Ultrasonography can discriminate between intrinsic and extrinsic foot muscle properties and has therefore gained considerable popularity as an indirect strength evaluation. However, an overview on the use of ultrasound for assessing intrinsic foot musculature (IFM) is currently lacking. What is the current evidence regarding (1) 2D ultrasonography protocols and its reliability? (2) Reference values for cross-sectional area and dorso-plantar thickness evaluation in asymptomatic and symptomatic persons? The PRISMA guidelines were used to conduct this systematic review. Eight databases (PubMed, Embase, Web of Science, Cochrane Library, Scopus, CINAHL, SPORTDiscus and EuropePMC) were searched up to November 1, 2021. Studies reporting quantitative 2D ultrasound findings of the intrinsic foot muscles with no limitation to sex, BMI, ethnicity or physical activity were included. Studies were assessed for methodological quality using the Downs and Black checklist. Fifty-three studies were retained. Protocols showed an overall good to great reliability, suggesting limits of agreement between 8 and 30% of relative muscle size with minimal detectable changes varying from 0.10 to 0.29 cm² for cross-sectional area and 0.03–0.23 cm for thickness. Reference values are proposed for both cross-sectional area and thickness measurements of the abductor hallucis, flexor digitorum brevis, flexor hallucis brevis and quadratus plantae in asymptomatic persons. This could not be performed in the symptomatic studies due to a limited number of relevant studies addressing the symptomatic population, therefore a clinical overview is outlined. Clinically, IFM properties have been studied in ten distinct pathological conditions, predominantly pointing towards decreased muscle properties of the abductor hallucis. We provide a clear and comprehensive overview of the literature regarding 2D ultrasonography of the IFM, making the available evidence more accessible to decision makers and researchers.

Background: Quantitative ultrasound imaging is a popular technique to assess the structural properties of the intrinsic and extrinsic foot muscles. Although several studies examined test-retest reliability, specific gaps remain in assessing inter-rater reliability, particularly distinguishing between image acquisition and muscle measurement. Additionally, these studies utilized equipment that may not be generalizable across both clinical and research settings and often involved small sample sizes without prior sample size calculations. This study aimed to investigate test-retest reliability as well as global and measurement-based inter-rater reliability (MIRR) using a low-end ultrasound device to measure intrinsic and extrinsic foot muscle sizes. Methods: This prospective reliability study included 21 active individuals. Five intrinsic muscles [abductor hallucis (AbH), flexor digitorum brevis (FDB), flexor hallucis brevis (FHB), quadratus plantae (QP), abductor digiti minimi (AbDM)], and three extrinsic muscles [peroneal (PER), flexor digitorum longus, tibialis anterior (TA)] were scanned. Three investigators independently acquired images on two occasions and measured cross-sectional area (CSA) and thickness in September and October 2023. Participants were assessed either at the Musculoskeletal Research Group laboratory (University of Leuven, Bruges) or in the Rehabilitation Sciences laboratory (Ghent University hospital). Test-retest (same investigator, one week in between), global inter-rater (each investigator measures own image set) and MIRR (three investigators measure one image set) was performed following intra-class correlation, standard error of the measurement (SEM) and coefficient of variation. Results: Test-retest reliability showed intraclass-correlation coefficients of 0.60–0.88 for the FDB and 0.38–0.73 for the TA. SEM ranged from 0.16 to 0.41 cm² (CSA) and from 0.05 to 0.31 cm (thickness) for the intrinsic, while they ranged from 0.19 to 1.13 cm² and from 0.12 to 0.44 cm for the extrinsic muscles. Global inter-rater correlation coefficients varied between 0.4 and 0.8 for the AbH and FDB. Measurement based inter-rater correlation coefficient varied between 0.50 and 0.96 for AbH, FDB, TA and PER muscles. SEM ranged from 0.14 to 0.89 cm² (CSA) and from 0.07 to 0.24 cm (thickness) for the intrinsic, while they ranged from 0.29 to 0.85 cm² (CSA) and from 0.12 to 0.51 cm (thickness) for the extrinsic muscles. Coefficients of variations were between 4% and 34%. For test-retest, they were consistently ≤10% for AbH thickness, FDB CSA, FHB and TA. FDB coefficients of variation were ≤10% across all inter-rater reliabilities. Conclusions: Most muscles demonstrated moderate to excellent test-retest reliability using a portable ultrasound device, supporting its generalizability. However, the greater variability in global inter-rater reliability suggests substantial variation during image acquisition. The same clinician should perform pre-intervention and follow-up assessments to minimize errors. If different clinicians are involved, caution is needed when comparing measurements.

Background: Lateral ankle sprain (LAS) is prevalent among runners, with many developing chronic ankle instability (CAI). While CAI is associated with many motor-behavioral, sensory-perceptual, and pathomechanical factors, its impact on gait biomechanics remains unclear. This cross-sectional study aimed to assess gait biomechanics and other factors contributing to CAI in runners.Methods: Seventy participants (47 men and 23 women) were categorized as healthy (n = 24), acute LAS (n = 17), CAI (n = 16) and copers (n = 13). Walking and running spatiotemporal, kinetic and kinematic parameters were collected on an instrumented treadmill. Rehabilitation-oriented assessment outcomes were also assessed. One-way ANOVA or Kruskal–Wallis tests were used, along with their corresponding post-hoc tests. Effect sizes (g or r according to normality) were reported.Results: Runners with CAI and acute LAS reported significantly greater perceived instability (r = 0.68–0.86) and worse self-reported function (r = 0.47–0.67) than healthy controls and copers. However, running biomechanics did not differ between groups, suggesting that traditional biomechanical assessments at comfortable speeds may not be sensitive to functional deficits in CAI. A notable finding was the lower mechanical work recovery during walking in copers compared to healthy controls (g = 0.98).Conclusion: These results highlight the importance of considering self-reported function and perceived instability when assessing LAS and CAI. The absence of gross running gait alterations suggests that rehabilitation could safely integrate running early in recovery. However, more demanding tasks or advanced biomechanical modeling techniques may be needed to identify residual gait impairments.

Introduction: Lateral ankle sprains commonly occur during directional changes and often lead to chronic ankle instability (CAI) associated with altered movement patterns, muscle weakness and atrophy. This study aimed to compare foot joint biomechanics during cutting among individuals with CAI, copers, and uninjured controls and to explore the relationship between intrinsic foot muscle sizes and functional status. Methods: Cross‐sectional study. Sixty‐nine recreationally active males (25.4 ± 4 years) were categorised by perceived instability and self‐reported function. Ankle and foot joint kinematics and kinetics were assessed using a multi‐segment foot model during 45° side‐step cutting at a mean approach speed of 3.5 m·s ⁻¹ . Groups were compared using ANOVA or Kruskal–Wallis tests, and continuous biomechanical data were analysed using statistical parametric or non‐parametric mapping. Cross‐sectional area and thickness of the abductor hallucis and flexor digitorum brevis were recorded using ultrasound imaging. Muscle sizes were explored using general linear models, and correlations with questionnaire scores and physical examination outcomes were calculated. Results: Foot joint kinematics and kinetics did not significantly differ between the groups. Individuals with CAI exhibited lower peak posterior ground reaction force than controls (mean difference: −6.2% of body weight, 95% CI: −11.9 to −0.4, p = 0.03). Statistical parametric mapping revealed lower posterior ground reaction forces from 22% to 46% of stance in CAI versus controls ( p < 0.05). Intrinsic foot muscle sizes showed weak, statistically significant negative correlations with perceived instability scores ( ρ = −0.31 to −0.36) and self‐reported function scores ( ρ = −0.26 to −0.33). Conclusion: Despite pronounced sensory‐perceptual impairments, individuals with CAI demonstrated few differences in foot joint biomechanics compared with copers and controls during a moderate‐speed anticipated cutting task. Reduced braking forces may reflect altered deceleration strategies. These findings suggest the importance of assessing deceleration abilities in people with CAI and indicate a potential role of foot muscles in CAI‐related functional outcomes. Trial Registration: ClinicalTrials.gov (NCT05815576)