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John J. Fraser and Jay Hertel

Context: Intrinsic foot muscle (IFM) exercises are utilized clinically in the treatment of foot and ankle conditions. However, the effectiveness of training on IFM motor function is unknown. Objective: To study the effects of a 4-week IFM exercise program on motor function, perceived difficulty, and IFM motor activation measured using ultrasound imaging (USI) during 3 IFM exercises. Design: Single-blinded randomized control trial. Setting: Laboratory. Participants: A total of 24 healthy, recreationally active young adults without history of ankle–foot injury who have never performed IFM exercises participated (12 males and 12 females; mean age = 21.5 [4.8] y; body mass index = 23.5 [2.9] kg/m2) Intervention: Following randomization, participants allocated to the intervention group received a 4-week progressive home IFM exercise program performed daily. Participants in the control group did not receive any intervention. Main Outcome Measures: Clinician-assessed motor performance (4-point scale: 0 = does not initiate movement and 3 = performs exercise in standard pattern), participant-perceived difficulty (5-point Likert scale: 1 = very easy and 5 = very difficult), and USI motor activation measures (contracted measurementresting measurement) of the abductor hallucis, flexor digitorum brevis, quadratus plantae, and flexor hallucis brevis were assessed during toe-spread-out, hallux-extension, and lesser-toe-extension exercises. Results: The intervention group demonstrated significant improvement in motor performance in the toe-spread-out exercise (pre = 1.9 [0.5], post = 2.6 [0.5], P = .008) and less perceived difficulty in the toe-spread-out (pre = 3.1 [1.3], post = 2.3 [1.2], P = .01), hallux-extension (pre = 3.2 [1.5], post = 2.0 [1.2], P = .005), and lesser-toe-extension (pre = 1.9 [0.7], post = 1.2 [0.4], P = .03) exercises. Both groups demonstrated increased USI motor activation in the abductor hallucis during the toe-spread-out exercise (intervention: pre = 1.07 [0.06], post = 1.11 [0.08] and control: pre = 1.08 [0.06], post = 1.11 [0.06]; P = .05). No other significant main effects or group by time interactions were observed. Conclusion: A 4-week IFM exercise intervention resulted in improved motor performance and decreased perceived difficulty when performing the exercises, but not changes in USI measures of IFM activation compared with a control group.

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John J. Fraser, Rachel Koldenhoven, and Jay Hertel

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 × 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) and BMI (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.