# Effects of Achilles Tendon Moment Arm Length on Insertional Achilles Tendonopathy
## 4 Discussion  
COR method and TE methods are mainly used for the measurement of the lever arm of Achilles tendon [5]. It has been reported that the TE method shows a shorter moment arm than the COR method [5]. However, it has also been reported that there is no difference in measurements between the two methods [20]. Thus, there is no unified view on this issue. Furthermore, the Achilles tendon moment arm has been shown to be influenced by weight-bearing [21], and an evaluation method using weight-bearing X-ray has been developed for taking images during weight-bearing [10]. It is also feasible to use radiographs to measure the Achilles tendon moment arm [10]. In the present study, we examined the group of patients with IAT before and after surgery, in comparison to a control group of healthy individuals. In order to measure the moment arm and evaluation of the morphology of the bone during weight-bearing in each group, we decided to use a weight-bearing X-ray for evaluation. Also, the usage of an X-ray is a simple method and allows us to perform anatomical calculations easily. Therefore, we determined that it was a good method in our study. A study using this measurement method reported a moment arm length of 64.3 mm [10]. In the present study, the average moment arm lengths of the healthy and IAT groups were 51.7 mm and 53.7 mm, respectively, both of which were shorter than the previously reported value. However, the length of the sole of subjects in the previous report was 185.7 mm, in contrast to 171 mm and 170.1 mm in healthy and IAT groups, respectively, in the present study. This suggests that the moment arm length may be influenced by the difference in the physical constitution. In addition, [PERSON] et al. [10] showed that the moment arm length and \(\alpha\) angle are influenced by the difference in the shape of the foot, such as flat and concave feet. In the present study, no significant difference in \(\alpha\) angle between the control and experimental groups was observed, indicating that the results are not influenced by the difference in the shape of the subject's foot, even in flat and concave feet. To date, there have been no reports comparing the Achilles tendon moment lengths between healthy individuals and patients with IAT, but many factors are known to influence the pathophysiology of IAT. [PERSON] et al. [22] stated that it is caused by the traction force of the Achilles tendon during ankle movement. In the present study, those with IAT had a significantly longer Achilles tendon moment arm than healthy individuals. However, there was no significant difference in the force applied to the Achilles tendon between two groups. This indicates that the traction force of the Achilles tendon, which is the cause of IAT, is not influenced simply by the force applied to the Achilles tendon. It has been shown that the Achilles tendon exerts approximately 4 to 6 times its weight during running [23; 24], stretching approximately 8% of the total length of the Achilles tendon [25]. It has also been shown that, despite the smaller force applied during walking than in running, approximately 7% of the total length of the Achilles tendon is stretched during walking [26]. This indicates that the insertion of the Achilles tendon is influenced not only by simple tension but also by the elastic energy due to the stretching of the Achilles tendon [27]. In fact, triceps surae eccentric exercise has been recommended as a conservative therapy for IAT [28; 29], and the extension of the Achilles tendon is a major cause of IAT. Our data revealed that individuals with IAT had a longer moment arm length than healthy individuals. This suggests that, with a long moment arm, the arc length associated with the ankle rotation lengthens, increasing the length displacement of the muscle-tendon complex, and further increasing the elastic energy consumption of the Achilles tendon [13; 14]. Therefore, the elasticity of the Achilles tendon is shown to be greatly responsible for the development of IAT.