The performance characteristics of the toe, such as toe flexor strength, are very important for various movements, including standing [1
] and walking [2
]. Menz et al. [4
] demonstrated that toe flexor strength was a significant independent predictor of balance and functional ability in older people. They also reported that people who experienced falls exhibited decreased toe flexor strength compared with those who did not fall [5
]. However, toe flexor strength has not been evaluated in a standard manner similar to that used to determine hand grip strength.
We previously assessed age-related changes in toe grip strength (TGS) and investigated relationships between TGS and sex, age, weight, and height using a toe grip dynamometer [6
]. We found that these factors contribute to the prediction of TGS; however, the association of other factors, such as foot structure characteristics, with TGS are unknown.
Structurally, toe deformities, such as hallux valgus, are often found clinically, regardless of patient age or sex. Mickle et al. [7
] demonstrated an association between toe deformity and reduced toe flexor strength. A relationship between the toe flexor strength and the medial longitudinal arch (MLA) height has also been reported [8
]. Weak plantar intrinsic or extrinsic muscles (i.e., toe flexors) that do not provide sufficient dynamic truss support for the MLA may be involved in the underlying aetiology of decreasing foot arch height (FAH) [8
]. Headlee et al. [10
] reported that repeated isotonic flexion of the metatarsophalangeal joints, through their full range of motion, resulted in navicular drop. A loss of foot intrinsic muscular function due to fatigue has also been reported to result in a loss of structural MLA support [11
]. However, some studies have reported that there is no association between TGS and the MLA height [12
]. Therefore, the association between TGS and FAH remains unclear. Additionally, although some researchers have reported that the range of toe flexion (a factor in toe curl motion) is related to TGS in younger women [9
] and in frail, elderly women [13
], the association between TGS and range of toe flexion remains to be clarified in other groups.
Factors including hallux valgus, toe curl ability, and FAH might also influence TGS; however, the associations between TGS and these characteristics are not well known. These characteristics can be easily measured in clinical practice, and might be amenable to interventions that improve these conditions and lead to improved TGS. Therefore, the present study investigated the association between TGS and foot characteristics, including hallux valgus, toe curl ability, and FAH. We hypothesised that reduced severity of hallux valgus, greater toe curl ability, and FAH were associated with greater TGS.
Participant characteristics are shown in Table 1
. A large proportion of the participants were over 60 years of age. Although significant differences were found between men and women with respect to foot length, flexed foot length, truncated foot length, and navicular height, there were no significant differences in HVAs, toe curl abilities, and FAHs between the sexes (Table 2
Age group (years)
| || || |
20 to 29
30 to 39
40 to 49
50 to 59
60 to 69
70 to 79
Toe grip strength (TGS) and foot structure characteristics
Foot length (cm)
Flexed foot length (cm)
Truncated foot length (cm)
Navicular height (cm)
Toe curl ability (%)
According to Pearson’s correlation coefficient, TGS was significantly correlated with age (p
< 0.01), height (p
< 0.05), toe curl ability (p
< 0.01), and FAH (p
< 0.05), but not with weight or HVA, in men (Table 3
). In women, TGS was significantly correlated with age (p
< 0.01), height (p
< 0.01), and toe curl ability (p
< 0.01), but not with weight, HVA, or FAH (Table 3
Correlation coefficients between toe grip strength and other factors, according to sex
In the stepwise multivariate linear regression analyses, age, height, toe curl ability, and FAH were applied as the explanatory variables, for men. TGS was shown to be correlated with age (standardised partial regression coefficient [β]=−0.339, p
–0.015) and toe curl ability (β–0.282, p
–0.042), but not with height or FAH; the TGS adjusted R2
was 0.22 (Table 4
). For women, age, height, and toe curl ability were applied as the explanatory variables. TGS was correlated with age (β–−0.397, p
< 0.001), height (β–0.167, p
–0.011), and toe curl ability (β–0.283, p
< 0.001); the adjusted R2
was 0.40 (Table 5
Stepwise multiple regression analysis of toe grip strength in men
Toe curl ability (%)
Stepwise multiple regression analysis of toe grip strength in women
Toe curl ability (%)
This study investigated the relationships between TGS and HVA, toe curl ability, and FAH, and indicated that there is an association between TGS and toe curl ability in both men and women. Further, the results indicate that improving toe curl ability, i.e., the range of toe flexion motion and arch contraction, may improve TGS. Therefore, range-of-motion or joint mobilisation exercises that improve the range of toe flexion or toe curl ability, as well as muscle strengthening exercises, might improve TGS. However, at present, a causal relationship between TGS and toe curl ability cannot be demonstrated. Whether or not improving toe curl ability leads to improved TGS will have to be investigated in a future study.
Based on the Pearson’s correlation coefficients, TGS was negatively correlated with age and positively correlated with height and toe curl ability, in both sexes. The correlations between TGS and the demographic and clinical characteristics, for both sexes, were similar to the results observed in our previous study [6
], except for weight. The correlation between TGS and toe curl ability suggests that the active range of toe and foot motion generated the TGS because toe curl ability positively correlated with TGS, in both sexes. In men, TGS negatively correlated with FAH, indicating that increased arch height resulted in decreased TGS. Moreover, in men, FAH was positively correlated with age and negatively correlated with height and toe curl ability (data not shown). Therefore, there might be other confounding factors affecting the relationship between TGS and FAH.
According to the stepwise multivariate linear regression analyses, TGS correlated with age and toe curl ability, in both sexes. In addition, height was shown to correlate with TGS in females. In our previous study, 31% of TGS was accounted for based on age, sex, height, and weight. In this study, the stepwise multivariate linear regression analyses were calculated based on age, height, and toe curl ability, in women, and on age, height, toe curl ability, and FAH, in men. As a result, 22% of the TGS was accounted for by age and toe curl ability, in men. In women, 41% of the TGS was accounted for by age, height, and toe curl ability. The reason for the differences in the adjusted R2 values for the stepwise multivariate linear regression analyses between the present and previous studies might be related to the inclusion of sex as one of the explanatory variables. In the future, factors other than foot characteristics will require more detailed investigation to clarify the factors contributing to TGS; factors such as exercise, sports history, and occupation may also influence TGS.
Age demonstrated the strongest correlation with TGS in both sexes. This result was consistent with that of our previous study [6
], indicating that age-related changes are the strongest contributing factors of TGS, regardless of body type or foot characteristics.
TGS correlated with toe curl ability, but not with HVA or FAH. Therefore, toe curl ability is an important contributing factor for TGS. In the present study, toe curl ability (percentage) was calculated as (foot length–flexed foot length)/foot length. This calculation considers the sum of the range of motion of the first metatarsophalangeal and interphalangeal joints, as well as some degree of intertarsal mobility. When measuring TGS, the participants were asked to pull the grip bar using their toes; in other words, they were required to curl their toes around the grip bar. Theoretically, flexible toes should generate higher TGS. However, the toe curl ability values, in the present study, did not represent the ability of all the toes because the toe curl ability was calculated based on the first toe. Since the muscle strength of the first toe has been reported to show the strongest association with TGS [17
], the first toe was used as a benchmark to calculate toe curl ability. Future studies will need to consider the function of the other toes, as well.
In the present study, HVA was not significantly associated with TGS, but the toe flexor strength of individuals with hallux valgus has been reported to be weaker than that in normal individuals [7
]. In this study, we did not analyse the individuals with severe hallux valgus who could not correctly grip the toe grip dynamometer bar. Therefore, the HVA might not have influenced the TGS in the present study.
The FAH was negatively correlated with TGS in men, but was not correlated with TGS in women. We hypothesised that arch height might be positively correlated with TGS because the flexor hallucis longus and flexor digitorum longus muscles and other intrinsic foot muscles support the MLA. As previously mentioned, weak plantar intrinsic or extrinsic muscles (i.e., toe flexors) cannot provide sufficient dynamic truss support for the MLA [8
]. In addition, the force produced by the intrinsic muscles was previously reported to contribute to the truss support of the MLA [8
]. Headlee et al. [10
] reported that plantar intrinsic foot muscle fatigue increases navicular drop. This report also indicated a relationship between toe flexor strength and the MLA. Jung et al. [18
] showed that toe curl exercises do not change the MLA angle formed by the line connecting the first metatarsal head and the navicular tuberosity and the line connecting the navicular tuberosity and the medial side of the calcaneal bone. However, they also reported that the MLA angle significantly decreased during short-foot exercises, which are performed to activate the foot’s intrinsic muscles by pulling the metatarsal heads toward the heel, while the long toe flexors are relaxed. These results suggest that toe curls, such as during TGS measurements, may not influence arch height.
This study has several limitations. First, the participants were healthy and free of major foot problems. Furthermore, the participants were 20 to 79-years-old and all were Japanese. Therefore, our results cannot be generalised beyond this population. Western people tend to wear shoes indoors, whereas Japanese do not. This cultural difference may have an effect on foot structure and function, and on TGS. Second, the number of male participants was relatively small compared to the number females. Additionally, more than the half the participants were 60 to 79 years of age. Future studies will require a larger and more evenly age-distributed population. Finally, the TGS and characteristics were measured in a sitting position, which is a non-weight bearing position. We measured TGS in the sitting position using the toe grip dynamometer because standing TGS measurements are difficult, especially for older participants. Thus, we will also investigate TGS and foot characteristics in the standing position in a future study.
The authors declare that they have no competing interests.
DU participated in the study design; data acquisition, analysis, and interpretation, and in drafting the manuscript. TF participated in the study design, data acquisition, and helped draft the manuscript. DM participated in data acquisition and helped to draft the manuscript. MS helped with the statistical analysis, data interpretation, and drafting of the manuscript. All authors read and approved the final version of the manuscript.