Relationship of Physical Factors to the Acceptability and Unacceptability of Short Foot Exercise: A Preliminary Study

ABSTRACT

Objectives: This study aimed to examine the physical characteristics of participants who can and cannot perform the short foot exercise (SFE).

Methods: This prospective observational study was conducted in a university laboratory in Japan. University students (aged ≥18 years) enrolled between April and December 2024 were recruited as participants. We measured height, weight, foot length, medial longitudinal arch height, and toe grip strength. Ultrasonography captured transverse images of the intrinsic foot muscles, including the abductor hallucis, flexor hallucis brevis, and flexor digitorum brevis. Participants received single-instruction SFE training in a sitting position and were classified into two groups: SFE possible and SFE not possible. Those in the SFE not possible group received an additional 10 min of instruction before reattempting SFE. Based on performance, participants were categorized into three groups: those who succeeded after initial instruction (SFE-A), after practice (SFE-PT), or remained incapable (SFE-NP). Statistical analysis was conducted using one-way analysis of variance (ANOVA) and multiple comparison tests for each measurement and analysis item, with the significance level set at P < 0.05.

Results: In female participants, toe grip strength was lower in the SFE-A group than in the SFE-PT and SFE-NP groups, whereas no significant differences were observed in male participants or in muscle morphology and the navicular drop index in either sex.

Conclusions: These findings suggest that immediate performance of the SFE may not necessarily be associated with greater toe grip strength or larger intrinsic muscle size but may relate to coordination between intrinsic and extrinsic muscles.

INTRODUCTION

The functional characteristics of the foot have been described by McKeon et al.¹⁾ in terms of the foot core system, which is based on the spinal stabilizing system model of Panjabi.²⁾ The foot core system comprises three subsystems: active, passive, and neural.¹⁾ The active subsystem consists of intrinsic muscles that maintain foot stability and extrinsic muscles that facilitate movement. Intrinsic muscles contribute to the functional connection of the longitudinal and transverse foot arches, whereas extrinsic muscles aid in shock absorption and propulsion during walking and running.¹⁾ A previous study reported that weakness of the abductor hallucis (ABH) leads to increased navicular drop (ND).³⁾ Additionally, another study found that ABH muscle fatigue also increases ND.⁴⁾ Based on these findings, intrinsic muscles—rather than extrinsic muscles—are believed to play a crucial role in maintaining the medial longitudinal arch, highlighting the importance of training intrinsic foot muscles to sustain and enhance the active subsystem.

Recently, short foot exercise (SFE) has gained attention as a training method for foot function. SFE targets intrinsic muscles such as the ABH, flexor hallucis brevis (FHB), and flexor digitorum brevis (FDB). Most previous studies have taught SFE in a seated position, guiding the metatarsal heads toward the calcaneus while preventing interphalangeal joint flexion.⁵⁾ SFE has been reported to enhance medial longitudinal arch structure in individuals with flatfoot^6,7) and to be effective in managing hallux valgus.^8,9) In addition, SFE has been shown to improve balance¹⁰⁾ and enhance walking and running performance.^11,12) Therefore, SFE is considered one of the most effective exercises for foot-related movement disorders.

Despite its widespread use and extensive research, SFE mastery varies among individuals; some participants quickly learn the exercise, whereas others struggle. Previous studies have explored various feedback methods, including ultrasound,¹³⁾ electromyography,¹⁴⁾ and air cushion-based feedback.¹⁵⁾ However, the characteristics of individuals who can or cannot immediately perform SFE remain unclear. Identifying these characteristics could improve instructional methods and optimize SFE implementation. Therefore, this study aimed to examine the physical characteristics of participants who can and cannot successfully perform SFE. In clinical practice, some patients are able to perform SFE simply by watching a video or demonstration, some can perform it after a brief period of practice, and some are unable to perform it immediately. Those who succeed after a short practice typically require about 5–10 min of SFE instruction. Clarifying the characteristics of these groups may help optimize instructional methods and facilitate integration with other therapeutic interventions. Therefore, participants were divided into three groups based on their ability to perform SFE, and measurements of toe grip strength, muscle thickness, and cross-sectional area (CSA) of the intrinsic foot muscles were compared between groups. We hypothesized that, in both male and female participants, greater toe grip strength would be associated with larger muscle CSA and thickness, as well as the immediate ability to perform SFE.

MATERIALS AND METHODS

Participants and Study Design

University students (minimum age 18 years), enrolled at our university from April 2024 to December 2024, were included as participants. Exclusion criteria included a history of neuropathy in the dominant lower limb, surgery on the dominant side within the past 6 months, or pain symptoms at rest. Before assessing outcome measures, participant characteristics—age, sex, height, weight, ND, and exclusion criteria—were recorded. ND was measured as the difference between sitting and standing medial arch heights. Participants with an ND greater than 8.1 mm were classified as having flatfoot. Written informed consent was obtained from all participants before enrollment. This prospective, observational study was approved by the Institutional Review Board of Hyogo Medical University (No. 4704).

Outcome Measurements

Medial Longitudinal Arch Height Ratio and Navicular Drop Index

Participants were seated with the ankle in 0° dorsiflexion and the knee flexed at 90°. The medial aspect of the first metatarsal head and the navicular tuberosity were palpated and marked. Foot length was defined as the distance between the medial side of the first metatarsal head and the heel. Medial longitudinal arch height in the sitting position was measured as the vertical distance from the floor to the navicular tuberosity. The same measurement was taken in the standing position. The medial arch height ratio was calculated as medial arch height divided by foot length in each position. ND was determined as the difference between sitting and standing medial arch heights. The navicular drop index (NDI) was calculated as the ratio of ND to foot length.

Toe Grip Strength

Toe muscle strength was assessed using the Toe Muscle Strength Analyzer II (Takei Kiki Kogyo, Tokyo, Japan). The toe bar was adjusted for optimal grip comfort, and participants sat with 90° of hip and knee flexion and 0° of ankle dorsiflexion. After confirming positioning, the heels were secured with a belt to prevent movement. The measuring instrument was stabilized, and participants were instructed to flex their toes and pull the bar while maintaining position. Measurements were taken twice, and the average value was recorded.

Muscle Thickness and Cross-sectional Area

Muscle thickness and CSA of the ABH, FHB, and FDB muscles were measured using an ultrasound imaging device (JS2; Medicare, Yokohama, Japan) with a linear probe (5–12 MHz). A single experienced examiner conducted all measurements following extensive practice and referencing previous studies and anatomy textbooks. Measurements were performed according to prior research.^16,17) Participants were positioned supine with the hip joint externally rotated and the knee joint slightly flexed. The ABH muscle was assessed from the medial tuberosity of the talus to the navicular tuberosity. The probe was then rotated 90° to obtain a transverse section at the site of greatest muscle thickness. The FHB muscle was examined along the long axis of the first metatarsal bone, followed by a 90° probe rotation to capture the transverse section at the thickest region. Similarly, the FDB muscle was evaluated along the line connecting the medial tuberosity of the talus and the third metatarsal bone, with the probe rotated 90° to obtain a transverse section after identifying the thickest region. Each image was transferred to a personal computer, where muscle thickness and CSA were analyzed using image analysis software (ImageJ; National Institutes of Health, Bethesda, MD, USA) (Fig. 1).

Fig. 1.

Examples of ultrasound images of intrinsic foot muscles. Top left, FHB; top right, ABH; lower left, FDB. Area surrounded by dotted line indicates the CSA. The length of each double arrow indicates muscle thickness. Scale is in centimeters.

Assessment of SFE Feasibility

Participants were instructed once on performing SFE through a video demonstration. Each participant was told to perform SFE in a sitting position and to move the heel to shorten the distance between the first metatarsal head and the heel.^5,7) Immediately after the instruction, participants attempted the SFE as a feasibility test. Participants were categorized into three groups according to their ability to perform SFE: those who could perform it immediately after watching a video or demonstration, those who could perform it after 5–10 min of practice, and those who could not perform it immediately. Those who successfully performed the SFE at least three out of five times were classified as the SFE-able (SFE-A) group. Participants unable to meet this criterion received 10 min of additional SFE training using a video. During training, participants were instructed to perform the SFE as demonstrated in the video, with verbal cues primarily consisting of the following: “Apply force as if bringing the base of the first metatarsal and the heel together,” “Exert force to elevate the medial longitudinal arch,” and “Without flexing the toes, press them against the floor.” Positive feedback was provided when the SFE was successfully performed. In contrast, when the SFE was not executed properly, corrective feedback was given to explain the reasons for failure. Manual facilitation, such as tactile guidance on the target muscles, was not applied. After training, those who successfully performed the SFE at least three out of five times were classified as the SFE-able post-training (SFE-PT) group, whereas those still unable to perform the SFE were designated as the SFE-not possible (SFE-NP) group (Fig. 2). SFE feasibility was determined by the ability to shorten the distance between the first metatarsal head and the heel without internal or external ankle rotation and to flex the metatarsophalangeal joint without flexing the distal or proximal interphalangeal joints.

Fig. 2.

Flowchart of group allocation. Dashed rectangle, criteria used for classification; dotted rectangle, content of the SFE training.

Statistical Analysis

Participants were divided by sex, and between-group comparisons were conducted separately for male and female participants. One-way analysis of variance (ANOVA) and multiple comparison tests were used to analyze each measurement and outcome variable with the significance level set at P < 0.05.

RESULTS

The characteristics of the participants in this study are shown in Table 1. Eighty participants (39 men, 41 women) were included. Group distribution was as follows: SFE-A (n=14; 7 men, 7 women), SFE-PT (n=25; 12 men, 13 women), and SFE-NP (n=41; 20 men, 21 women). No significant between-group differences in baseline characteristics, including age, height, weight, and body mass index, were observed in either sex. Flatfoot, defined as ND greater than 8.1 mm,¹⁸⁾ was identified in one male participant and in one female participant. The results of outcome measurements are shown in Table 2. In female participants, toe grip strength in the SFE-A group (mean ± standard deviation: 8.0 ± 3.1 kg) was significantly lower than that in both the SFE-PT (15.7 ± 5.1 kg, P < 0.05) and the SFE-NP (14.0 ± 5.9 kg, P < 0.05) groups (Fig. 3). This difference was not observed among male participants, in whom toe grip strength values were comparable across groups (SFE-A, 16.9 ± 6.0 kg; SFE-PT, 22.1 ± 6.2 kg; SFE-NP, 19.7 ± 5.9 kg) (Fig. 3). For all other measured variables, no significant between-group differences were detected in either sex (Table 2).

Table 1. Participant characteristics

Characteristic	Male participants					Female participants
	All participants (n=39)	SFE-A (n=7)	SFE-PT (n=12)	SFE-NP (n=20)		All participants (n=41)	SFE-A (n=7)	SFE-PT (n=13)	SFE-NP (n=21)
Age, years	20.4 ± 1.1	20.1 ± 0.9	20.7 ± 1.3	20.4 ± 0.9		20.3 ± 1.1	20.6 ± 0.8	20.4 ± 1.4	20.1 ± 1.0
Height, cm	171.6 ± 5.6	173.0 ± 7.0	173.5 ± 3.2	170.0 ± 6.0		159.2 ± 4.8	157.4 ± 4.2	159.4 ± 6.3	159.6 ± 4.0
Body weight, kg	63.4 ± 6.1	63.0 ± 8.2	63.2 ± 5.1	63.7 ± 6.2		51.5 ± 5.6	49.2 ± 4.6	51.9 ± 6.1	52.0 ± 5.6
Foot length, cm	25.0 ± 1.3	24.9 ± 1.3	25.1 ± 1.2	24.9 ± 1.3		23.0 ± 1.1	22.8 ± 0.7	23.1 ± 1.4	23.0 ± 1.0
Arch height, cm
Sitting	5.43 ± 0.67	5.64 ± 0.82	5.56 ± 0.65	5.28 ± 0.62		5.04 ± 0.47	5.00 ± 0.28	4.88 ± 0.65	5.15 ± 0.37
Standing	5.20 ± 0.58	5.44 ± 0.80	5.35 ± 0.43	5.03 ± 0.55		4.76 ± 0.49	4.76 ± 0.43	4.59 ± 0.65	4.86 ± 0.37
Navicular drop, mm	2.26 ± 2.71	2.00 ± 2.08	2.08 ± 3.84	2.45 ± 2.16		2.83 ± 2.51	2.42 ± 3.10	2.92 ± 2.78	2.90 ± 2.23
Flatfoot	1	0	1	0		1	0	1	0

Data presented as mean ± standard deviation or as number.

Table 2. Outcome measurements

Measurement	Male participants					Female participants
	All participants (n=39)	SFE-A (n=7)	SFE-PT (n=12)	SFE-NP (n=20)		All participants (n=41)	SFE-A (n=7)	SFE-PT (n=13)	SFE-NP (n=21)
Arch height ratio, %
Sitting	21.9 ± 3.0	23.0 ± 3.7	22.2 ± 2.5	21.4 ± 3.1		22.1 ± 2.1	22.0 ± 1.0	21.3 ± 2.9	22.5 ± 1.8
Standing	20.9 ± 2.8	21.9 ± 3.6	21.4 ± 1.9	20.3 ± 3.0		20.7 ± 2.0	20.9 ± 1.6	19.9 ± 2.6	21.1 ± 1.61
Navicular drop index	0.01 ± 0.01	0.01 ± 0.01	0.01 ± 0.02	0.01 ± 0.01		0.02± 0.01	0.01± 0.01	0.01 ± 0.01	0.01 ± 0.01
Muscle thickness, cm
ABH	1.05 ± 0.16	1.00 ± 0.18	1.03 ± 0.14	1.09 ± 0.17		0.90 ± 0.16	0.86 ± 0.19	0.94 ± 0.12	0.88 ± 0.17
FHB	1.00 ± 0.17	0.99 ± 0.12	0.95 ± 0.16	1.03 ± 0.19		0.88 ± 0.16	0.88 ± 0.11	0.91 ± 0.17	0.86 ± 0.16
FDB	0.89 ± 0.17	0.83 ± 0.15	0.87 ± 0.13	0.93 ± 0.19		0.74 ± 0.15	0.76 ± 0.15	0.74 ± 0.15	0.73 ± 0.15
Muscle cross sectional area, cm2
ABH	2.30 ± 0.48	2.35 ± 0.72	2.24 ± 0.46	2.32 ± 0.41		1.77 ± 0.51	1.62 ± 0.51	1.84 ± 0.39	1.77 ± 0.59
FHB	1.86 ± 0.38	1.86 ± 0.25	1.85 ± 0.38	1.87 ± 0.43		1.50 ± 0.29	1.54 ± 0.30	1.54 ± 0.32	1.46 ± 0.27
FDB	2.15 ± 0.45	1.98 ± 0.33	2.20 ± 0.37	2.17 ± 0.53		1.46 ± 0.34	1.52 ± 0.40	1.52 ± 0.34	1.40 ± 0.33
Toe grip strength, kg	19.9 ± 6.1	16.9 ± 6.0	22.1 ± 6.2	19.7 ± 5.9		13.5 ± 5.8	8.0 ± 3.1a	15.7 ± 5.1b	14.0 ± 5.9b

Different lowercase letters for toe grip strength indicate significant difference between groups (P < 0.05).

Fig. 3.

Results for toe grip strength for male participants (left) and female participants (right). Asterisk indicates significant difference (P < 0.05).

DISCUSSION

SFE has been widely implemented in both clinical practice and research as a means of strengthening the intrinsic foot muscles; however, it is recognized as a technically challenging task. Previous investigations have highlighted the necessity of various forms of feedback to facilitate its proper execution, including ultrasound-based feedback,¹³⁾ electromyographic feedback,¹⁴⁾ and feedback using an air cushion device.¹⁵⁾ Despite these reports, some individuals are able to successfully perform the SFE following only a brief orientation, without the need for specialized feedback modalities. In light of this discrepancy, this study aimed to examine the characteristics that differentiate those who can perform the SFE immediately as instructed from those who have difficulty or cannot perform SFE. Specifically, this study compared toe grip strength, muscle thickness, CSA of the FHB, FDB, and ABH, and NDI between these groups. Our findings demonstrated that, among female participants, toe grip strength was significantly lower in the SFE-A group than in the SFE-PT and SFE-NP groups. In contrast, no significant differences were identified among male participants with respect to toe grip strength. Furthermore, for both sexes, no statistically significant group differences were observed in the muscle thickness or CSA of the intrinsic foot muscles examined, nor in NDI.

A previous study using magnetic resonance imaging (MRI) reported positive correlations between toe grip strength and the CSA of intrinsic foot muscles, including the medial group (FHB, FDB, quadratus plantae, lumbricals, and ABH) and the lateral group (abductor digiti minimi, flexor digiti minimi brevis, and the dorsal and plantar interossei).¹⁹⁾ Another MRI-based study also demonstrated an association between toe grip strength and the CSA of ABH.²⁰⁾ Therefore, toe grip strength is generally considered to be related to the CSA of intrinsic foot muscles. However, in the present study, no significant relationship between toe grip strength and muscle CSA was identified. Three possible explanations can be considered.

First, the CSA and thickness of the intrinsic foot muscles of the female participants in the present study may have been lower than those generally reported in the literature. For instance, a systematic review of ultrasound measurements of intrinsic foot muscles reported pooled mean values of CSA and thickness for each muscle, based on 52 studies.²¹⁾ The reported reference values were as follows: CSA of ABH (2.43 ± 0.78 cm²), FDB (2.01 ± 0.34 cm²), FHB (2.36 ± 1.20 cm²), and quadratus plantae (1.75 ± 0.54 cm²), with respective thickness values of 1.11 ± 0.28 cm, 0.87 ± 0.28 cm, 1.38 ± 0.44 cm, and 0.98 ± 0.12 cm. Compared with these reference data, the male participants in the present study showed no substantial deviations in CSA or thickness, which is consistent with the lack of significant group differences in toe grip strength among male participants. In contrast, female participants demonstrated lower values for some muscles, such as the CSA and thickness of the ABH and CSA of the FHB, than the reference values. These reduced values may have contributed to the significantly lower toe grip strength observed in the SFE-A group among female participants.

Second, intrinsic foot muscles do not act in complete isolation but rather are often involved in movements related to both the hallux and the lesser toes. An MRI study investigating intrinsic muscle activity during foot exercises, including SFE, reported that not only the ABH, FDB, and FHB were active, but also other intrinsic muscles, such as the abductor digiti minimi and flexor digiti minimi.²²⁾ In addition, the MRI study on the relationship between toe grip strength and the CSA of intrinsic and extrinsic muscles¹⁹⁾ classified the intrinsic muscles collectively into medial and lateral groups, which may have better reflected the muscles contributing to specific tasks. In contrast, the present study measured each muscle individually, without grouping multiple muscles, which may explain the absence of a significant relationship between toe grip strength and muscle CSA.

Third, the possibility of anatomical connections between the extrinsic muscles, specifically the tendons of the flexor hallucis longus (FHL) and flexor digitorum longus (FDL), and the soft tissues of the foot should be considered. An anatomical study has shown that the FHL frequently exhibits tendinous connections with the FDL and branches to the second and third toes, thereby contributing to multi-toe flexion at the peripheral structural level.²³⁾ Moreover, a study on finger force production has described the “enslaving effect,” in which involuntary force is produced by non-task fingers even when attempting to use a single finger, reflecting neural co-activation at the central level.²⁴⁾

Taken together, these results suggest that the significantly lower toe grip strength observed in female participants in the SFE-A group than that in female participants in the SFE-PT and SFE-NP groups may not be fully explained by muscle CSA alone. Rather, anatomical interconnections and neural coupling mechanisms may have contributed to this phenomenon, although such an explanation remains speculative.

A previous study reported that an ND in the range of 6.0–8.1 mm indicates a neutral foot position.¹⁸⁾ When participants in this study were classified using this criterion, most had neutral or supinated foot postures, with only two exhibiting pronation. Previous research has shown that individuals with flatfoot have smaller ABH and FHB muscle thickness and CSA than those without flatfoot.²⁵⁾ Given that nearly all participants in this study did not have flatfoot, it is likely that no significant differences in ABH, FHB, and FDB muscle thickness or CSA were observed. However, this study did not assess flatfoot using established evaluation methods, such as X-ray imaging or the Foot Posture Index, which have shown strong correlations with radiographic findings in prior research. Therefore, future studies should incorporate ND and arch height ratio assessments alongside foot morphology and posture evaluations.

In summary, this study revealed sex-specific differences in the relationship between toe grip strength and the ability to perform SFE. Among female participants, the toe grip strength in the SFE-A group was lower than that in the SFE-PT and SFE-NP groups, whereas no significant differences were observed in male participants. Moreover, intrinsic foot muscle morphology and the NDI did not differ among groups in either sex. These results suggest that successful immediate performance of SFE may not be determined solely by muscle strength or size, but rather by the coordination between intrinsic and extrinsic muscles. Further investigations incorporating direct assessments of muscle activity and evaluations of the passive and neural subsystems are warranted to clarify these mechanisms.

Study Limitations

This study has some limitations. First, although this study focused on the active subsystem of the foot core system, the passive (bones, plantar aponeurosis, ligaments) and neural (sensory receptors) subsystems were not assessed. Second, because muscle activity and elasticity were also not measured, the interpretations of intrinsic–extrinsic coordination remain speculative. Third, SFE feasibility was determined by a single examiner, and inter-rater reliability was not assessed. Although this ensured internal consistency, reproducibility across examiners or institutions remains uncertain. Fourth, several outcome measures, including toe grip strength in male participants, showed no significant differences among groups. These results could not be fully explained by the present data, and future studies incorporating qualitative assessments or kinematic analyses may help clarify them. Fifth, because participants were young adults, generalization to older or clinical populations is limited. Sixth, the present study did not include radiographic imaging or the Foot Posture Index to evaluate flatfoot. Furthermore, no assessment of hallux valgus was conducted. Finally, because this was a cross-sectional study, causal relationships between SFE performance, muscle morphology, and foot posture could not be determined. Future studies should incorporate direct measurements of muscle activity and investigate diverse populations using longitudinal designs.

CONCLUSION

This study investigated the physical characteristics of healthy participants who were able or unable to perform the SFE. Among female participants, toe grip strength was significantly lower in the SFE-A group than in the SFE-PT and SFE-NP groups, whereas no significant differences were observed among male participants. For each sex, no significant differences were observed between groups for intrinsic foot muscle morphology or the NDI. These results suggest that immediate performance of the SFE may not necessarily be associated with greater toe grip strength or larger intrinsic muscle size but may instead relate to the coordination between intrinsic and extrinsic muscles. Further research incorporating direct assessments of muscle activity and evaluations of the passive and neural subsystems is warranted to clarify the mechanisms underlying these differences.

ACKNOWLEDGMENTS

This work was supported by a Hyogo Medical University Grant for Research Promotion (2024). We thank Editage (www.editage.jp) for English language editing.

CONFLICTS OF INTEREST

The authors declare no conflict of interest.

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