STRETCH TOLERANCE IS LINKED WITH ENDOGENOUS MODULATION OF PAIN

Støve MP1, Hirata RP2, Palsson TS21University College of Northern Denmark, Department of Physiotherapy, Aalborg, Denmark, 2Aalborg University, Department of Health Science and Technology (SMI), Aalborg, DenmarkBackground: Stretching is known to increase joint range of motion and is extensively used in relation to rehabilitation and exercise to improve function or prevent injury. Evidence suggests that changes in joint range of motion following stretching are caused by modifications in subject sensations resulting in changes in the tolerance to stretch. Although an analgesic effect has been considered to play a role in higher tolerance to passive tension, it is unknown to which extend pain affects stretch tolerance. Taken together, it is possible that stretch tolerance is linked with modulation of somatosensory input. The conductance of nociceptive afferent signals can be modulated via descending inhibition and facilitation by engaging supraspinally-mediated endogenous mechanisms. It is possible that engaging these modulating mechanisms may affect stretch tolerance. More knowledge regarding this may provide valuable information about the mechanisms underlying the effect of stretching.Purpose: To measure the passive range of motion in healthy individuals where endogenous pain inhibition (exercise-induced hypoalgesia (EIH) and conditioned pain modulation (CPM)) are induced.Methods: Nineteen healthy male participants were included in this randomized, repeated-measures cross-over study conducted in two sessions (EIH & CPM) on two separate days. The order of sessions (EIH or CPM first) was randomized and counterbalanced. Subjects were seated in a Biodex System 4 Pro (Biodex Medical Systems, New York, USA) with their upper body, thigh and pelvis securely fixated. The Biodex slowly moved the lower leg from flexion towards extension of the knee and subjects were asked to indicate when they reached the threshold between stretch and pain (stretch tolerance) upon which the movement stopped, and the joint angle was registered. Passive knee extension range of motion (PROM) and pressure pain thresholds (PPT) were assessed before and after the induction of an EIH and CPM response. The EIH was assessed following a 3-minute isometric muscle contraction of the hand flexors and CPM was assessed after the subjects had submerged their hand in ice water (1°-4°C) for two minutes. All subjects were blind to the outcome after each session. The difference in ROM and PPTs between measurements was extracted for data analysis.Results: A statistically significant increase in PROM (p < 0.034) and PPT (p < 0.027) was found after EIH and CPM with large effect sizes (d > 0.9). Moderate and significant correlations in pain sensitivity responses were also found between EIH and CPM (Rho = 0.562, p < 0.001).Conclusion(s): By engaging endogenous pain inhibitory mechanisms (through exercise or by introducing a competing painful stimulus) the tolerance to stretch increases, resulting in an increase in passive knee extension range of motion.Implications: These findings have clinical relevance as they indicate that the modulation of pain has significance for the tolerance to stretch. This is an important finding considering that stretching is often prescribed as part of the rehabilitation of different musculoskeletal pain conditions where reduced endogenous pain inhibition is frequently seen.Keywords: Stretching, Endogenous pain modulation, Stretch toleranceFunding acknowledgements: University College North Denmark, Department of Health Science and Technology (SMI), Aalborg University. The authors have no conflicts of interest.Topic: Sport & sports injuries; Pain & pain management; MusculoskeletalEthics approval required: YesInstitution: RECEthics committee: North Denmark Regional Ethics CommitteeEthics number: (N-20160019)All authors, affiliations and abstracts have been published as submitted.