Wind-Up Pain: When Repeated Stimuli Hurt More Each Time

In normal pain processing, the same stimulus delivered repeatedly at the same intensity should produce roughly the same pain response each time. If someone taps your arm with the same force ten times, the tenth tap should feel about the same as the first. Wind-up is what happens when it does not: each successive tap produces a progressively larger pain response, even though the stimulus has not changed. This is not an illusion or an overreaction. It is a measurable change in how spinal cord neurons respond to repeated input [1].

Wind-up was first described by Mendell and Wall in 1965, working with dorsal horn neurons in the spinal cord [1]. They found that when C-fibers (the slow, unmyelinated nerve fibers that carry dull, aching pain signals) were stimulated repeatedly at frequencies above approximately 0.3 Hz (once every three seconds or faster), the dorsal horn neurons produced progressively larger responses to each stimulus. The neurons were summing the inputs over time, rather than treating each one independently.

The finding was significant for a reason that was not fully appreciated at the time: it demonstrated that the spinal cord is not a passive relay station. It processes, modifies, and in the case of wind-up amplifies the signals that pass through it. The dorsal horn is an active participant in pain processing, not a cable carrying messages from the periphery to the brain.

The Mechanism

The spinal cord mechanism behind wind-up involves a specific receptor: the NMDA (N-methyl-D-aspartate) receptor. Under normal conditions, NMDA receptors on dorsal horn neurons are blocked by a magnesium ion sitting in the receptor channel. A single C-fiber stimulus releases glutamate, which activates AMPA receptors and produces a normal, proportional response. The NMDA receptor stays blocked [4].

When stimuli arrive repeatedly and fast enough, the cumulative depolarization from repeated AMPA receptor activation dislodges the magnesium block from the NMDA receptor. Once unblocked, the NMDA receptor allows calcium to flow into the neuron, producing a much larger response and triggering intracellular changes that further increase excitability. Each subsequent stimulus now produces a bigger response than the last [4].

The intracellular changes triggered by NMDA receptor activation are worth specifying. Calcium influx activates protein kinases, including protein kinase C, which phosphorylate the NMDA receptor itself, making it easier to activate in the future. This is a form of short-term synaptic plasticity: the synapse is literally learning to be more responsive. The same type of plasticity, long-term potentiation, underlies memory formation in the hippocampus. In the dorsal horn, it underlies the amplification of pain [6].

In healthy people, wind-up is self-limiting. When the stimulation stops, the neuron returns to its baseline state within seconds to minutes. The NMDA receptors are re-blocked, the intracellular signaling cascades subside, and the system resets. Wind-up in a normal nervous system is a protective amplification: if a stimulus is being repeated rapidly, it might be something dangerous, and amplifying the signal ensures it gets attention. The key feature of normal wind-up is that it resolves. The system amplifies a repeated signal, the person attends to it and acts, and then the amplification subsides. It is a temporary state with a clear adaptive purpose.

What the Research Shows in Fibromyalgia

Staud and colleagues conducted the key studies of wind-up in fibromyalgia. In a 2001 study comparing 36 people with fibromyalgia to healthy controls, they found three specific abnormalities [2]:

Wind-up was enhanced: it began at lower stimulus intensities in people with fibromyalgia. A stimulus that did not produce wind-up in controls produced progressive amplification in fibromyalgia. The threshold at which the spinal cord began summing and amplifying was lower, which means everyday stimuli that would not trigger wind-up in a healthy system, including the normal mechanical stimuli of walking, sitting, or light touch, could trigger amplification in a sensitized system.

Wind-up was accelerated: the pain rating increased faster with each successive stimulus. By the fifth or sixth repetition, pain ratings in the fibromyalgia group had increased substantially more than in controls. The slope of the summation curve was steeper, meaning the amplification built more rapidly.

Wind-up was prolonged: after stimulation stopped, the elevated pain response took longer to decay in the fibromyalgia group. Where controls returned to baseline within seconds, people with fibromyalgia maintained elevated pain responses for significantly longer [2]. This prolonged after-discharge is clinically important: it means that even brief activities can produce pain that outlasts the activity by minutes or longer, contributing to the impression that “everything I do causes pain that won’t stop.”

A later study by Staud and colleagues showed that this enhanced wind-up was maintained by ongoing peripheral input from muscles [3]. When muscle afferent input was temporarily blocked, wind-up responses normalized. This suggests that while the amplification occurs in the spinal cord, it depends on a continuous stream of input from the periphery to sustain it. The sensitized spinal cord is not operating in isolation; it is being fed by peripheral signals that it then amplifies. This finding has implications for treatment: reducing peripheral input (through appropriate exercise, muscle relaxation, or other strategies) may help reduce the drive that maintains central amplification.

Wind-Up and Central Sensitization

Wind-up and central sensitization are related but not identical. Wind-up is a short-term phenomenon, a temporary increase in excitability during and briefly after repeated stimulation. Central sensitization is a longer-lasting state in which the nervous system remains sensitized even without ongoing stimulation [6].

The relationship between them is developmental: wind-up is one of the mechanisms through which central sensitization is established. Repeated episodes of wind-up, particularly when descending inhibitory pathways are not functioning well enough to reset the system, can lead to the more persistent changes that characterize central sensitization, including alterations in gene expression, receptor distribution, and synaptic architecture in the dorsal horn. Woolf and Thompson showed that NMDA receptor activation is necessary for both wind-up and the maintenance of central sensitization, establishing the molecular link between the two processes [4].

In fibromyalgia, this distinction may be somewhat academic, because the system does not reset. Enhanced wind-up and central sensitization coexist and reinforce each other: the sensitized background makes wind-up occur more readily, and the wind-up that occurs further maintains the sensitized state. It is a self-sustaining loop.

Why This Matters Clinically

Enhanced wind-up provides a mechanistic explanation for a common clinical observation: activities that start off tolerable become progressively more painful the longer they continue. A person with fibromyalgia might begin a walk feeling reasonable but find the pain escalating steadily over the course of 20 minutes, even though the biomechanical demands of each step have not changed. The spinal cord is summing each repetitive input and producing a progressively amplified output.

This has direct implications for pacing. The goal of staying within a sustainable activity level is, in mechanistic terms, staying below the threshold at which wind-up significantly amplifies the pain signal. The recommendation to stop before pain escalates, rather than pushing through to the point of significant pain increase, is grounded in the wind-up mechanism: once temporal summation is fully activated, the heightened state takes time to resolve, and that resolution time is prolonged in fibromyalgia.

Understanding wind-up also explains why the “push through the pain” advice that works for many acute conditions can backfire in fibromyalgia. In a normal nervous system, continuing an activity despite mild pain does not cause the pain to escalate in the same way. In a sensitized system, the repeated stimuli of continued activity trigger progressive amplification, and the resulting pain is both more intense and longer-lasting than the activity warrants. Activity is still the treatment, but the dose matters: graded, paced, and stopped before significant escalation.

What Remains Uncertain

Whether enhanced temporal summation is a cause of fibromyalgia, a consequence of it, or both is not resolved. Longitudinal studies tracking whether enhanced wind-up precedes the development of fibromyalgia symptoms are limited. If enhanced wind-up could be identified before widespread pain develops, it might serve as an early marker for intervention, but the evidence to support this possibility does not yet exist.

Whether treatments that specifically target the NMDA receptor (such as ketamine, which blocks it) can reduce wind-up in fibromyalgia has shown some preliminary results, but clinical evidence is insufficient for routine use. The relationship between NMDA receptor activation in pain pathways and its function in other parts of the nervous system (including memory and learning) makes pharmacological targeting of this receptor complex and potentially problematic.

And whether the degree of enhanced wind-up in an individual patient could predict which treatments are most likely to help is a question that research has not yet answered. The measurement tools exist (quantitative sensory testing), but their role in guiding individual treatment decisions is not established. A future in which a clinician measures a patient’s temporal summation profile and uses it to select between pregabalin, duloxetine, exercise protocols, or combinations is plausible but not yet supported by the evidence needed to implement it.

Getting support for chronic pain. Chronic pain, including fibromyalgia, is a complex medical condition that deserves proper clinical attention. A GP, rheumatologist or pain specialist can help develop a management plan. The American Chronic Pain Association (theacpa.org) and the National Fibromyalgia Association (fmaware.org) have patient resources and practitioner directories.