A major finding obtained here was that TFS was significantly correlated with comfortable walking speed in the pronated and supinated groups. In addition, TFS tended to negatively correlate with the score of timed up-and-go in the pronated and supinated groups and positively correlate with the score of chair stand in the pronated group. The corresponding relationships were not found in the neutral group. These results indicate that TFS is associated with mobility, walking speed in particular, in older women with pronated and supinated feet but not with neutral feet.
A previous study [13] suggested that foot posture does not influence the relationships between TFS and functional performances in older adults, which appears to contradict with our findings. The discrepancy may be due to the differences between the present and prior studies in the procedure for the TFS measurement and analytical approach to examining the association of TFS with functional performances. First, the previous study evaluated TFS by a paper grip test [13], which is not quantitative. On the other hand, the present study quantitatively determined TFS by using a toe grip dynamometer and examined the relationships with the quantitative variables. Second, the previous study adopted not only TFS and functional performance but also FPI as parametric variables and examined the relationships between them [13]. The current study used FPI as a categorical variable rather than a parametric variable and examined the relationship between TFS and functional performance in each group based on the classification of FPI. These differences in the methodological and analytical approaches may explain the discrepancy between the present and previous studies in the results of the influences of foot posture on the association of TFS with functional performance.
For the observed significant relationships in the pronated and supinated groups, ANCOVA showed no significant difference between the two groups in the slopes of the regression lines. This implies that the relative contribution of TFS on comfortable walking speed does not differ between the pronated and supinated groups. The reason why the significant associations were found in older women with pronated and supinated feet is unknown but might partially involve the possible influences of foot posture on the biomechanical profiles of a foot during the gait cycle.
First, pronated feet compared to neutral feet demonstrate greater rearfoot inversion angle [30], and higher activity of tibialis anterior and lower activity of peroneus longus [5] at the initial contact of the gait cycle. At the midstance of the gait cycle, young adults with pronated feet have an increased rearfoot eversion angle [30], and increased activity of the tibialis posterior and decreased activity of the peroneus longus [5]. In the stance phase of the gait cycle, furthermore, pronated feet would require more prolonged activity of intrinsic muscles to stabilize the transverse tarsal joint, which contributes to propulsive force generation, compared to neutral feet [31]. Impairment of functions of the intrinsic foot muscles by the nerve block [32] and smaller size of these muscles [33] are reported to be associated with foot pronation. The cross-sectional area of the intrinsic foot muscles is a determinant factor for TFS [21]. Thus, the intrinsic foot muscles appear to have an important role for developing TFS [21] and for maintaining the medial longitudinal arch, contributing to the support of foot posture [32] [33]. Taking these aspects into account together with the aging effects on TFS [15], it is assumed that for older adults with pronated feet, activating the intrinsic foot muscles more, which directly leads to greater TFS development, would be required to compensate for the aforementioned abnormal joint kinematics and muscle activities of the lower limbs during walking, and consequently it might have resulted in a significant association between TFS and comfortable walking speed.
As compared to individuals with pronated feet, however, those with supinated feet have a decreased peak rearfoot eversion angle and midfoot eversion angle during walking [3]. In addition, supinated feet show greater peak planter pressure at the 2nd, 3rd, and 4th metatarsal head and smaller peak planter pressure at the hallux during walking, compared to neutral feet [34]. These findings suggest that walking biomechanics in supinated feet would differ from that of either pronated or neutral feet. At the same time, it implies that the aforementioned reason assumed for the significant association between TFS and comfortable walking speed in the pronated group cannot be applied to the supinated foot group. However, this somewhat differs from the assumption derived from the ANCOVA results indicating a similar contribution of TFS development to comfortable walking speed between pronated and supinated foot groups. In any case, there is less information on biomechanical profiles during gait cycle in individuals with supinated feet. Further study is needed to clarify this concern.
In contrast to the pronated and supinated foot groups, the neutral foot group did not show a significant relationship between TFS and comfortable walking speed. Individuals with neutral feet do not have abnormal walking biomechanics, observed in those with pronated and supinated feet. In other words, possessing high TFS may not be an advantage for individuals with neutral feet to walk faster, as there is no abnormal walking biomechanics to compensate for. As a general observation, hip extension [35] and hip abduction [36] strength are associated with comfortable walking speed in older adults. In addition, it has been shown that older adults produce net positive work more at the hip joint than at the ankle joint during walking [37]. Considering these aspects, it seems that for older adults with neutral foot posture, muscle strength around the hip joint rather than TFS would play greater contribution to propelling the body in the forward direction during walking.
In the current results, there was no significant relationship between TFS and static balance regardless of the groups. For older adults, no study has examined the association of TFS, determined quantitatively using a dynamometer, with postural static balance. For older adults, many physiological factors are associated with postural balance [38], such as weakness of hip strength and knee extensor strength [39]. From a systematic review, however, evidence indicating the contribution of muscle weakness to postural instability in healthy older adults is limited [40]. Combining this with the current results, it is likely that muscle strength, including TFS, may not be an influential factor for postural balance regardless of the difference of the foot posture.
The present study has some limitations. First, this study examined only older women. It is known that the age-related reduction in TFS is different between men and women [15]. Thus, whether the current findings can be applied to older men is yet to be investigated. Second, the present study did not measure body kinematics and activities of the related muscles during the execution of the functional tasks. Future studies should be directed towards including measurements of these biomechanical parameters to elucidate the physiological backgrounds of the influences of foot posture on the relationship between TFS and mobility in older adults.