Deep hip external rotator muscle size in ballet dancers compared to non-dancing athletes, and associations to pain

Objectives: To compare the deep hip external rotator (DHER) muscle (piriformis, gemelli, quadratus femoris (QF)) cross-sectional area (CSA) in dancers and athletes and evaluate the relationship between DHER size and hip pain. Study design: Cross-sectional study. Setting: Elite ballet and sport. Participants: 33 professional ballet dancers and 33 age and sex-matched athletes. Main outcome measures: CSAs of piriformis, gemelli and QF on magnetic resonance imaging (MRI). Hip pain scored with the Copenhagen Hip and Groin Outcome Score (HAGOS): Hip pain was categorised as ‘yes’ if HAGOS pain score was <100, ‘no’ if HAGOS pain was = 100. Results: Estimated marginal mean CSA of piriformis, gemelli and QF muscles was similar in athletes and dancers (p > 0.05), and CSAs were not associated with hip pain. Male and female dancers had similar sized DHER muscles. In athletes, CSA of piriformis was 55% (p = 0.02, Cohen's d(95%CI) = 0.98 (0.26,1.71)) and the gemelli were 34% (p = 0.03, Cohen's d(95%CI) = 0.98 (0.26,1.70)) larger in men than women; QF was 36% larger (p = 0.08, Cohen's d(95%CI) = 0.77 (0.06,1.48)). Conclusion: Although ballet dancers perform in external rotation, their DHER are no larger than athletes and muscle size was not associated with hip pain.