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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. 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? 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. 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). A single leg swing exercise may be effective for self-exercise of intrinsic foot muscles, particularly when intensive supervised physiotherapy is not possible. • Abductor hallicus longus activated in single leg swing without lengthy practice. • The minimum longitudinal arch angle was larger in standing than in sitting. • Standing single leg swing may be effective exercise for foot intrinsic muscles. [ABSTRACT FROM AUTHOR]

The intrinsic and extrinsic foot softtissue structures that apply force and support the medial longitudinal arch (MLA) have been implicated in the development of flat feet. However, the relationship between the changes in MLA height under increasing load and the morphological and mechanical properties of individual intrinsic and extrinsic foot soft tissue structures is not fully understood. To examine the morphological and mechanical characteristics of the foot soft tissue structures in flat feet when subjected to loading. This study consisted of two studies focusing on the extrinsic foot muscles (10 normal feet/11 flat feet) and intrinsic foot muscles (14 normal feet/13 flat feet). Images of the extrinsic and intrinsic foot muscles and plantar fascia (PF) under 10%, 50%, and 90% body weight conditions were obtained using ultrasound-based shear-wave elastography. The cross-sectional area (CSA) of the peroneus brevis was larger in the flat-foot group than in the normal-foot group under all loading conditions. The CSAs of the intrinsic foot muscles (abductor hallucis, flexor digitorum brevis, and quadratus plantae) and thickness of the PF in the flat-foot group decreased significantly with increasing load. As for mechanical characteristics, the stiffness of the flexor digitorum longus and abductor hallucis was higher in the flat-foot group than in the normal group under high loading conditions. In addition, flat feet with greater flexibility tended to exhibit a greater decrease in PF thickness and smaller increase in stiffness. Excessive stretching of the intrinsic foot muscles and PF occurs in flat feet, and excessive contraction of the flexor digitorum longus may counteract the excessive lowering of the foot arch. Therefore, it is necessary to promote the contraction of the intrinsic foot musculature in feet with greater flexibility of the MLA during loading. • The properties of foot soft tissue structures during loading are not fully understood. • Clarified the morphological and mechanical characteristics of soft tissue structures in flat feet. • Excessive stretching of the intrinsic foot muscles and the plantar fascia occurs in flat feet. • Excessive contraction of the flexor digitorum longus may counteract the lowering of the foot arch. [ABSTRACT FROM AUTHOR]

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. Are the activities of intrinsic foot muscles affected by standing and sitting postures during gradual force generation? 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. 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). 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 • Foot muscle activity during force generation differed between standing and sitting. • Such activity was greater in standing posture when exerting force at high intensity. • Posture selection is important for exercising the intrinsic foot muscles. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• A protocol for sEMG placement is defined for intrinsic foot muscles. • Four of six intrinsic muscles showed good correlation for the validity of the protocol. • Flexor hallucis Brevis and Abductor Hallucis muscles did not show good validity. • Improved protocols and technology of EMG of the intrinsic foot muscles are required. 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. This paper looks at the potential reliability of a sEMG protocol for assessing the intrinsic foot muscles. 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. 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. 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. [ABSTRACT FROM AUTHOR]

Foot-related complications including impaired peripheral circulation and lower limb ulceration are severe consequences for those with diabetes mellitus. This study aimed to assess differences in tissue oxygenation and oxygen utilization of the plantar surface intrinsic foot muscles between diabetic participants and healthy comparisons following short foot exercise and a standard walking protocol. Eighteen participants, 9 with diabetes and 9 healthy age- and sex-matched comparisons, completed two interventions in a randomized order. For the short foot exercise intervention, participants completed 5 sets of 15 intrinsic foot muscle contractions. For the walking intervention, participants completed a modified six-minute walk test. Tissue oxygenation variables including oxygenated hemoglobin, deoxygenated hemoglobin, and tissue saturation index were measured using near-infrared spectroscopy in quiet stance and during intrinsic foot muscle contraction cycles following each intervention. Means, standard deviations, 95 % confidence intervals, mean differences, and Cohen's d effect sizes were calculated for each tissue oxygenation variable. The results of this study indicated no significant group differences in quiet standing tissue oxygenation measures at baseline and following each intervention. Participants in the diabetic group had significantly less change in tissue saturation index during intrinsic foot muscle contractions compared to healthy participants after the short foot exercise intervention (ES= 4.00, P =.0002) and walking intervention (ES= 1.33, P =.015). By utilizing wireless NIRS and novel research methodology, this study was able to explore changes in plantar surface tissue oxygenation of the intrinsic foot muscles following a targeted short foot exercise intervention as well as a standard walking protocol in patients diagnosed with diabetes compared to age- and sex- matched individuals without diabetes. We identified that diabetic participants presented with less oxygen utilization during intrinsic foot muscle contractions performed following both exercise interventions compared to their healthy age- and sex- matched comparisons. • Weight-bearing assessment of foot muscle oxygenation by near-infrared spectroscopy. • Diabetics utilize less oxygen than nondiabetics to perform short foot contractions. • Diabetics may benefit from targeted short foot exercise to reduce foot complications. [ABSTRACT FROM AUTHOR]

Highlights • Neuromuscular electrostimulation has no influence on parameters of intrinsic foot muscle strength. • A running program in minimal shoes increases the cross sectional area of the abductor hallucis muscle. • An increase in cross sectional area has no influence on longitudinal arch stability. Abstract 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: Background: This prospective study explored the effects of endurance running (ER) in minimal versus standard running shoes on the foot's superficial layer intrinsic muscles and the function of the longitudinal arch. Our hypothesis was that running in minimal shoes would cause hypertrophy in these muscles and lead to higher, stronger, stiffer arches. Methods: The hypothesis was tested using a sample of 33 healthy runners randomized into two groups, a control group shod in traditional running footwear and an experimental group shod in minimal support footwear, whose feet were scanned in an MRI before and after a 12-week training regime. Running kinematics as well as arch stiffness and height were also assessed before and after the treatment period. Results: Analysis of anatomical cross-sectional areas and muscle volumes indicate that the flexor digitorum brevis muscle became larger in both groups by 11% and 21%, respectively, but only the minimally shod runners had significant areal and volumetric increases of the abductor digiti minimi of 18% and 22%, respectively, and significantly increased longitudinal arch stiffness (60%). Conclusion: These results suggest that endurance running in minimal support footwear with 4 mm offset or less makes greater use of the spring-like function of the longitudinal arch, thus leading to greater demands on the intrinsic muscles that support the arch, thereby strengthening the foot. [Copyright &y& Elsevier]

Abstract: Although critical for effective human locomotion and posture, little data exists regarding the segmentation, architecture and contraction time of the human intrinsic foot muscles. To address this issue, the Abductor Hallucis (AH), Abductor Digiti Minimi (ADM), Flexor Digitorum Brevis (FDB) and Extensor Digitorum Brevis (EDB) were investigated utilizing a cadaveric dissection and a non-invasive whole muscle mechanomyographic (wMMG) technique. The segmental structure and architecture of formaldehyde-fixed foot specimens were determined in nine cadavers aged 60–80years. The wMMG technique was used to determine the contraction time (Tc) of individual muscle segments, within each intrinsic foot muscle, in 12 volunteers of both genders aged between 19 and 24years. While the pattern of segmentation and segmental –architecture (e.g. fibre length) and –Tc of individual muscle segments within the same muscle were similar, they varied between muscles. Also, the average whole muscle Tc of FDB was significantly (p <0.05) shorter (faster) (Tc=58ms) than in all other foot muscles investigated (ADM Tc=72ms, EDB Tc=72ms and ABH Tc=69ms). The results suggest that the architecture and contraction time of the FDB reflect its unique direct contribution, through toe flexion, to postural stability and the rapid development of ground reaction forces during forceful activities such as running and jumping. [Copyright &y& Elsevier]

Abstract: Background: The aim of this study was to determine the difference in activation patterns of the plantar intrinsic foot muscles during two quiet standing tasks with increasing postural difficulty. We hypothesised that activation of these muscles would increase with increasing postural demand and be correlated with postural sway. Methods: Intra-muscular electromyographic (EMG) activity was recorded from abductor hallucis, flexor digitorum brevis and quadratus plantae in 10 healthy participants while performing two balance tasks of graded difficulty (double leg stance and single leg stance). These two standing postures were used to appraise any relationship between postural sway and intrinsic foot muscle activity. Findings: Single leg stance compared to double leg stance resulted in greater mean centre of pressure speed (0.24ms−1 versus 0.06ms−1, respectively, P≤0.05) and greater mean EMG amplitude for abductor hallucis (P ≥0.001, ES=0.83), flexor digitorum brevis (P ≤0.001, ES=0.79) and quadratus plantae (P ≤0.05, ES=0.4). EMG amplitude waveforms for all muscles were moderate to strongly correlated to centre of pressure (CoP) medio-lateral waveforms (all r ≥0.4), with muscle activity amplitude increasing with medial deviations of the CoP. Intra-muscular EMG waveforms were all strongly correlated with each other (all r ≥0.85). Interpretations: Activation of the plantar intrinsic foot muscles increases with increasing postural demand. These muscles are clearly important in postural control and are recruited in a highly co-ordinated manner to stabilise the foot and maintain balance in the medio-lateral direction, particularly during single leg stance. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

• The evidence is available on the functional capacity and morphological characteristics of the intrinsic foot muscles in subjects with HV deformity. • There is association between intrinsic foot muscle weakness and HV deformity. • Subjects with HV deformity have different intrinsic foot muscle performances compared to subjects without deformity. 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. 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. 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. This review found scientific evidence on muscle performance impairment in the abductor hallucis and flexor hallucis brevis in subjects with HV deformity. [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]

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. [ABSTRACT FROM AUTHOR]