With regard to human foot function in the growth process, the foot arch develops in the first decade (3). There is a wide range of variability in foot arch flexibility within the human population. Accordingly, it has been discussed whether the foot arch height (FAH) is responsible for motor skills and physical performance in children (11) or not (1,12,21). In clinical settings, the medial longitudinal arch is given more attention because it seems to play an important role in supporting the body weight during static and dynamic movements. Lin et al. (11) have shown that flatfoot is related to poor motor skills and physical performance in children. On the other hand, Tudor et al. (21) have found no correlation between FAH and motor skills in children. In addition, it has been reported that FAH is not significantly related to the explosive power of the lower limb muscles (12). The relation between FAH and physical performance in children remains a subject of discussion and controversy.
Muscle strength is remarkably important in various stages of physical growth (23) and prevention against premature death (16). The foot muscles are a unit that produces force for postural control during locomotion. Muscles of the foot generate force during the ground contact phase of human locomotion as the metatarsophalangeal joint is dorsiflexed (13). Although vertical ground reaction forces during these movements are absorbed or stored by the medial longitudinal arch (9), these forces also increase plantar fascia tension to maintain the integrity of the truss of the foot (10). Recently, the toe flexor muscle strength (TFS) has been shown as an important factor in enhancing jump performance (6). Because many physical activities are performed in a standing position, the foot arch plays an important role in reducing the impact forces on the load in running or repetitive jumping-like movements (18); at the same time, TFS may help support the arch structure and enhance initiation of dynamic movements. It can be speculated that the foot arch and foot muscle strength might enhance specific types of lower limb physical performance in different ways.
Accordingly, foot function should be evaluated with both the muscle strength and arch structure of the foot in the standing position. However, many studies evaluate only the foot arch in the no standing condition, and no studies have investigated the muscle strength of the foot in children. To better understand the specific roles of the muscle strength and arch height of the foot, it should be useful to consider how these indices of the foot in the standing condition are related to different types of physical performance of the lower limbs of children. Therefore, the aim of this study was to investigate the muscle strength and arch height of the foot in the standing position and the relations between these indices and different types of physical performance involving the lower limbs in children.
A total of 301 healthy elementary school children (third grade: n = 158, age = 8.6 ± 0.5 yr; fifth grade: n = 143, age = 10.6 ± 0.5 yr; means ± SD) participated in this study. The methods and all procedures used this investigation were in accordance with current local guidelines and the Declaration of Helsinki and were approved by the Ethical Committee for Human Experiments, Hokkaido University of Education. All subjects were informed about the experimental procedure and the purpose of the study before the onset of the study. Details of the testing procedure and parental consent forms were explained to the parents or guardians before the onset of the study, and an informed consent was obtained from all participants and their parents or guardians.
All measurements were performed in physical education classes. Standing height and body mass (BM) were measured with lightweight clothes and no shoes. Before the physical performance tests, children were instructed to perform warming-up exercises and to stretch their leg and foot muscles. All physical performance tests and morphological measurements were performed in a random order.
TFS was measured using a specifically designed toe grip dynamometer (T. K. K. 3361; Takei Scientific Instruments Co., Niigata, Japan). The range of force represented by this dynamometer is about 1–400 N (0.5–40 kg on the display monitor of the device). The toe grip dynamometer was calibrated to ensure its accuracy before the experiment, and it was used for all measurements, unless unexpected outcomes were displayed. The experimental setup for the TFS measurements is shown in Figure 1. The first proximal phalanx of the foot was positioned on the grip bar, and the heel was adjusted to the heel stopper on the dynamometer. Because of the wide range of toe lengths within the human population, some of the shortest toes could not grip the bar. During the measurements, the child placed his/her arms in front of his/her chest and was instructed to perform the TFS measurement without pulling the grip bar by extension of the hip in the standing position. Before the maximum TFS measurement, the child performed 3–5 trials at a submaximum level of isometric force. To measure TFS, the child optimally gripped the bar with his/her toes and exerted the maximum force for approximately 3 s. The measured force was recorded on the device itself (Fig. 1B). Measurements of maximum TFS were performed three times in both the right and left foot in a randomized order, and the mean value of the highest value among the three measurements for each foot was used for further analysis. The maximum TFS was also adjusted by BM (relative TFS (rTFS)). Because muscle strength was largely affected by body size (7,14), the absolute value of the TFS was divided by BM and the following equation was used: rTFS = TFS (N)/BM (kg).
Foot arch index.
Foot length (FL) and FAH were measured in the standing position using a ruler (Fig. 1C). FL was measured as the distance between the posterior heel and the longest toe in the standing position. FAH was measured as the vertical distance between the tuberosity of the navicular (scaphoid) bone of the foot and the floor in the standing position, and FAH relative to FL was represented by the foot arch index (FAI) (FAH/FL × 100). The vertical height of the navicular bone has been used as a noninvasive clinical measure of the medial longitudinal arch (5).
Physical performance tests for lower limbs.
Figure 2 shows four types of physical performance tests involving the lower limbs. Physical performance was evaluated with a 50-m sprint (Fig. 2A), a standing broad jump (Fig. 2B), a repeated side step (Fig. 2C), and a rebound jump (Fig. 2D). During these tests, all children wore rubber-soled shoes.
The child prepared to run from the standing position. After a signal, they started to sprint on a 50-m dirt track. Time was measured twice using a manual stopwatch, and the best sprinting time was used for further analysis.
Standing broad jump.
The child was asked to jump horizontally as far as possible. To familiarize the performance of standing broad jump using double leg with arm swing, two trials at a submaximal level were performed. Then, maximum jump was performed and a distance between the start line and the landing point on the ground was measured. The longest jump distance among three trials was used for further analysis.
Repeated side step.
This agility test required the child to move laterally among three lines as quickly as possible for 20 s. Three parallel lines were set 1 m apart from each other, and the child stepped sideways from one line to the other line. The number of times the subject crossed the lines was counted. The highest value among two trials was used for further analysis.
Five consecutive rebound jumps were used for evaluating ballistic stretch–shortening cycle movement ability. Rebound jumps were repeated vertical jumps with rebound movement similar to a bouncing ball. Before the actual measurement, 3–5 trials at a submaximum level of vertical rebound jump were performed. Then, the child was instructed to perform five consecutive jumps at maximum height and minimum ground contact time. The child placed his/her hands on his/her hips throughout the entire jumps and kept his/her torso in an upright position to eliminate the effects of arm swing and emphasize the use of the leg muscles. The repeated rebound jumps were performed on a sensor mat that was connected to a personal computer with an analog-to-digital converter (Multi Jump Tester, PH-1260; DKH, Tokyo, Japan). The rebound jump index (RJI) for five consecutive rebound jumps was calculated by the following equation (19): RJI (m·s-1) = (1/8) (g × Tf2)/Tc, where g is the acceleration of gravity (9.81 m·s-2), Tf is time of the flight of the jump (s), and Tc is time of the foot contact on the ground (s). The best record of the RJI in five consecutive jumps was used for further analysis (Fig. 2D).
All data were presented as means ± SD. The number of data was noted in each analysis because of failure to execute performance tests or lack of test score. Relations between all variables were examined by Pearson correlation and multiple linear regression analysis. Reproducibility of the TFS among three trials was evaluated using intraclass correlation coefficients. Gender differences in all parameters were examined by an unpaired t-test. The level of statistical significance was set at P < 0.05.
TFS and rTFS were 81.2 ± 22.3 N and 2.9 ± 0.8 N·kg-1 in third graders and 103.7 ± 27.4 N and 2.8 ± 0.8 N·kg-1 in fifth graders. Intraclass correlation coefficient (1,3) values for TFS in third and in fifth graders were 0.830 (P < 0.001) and 0.838 (P < 0.001), respectively. Morphological characteristics and physical performance of the foot and lower limbs are summarized in Table 1. Gender differences were found in TFS, 50-m sprint, standing broad jump, and repeated side step for third grade children and in body mass index, arch height, FAI, and standing broad jump for fifth grade children. Boys showed higher values in most of parameters compared with girls, whereas FAH and FAI were greater for fifth grade girls than those for fifth grade boys.
Figure 3 shows the correlation between TFS and FAI and between rTFS and FAI. There were no significant correlations between TFS and FAI for third graders (overall, r = 0.002; boys, r = 0.032; girls, r = -0.052) and fifth graders (overall, r = -0.006; boys, r = -0.092; girls, r = 0.056) and between rTFS and FAI for third graders (overall, r = 0.050; boys, r = 0.045; girls, r = 0.041) and fifth graders (overall, r = -0.043; boys, r = -0.137; girls, r = 0.038), indicating that TFS and FAI were independent variables.
Table 2 shows correlation coefficients between the foot parameters (TFS, rTFS, and FAI) and physical performances involving the lower limbs. TFS was correlated with 50-m sprint in overall third graders, overall fifth graders, and fifth grade girls, with standing broad jump in overall third graders, overall fifth graders, and fifth grade boys and girls, and with repeated side step in overall third graders, overall fifth graders, third grade boys, and fifth grade girls but not with RJI in the children from either grade. rTFS was correlated with all parameters of physical performances in overall third graders and overall fifth graders, with 50-m sprint in fifth grade boys and girls, with standing broad jump in third grade girls and fifth grade boys and girls, with repeated side-step in third grade boys and fifth grade boys and girls, and with RJI in third grade girls and fifth grade boys. No significant correlations between FAI and any parameters of physical performances were found, except for RJI in overall fifth graders, repeated side step in third grade girls, and 50-m sprint in fifth grade girls.
Table 3 shows the results from multiple linear regression analyses. After adjusting for confounding factors (gender and BM), TFS was the only significant correlating factor for all lower-limb performance measures in overall third and fifth graders; FAI was not a factor, except for the RJI in overall fifth graders.
This study showed that TFS was associated with enhancement of some measures of lower limb physical performance in children. Our main findings were that TFS and rTFS were not significantly related to FAI and that rTFS was related to all dynamic muscular performance measures of the lower limbs in children. FAI was related with only repetitive dynamic movements such as rebound jump ability in fifth grade children. These results suggest that the foot function should be evaluated in terms of both the muscle strength and arch structure of the foot in children.
Our results indicate that dynamic performance involving the lower limbs is supported by the muscle strength of the foot. Many kinds of physical performance require high force production by the muscles. Sprinting and jumping correlate with the maximum force-generating capacity of the lower limbs (25,26). Vertical jump performance is enhanced by multisegment force transmission to generate large force rapidly on the ground (4). The large force reacting on the ground during the dynamic performance of the lower limbs is explained by the net moment of muscles produced by the hip extensors, knee extensors, ankle plantarflexors, and toe flexors. Therefore, larger TFS might be required to enhance the muscular performance of the lower limbs in children to some extent. This is supported by the finding that exercise training for the toe flexor muscles resulted in improvement of jump performance in young men (6,22).
The muscle and tendon complex of the foot helps support the foot arch and generates force during movement. The foot arch might help enhance some physical performance. It has been reported that the midtarsal joint of the foot enhances the efficiency of bipedal locomotion by storage and release of elastic strain energy in the longitudinal arch (9). These might explain our results that FAI was related to the RJI in fifth grade children. The foot arch may have an important role in storing and releasing elastic strain energy during repetitive dynamic movements of the foot and leg.
There are gender differences in pubertal development (2,8,24), and the onset of puberty is earlier in girls than that in boys (15). These facts might account for our results that TFS and some physical performance tests showed gender differences in third grade children whereas there was no difference in TFS and some physical performance tests in fifth grade children. Although it was unclear why there were gender differences in TFS in third grade children, these differences might be due to the neurological development and size of the plantar muscles. The force-generating capacity of muscle is generally determined by both neurological factors and muscle size. At the same time, specific tension, which is the muscle strength adjusted by muscle size, does not differ between genders. During growth, boys increase their vertical jump height, whereas girls decrease their vertical ground reaction force during the take-off of jump (17,20). Thus, gender differences in the neuromuscular power of the lower limbs emerge during puberty, yet these differences may be offset by the muscular development of girls in fifth grade. Further studies are needed to understand the neurological and morphological factors of toe flexor muscles in the growth stage of children using noninvasive techniques, such as EMG, magnetic resonance imaging, and ultrasound analyses.
From a practical point of view, to improve the physical performance of the lower limbs, exercise training for toe flexor muscles may be useful. Although it seems that the ability of physical performance is primarily determined by the muscle functions of large muscles in the lower limb, there may be potential to enhance physical performance with increases in the muscle strength of the foot in children.
Our results suggest that TFS is independent from FAI, such that measurement of TFS is another important parameter for evaluating lower limb and foot functions. Measurement of TFS can be a useful predictor of physical performance involving the lower limbs of children.
We thank the all children and teachers who cooperated with us for this study.
This study was funded by Grant-in-Aid for Scientific Research (C; #22500619 for H. S.), for Young Scientists (B; #24700631 and #23700688 for M. O., and T. K., respectively); for Young Scientists (A; #23680066 for J. Y.); and for Exploratory Research (#24650408 for J. Y.) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.
N. M. and J. Y. contributed equally to this work.
The authors declare no conflicts of interest.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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