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This study aimed to develop an insertion technique for intramuscular EMG recording of the oblique head of adductor hallucis (AddH) and first dorsal interosseous (FDI) muscles in humans via the dorsum of the foot, and report feasibility of intramuscular EMG data acquisition during walking in shoes. In eight individuals without musculoskeletal pain or injury (5 males; 32 ± 8 years), intramuscular electrodes were inserted into AddH (oblique head) and FDI through the right foot's dorsum (between metatarsals I-II) with ultrasound guidance. The ultrasound transducer was positioned on the plantar surface. Intramuscular EMG was also recorded from abductor hallucis, tibialis posterior, flexor digitorum longus and peroneus longus. Participants performed six overground walking trials wearing modified shoes, and rated pain associated with the intramuscular electrodes during walking (numerical rating scale, 0-10). High-quality EMG recordings were obtained from intrinsic and extrinsic foot muscles. Analyses of power spectral densities indicated that movement artefacts commonly observed during gait were removed by filtering. Pain associated with AddH/FDI electrodes during walking was low (median[IQR] 1[2]; range 0-4) and similar to other sites. Findings demonstrate that intramuscular EMG recording from AddH (oblique head) and FDI using this insertion technique is feasible and associated with minimal pain when walking in shoes., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)

This study investigated the local, remote, and contralateral effects of a four-week intrinsic foot muscle exercise intervention in recreationally active participants on foot parameters, flexibility, and performance of the posterior chain (PC). Twenty-eight healthy participants (12f, 16m) were randomly assigned to a control group or performed 2 × 6 min of foot exercises twice daily unilaterally at least five days/week for four weeks. At baseline (M1), after the intervention (M2), and after a four-week wash-out period (M3), we assessed bilateral Foot Posture Index-6, medial longitudinal arch mobility, single-leg stance balance, range of motion (ROM) (first metatarsophalangeal joint and ankle), and flexibility and performance of the PC. The FPI-6 score changes over time differed significantly between groups for both legs, improving by 26% in the trained- (p < .001) and 11% in the untrained leg (p = .02) in the intervention group from M1 to M2. Improvements were maintained at M3 for the trained leg (p = .02). Ankle range of motion and balance of the trained leg improved from M1 to M2, yet only became significant at M3 (ROM: p = .02; balance: p = .007). The other parameters did not change significantly. A four-week foot exercise intervention might have local but no remote effects in healthy young adults.German Clinical Trial Register (DRKS00027923) (24/08/2022)., (© 2024. The Author(s).)

Background: Intrinsic foot muscles are known to support the medial longitudinal arch (MLA) and stabilize the foot, and they are activated with weight bearing and increased postural demand. Various types of intrinsic foot muscle training have been reported, but one of the most useful of these, the short foot exercise, is challenging to perform effectively and requires practice, making it difficult to implement in ordinary clinical settings., Research Question: What are the differences in abductor hallucis longus (ABH) muscle activity and MLA angle during intrinsic foot muscle exercises that employ weight bearing and balancing conditions when they are performed with minimal practice?, Methods: Sixteen healthy volunteers performed nine different intrinsic foot muscle exercises, practiced once or twice. The exercises consisted of toe curl, short foot without pushing, short foot with pushing and toe spread exercises in sitting and standing positions, and single leg swing in a standing position. Each exercise was performed three times for five seconds. The activities of the ABH muscles were measured using surface electromyographic (EMG) sensors and the MLA angles during the exercises were captured using an optical motion tracking system. The integrals of the ABH EMG signals were calculated., Results: Differences in the integral and maximum of the ABH EMG signal were found between the exercises (p < 0.001). Post-hoc pair-wise analysis revealed that the EMG activity was larger during the swing exercise than in exercises other than toe spread, both in sitting and standing positions, and short foot exercise with pushing while standing. The minimum MLA angle during each exercise was smaller for the toe spread exercise in a sitting position than other exercises (p < 0.023)., Significance: A single leg swing exercise may be effective for self-exercise of intrinsic foot muscles, particularly when intensive supervised physiotherapy is not possible., Competing Interests: Declaration of Competing Interest None., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

Background: Training the plantar intrinsic foot muscles (PIFMs) has the potential to benefit patients with lower extremity musculoskeletal conditions as well as the aged population. Isolated foot exercises, often standard in clinical practice, are difficult to perform, whereas functional exercises are much easier to accomplish. However, it is unclear whether functional exercises are comparable to isolated foot exercises in activating the PIFMs., Objective: This study aims to compare the activation of PIFMs between functional exercises versus isolated foot exercises., Methods: Using surface electromyography (EMG), muscle activation of three PIFMs was measured in four functional exercises (i.e. normal/unstable toe stance, toe walking, and hopping) versus a muscle-specific isolated foot exercise in 29 younger adults, resulting in 12 comparisons., Results: Functional exercises showed larger mean EMG amplitudes than the isolated foot exercises in 25% of the 12 comparisons, while there was no difference in the remaining 75%., Conclusion: Functional exercises provoked comparable or even more activation of the PIFMs than isolated foot exercises. Given that functional exercises are easier to perform, this finding indicates the need to further investigate the effectiveness of functional exercises in physical therapy to improve muscle function and functional task performance in populations that suffer from PIFM weakness or dysfunction.

There is growing evidence to support a role for the abductor hallucis (AH) in standing balance control; however, functional properties of the muscle that may provide more insight into AH's specific contribution to upright posture have yet to be characterized. This study was conducted to quantify functional neuromechanical properties of the AH and correlate the measures with standing balance variables. We quantified strength and voluntary activation during maximal voluntary isometric contractions of the great toe abductor in nine (3 females and 6 males) healthy, young participants. During electrically evoked twitch and tetanic contractions, we measured great toe abduction peak force and constructed a force-frequency curve. We also evaluated peak abduction force, contraction time (CT), half-relaxation time (HRT), rate of force development (RFD), and relaxation rate (RR) from twitch contractions evoked using doublet stimuli. Strength, VA, CT, HRT, RFD, and RR were correlated to centre of pressure standard deviation (COP SD) and velocity (COP VEL) variables of the traditional COP trace and its rambling and trembling components during single-legged stance. AH twitch properties (e.g., CT: 169.8 ± 32.3 ms; HRT: 124.1 ± 29.2 ms) and force-frequency curve were similar to other slow contractile muscles. Contractile speed related negatively with COP VEL, suggesting AH may be appropriate for slow, prolonged tasks such as ongoing postural balance control. Correlation coefficient outcomes for all variables were similar between rambling and trembling components. Our results provide further evidence for the importance of AH neuromechanical function for standing balance control, at least during a challenging single-legged posture., Competing Interests: The authors declare there are no competing interests.

Background: Intrinsic foot muscle exercises are used in clinical and sports practice to improve performance. Force generation during toe flexion is greater in the standing posture than in the sitting posture; nonetheless, the mechanism underlying the activation of intrinsic foot muscles during force generation and whether there exists a difference between these two postures still remain unclear., Research Question: Are the activities of intrinsic foot muscles affected by standing and sitting postures during gradual force generation?, Methods: Seventeen men participated in the laboratory based cross-sectional study. Each participant performed a force ramp-up toe flexion task from 0% to 80% of the maximal toe flexor strength (MTFS) in sitting and standing postures. High-density surface electromyography signals obtained during the task were determined by calculating the root mean square (RMS). Additionally, modified entropy and coefficient of variation (CoV) were calculated at 20-80 % MTFS for each 10 % MTFS., Results: The RMS between the two postures indicated an interaction effect (p < 0.01). Post-hoc analyses revealed that intrinsic foot muscle activity during the ramp-up task was significantly higher in the standing posture than in the sitting posture at 60 % MTFS (67.53 ± 15.91 vs 54.64 ± 19.28 % maximal voluntary contraction [MVC], p = 0.03), 70 % MTFS (78.11 ± 12.93 vs 63.28 ± 18.65 % MVC, p = 0.01), and 80 % MTFS (81.78 ± 14.07 vs 66.90 ± 20.32 % MVC, p = 0.02). In the standing posture, the modified entropy at 80 % MTFS was lower than that at 20 % MTFS (p = 0.03), and the CoV at 80 % MTFS was higher than that at 20 % MTFS (p = 0.03)., Significance: These results indicated that posture selection is important for high-intensity exercises of the intrinsic foot muscles, such as resistance training. Thus, improving performance related to toe flexor strength might be more effective when conducted under adequate weight-bearing situations, such as in the standing posture., Competing Interests: Declaration of interest The authors declare no conflicts of interest related to this work., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Background: The characteristics of foot structure in adolescents and adults are different, affecting sports performance and leading to the progression of foot and lower extremity disorders., Research Question: This study aimed to investigate the relationship between the intrinsic foot muscles (IFM) and plantar fascia morphology and the repetitive rebound jumping and jump landing ability in adolescent athletes., Methods: A total of 60 adolescent athletes (35 boys and 25 girls) participated in this study. B-mode ultrasonography was used to obtain images of the IFM and plantar fascia morphology [thickness and cross-sectional area (CSA) of the abductor hallucis (AbH), flexor hallucis brevis (FHB), flexor digitorum brevis (FDB), and thickness of the plantar fascia]. The repetitive rebound jump performance was evaluated using the Optojump™ system. Participants were instructed to jump five times continuously with one leg, jumping as high as possible with minimal ground contact time. The jump landing was assessed by measuring the dynamic posture stability index (DPSI) using forward one-legged jump landings., Results: The thickness and CSA of the AbH and FDB were positively correlated with the jump height and reactive jump index. The DPSI score was significantly correlated with the thickness of the AbH, but not with other IFMs or plantar fascia. In the multiple regression analysis, only the thickness of the FDB was associated with the jump height and reactive jump index, indicating that FDB thickness might facilitate adolescent athletes to jump higher with minimal contact time in repetitive rebounding movements., Significance: The IFM (especially FDB) should be focused on when examining sports performance in adolescent athletes., Competing Interests: Declaration of Competing Interest None., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Context: Mounting evidence suggests neuromuscular electrical stimulation (NMES) as a promising modality for enhancing lower limb muscle strength, yet the functional effects of a single electrical stimulation session for improving the function of the intrinsic foot muscles (IFM) has not been evaluated., Objective: To investigate the immediate effects of an NMES session compared with a sham stimulation session on foot force production, foot dome stability, and dynamic postural control in participants with static foot pronation., Design: Randomized controlled clinical trial., Setting: Laboratory., Patients or Other Participants: A total of 46 participants (23 males, 23 females) with static foot pronation according to their Foot Posture Index (score ≥ 6) were randomly assigned to an NMES (n = 23) or control (n = 23) group., Intervention(s): The NMES group received a single 15-minute NMES session on the dominant foot across the IFM. The control group received a 15-minute sham electrical stimulation session., Main Outcome Measure(s): All outcome measurements were assessed before and after the intervention and consisted of foot force production on a pressure platform, foot dome stability, and dynamic postural control. Statistical analysis was based on the responsiveness of the outcome measures and responder analysis using the minimum detectable change scores for each outcome measure., Results: In the NMES group, 78% of participants were classified as responders for at least 2 of the 3 outcomes, compared with only 22% in the control group. The relative risk of being a responder in the NMES group compared with the control group was 3.6 (95% CI = 1.6, 8.1]. Interestingly, we found that all participants who concomitantly responded to foot strength and navicular drop (n = 8) were also responders in dynamic postural control., Conclusions: Compared with a sham stimulation session, a single NMES session was effective in immediately improving foot function and dynamic postural control in participants with static foot pronation. These findings support the role of NMES for improving IFM function in this population., (© by the National Athletic Trainers' Association, Inc.)

To preserve motion, humans must adopt actuator-like dynamics to replace energy that is dissipated during contact with damped surfaces. Our ankle plantar flexors are credited as the primary source of work generation. Our feet and their intrinsic foot muscles also appear to be an important source of generative work, but their contributions to restoring energy to the body remain unclear. Here, we test the hypothesis that our feet help to replace work dissipated by a damped surface through controlled activation of the intrinsic foot muscles. We used custom-built platforms to provide both elastic and damped surfaces and asked participants to perform a bilateral hopping protocol on each. We recorded foot motion and ground reaction forces, alongside muscle activation, using intramuscular electromyography from flexor digitorum brevis, abductor hallucis, soleus, and tibialis anterior. Hopping in the Damped condition resulted in significantly greater positive work and contact-phase muscle activation compared with the Elastic condition. The foot contributed 25% of the positive work performed about the ankle, highlighting the importance of the foot when humans adapt to different surfaces. NEW & NOTEWORTHY Adaptable foot mechanics play an important role in how we adjust to elastic surfaces. However, natural substrates are rarely perfectly elastic and dissipate energy. Here, we highlight the important role of the foot and intrinsic foot muscles in contributing to replacing dissipated work on damped surfaces and uncover an important energy-saving mechanism that may be exploited by the designers of footwear and other wearable devices.

Magnetic resonance imaging (MRI) can be used to quantify intrinsic foot muscle morphology and composition. Due to the high spatial resolution required to adequately capture the architecturally complex anatomy, manual segmentation is time consuming and not clinically feasible. The aim of this study was to evaluate if a reduced number of MRI slices can be used to accurately estimate intrinsic foot muscle volume and composition. A three-dimensional 2-point Dixon sequence of the whole foot was acquired at 7-Tesla for thirteen asymptomatic individuals and twenty individuals with plantar heel pain. Slice intervals of 2, 3, 5, 10, 15 and 30 were used to calculate alternative muscle volume and composition, and were compared to reference values calculated from every available slice. Agreement between methods was assessed by calculating mean differences and 95% limits of agreement, and inspection of Bland -Altman plots. In both groups, slice intervals of 2, 3 and 5 provided excellent precision for all muscles (measurement error < 1%). Larger slice intervals of 10, 15 and 30 provided excellent precision for some muscles, but for other muscles (e.g. small forefoot muscles), error was up to 7.3%. Bland-Altman plots showed no systematic measurement bias. This study provides a quantitative basis for selecting a reduced number of slices to measure intrinsic foot muscle volume and composition from MRI. A slice interval of 10 may provide a balance between efficiency (36 mins vs. 6 h) and accuracy (error < 2.4%) across all intrinsic foot muscles in asymptomatic individuals and those with plantar heel pain., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

The central nervous system utilizes tendon compliance of the intrinsic foot muscles to aid the foot's arch spring, storing and returning energy in its tendinous tissues. Recently, the intrinsic foot muscles have been shown to adapt their energetic contributions during a variety of locomotor tasks to fulfil centre of mass work demands. However, the mechanism by which the small intrinsic foot muscles are able to make versatile energetic contributions remains unknown. Therefore, we examined the muscle-tendon dynamics of the flexor digitorum brevis during stepping, jumping and landing tasks to see whether the central nervous system regulates muscle activation magnitude and timing to enable energy storage and return to enhance energetic contributions. In step-ups and jumps, energy was stored in the tendinous tissue during arch compression; during arch recoil, the fascicles shortened at a slower rate than the tendinous tissues while the foot generated energy. In step-downs and landings, the tendinous tissues elongated more and at greater rates than the fascicles during arch compression while the foot absorbed energy. These results indicate that the central nervous system utilizes arch compression to store elastic energy in the tendinous tissues of the intrinsic foot muscles to add or remove mechanical energy when the body accelerates or decelerates. This study provides evidence for an adaptive mechanism to enable the foot's energetic versatility and further indicates the value of tendon compliance in distal lower limb muscle-tendon units in locomotion., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2022. Published by The Company of Biologists Ltd.)

Background: Weakness and disuse of intrinsic foot muscles contributes to dysfunction in foot and toe alignment and sensory input, which may lead to instability and falls in older adults. The aim of this systematic review was to report the effects of intrinsic foot muscle strengthening (IFMS) interventions on functional mobility in adults aged ≥65 years., Methods: A systematic review was performed with searches from December 2019-February 2021 using MEDLINE, CINAHL, SPORTDiscus, Rehab and Sports Medicine Source, Cochrane Database of Systematic Reviews, and Cochrane Central Register of Controlled Trials. Additional sources were sought using reference scanning. Eligible sources analyzed adults aged ≥65 years (n = 1674) who were ambulatory, used a functional mobility outcome measure, and contained foot and ankle interventions that included IFMS. Literature studies regarding neurological, vestibular, cognitive, amputation, or post-surgical conditions were excluded. Studies that did not specify intrinsic foot muscle involvement were excluded. Two authors extracted relevant studies and appraised them using the Physiotherapy Evidence Database (PEDro) scale., Results: A total of 1420 articles were screened for relevance, and 16 were extracted. Five additional sources were obtained through reference scanning. Nine articles were eligible for review. PEDro scores ranged from 3 to 7 (out of 10), indicating "fair" quality of evidence. Heterogeneity of methods and data did not allow for statistical comparison. Themes extracted from sources were types of intrinsic foot strengthening interventions and parameters; outcomes on falls, balance, functional mobility; and subjective reports regarding functional mobility., Conclusion: Evidence reviewed was of fair quality. IFMS interventions contributed to improvements in strength, balance, mobility, and possibly reduced fall risk. There was little effect on gait. Subjective reports indicate a possible mechanism for improved mobility may be from increased proprioception and sensation., (© 2021 The American Geriatrics Society.)

M. abductor hallucis (AbH) is the strongest intrinsic foot muscle and its dysfunction underlies various foot disorders. Attempts to strengthen the muscle by voluntary exercises are constrained by its complex morphology and oblique mechanical action, which leads to an inability even in asymptomatic individuals to fully activate AbH. This study investigated the extent and magnitude of this inability whilst also providing preliminary evidence for the virtue of targeted sub-maximum neuromuscular electrical stimulation (NMES) as a countermeasure for an AbH activation deficit. The voluntary activation ratio (VAR) was assessed via the twitch interpolation technique in the left AbH of 13 healthy participants during maximum voluntary 1st metatarsophalangeal joint flexion-abduction contractions (MVC). Participants were grouped ("able" or "unable") based on their ability to fully activate AbH (VAR ≥ 0.9). 7 s-NMES trains (20 Hz) were then delivered to AbH with current intensity increasing from 150% to 300% motor threshold (MT) in 25% increments. Perceived comfort was recorded (10 cm-visual analogue scale; VAS). Only 3 participants were able to activate AbH to its full capacity (able, mean (range) VAR: 0.93 (0.91-0.95), n = 3; unable: 0.69 (0.36-0.83), n = 10). However, the maximum absolute forces produced during the graded sub-maximum direct-muscle NMES protocol were comparable between groups implying that the peripheral contractility of AbH is intact irrespective of the inability of individuals to voluntary activate AbH to its full capacity. These findings demonstrate that direct-muscle NMES overcomes the prevailing inability for high voluntary AbH activation and therefore offers the potential to strengthen the healthy foot and restore function in the pathological foot., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

The foot-sole cutaneous receptors (section 2), their function in stance control (sway minimisation, exploratory role) (2.1), and the modulation of their effects by gait pattern and intended behaviour (2.2) are reviewed. Experimental manipulations (anaesthesia, temperature) (2.3 and 2.4) have shown that information from foot sole has widespread influence on balance. Foot-sole stimulation (2.5) appears to be a promising approach for rehabilitation. Proprioceptive information (3) has a pre-eminent role in balance and gait. Reflex responses to balance perturbations are produced by both leg and foot muscle stretch (3.1) and show complex interactions with skin input at both spinal and supra-spinal levels (3.2), where sensory feedback is modulated by posture, locomotion and vision. Other muscles, notably of neck and trunk, contribute to kinaesthesia and sense of orientation in space (3.3). The effects of age-related decline of afferent input are variable under different foot-contact and visual conditions (3.4). Muscle force diminishes with age and sarcopenia, affecting intrinsic foot muscles relaying relevant feedback (3.5). In neuropathy (4), reduction in cutaneous sensation accompanies the diminished density of viable receptors (4.1). Loss of foot-sole input goes along with large-fibre dysfunction in intrinsic foot muscles. Diabetic patients have an elevated risk of falling, and vision and vestibular compensation strategies may be inadequate (4.2). From Charcot-Marie-Tooth 1A disease (4.3) we have become aware of the role of spindle group II fibres and of the anatomical feet conditions in balance control. Lastly (5) we touch on the effects of nerve stimulation onto cortical and spinal excitability, which may participate in plasticity processes, and on exercise interventions to reduce the impact of neuropathy., (© 2021 Peripheral Nerve Society.)

Background: The intrinsic foot muscles are vital for maintaining normal foot function. This study was conducted to systematically review the literature on the functional capacity and morphological characteristics of the intrinsic foot muscles in subjects with Hallux Valgus (HV) deformity., Methods: A search was carried out in all available electronic databases, including Pub Med, Scopus, Embase and Web of Science, for identifying any relevant studies published from 1990 to October 2018., Results: Three studies had investigated intrinsic foot muscle size using ultrasound imaging; two had reported electromyography parameters and four had measured the muscle force capacity. The results of the present review suggested that the functional capacity and morphological characteristics of intrinsic foot muscles are different in subjects with HV compared to those without this deformity., Conclusion: This review found scientific evidence on muscle performance impairment in the abductor hallucis and flexor hallucis brevis in subjects with HV deformity., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Background: Foot muscle weakness can produce foot deformity, pain and disability. Toe flexor and foot arch exercises focused on intrinsic foot muscle strength and functional control may mitigate the progression of foot deformity and disability. Ensuring correct exercise technique is challenging due to the specificity of muscle activation required to complete some foot exercises. Biofeedback has been used to improve adherence, muscle activity and movement patterns. We investigated the feasibility of using a novel medical device, known as "Archercise", to provide real-time biofeedback of correct arch movement via pressure change in an inflatable bladder, and foot location adherence via sensors embedded in a footplate during four-foot exercises., Methods: Thirty adults (63% female, aged 23-68 years) performed four-foot exercises twice on the Archercise sensor footplate alone and then with biofeedback. One-way repeated measures ANOVA with pairwise comparisons were computed to assess the consistency of the exercise protocol between trial 1 and trial 2 (prior to biofeedback), and the effectiveness of the Archercise biofeedback device between trial 2 and trial 3 (with biofeedback). Outcome measures were: Arch movement exercises of arch elevation and lowering speed, controlled arch elevation, controlled arch lowering, endurance of arch elevation; Foot location adherence was determined by percentage of time the great toe, fifth toe and heel contacted footplate sensors during testing and were analysed with paired sample t-tests. Participant survey comments on the use of Archercise with biofeedback were reported thematically., Results: Seventeen (89%) arch movement and foot location variables were collected consistently with Archercise during the foot exercises. Archercise with biofeedback improved foot location adherence for all exercises (p = 0.003-0.008), coefficient of determination for controlled arch elevation (p < 0.0001) and endurance area ratio (p = 0.001). Twenty-nine (97%) participants reported Archercise with biofeedback, helped correct exercise performance., Conclusions: Archercise is a feasible biofeedback device to assist healthy participants without foot pathologies perform foot doming exercises., Trial Registration: Australian New Zealand Clinical Trials Registry (ANZCTR): 12616001559404. Registered 11 November 2016, http://www.ANZCTR.org.au/ACTRN12616001559404p.aspx.

Running is an activity with a consistently high injury rate. Running footwear design that mimics barefoot running has been proposed to reduce injury rate by increasing the strength of foot structures. However, there is little evidence to support this. The purpose of the current study is to use shear wave ultrasound elastography to examine material properties (shear modulus) of intrinsic foot structures in experienced minimally and traditionally shod runners. It is hypothesized that minimalist runners will exhibit increased stiffness compared to controls demonstrating the strengthening of these structures. Eighteen healthy runners (8 minimalist and 10 traditionalist), running a minimum of 10 mi · wk -1 , participated. Elastography scans were performed on the left foot of each participant. There is no apparent stiffening of foot structures associated with wearing minimalist shoes. Only the FHB tendon is different between shoe types and, contrary to the hypothesis, was stiffer in traditionalist compared to minimalist runners (257.26 ± 51.64 kPa vs 160.88 ± 27.79 kPa, respectively). A moderate positive (r = 0.7) relationship between training load and tendon stiffness suggests strengthening of tendon when running in traditional shoes. If running in minimalist shoes increases loading on these structures without resulting in stronger tissues, it is possible that minimalist footwear may increase injury risk.

Our current knowledge on the neurophysiological properties of intrinsic foot muscles is limited, especially at high forces. This study therefore aimed to investigate the discharge characteristics of single motor units in an intrinsic foot muscle, namely flexor hallucis brevis, during voluntary contractions up to 100% of maximal voluntary contraction. We measured the recruitment threshold and discharge rate of flexor hallucis brevis motor units using indwelling fine-wire electrodes. Ten participants followed a target ramp up to maximal voluntary contraction by applying a metatarso-phalangeal flexion torque. We observed motor unit recruitment thresholds across a wide range of isometric forces (ranging from 10 to 98% of maximal voluntary contraction) as well as across a wide range of discharge rates (ranging from 4.8 to 23.3 Hz for initial discharge rate and 9.5 to 34.2 Hz for peak discharge rate). We further observed patterns of high variability in recruitment threshold and discharge rate as well as crossover in discharge rate between motor units within the same participant. These findings suggest that the force output of a muscle is generated through a mechanism with substantial variability rather than relying on a rigid organization, which is in contrast to the proposed onion-skin theory. The demands placed on the plantar intrinsic foot muscles during high- and low-force tasks may explain these observed neurophysiological properties. NEW & NOTEWORTHY We recorded for the first time single motor unit action potential trains in the flexor hallucis brevis, a short toe muscle, over the full range of maximum voluntary contraction. Its motor units are recruited up to very high (98%) recruitment thresholds with a substantial range of discharge rates. We further show high variability with crossover of discharge rates as a function of recruitment threshold both between participants and between motor units within participants.

The purpose of this study was to investigate the effect of thickness, cross-sectional area and stiffness of intrinsic foot muscles on performance in single-leg stance balance tasks in healthy sedentary young females. This study included a total of 40 healthy sedentary young females between the ages of 19 and 35 years. Single-leg stance balance assessments were carried out using Biodex Balance Systems (Biodex Medical Systems, Shirley, NY, USA). Performance in the single-leg stance balance tests was assessed using the overall stability index (OSI), mediolateral stability index (MLSI) and the anteroposterior stability index (APSI). Lower scores indicated better postural stability. Stiffness, thickness and cross-sectional area measurements of the abductor hallucis (AbH), flexor digitorum brevis (FDB) and flexor hallucis brevis (FHB) muscles were performed using an ultrasonography device. Larger AbH and FHB muscles were correlated with higher OSI, APSI, and MLSI (r = 0.31-0.46, p < 0.05), whereas larger FDB muscle was correlated with higher OSI and MLSI (r = 0.28-0.38, p < 0.05). Higher stiffness of the AbH and FHB muscles were correlated with lower OSI, APSI, and MLSI (r = -0.32 to 0.58, p < 0.05), but stiffness of the FDB muscle was not significantly correlated with OSI, APSI, and MLSI (r = 0.03-0.22, p ˃ 0.05). These results suggest that larger AbH, FDB and FHB muscles are related to reduced performance in single-leg stance balance tests, whereas higher AbH and FHB stiffness are related to better performance in single-leg stance balance tests in healthy sedentary young females., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Background: Evaluating the strength of the small muscles of the foot may be useful in a variety of clinical applications but is challenging from a methodology standpoint. Previous efforts have focused primarily on the functional movement of toe flexion, but clear methodology guidelines are lacking. A novel foot doming test has also been proposed, but not fully evaluated. The purposes of the present study were to assess the repeatability and comparability of several functional foot strength assessment techniques., Methods: Forty healthy volunteers were evaluated across two testing days, with a two-week doming motion practice period between them. Seven different measurements were taken using a custom toe flexion dynamometer (seated), custom doming dynamometer (standing), and a pressure mat (standing). Measurements from the doming dynamometer were evaluated for reliability (ICCs) and a learning effect (paired t-tests), while measurements from the toe flexion dynamometer and pressure mat were evaluated for reliability and comparability (correlations). Electromyography was also used to descriptively assess the extent of muscle isolation in all measurements., Results: Doming showed excellent within-session reliability (ICCs > 0.944), but a clear learning effect was present, with strength (p < 0.001) and muscle activity increasing between sessions. Both intrinsic and extrinsic muscles were engaged during this test. All toe flexion tests also showed excellent reliability (ICCs > 0.945). Seated toe flexion tests using the dynamometer were moderately correlated to standing toe flexion tests on a pressure mat (r > 0.54); however, there were some differences in muscle activity. The former may better isolate the toe flexors, while the latter appeared to be more functional for many pathologies. On the pressure mat, reciprocal motion appeared to display slightly greater forces and reliability than isolated toe flexion., Conclusions: This study further refines potential methodology for foot strength testing. These devices and protocols can be duplicated in the clinic to evaluate and monitor rehabilitation progress in clinical populations associated with foot muscle weakness.

Background: The use of surface EMG (sEMG) to record muscle activity is common place yet due to restrictions in technology studies on the intrinsic foot muscles have been limited or only fine wire instruments have been used., Aim: This paper looks at the potential reliability of a sEMG protocol for assessing the intrinsic foot muscles., Methods: Six intrinsic muscles were defined using ultrasound and muscle function testing. A protocol for sensor placement was created with repeatability and reliability testing of the protocol conducted by three separate testers on three subjects over two different time frames. Inter tester and Inter session repeatability and reliability was measured with ICC and percentage standard error of measurement., Results: Although there was good correlation between Extensor Digitorum Brevis, Dorsal Interossei, Abductor Digiti Minimi and Flexor Digitorum Brevis there was increased variability and poor correlation for Flexor hallucis Brevis and Abductor Hallucis. The percentage standard error of measurement did not support the high ICC values indicating a lower precision of measurement., Significance: Variability between testers and sessions shows an inconsistent reliability of sEMG and further work is required with protocols focussing on grouping muscles to improve the understanding of the intrinsic foot muscles., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The foot has been considered both as an elastic mechanism that increases the efficiency of locomotion by recycling energy, as well as an energy sink that helps stabilize movement by dissipating energy through contact with the ground. We measured the activity of two intrinsic foot muscles, flexor digitorum brevis (FDB) and abductor hallucis (AH), as well as the mechanical work performed by the foot as a whole and at a modelled plantar muscle-tendon unit (MTU) to test whether these passive mechanics are actively controlled during stepping. We found that the underlying passive visco-elasticity of the foot is modulated by the muscles of the foot, facilitating both dissipation and generation of energy depending on the mechanical requirements at the centre of mass (COM). Compared to level ground stepping, the foot dissipated and generated an additional -0.2 J kg -1 and 0.10 J kg -1 (both p < 0.001) when stepping down and up a 26 cm step respectively, corresponding to 21% and 10% of the additional net work performed by the leg on the COM. Of this compensation at the foot, the plantar MTU performed 30% and 89% of the work for step-downs and step-ups, respectively. This work occurred early in stance and late in stance for stepping down respectively, when the activation levels of FDB and AH were increased between 69 and 410% compared to level steps (all p < 0.001). These findings suggest that the energetic function of the foot is actively modulated by the intrinsic foot muscles and may play a significant role in movements requiring large changes in net energy such as stepping on stairs or inclines, accelerating, decelerating and jumping.

Human feet have evolved to facilitate bipedal locomotion, losing an opposable digit that grasped branches in favor of a longitudinal arch (LA) that stiffens the foot and aids bipedal gait. Passive elastic structures are credited with supporting the LA, but recent evidence suggests that plantar intrinsic muscles (PIMs) within the foot actively contribute to foot stiffness. To test the functional significance of the PIMs, we compared foot and lower limb mechanics with and without a tibial nerve block that prevented contraction of these muscles. Comparisons were made during controlled limb loading, walking, and running in healthy humans. An inability to activate the PIMs caused slightly greater compression of the LA when controlled loads were applied to the lower limb by a linear actuator. However, when greater loads were experienced during ground contact in walking and running, the stiffness of the LA was not altered by the block, indicating that the PIMs' contribution to LA stiffness is minimal, probably because of their small size. With the PIMs blocked, the distal joints of the foot could not be stiffened sufficiently to provide normal push-off against the ground during late stance. This led to an increase in stride rate and compensatory power generated by the hip musculature, but no increase in the metabolic cost of transport. The results reveal that the PIMs have a minimal effect on the stiffness of the LA when absorbing high loads, but help stiffen the distal foot to aid push-off against the ground when walking or running bipedally., Competing Interests: The authors declare no conflict of interest., (Copyright © 2019 the Author(s). Published by PNAS.)

Background: Weakness of the intrinsic foot muscles is thought to produce deformity, disability and pain. Assessing intrinsic foot muscles in isolation is a challenge; however ultrasound might provide a solution. The aims of this study were to assess the reproducibility of assessing the size of abductor halluces (AbH) and the medial belly of flexor hallucis brevis (FHBM) muscles, and identify their relationship with toe strength, foot morphology and balance., Methods: Twenty one participants aged 26-64 years were measured on two occasions for muscle cross-sectional area using a Siemens Acuson X300 Ultrasound System with 5-13 MHz linear array transducer. Great toe flexor strength was measured by pedobarography, the paper grip test and hand-held dynamometry. Foot morphology was assessed by foot length, truncated foot length, Foot Posture Index (FPI) and dorsal arch height. Balance was measured by the maximal step test. Intra-class correlation coefficients (ICC 3,1 ) were used to evaluate intra-rater reliability. Pearson's correlation coefficients were performed to assess associations between muscle size and strength, morphology and balance measures. To account for the influence of physical body size, partial correlations were also performed controlling for truncated foot length., Results: Intra-rater reliability was excellent for AbH (ICC 3,1  = 0.97) and FHBM (ICC 3,1  = 0.96). Significant associations were found between cross-sectional area of AbH and great toe flexion force measured standing by pedobarography ( r  = .623, p  = .003),), arch height measured sitting ( r  = .597, p  = .004) and standing ( r  = .590, p  = .005), foot length ( r  = .582, p  = 006), truncated foot length ( r  = .580, p  = .006), balance ( r  = .443, p  = .044), weight ( r  = .662, p  = .001), height ( r  = .559, p  = .008), and BMI ( r  = .502, p  = .020). Significant associations were found between cross-sectional area of FHBM and FPI ( r  = .544, p  = .011), truncated foot length ( r  = .483, p  = .027) and foot length ( r  = .451, p  = .040). Significant partial associations were found between AbH and great toe flexion force in standing by pedobarography ( r  = .562, p  = .012) and FHBM and the FPI ( r  = .631, p  = .003)., Conclusions: Measuring the cross-sectional area of AbH and FHBM with ultrasound is reproducible. Measures of strength, morphology and balance appear to relate more to the size of AbH than FHBM. After controlling for physical body size, cross-sectional area of AbH remained a significant correlate of great toe flexor strength and might be a useful biomarker to measure early therapeutic response to exercise., Competing Interests: The Human Research Ethics Committee of the University of Sydney approved the study (Protocol No. 2012/2849) and participants provided written informed consent.The authors declare that they have no competing interests.Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Purpose: The purpose of this study was to investigate the effect of an eight-week neuromuscular electrostimulation program on the intrinsic foot muscle strength. The results were compared with those from a passive and an active control group., Methods: 74 healthy participants were recruited and divided into three groups: a neuromuscular electrostimulation group (n=19), a passive control group (n=15) with no further intervention, and an active control group following a running protocol with minimal shoes (n=40). The electrostimulation and running groups followed a training protocol consisting of two sessions per week over a period of eight weeks. Three characteristics of intrinsic foot muscle strength were investigated: cross sectional area of the abductor hallucis muscle, longitudinal arch stability, and intrinsic foot muscle fatigue., Results: After eight weeks of intervention, the cross sectional area increased by 16.3% for the running group with a large effect size (0.801) according to Cohen's d. The electrostimulation group showed no such effect. The increase in the cross sectional area had no impact on longitudinal arch stability or intrinsic foot muscle fatigue results., Conclusion: This study investigated neuromuscular electrostimulation as a prevention and rehabilitation strategy. The results indicate that, compared to minimally shod running, the effects of electrostimulation on healthy participants might be too small to be detected. Further, the results provide evidence that the static navicular drop test is not sensitive enough to indicate intrinsic foot muscle strength. This appears clinically relevant, as this test is often used by therapists to evaluate patients' longitudinal arch function., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Maintaining standing balance involves multisensory processing and integration to produce dynamic motor responses. Electrical vestibular stimulation (EVS) delivered over the mastoid processes can be used to explore the vestibular control of balance. The purpose of this study was to determine whether intrinsic foot muscles exhibit vestibular-evoked balance responses and to characterize the traits associated with these responses. Electromyography (EMG) of the abductor hallucis (AH), abductor digiti minimi (ADM) and medial gastrocnemius (MG) and anterior-posterior (AP) forces were sampled while quietly standing participants were subjected to a random continuous EVS signal (peak-to-peak amplitude = ±3 mA). The relationship between EVS input and motor output was characterized in both the frequency (coherence) and time (cumulant density) domains. When head orientation was rotated in yaw from left to right, the biphasic cumulant density function was inverted for all muscle (EVS-EMG) and whole-body (EVS-AP forces) balance responses. When vision was occluded, the EVS-EMG and EVS-AP forces coherence function amplitude increased at low frequencies (<2 Hz) and was accompanied by a heightened medium-latency peak amplitude for all muscles as well as the whole-body balance response (AP forces) compared to when static visual cues were present. The enhanced coherence amplitudes at lower frequencies may highlight a mechanism for the increase in postural sway from vision to occluded vision. The current findings indicate that the vestibular control of standing balance can be represented by the intrinsic foot muscles and implicate a postural role for these muscles in modulating quiet standing., (Copyright © 2018 IBRO. Published by Elsevier Ltd. All rights reserved.)

Intrinsic foot muscles (IFM) are a crucial component within the human foot. Investigating their functioning can help understand healthy and pathological behaviour of foot and ankle, fundamental for everyday activities. Recording muscle activation from IFM has been attempted with invasive techniques, mainly investigating single muscles. Here we present a novel methodology, to investigate the feasibility of recording physiological surface EMG (sEMG) non-invasively and quantify patterns of activation across the whole plantar region of the foot. sEMG were recorded with a 13 × 5 array from the sole of the foot (n = 25) during two-foot stance, two-foot tiptoe and anterior/posterior sways. Physiological features of sEMG were analysed. During anterior/posterior epochs within the sway task, sEMG patterns were analysed in terms of signal amplitude (intensity) and structure (Sample Entropy) distribution, by evaluating the centre of gravity (CoG) of each topographical map. Results suggest signals are physiological and not affected by loading. Both amplitude and sample entropy CoG coordinates were grouped in one region and overlapped, suggesting that the region with highest amplitude corresponds with the most predictable signal. Therefore, both spatial and temporal features of IFM activation may be recorded non-invasively, providing opportunity for more detailed investigation of IFM function in healthy and patient populations., (Copyright © 2018. Published by Elsevier Ltd.)

Over-pronated feet are common in adults and are associated with lower limb injuries. Studying the foot muscle morphology and foot kinematic patterns is important for understanding the mechanism of over-pronation related injuries. The aim of this study is to compare the foot muscle morphology and foot inter-segmental kinematics between recreational runners with normal feet and those with asymptomatic over-pronated feet. A total of 26 recreational runners (17 had normal feet and 9 had over-pronated feet) participated in this study and their foot type was assessed using the 6-item Foot Posture Index. Selected foot muscles were scanned using an ultrasound device and the scanned images were processed to measure the thickness and cross-sectional area of the muscles. Muscles of interest include abductor hallucis, abductor digiti minimi, flexor digitorum brevis and longus, tibialis anterior and peroneus muscles. Foot kinematic data during walking was collected using a 3D motion capture system incorporating the Oxford Foot Model. The results show that individuals with over-pronated feet have larger size of abductor hallucis, flexor digitorum brevis and longus and smaller abductor digiti minimi than controls. Higher rearfoot peak eversion and forefoot peak supination during walking were observed in individuals with over-pronated feet. However, during gait the forefoot peak abduction was comparable. These findings indicate that in active asymptomatic individuals with over-pronated feet, the foot muscle morphology is adapted to increase control of the foot motion. The morphological characteristics of the foot muscles in asymptomatic individuals with over-pronated feet may affect their foot kinematics and benefit prevention from injuries., (Copyright © 2017 Elsevier B.V. All rights reserved.)

The capacity to store and return energy in legs and feet that behave like springs is crucial to human running economy. Recent comparisons of shod and barefoot running have led to suggestions that modern running shoes may actually impede leg and foot-spring function by reducing the contributions from the leg and foot musculature. Here we examined the effect of running shoes on foot longitudinal arch (LA) motion and activation of the intrinsic foot muscles. Participants ran on a force-instrumented treadmill with and without running shoes. We recorded foot kinematics and muscle activation of the intrinsic foot muscles using intramuscular electromyography. In contrast to previous assertions, we observed an increase in both the peak (flexor digitorum brevis +60%) and total stance muscle activation (flexor digitorum brevis +70% and abductor hallucis +53%) of the intrinsic foot muscles when running with shoes. Increased intrinsic muscle activation corresponded with a reduction in LA compression (-25%). We confirm that running shoes do indeed influence the mechanical function of the foot. However, our findings suggest that these mechanical adjustments are likely to have occurred as a result of increased neuromuscular output, rather than impaired control as previously speculated. We propose a theoretical model for foot-shoe interaction to explain these novel findings., (© 2016 The Author(s).)

Minimalist shoes are proposed to prevent injury and enhance performance by strengthening intrinsic foot muscles, yet there is little consensus on the effectiveness of minimalist shoes in increasing muscle strength or size. This systematic review assesses using minimalist shoes as an intervention on changes in plantar intrinsic foot muscle size and strength. PubMed, CINHAL, Scopus, and SPORT Discus were systematically searched for articles from January 2000 to March 2022. Studies were included if they had an intervention of at least 2 weeks with a control group and examined the effect of minimalist shoes on plantar intrinsic foot strength or size (either volume, cross-sectional area, or thickness). Nine studies were included. There were significant increases and percent changes in foot muscle strength, volume, cross-sectional area, and thickness. Strength increased between 9–57%, and size increased between 7.05–10.6%. Minimalist shoes may effectively increase intrinsic foot muscle size and strength in healthy individuals, and they may also be more convenient than implementing time-intensive physical therapy programs. Future research is needed to explore using minimalist shoes as an intervention in clinical populations who would benefit from increased IFM strength and function. It will also be vital to improve upon IFM assessment methods. [ABSTRACT FROM AUTHOR]

Purpose: It is not well established how motion and muscle activation of the medial longitudinal arch (MLA) of the foot vary under different loading conditions. Intrinsic and extrinsic foot muscles may play a role in postural control, which may be investigated by comparing loading tasks with differing postural demands. The objective of this study was to investigate the interaction of MLA flexibility and loading task on muscle activation. Methods: Twenty healthy adults completed two instrumented single-foot loading tasks: controlled external load of 50% body weight while sitting and bilateral standing. Fine-wire intramuscular and surface electromyography collected flexor hallucis brevis, abductor hallucis, tibialis posterior, flexor hallucis longus, tibialis anterior, and peroneus longus activation. MLA deformation was measured as a percent change in navicular height with loading. Results: During seated external loading, greater MLA deformation was associated with greater muscle activation for all instrumented muscles (R2 = 0.224-0.303, p < 0.05) except for tibialis anterior. During bilateral stance, there were no correlations between MLA deformation and muscle activation. Activation of all extrinsic muscles except for tibialis anterior were greater during bilateral standing than during external loading (p = 0.002-0.013), indicating activation of these muscles was caused by postural demands of the standing task, not simply load. Conclusions: MLA deformation and muscle activation are strongly task-dependent. [ABSTRACT FROM AUTHOR]

Lateral ankle sprains (LAS) and chronic ankle instability (CAI) are common musculoskeletal injuries that are a result of inversion injury during sport. The midfoot and forefoot is frequently injured during a LAS, is often overlooked during clinical examination, and maybe contributory to the development of CAI. The purpose of part two of this clinical commentary and current concept review is to increase clinician's awareness of the contribution of midfoot and forefoot impairment to functional limitation and disability of individuals who experience LAS and CAI and to facilitate future research in this area. The importance of multisegmented foot and ankle assessment from a clinical and research perspective is stressed. Select physical assessment and manual therapeutic techniques are presented to assist the clinician in examination and treatment of the ankle-foot complex in patients with LAS and CAI. [ABSTRACT FROM AUTHOR]

Human legs operate like springs with adjustable stiffness during locomotion, improving movement economy and versatility. The potential for the foot to contribute to this spring-like mechanism has been established. However, due to previous modelling approaches assuming a rigid-foot segment, it is unknown if ankle and foot quasi-stiffness can be actively regulated, quasi-stiffness being a measure of joint deformation under an applied load. In this study, we sought to determine if midfoot quasi-stiffness was increased in a similar manner as the ankle with increasing load demands, and if these changes were mediated by increased activation of the intrinsic foot muscles. We also quantified differences in measures of ankle joint quasi-stiffness attributable to the modelling assumption of a rigid-foot segment versus a multi-segment foot. Twenty-two healthy individuals performed a single-leg hopping task at three frequencies (2.0 Hz, 2.3 Hz, and 2.6 Hz) while measuring lower limb kinematics, kinetics and muscle activation (EMG). Kinetics and kinematics were calculated at the midfoot and using two different ankle models: rigid-foot model (shank-foot) and an anatomical ankle (shank-calcaneus). Midfoot quasi-stiffness increased with hopping-frequency (p < 0.05), while contact phase intrinsic foot muscle activation decreased (p < 0.05). The assumption of a rigid-foot model overestimated ankle ROM by ~10° and underestimated ankle quasi-stiffness by 45–60%. This study demonstrates that midfoot quasi-stiffness increases with hopping-frequency; however, the mechanism for these changes remains unclear. Furthermore, this study demonstrates the need to assess the ankle and foot independently, using appropriate models, to avoid significant inaccuracies in basic ankle kinematic and kinetic outputs, such as range of motion and joint quasi-stiffness. [ABSTRACT FROM AUTHOR]