Intrinsic foot muscle size can be measured reliably in weight bearing using ultrasound imaging

Highlights

• Ultrasound image measurement of foot muscles is feasible in weight bearing.

• Measurements are highly reliable between operators and test sessions.

• Changes in morphology of 10–18% exceed measurement error.

• Consider participant characteristics when comparing between individuals.

Abstract

Background

The intrinsic foot muscles (IFMs) are important contributors to optimal foot function. While assessment of IFM morphology using ultrasound imaging in non-weight bearing has been established, this does not evaluate the foot in its primary functional position of weight bearing.

Research question

Is ultrasound imaging a reliable and clinically feasible method of measuring IFM morphology in weight bearing, do these measures differ to those from non-weight bearing and are they associated with participant characteristics?

Methods

Ultrasound images were obtained by a single rater from twenty-four healthy participants on two occasions, one week apart. Images were taken in weight bearing (bilateral stance) and non-weight bearing (seated). Cross-sectional area and thickness of the abductor hallucis muscle, and dorsoplantar thickness of the muscles of the first interstitium were measured from acquired images. A second rater also acquired images at the first session. Participant characteristics included age, height, weight, sex, foot posture and foot mobility.

Results

Measurements of IFM morphology demonstrated high reliability within and between test sessions, as well as between raters (ICCs > 0.8). Our findings suggest that changes of 10–18% could be considered to exceed measurement error. Larger IFM size was related to larger body size (taller, heavier), foot posture (longer foot, higher arch, wider midfoot) and male sex.

Significance

This study is the first to describe a reliable and clinically feasible method of measuring IFM morphology in weight bearing. These measurements could be used in future studies to assess IFM morphology in patient populations and to evaluate the effect of intervention. Body size and foot posture explained between 20 and 41% of the variance in measurements and should be considered when comparing IFM morphology between individuals. The establishment of reliable measurements in weight bearing provides a crucial step towards the future evaluation of IFM function using ultrasound imaging.

Introduction

The intrinsic muscles of the human foot play an integral role in foot stability, shock absorption and force transmission during functional activities [1,2]. In contrast to extrinsic muscles that originate in the leg, intrinsic foot muscles (IFMs) both originate and insert in the foot and have the anatomical configuration to provide active support the foot’s arches [3,4], particularly the medial longitudinal arch. In support, electromyography studies demonstrate that abductor hallucis, flexor digitorum brevis and quadratus plantae muscles contribute to control of foot posture during standing [5,6], and are active during stance phase of gait; actively lengthening during early stance and shortening during late stance [1].

Considering the importance of IFMs for optimal foot function, impairments of IFM structure and function have been implicated in lower limb pain and injury [7]. Cross-sectional studies demonstrate differences of IFMs between individuals with and without chronic plantar heel pain [8], hallux valgus [9], chronic ankle instability [10] and diabetes [11]. Thus, it is plausible that assessment and training of IFM function may contribute to effective management of individuals with lower limb disorders.

Measurement of IFM structure and activation is challenging, particularly in the clinical setting. Although several methods to measure strength have been described, these tests recruit both the intrinsic and extrinsic muscles of the foot [12]. Other methods such as computed tomography, magnetic resonance imaging, electromyography and muscle biopsy can differentiate extrinsic and intrinsic muscles, and identify individual IFMs [12]. However, the clinical utility of these methods is limited due to cost, accessibility and invasiveness. Ultrasonography is a non-invasive, widely available method to objectively measure muscle structure [13] and estimate activation [14], and can be applied to measurement of IFMs. Ultrasound measurements of IFM thickness and cross-sectional area (CSA) are reliable [[15], [16], [17], [18], [19], [20]] and are correlated with strength measures using toe dynamometry [20,21]. Unfortunately, imaging has predominantly been performed with the patient in non-weight bearing positions, which does not replicate foot function. Only one study has demonstrated reliability in a weight bearing position [16], but required use of customised equipment to enable access for measurement. Finally, the reliability of ultrasound measurements has not been reported for all IFMs; no studies have evaluated adductor hallucis - the second largest IFM and substantial mechanical advantage to control the arch [4].

This study aimed to: (i) develop a clinically feasible method to measure IFM morphology in weight bearing using ultrasound imaging; (ii) establish the reliability, measurement error and minimal detectable change for measurements in both weight bearing and non-weight bearing; and (iii) examine differences between measurements from weight bearing and non-weight bearing positions. A secondary aim was to explore the relationship between measures of IFM morphology and participant characteristics such as age, sex, body size, foot posture and mobility.