The primary finding of this study was that a higher ratio of the tibia length to femoral length correlated with better IAAF score in 400-m sprinters, but not in 100-m sprinters. Maintaining a consistent step frequency during 400-m sprinting is required for achieving superior long sprint performance [16, 23]. Morphological factors may contribute to the maintenance of step frequency, potentially by sustaining economical sprinting [5, 6]. The ratio between the leg bone lengths may be useful in reducing the leg’s moment of inertia and the positive work of the hip flexors during the swing phase while sprinting. Thus, this favorable morphology may help achieve superior long sprint performance, potentially by maintaining step frequency and allowing for economical sprinting in 400-m specialized sprinters.
This study showed no correlation between the ratio of the tibia length to femur length and IAAF score in 100-m sprinters. In general, superior 100-m sprint performance may not require economical movement, because sprint velocity during 100-m sprinting does not decrease significantly compared to that during 400-m sprinting [11, 24]. Furthermore, superior 100-m sprint performance is related to greater ground reaction force during 100-m sprinting [15, 40]. An increase in the ground reaction force during 100-m sprinting may be associated with greater sizes of some leg muscles because of the positive relationships between these muscle sizes and 100-m sprint performance [1, 3, 7]. In particular, previous studies determined that greater thigh muscles, including the quadriceps femoris and hamstring, correlated with better 100-m sprint performance in sprinters [1, 3, 7]. When having a higher ratio of the tibial length to femoral length in 100-m sprinters, this morphology may be modeling smaller thigh muscles due to a necessary shortening of the thigh length. Therefore, the ratio between the leg bone lengths may not relate to 100-m sprint performance in 100-m specialized sprinters.
We previously reported using MRI that relative lengths of the leg bones (i.e., tibial length and total length of the femur and tibia) normalized to body height correlated with running performance in endurance runners [14]. By contrast, in the present study, absolute and relative individual and total lengths of the leg bones did not correlate with sprint performance in either 100-m or 400-m sprinters. Although the longer leg is associated with an increase in step length during sprinting [9, 10], step length may be a less important kinematic factor for superior sprint performance than step frequency during both 100-m and 400-m sprinting [15-17]; specifically, an increase in step length is not required for achieving superior sprint performance. Only one study by Morin et al. [15] reported that relative length of the leg (i.e., the distance from the great trochanter to the ground) normalized to body height did not correlate with 100-m sprint velocity. Therefore, the present findings corroborate with this result by showing an absence of a relationship between the leg length and sprint performance in 100-m specialized sprinters. Furthermore, the present study is the first to determine that longer leg may not be required for achieving superior long sprint performance in 400-m specialized sprinters.
This study showed that all bone length variables did not differ between 100-m and 400-m sprinters; thus, characteristics of the leg bone length are similar between the two groups. In additional analyses of the present study, absolute total and individual lengths of the femur and tibia in sprinters (a combined group of 100-m and 400-m sprinters) were higher than those in 5000-m endurance runners (e.g., 431.4 ± 13.9 vs. 420.8 ± 20.2 mm for the femoral length and 358.8 ± 13.8 vs. 351.3 ± 18.2 mm for the tibial length; P < 0.05 for both) observed in our previous study [14], which may be due to greater body height for sprinters than that for endurance runners (172.8 ± 4.0 vs. 169.6 ± 5.6 cm, P = 0.01). By contrast, relative individual and total lengths of the femur and tibia and a ratio of the tibial length to femoral length did not differ between sprinters and endurance runners (25.0 ± 0.4 vs. 24.8 ± 0.7 % of both height for the relative femoral length, 20.8 ± 0.6 vs. 20.7 ± 0.6 % of body height for the relative tibial length, 0.83 ± 0.02 vs. 0.84 ± 0.02; P > 0.05 for all). Similar results were also observed for the leg bone length variables between 100-m and 400-m sprinters and endurance runners (data not shown). Therefore, characteristics of the leg bone lengths relative to body size and the ratio between the leg bone lengths may be similar among athletes competing in distances from 100-m to 5000-m.
Our previous studies determined that longer forefoot bones and greater knee extensor moment arm correlated with better long sprint performance in 400-m sprinters [5, 6]. To the best of our knowledge, no other researchers have reported morphological factors for superior long sprint performance in 400-m sprinters. These favorable morphological factors of superior 400-m sprint performance obtained in our previous studies are also determinants for achieving superior 100-m sprint performance [2, 4]. By contrast, a higher ratio of the tibia length to femoral length may be a determinant only of 400-m sprinters. Therefore, the present study is the first to find a specific morphological factor contributing to superior long sprint performance in 400-m specialized sprinters. The information may be helpful to selecting sprint events and understanding individual features in sprinters, particularly in 400-m specialized sprinters.
Limitations
We hypothesized that a higher ratio of the tibial length relative to femoral length may help achieve superior long sprint performance, potentially by maintaining step frequency and performing economical sprinting in 400-m specialized sprinters. However, we did not measure kinetic (e.g., ground reaction force) and kinematic (e.g., step frequency) data during 400-m sprinting. Further studies are needed to examine the relationships between the leg bone length variables and kinetic and kinematic data during 400-m sprinting.