# A Novel Method for Field Measurement of Ankle Joint Stiffness in Hopping
## 4 Discussion  
The aim of this study was to introduce a novel low-cost method for measuring AJS during hopping. For this purpose, a simple method was considered to simulate GRF and CoM displacement during hopping. Furthermore, inverse kinematic method was used to estimate joint angles. In addition, ankle joint moment was calculated using inverse dynamic method. Finally, AJS was obtained as the slope of ankle joint's moment-angular displacement data. The capability of new method to calculate AJS was different between participants, but overall there was a significant correlation (r = 0.752) between ankle stiffness obtained using the two methods.  
As the proposed method is new, there is no study in literature to compare the results against. However, appropriate agreement was observed between the results of the conventional method used in this study as reference and those reported in the previous literature. The purposes of previous studies were different than this study and each of those assessed correlation or reliability of ankle joint stiffness with different factors such as leg stiffness, performance, or risks of injury. However, the method of measuring the moment and stiffness of ankle joint in hopping are similar in all of these studies, which are the same as reference method in this study. Therefore, the data extracted using the reference method in this study could be compared with previous data, where the net moment in ankle joint closely represents those reported in previous studies [4; 19; 32]. Also, the stiffness of ankle joint, 642.39 N\(\cdot\)m/rad from the reference method is comparable to 433.37 N\(\cdot\)m/rad  
Figure 6: Bland–Altman plots displaying the difference between joint angle (**top**) and joint stiffness (**bottom**) measured with the new method and reference method in ankle joints.  
or 553.77 N\(\cdot\)m/rad that have been reported in the literature [17; 32; 33]. In a similar study proposed a simple method to calculate the leg stiffness, the results were similar to the reference method [25].  
The difference between stiffness values obtained from two methods could be due to underestimation of the propulsive time in the new method, because the ground reaction force was modeled as a sine wave, it was assumed that propulsive time is half of contact time [25]. However, the propulsive time was observed to be over 50% of the contact time in experiment that was due to the asymmetry of the landing and take-off phase duration within the contact time [34]. In the inverse kinematic method, the movement of end effector assumed to be completely vertical, but the horizontal velocity is not zero in kinematic data obtained from motion analyses, which could be a cause of discrepancy compared to the actual ankle joint stiffness. Another reason for the observed differences between the AJS obtained from the proposed new method vs. those obtained from conventional methods could be the position of center of mass, which is supposed to be on the hip joint.  
The method was capable of measuring ankle joint stiffness with a significant correlation with the stiffness measured using conventional method. The proposed method does not require lab equipment, such as force-plate and motion analyses systems, that makes it suitable in the field and in low resource settings.