Tonic pain does not influence the modulation of the nociceptive withdrawal reflex during walking, suggesting that the need to maintain balance overrides any effects of endogenous pain modulation.
Key Findings
Results
Gait phase significantly affected reflex response in most muscles during walking.
There was a significant effect of gait phase on reflex response in all muscles except ipsilateral biceps femoris.
Sixteen healthy participants (mean ± SD age: 29 ± 9 years, 3 males) walked on a treadmill while lower-limb kinematics and EMG from bilateral biceps femoris, vastus lateralis, soleus and tibialis anterior were recorded.
A brief electrical stimulus was applied to the arch of the right foot at predefined points in the gait cycle to elicit an NWR.
Reflex responses were calculated from EMG as a percentage difference to the EMG present without any stimulation.
Results
Tonic pain produced only one significant effect on reflex response: a small decrease in contralateral tibialis anterior NWR.
When accounting for the intensity of tonic pain, there was a 0.64 decrease in normalised reflex response in the contralateral tibialis anterior with tonic pain versus control (p = 0.009).
There were no other significant effects of tonic pain on reflex response in any other muscle (p > 0.05).
Tonic pain was applied to the left arm during treadmill walking.
Results
Tonic pain did not alter the phase-dependent modulation of the nociceptive withdrawal reflex during walking.
The study directly compared NWR modulation across the gait cycle with and without tonic pain.
Despite tonic pain being known to inhibit the NWR in static conditions, this inhibitory effect did not extend to meaningful changes in the gait-phase-dependent modulation of the NWR.
The authors propose that the need to maintain balance overrides any effects of endogenous pain modulation.
Results
Electrical stimulation during the loading response produced an approximately 3° increase in dorsiflexion in the contralateral leg.
Stimulation during loading response saw around a 3° increase (p = 0.007) in dorsiflexion in the contralateral leg.
There was an increase in knee flexion on the stimulated leg when it was unloading, however there was large variability in the kinematics.
Kinematic data were recorded alongside EMG to capture locomotor responses to the nociceptive stimulus.
Discussion
The authors propose that balance maintenance requirements during walking override endogenous pain modulation of the NWR.
The tonic pain-induced inhibition of the NWR, previously documented in static conditions, was largely absent during locomotion.
The authors suggest this is because withdrawing a limb during gait poses a postural stability threat that the nervous system prioritizes over pain-related reflex suppression.
This interpretation aligns with the concept that spinal reflex circuits are reorganized during locomotion to serve balance and movement continuity.
What This Means
This research suggests that when people experience ongoing (tonic) pain, it does not meaningfully change how the body's automatic withdrawal response to a painful foot stimulus is regulated during walking. The nociceptive withdrawal reflex (NWR) is the automatic movement — like pulling away — that happens when a painful stimulus is detected. Previous research showed that tonic pain can dampen this reflex when a person is standing still, but this study tested whether the same dampening occurs during the more complex task of walking on a treadmill. Sixteen healthy volunteers walked on a treadmill both with and without tonic pain applied to their arm, while brief electrical shocks to the foot triggered the reflex at different points in the walking cycle.
The study found that where a person is in their walking cycle — for example, whether the foot is on the ground or in the air — has a strong influence on the reflex response, confirming what was already known about gait-phase-dependent modulation. However, tonic pain had almost no effect on this pattern. The only statistically significant pain-related finding was a very small reduction in reflex activity in one muscle on the opposite leg. The researchers also observed some changes in leg movement angles following stimulation, but these were highly variable across participants.
This research suggests that during walking, the brain and spinal cord prioritize keeping balance over modifying pain reflexes. In other words, the nervous system appears to suppress the pain-dampening effect of tonic pain during locomotion, likely to ensure that any protective withdrawal response needed to prevent a fall can still occur. This has potential implications for understanding how chronic pain might or might not disrupt normal walking patterns, and why studying pain reflexes in static conditions may not fully capture what happens during real-world movement.
Check Your Own Numbers
Upload your bloodwork. We'll cross-reference your results against this study and 4,700 others.
Reeves J, Thacker C, Cunningham G, Hughes S, Davies J. (2026). Modulation of the nociceptive withdrawal reflex in walking is not changed by the presence of tonic pain.. Experimental brain research. https://doi.org/10.1007/s00221-026-07379-7