This is the first study which identified the intersegmental coordination difference between athletes with and without MTSS history. As it was hypothesized that not only single segment motion but also intersegmental coordination showed significant difference between groups. The no history group showed that approximately 60% of intersegmental coordination patterns in the sagittal and coronal planes between the rearfoot and midfoot were in-phase, which the rearfoot and midfoot rotate toward same direction with similar amplitude [17
]. However, segment motions dominantly occurred in the rearfoot compared with the midfoot in the MTSS history group (52% in sagittal plane and 40% in coronal plane).
There are two possible reasons which lead to the rearfoot dominant motion compared with the midfoot in the MTSS history group; the rearfoot motion was excessive or the midfoot motion was restricted. Excursions in sagittal plane were significantly higher in the rearfoot and lower in the midfoot in sagittal plane in the MTSS history group. This means that both excessive rearfoot and insufficient midfoot motions might be related to the rearfoot dominant motion in sagittal plane in the MTSS history group. These can be important findings because coronal plane alignment and motion abnormalities have been mainly reported as risk factor for MTSS by previous studies [4
]. Because this study employed single-leg drop jump, which required larger ranges of motion in each segment compared to running or walking tasks, this may have highlighted different kinematic in the sagittal plane between the groups. The results differ from the hypothesis, the single segment angles in the coronal plane did not show any difference in the rearfoot and midfoot. This indicates that amount of motion in the rearfoot and midfoot in the coronal plane was similar; however, coordinated motion between the rearfoot and midfoot were different between groups. Taken together, not only instant single segment kinematic data, such as peak angle, but also coordination pattern in coronal plane throughout the task should be assessed to detect abnormal kinematics in MTSS patients.
Navicular drop have been considered as risk factors for MTSS and therapists often try to prevent from lowering navicular height by using insoles or taping for treatment and prevention [4
]. However, this study indicates that the rearfoot motion should be stabilized rather than the midfoot or the midfoot motion should be facilitated to correct coordination between segments. Insoles or taping may lead to too much restriction for the midfoot motion. A systematic review revealed that prior use of orthotics was one of risk factors for MTSS [4
]. Use of insoles or taping may result in increased rearfoot motion as a compensation. Rearfoot motion is controlled by the calf muscles, including the gastrocnemius, soleus, tibialis posterior, tibialis anterior, flexor digitorum longus, and flexor hallucis longus [9
]. One of the possible causes of MTSS is traction force induced longitudinal periostitis produced by the soleus, flexor digitorum longus, and tibialis posterior [13
]. It is speculated that excessive motion of the rearfoot leads to higher eccentric contraction level of these muscles and produces higher stress on the medial tibia. A previous study showed that running for 30 min increased the tibialis posterior and flexor digitorum longus stiffness [32
]. Also, runners with an MTSS history demonstrated higher tibialis posterior and flexor digitorum longus stiffness than that in no MTSS history [33
]. Rearfoot dominant pattern may increase eccentric muscle contraction and traction force to the medial border of the tibia and related to MTSS. Thus, the midfoot support with insoles or taping should be carefully provided not to excessively restrict the midfoot motion. In addition, strengthening training for the extrinsic and intrinsic foot muscles, which have important function to control each foot segmental motion, may be important for dynamic intersegmental support [9
]. A previous study showed that there was a positive correlation between muscle activity level of the tibialis posterior and percentage of in-phase between rearfoot and midfoot in single-leg drop jump task [35
There are some limitations in this study. Firstly, this study is a cross-sectional study which cannot identify MTSS risk factor. This study could only present feature of intersegmental coordination pattern of the foot for female lacrosse players with MTSS history. Secondly, since the participants in this study were only female lacrosse players, it is unclear whether same results can be seen in male athletes. Thirdly, because reflective markers were attached on the skin, skin motion artefact can influence the measurement reliability [36
]. Even though repeatability of the measurements was good to excellent in this study, it should be carefully understood that these results did not present joint motions. Finally, this study measured barefoot motion because it is difficult to measure segmental foot motions with wearing a footwear. Care must be taken that footwear may influence on the foot kinematics. However, we believe that motion in barefoot most represents individual motion pattern.