5 minutes.

Figure 2.

Change in pressure pain threshold (PPT) pre- and postisometric exercise or rest (control group) (A). Delta PPT pre- and postexercise of individual participants (B). The significant changes in PPT pre- and postexercise among the exercise-induced hypoalgesia (EIHypo) and exercise-induced hyperalgesia (EIHyper) subgroups (C). Values for (A and C) are group average ± SE. *P < 0.05, ***P < 0.001.

Figure 3.

The change in offset analgesia (OA) pre- and postisometric exercise; the exercise-induced hypoalgesia (EIHypo) subgroup exhibited a slight increase in OA efficacy (greater inhibition), whereas the exercise-induced hyperalgesia (EIHyper) subgroup exhibited a significant decrease in OA efficacy (less inhibition) (*1). Furthermore, OA efficacy at baseline was significantly greater in the EIHyper subgroup (*2) (A). The difference between the subgroups in the magnitude of OA change was significant (B). Values are group average ± SE. *P < 0.05.