Are Jumping Asymmetries Associated with Prospective Injury Risk in Pre-Professional Ballet?

Background: Pre-professional ballet dancers are exposed primarily to injury risk in the lower extremities with most injuries occurring during jumping and landing activities. Inter-limb asymmetry during jumping and landing activities has been associated with injury risk in adolescent athletes but this has not been examined in dancers. Purpose: To investigate associations between interlimb asymmetry in double-leg (DL-CMJ) and single leg (SLJ), countermovement jump performance and prospective injury risk in pre-professional adolescent ballet dancers. Study Design: Cohort-Study. Methods: Pre-professional adolescent ballet dancers (n=255) performed 3 DL-CMJ’s and 3 SLJ’s on force plates during of annual profiling. Absolute and directional (separate values for left and right dominance) asymmetries in a range of DL-CMJ kinetic variables and in SLJ height were calculated. Each variable was dichotomised as “high” or “normal” asymmetry according to whether % asymmetry was > or ≤ mean + 0.5 SD, based on the present sample. Risk ratios (RR) and 95% confidence intervals (95% CI) were calculated based on injury incidence in the subsequent academic year. Results: Of 242 dancers, 128 injuries were observed in the subsequent academic year. In the full sample, two absolute, six left limb dominant and one right limb dominant kinetic asymmetries across eccentric, concentric and landing phases of the DL-CMJ, and left limb dominant jump height asymmetry in the SLJ were associated with a significant (p=<0.001) increase in injury risk (RR= between 1.48 and 1.71, 95% CI = 1.01 to 2.48). Separating by sex, eccentric DL-CMJ asymmetries were not significant in boys, while in girls RR’s for eccentric asymmetries increased and SLJ height was not significant. Conclusions: Higher asymmetries in specific DL-CMJ kinetic variables and in SLJ height were associated with an elevated risk of injury in elite pre-professional ballet dancers with some specific sex differences. Associations were mainly identified for high left limb dominant asymmetry in the take-off phase suggesting that risk may be specific to a relative right limb deficit. Clinical Relevance: This study provides detailed and thorough initial research investigating associations between jumping asymmetry and prospective injury risk in pre-professional ballet dancers. This may lead to the introduction of more proactive injury reduction strategies in the future. This research also highlights that jump-land asymmetry-risk analyses should not only consider absolute values, but also left and right limb dominant asymmetries separately as associations are missed if directional asymmetries are not considered. Key Words: Injuries, Jumping, Limb asymmetry, Dance, Risk Factors, Biomechanics, Knee Injury, Ankle Injury, Foot Injury What is already known on the topic: Pre-professional adolescent ballet dancers face significant exposure to injury in the lower extremities and jumping and landing during dance is the most common mechanism of injury. Associations between SLJ height asymmetry and injury risk have been reported in team sports. What this study adds: High (relative to population norms), jump-land double leg CMJ and single leg jump height asymmetries, predominantly left dominance (right limb deficits) are associated with prospective injury risk in pre-professional ballet dancers. In addition, using internal descriptive statistics to classify asymmetry and analysis of directional asymmetries may provide a useful method to investigate interactions between asymmetry and injury.