Differences in the Size of Individual Plantar Intrinsic Foot Muscles Between Ballet Dancers and Non-Dancers

Introduction: In classic ballet, choreography often involves tiptoe standing. Tiptoe standing requires a high and stable foot arch structure, which is achieved by contraction of the plantar intrinsic foot muscles (PIFMs). Long-term repetitive loading with a specific movement can induce hypertrophic adaptation of the associated muscles. For dancers, however, limited information on the size of individual PIFMs is available from previous studies. The purpose of this study was to determine the differences in the sizes of 10 individual PIFMs between dancers and non-dancers.Methods: Muscle volumes (MVs) of 10 individual PIFMs were measured using magnetic resonance imaging in 15 female dancers and 15 female non-dancers. Muscles analyzed included abductor hallucis, flexor digitorum brevis, abductor digiti minimi, quadratus plantae, lumbricals, flexor hallucis brevis, adductor hallucis oblique head, adductor hallucis transverse head, flexor digiti minimi, plantar/dorsal interossei. In addition to absolute MVs, relative MVs normalized to body mass (rMVBM) and the percentage of individual MVs relative to the sum of 10 individual PIFM MVs (%MVWHOLE) were calculated.Results: The absolute MVs of 6 individual PIFMs, including the flexor digitorum brevis and lumbricals, were +16% to 59% larger in dancers than in non-dancers (P <= .048). The rMVBM of all individual PIFMs were +35% to 95% larger in dancers than in non-dancers (P <= .019). The %MVWHOLE of the flexor digitorum brevis and lumbricals were +10% to 36% higher (P <= .014) and those of the abductor digiti minimi and adductor hallucis oblique head were +8% to 11% lower (P <= .037) in dancers than in non-dancers.Conclusions: For all 3 MV measures, only the flexor digitorum brevis and lumbricals, which are functionally specialized for flexion of the second to fifth metatarsophalangeal joints, were consistently larger in dancers than in non-dancers. This may be due to long-term repetitive loading on these PIFMs during ballet training involving tiptoe standing.