This study assessed the RFD of toe pressure strength in healthy adults and older adults and compared the results with those of different age groups. The association between RFD of toe pressure strength and TUG was also examined. Results showed that the RFD of toe pressure strength in older adults was significantly lower than in healthy adults. Multiple regression analyses adjusted for covariates revealed that RFD of toe pressure strength was associated with TUG only in older adults.
A comparison of the RFD of toe pressure strength in healthy and older adults showed that, as we hypothesized, the RFD of toe pressure strength was lower in older adults than in healthy adults, similar to other RFDs previously reported [28, 29]. Subsequent analysis was performed with sex entered as a covariate, which remained unchanged, and significant. In other words, the results suggest that the RFD of toe pressure strength may also be affected by aging. In the RFD of the lower limb compared by age group, 36–68% lower knee flexion and extension strength and 25–50% lower ankle plantar flexion and dorsiflexion strength were reported in older adults than in healthy adults [30]. The RFD of toe pressure strength in older adults was similar to roughly 51% in healthy adults in the present study of RFD of toe pressure strength. The degree of difference was also assessed using Cohen's d, which was found to be “large” at 0.92 [31, 32]. On the other hand, the RFD of toe pressure strength was the smallest when compared with Cohen's d of toe pressure strength, hand grip strength, and knee extension strength measured in this study. Previous studies on ankle dorsiflexion muscle strength found that the degree of reduction in RFD was greater than that of maximal muscle strength, which differed from the results of this study [8]. This suggests that the degree of reduction in RFD may differ between the ankle and toe, albeit this is speculative in the present study.
A correlation analysis was conducted by age group with TUG as the dependent variable. The results showed a significant negative correlation between TUG and RFD of toe pressure strength only in older adults. In other words, the results showed a tendency to perform the TUG faster if the toe muscles were stronger and could be exerted more quickly. In contrast, no correlation was found between TUG and RFD of toe pressure strength in healthy adults. A study in healthy adults reported an association between RFD in the leg press and the put-a-shot test [33]. The RFD of the shoulder joint in tennis players has also been shown to affect serve speed [34]. Thus, the majority of studies on RFD in healthy adults have reported an association with high performance. In this study, TUG was the dependent variable. TUG is instructed to sit down, stand up, and walk as quickly as possible. In other words, we hypothesize that the TUG was less difficult for the healthy adults in the study and did not correlate with RFD of toe pressure strength due to the ceiling effect of the assessment.
Furthermore, TUG was used as the dependent variable in regression analysis. Neither a single regression analysis with RFD of toe pressure strength as an independent variable nor a multiple regression analysis with covariates entered showed a significant association in healthy adults. On the other hand, RFD of toe pressure strength was associated with TUG in older adults. This association was also found in multiple regression analysis models with covariates as inputs. Previous studies have shown that a decrease in the ratio of the size of type II muscle fibers to the size of type I muscle fibers results in a reduction of rapid force capacity (RFD) [35]. Age-related muscle atrophy is more pronounced in type II fibers than in type I fibers [36, 37]. We believe that these physiological characteristics were one factor in the association between TUG and RFD of toe pressure strength in older adults. It has also been reported that neurogenic factors are more important for RFD than muscular factors [6]. Factors affecting neural input with age include reduced motor unit discharges [38], reduced maximal firing rates [39], and prolonged neurotransmission times [40], with factors associated with fast movements being biasedly reduced [38]. We believe that the older adults analyzed in this study had reduced RFD due to the abovementioned mechanisms and were associated with TUGs that made them sit down, stand up, and walk as quickly as possible. Indeed, RFD of the lower limb muscles is associated with walking speed and stand-up movements, supporting the validity of the study results [41, 42]. Our previous study of older adults found that toe pressure strength was associated with the presence or absence of assistance standing up from a chair [43]. It has been shown that the toes, even though they are small muscles located in the periphery of the body, are involved in standing up movements, so it is valid and very interesting that toe elements were also involved in the TUG with standing up movements in this study.
The strength of this study is that it focused on RFD of the toes, which has been rarely reported, and revealed that RFD of toe pressure strength changes with age. Another point is that RFD of toe pressure strength in older adults was associated with TUG, which reflects physical function, and health status. Studies assessing RFD of ankle plantar flexor strength in older adults have shown an association between TUG and RFD of ankle plantar flexor strength. However, the study involved a small sample size of 19 older adults, which has been cited as one of its limitations [44]. We believe that one of the strengths of this study is that it solved the sample size problem and showed a correlation between TUG and RFD. Interventions to improve RFD have been shown to benefit from high-intensity[45] and speeded-up strength training [46]. Although the present study cannot address the effects of the intervention, it was possible to report primary results showing that training for toe RFD may also be necessary to improve daily activities, such as standing, sitting, and walking, which are included in the TUG.
Some limitations of this study must be taken into account. The RFD in this study is determined by the maximum muscle strength and the time taken to exert maximum muscle strength. Some previous studies have used RFD at earlier stages before maximal muscle strength as a representative [47, 48]. Some studies have also shown that different time intervals have different physiological significances [49]. The second point is that the variables used in this study were maximal muscle strength, except for the RFD of toe pressure strength. Therefore, comparing RFDs with those of other body sites was impossible. The contribution to physical function and performance must be compared with maximal muscle strength and RFD in other body parts. Finally, because this study is cross-sectional, causal relationships cannot be indicated.