In this study, we first determined that a larger US-estimated plantar flexor MV is correlated with less dorsiflexion flexibility, as evaluated using three parameters. Thereafter, to verify this finding, we examined the correlations between plantar flexor MV and dorsiflexion flexibility parameters using MRI and demonstrated that a larger MRI-measured plantar flexor MV is correlated with less dorsiflexion flexibility. Several previous studies have reported that a larger muscle size, as measured using US or MRI, is related to less joint flexibility (i.e., lower ROM and higher passive stiffness) [7, 10]. However, prior to the present study, no study has examined the relationship between MRI-measured MV and joint flexibility. Therefore, the present study is the first to determine this relationship.
Most previous studies examining the relationship between muscle size and joint flexibility employed passive stiffness as a parameter of joint flexibility [7, 9, 10, 12]. Only one study, by Magnusson et al. , determined that a larger biceps femoris CSA is correlated with lower passive knee extension ROM. The present study corroborates their results by showing the relationship between larger plantar flexor MV and lower passive dorsiflexion ROM. Furthermore, in addition to passive ROM, the present study assessed active ROM and demonstrated the relationship between larger plantar flexor MV and lower active dorsiflexion ROM. The magnitude of the passive ROM may affect pain sensitivity (i.e., stretch tolerance) when musculotendinous tissues are stretched among subjects [20, 21]. In contrast, the magnitude of active ROM does not appear to have this effect. Therefore, active ROM in addition to passive ROM and passive stiffness is a useful parameter to assess joint flexibility.
This study determined that the plantar flexor MV is inversely related to three dorsiflexion flexibility parameters. Nevertheless, the strength of these relationships was moderate; thus, the plantar flexor MV alone can explain only a part of dorsiflexion flexibility. Recently, shear wave ultrasound elastography has been used to assess the stiffness of tissues, including muscle [17, 19,20,21, 24]. Previous studies have determined that higher plantar flexor muscle stiffness measured using shear wave ultrasound elastography is correlated with lower dorsiflexion ROM in healthy young adults [20, 21]. To further clarify the present findings, further studies are needed to comprehensively determine the effects of the morphological and mechanical properties of muscle tissue on joint flexibility.
In the findings of study 1, correlation coefficients with all three dorsiflexion flexibility parameters were relatively higher for US-estimated plantar flexor MV than for plantar flexor MT. The findings of study 2 also showed that although correlation coefficients with passive dorsiflexion ROM and stiffness were similar between plantar flexor MV and ACSA, the correlation coefficient with active dorsiflexion ROM was relatively higher for plantar flexor MV than for plantar flexor ACSA. These present findings suggest that MV may be a better marker to evaluate the relationship between muscle size and joint flexibility than MT and CSA (i.e., ACSA). Nevertheless, in both studies 1 and 2, we found that the correlation coefficients with all three dorsiflexion flexibility parameters were relatively higher for lower leg circumference than for plantar flexor MV. The lower leg circumference includes not only muscle tissue properties but also properties of other tissues, including the fascia, adipose, and skin tissues. Of those tissues, Yoshitake et al.  determined that the skin may affect shear wave ultrasound elastography-measured muscle stiffness; the authors showed that the muscle stiffness; decreased by 50 % after the skin covering the gastrocnemius medialis was removed in cadavers. In the clinical setting, MV is often inconvenient to measure, especially using MRI, owing to the large clinical demand and considerable costs involved. Therefore, the findings of the present and previous studies suggest that the lower leg circumference may be useful as a sufficient and surrogate marker to assess joint flexibility in various large populations.
This study recruited only healthy young males, and therefore, it remains unclear whether the present findings can be generalized to populations of other ages (i.e., older individuals) and with different health states (i.e., patients with chronic diseases) or females. In particular, older individuals have less joint flexibility than do young individuals [8, 25], despite reductions of muscle size, because of age-related physiological processes . Thus, factors other than muscle size may play an important role in determining joint flexibility in older individuals. Indeed, previous studies have demonstrated that the sciatic nerve may potentially limit dorsiflexion ROM [17, 20]. In a recent study, Hirata et al.  found that stiffness of the sciatic nerve (i.e., measured at 15º dorsiflexion) measured using shear wave ultrasound elastography is correlated with passive dorsiflexion ROM in older individuals but not young individuals. This finding may be due to changes in the sensitivity to tension applied to the sciatic nerve that occur with aging because sciatic nerve stiffness is lower in older individuals than in young individuals . In addition, we and others previously determined that higher passive dorsiflexion stiffness is correlated with better race performance (i.e., 100-m or 5000-m personal best times) in both sprinters and endurance runners [2,3,4,5]; thus, this result may be simply due to the relationship between muscle size and race performance in both athlete groups. Although several studies have reported a potential relationship between plantar flexor muscle size and sprint performance [27, 28], we and others determined the lack of this relationship [23, 29, 30]. Furthermore, smaller, rather than larger, plantar flexor muscles may be more favorable for long-distance running among endurance runners [31, 32]. Therefore, it is hypothesized that the relationship between plantar flexor muscle size and dorsiflexion flexibility is unique for sprinters and/or endurance runners. To aid in the application of the present findings to the clinical setting, further studies are needed to determine the effect of plantar flexor muscle size on dorsiflexion flexibility in various populations.
The present study has several limitations. First, in this study, we instructed the subjects to refrain from performing stretching exercises for at least 2 hours before the measurements of dorsiflexion flexibility parameters, as in our previous studies [4, 5]. This decision was based on findings of previous studies showing that the level of dorsiflexion flexibility increased after acute plantar flexor stretching exercises and reversed to that before stretching exercises within 30 min [11, 19, 22]. Nevertheless, some previous studies strictly prohibited stretching exercises for more than 2 hours [17, 18]. Additionally, we instructed the subjects to avoid strenuous physical activity within 24 hours before the dorsiflexion flexibility measurements, as in previous studies [20, 21]. However, this interval might be insufficient to measure joint flexibility correctly because some previous studies strictly prohibited strenuous physical activity for more than 24 hours (i.e., > 48 hours) [17, 18]. Next, although we measured the maximal angle achieved during voluntary dorsiflexion as active dorsiflexion ROM, this measure may be affected by dorsiflexor strength. Furthermore, Kubo et al.  reported a positive correlation between plantar flexor maximal voluntary torque and passive dorsiflexion stiffness. Thus, plantar flexor and dorsiflexor strength might be related to the joint flexibility parameters measured in the present study. However, we did not measure plantar flexor or dorsiflexor strength. Further studies are needed to comprehensively determine the relationships of the muscle size and strength of the plantar flexors and dorsiflexors with dorsiflexion flexibility.