Physiology and pathophysiology of low back pain in ballet dancers

Abstract

Classical ballet dancers combine artistry and physical skills to perform graceful and exciting movements. Lengthy, exacting dance training potentially promotes unique adaptation of motor control to; execute these movements; maintain optimal physiological function, and provide stability to avoid injury and pain. Yet low back pain (LBP) is common in dancers. Differences in motor control are frequently observed between non-dancers with and without LBP. This often involves reduced and delayed activity of deep trunk muscles and augmented activity of superficial muscles. These changes are proposed to affect the control of movement and stiffness of the trunk. LBP is also associated with deficits in postural control, an aspect of motor control, which depends on control of the trunk. Some features of adaptation in motor control are relatively common; but there are some features that are specific to individuals or different populations. It is essential to understand how motor control is adapted in elite classical ballet dancers with LBP as less optimal motor control has potential to impair performance and limit the capacity to dance. The overall objective of this thesis was to investigate aspects of motor control in dancers with and without LBP, with specific attention to trunk muscle morphology and to postural stability. Studies I and II investigated trunk muscle size, symmetry and function, in dancers with and without LBP, at rest and during simple manoeuvres, with magnetic resonance imaging (MRI). Reduced size of the multifidus muscle was observed in dancers with LBP and dancers with both back and hip/pelvic region pain. This finding is similar to that for non-dancers. Study II provided preliminary evidence of a behavioural change in transversus abdominis (the deepest abdominal muscle); expressed as reduced length change of this muscle measured with MRI, in dancers with LBP. Thickness of transversus abdominis, obliquus internus abdominis and multifidus muscles were asymmetrical in dancers but this was not related to LBP. This may be related to repetitive performance of asymmetrical movements. Studies III and IV investigated postural control. In Study III, the trunk was perturbed in order to measure the dynamic properties of stiffness, damping and mass as an indication of control of the trunk. Dancers with LBP had less damping (control of velocity) than dancers without LBP, but this could be changed with motor imagery in the dancers with LBP. Study IV used linear and non-linear measures of centre of pressure trajectories to investigate standing balance in dancers with and without LBP and non-dancers. Balance was measured with the feet in parallel and the dance-specific turned out ‘first’ position; which increases the dependence on trunk motion for balance in the anteroposterior direction. Dancers without LBP used more movement to control balance than non-dancers. These findings suggest that “less” movement does not define optimal balance in dancers. Dancers with a history of LBP used strategies that were more similar to non-dancers than to their LBP-free counterparts. This compromised balance in dancers with LBP has potential to impact on performance. Each of these studies has identified differences in aspects of motor control between dancers with and without LBP and, in the case of balance, between non-dancers and the dancers’ groups. These changes in motor control associated with LBP are potentially modifiable and may provide a basis for the development of prevention or treatment programs to reduce the morbidity related to LBP in professional classical ballet dancers.