Pain induced adaptions of the neuromuscular system

People move differently when they have pain. Due to the heterogeneity of individuals’ daily activities (e.g. working habits, training), movement quantification may not suffice at describing the motor consequences of pain. The quality of movement, on the other hand, may be a more promising indicator of ongoing neuromuscular dysfunction in people with chronic pain.

The results from several of our studies indicate that people with pain may move with less variability. This is compatible with the hypothesis that pain induces a reduction of the available kinematic trajectories and degrees of freedom during natural movements. Whilst this adaptation may prove to be immediately beneficial (e.g. to avoid pain provocation) the long-term consequences may be harmful, leading to perpetration or recurrence of pain. We are exploring different methods to quantify the complexity and variability of spinal movement in people with pain. These measurements may be useful in future studies to evaluate the effects of interventions, including exercise, to enhance movement control in people with chronic pain.

Numerous studies and reviews have suggested that pain can trigger neuromuscular adaptations following an initial episode of pain. In this project we are investigating whether persistent neuromuscular adaptations predict future painful episodes in people with recurrent pain when evaluated during a period of remission. This project will provide information on which, if any, neuromuscular adaptations are associated with a higher likelihood of pain recurrence; this information is relevant to develop preventative exercise interventions, setting the basis for future clinical trials to tackle the recurrence of spinal pain.

Skilled motor learning is a continuous process throughout life. People continue learning new skills to accomplish new tasks or develop proficiency in previously acquired skills through repetition and practice. Traditionally, it was suggested that the acquisition of new skills or improvement of performance is associated with neuroplastic changes in the brain only. However, recent studies suggest that the spinal cord is actively involved in motor skill acquisition, and its role goes beyond what was assumed previously. Some previous studies have suggested an association between conditioned fear and lack of presynaptic inhibition. However, no study has investigated the relationship between modification of spinal mechanisms as the result of the acquisition of pain-related fear and acquisition of new motor skills, retention of previously acquired skills and proficiency in humans.

In a series of studies, we are aiming to 1-investigate the relationship between pain-related fear and modification of spinal mechanisms involved in a single-joint movement, 2-investigate the effect of pain and related fear on the acquisition of new motor skills and on the development of proficiency after skill acquisition, and 3- investigate the neural mechanisms underlying the interference between pain-related fear and motor skill acquisition using functional MRI of the spinal cord.

Compared to other experimental pain models, electrical stimulation has a number of advantages, such as its non-invasive nature and the possibility to precisely control the timing and intensity of the painful stimulation. We are currently exploring the use of noxious electrical stimulation in the context of how our bodies adapt to pain, and specifically to test: i) the effect of task-relevant pain on motor adaptations; ii) pathways in the central nervous system involved in responses to noxious stimulation.

Our interest in task-relevant pain, meaning a painful electrical stimulation whose intensity depends on how the participant moves, is justified by the idea that this kind of stimulus should be a stronger motivator to change motor behaviour compared to continuous pain that is not modulated by movement. We believe this task-relevant experimental model is also more representative of clinical musculoskeletal disorders, where pain intensity changes with people’s posture and movement. Current efforts are directed towards the optimization of the stimulation parameters to induce stable pain levels, while also investigating how whether task-relevant experimental pain influences motor behaviour differently from other models. The development of this task-relevant pain model will enable us to understand how people change their motor behaviour when pain is elicited by a specific movement.

One mechanism that is proposed to play a role in the development and/or persistence of pain and dysfunction is the alteration of patterns of movement caused by changes in motor control. A key question relates to the mechanisms that underlie the motor control changes reported in musculoskeletal conditions. Re-organisation of the motor cortex and changes in corticospinal excitability, observed using transcranial magnetic stimulation (TMS), have been described as possible mechanisms involved in chronic pain. For instance, studies on chronic low back and neck pain show a complex relationship between changes in motor behaviour and cortical organisation, likely to be influenced by characteristics of individual patients and their underlying mechanisms of pain.

When studying the organisation of the motor cortex representation and corticospinal excitability of the spinal muscles, findings are often complicated by the large inter-subject variability. The use of new tools to asses motor output may contribute to understand the source of this variability. By combining TMS and high-density electromyography, we are performing an extensive characterisation of the organisation of the motor cortex with respect to the activation of regions within the lumbar muscles. By describing altered function of corticospinal projections to different regions of the lumbar muscles, we aim to provide a better understanding of the mechanisms that underlie changes in motor control in chronic musculoskeletal spinal pain, which will improve prevention strategies and rehabilitation interventions.