Background and aims: The nociceptive flexion reflex (NFR) is a withdrawal reflex occurring in response to noxious stimuli. The NFR is considered to be an objective physiological correlate of spinal nociception. Although it is well documented that the NFR is subject to powerful modulation of several modifiable and non-modifiable factors, the effects of experimentally induced fatigue on the NFR have not yet been examined. Hence, this study aimed to characterize fatigue-related changes in spinal nociception in healthy adults.
Methods: Fifty-eight healthy people their NFR threshold was measured prior and following a rest period (control condition) and two fatigue inducing tasks performed in randomized order. The NFR was elicited by transcutaneous electrical stimulation of the sural nerve and objectified by electromyographic recordings from the biceps femoris muscle contraction. The stimulus intensity at which the NFR was elicited was used as outcome measure. An isokinetic fatiguing protocol was used as the physical task aimed at inducing muscular fatigue of the hamstrings. The cognitive fatiguing task intended to provoke mental fatigue existed out of a modified incongruent Stroop-word task. A linear mixed model analysis was used to assess the influence of fatigue on the NFR.
Results: Experimentally induced fatigue led to an inhibition in NFR thresholds. Post-hoc analysis revealed that this inhibition was similar in both fatiguing tasks. 
Conclusions: Spinal nociception is modulated by muscular and mental fatigue in healthy adults. These results highlight the importance of considering fatigue levels when evaluating nociceptive processing given the modulation of spinal nociception by fatigue.

THE INFLUENCE OF FATIGUE ON THE NOCICEPTIVE FLEXION REFLEX: A RANDOMIZED CROSS-OVER STUDY