New insight into motor adaptation to pain revealed by a combination of modelling and empirical approaches

BACKGROUND: Movement changes in pain. Unlike the somewhat stereotypical response of limb muscles to pain, trunk muscle responses are highly variable when challenged by pain in that region. This has led many to question the existence of a common underlying theory to explain the adaptation. Here, we tested the hypotheses that (1) adaptation in muscle activation in acute pain leads to enhanced spine stability, despite variation in the pattern of muscle activation changes; and (2) individuals would use a similar 'signature' pattern for tasks with different mechanical demands. METHODS: In 17 healthy individuals, electromyography recordings were made from a broad array of anterior and posterior trunk muscles while participants moved slowly between trunk flexion and extension with and without experimentally induced back pain. Hypotheses were tested by estimating spine stability (Stability Index) with an electromyography-driven spine model and analysis of individual and overall (net) adaptations in muscle activation. RESULTS: The Stability Index (P < 0.017) and net muscle activity (P < 0.021) increased during pain, although no two individuals used the same pattern of adaptation in muscle activity. For most, the adaptation was similar between movement directions despite opposite movement demands. CONCLUSIONS: These data provide the first empirical confirmation that, in most individuals, acute back pain leads to increased spinal stability and that the pattern of muscle activity is not stereotypical, but instead involves an individual-specific response to pain. This adaptation is likely to provide short-term benefit to enhance spinal protection, but could have long-term consequences for spinal health.