Ultrasound Measures of Intrinsic Foot Muscle Size and Activation Following Lateral Ankle Sprain and Chronic Ankle Instability

Context: Tibial nerve impairment and reduced plantarflexion, hallux flexion, and lesser toe flexion strength have been observed in individuals with recent lateral ankle sprain (LAS) and chronic ankle instability (CAI). Diminished plantar intrinsic foot muscles (IFMs) size and contraction are a likely consequence. Objectives: To assess the effects of ankle injury on IFM size at rest and during contraction in young adults with and without LAS and CAI. Setting: Laboratory. Design: Cross-sectional. Patients: A total of 22 healthy (13 females; age = 19.6 [0.9], body mass index [ BMI] = 22.5 [3.2]), 17 LAS (9 females; age = 21.8 [ 4.1], BMI = 24.1 [3.7]), 21 Copers (13 females; age = 20.8 [2.9], BMI = 23.7 [2.9]), and 20 CAI (15 females; age = 20.9 [4.7], BMI = 25.1 [4.5]). Main Outcome Measures: Foot Posture Index (FPI), Foot Mobility Magnitude (FMM), and ultrasonographic cross-sectional area of the abductor hallucis, flexor digitorum brevis, quadratus plantae, and flexor hallucis brevis were assessed at rest, and during nonresisted and resisted contraction. Results: Multiple linear regression analyses assessing group, sex, BMI, FPI, and FMM on resting and contracted IFM size found sex (B = 0.45; P <.001), BMI (B = 0.05; P = .01), FPI (B = 0.07; P = .05), and FMM x FPI interaction (B = - 0.04; P = .008) accounted for 19% of the variance (P = .002) in resting abductor hallucis measures. Sex (B = 0.42, P <.001) andBMI ( B = 0.03, P = .02) explained 24% of resting flexor digitorum brevis measures (P < .001). Having a recent LAS ( B = 0.06, P = .03) and FMM (B = 0.04, P = .02) predicted 11% of nonresisted quadratus plantae contraction measures (P = .04), with sex (P <.001) explaining 13% of resting quadratus plantae measures (B = 0.24, P = .02). Both sex (B = 0.35, P = .01) and FMM(B = 0.15, P =.03) predicted 16% of resting flexor hallucis brevis measures (P = .01). There were no other statistically significant findings. Conclusions: IFM resting ultrasound measures were primarily determined by sex, BMI, and foot phenotype and not injury status. Routine ultrasound imaging of the IFM following LAS and CAI cannot be recommended at this time but may be considered if neuromotor impairment is suspected.