# A Novel Method for Field Measurement of Ankle Joint Stiffness in Hopping
###### Abstract  
Stiffness of ankle joint has been investigated in a wide range of biomechanical studies with a focus on the improvement of performance and reduction in the risk of injury. However, measuring ankle joint stiffness (AJS) using the existing conventional methodologies requires sophisticated equipment such as force plate and motion analyses systems. This study presents a novel method for measuring AJS during a hopping task with no force or motion measurement system. Also the validity of the proposed new method was investigated by comparing the results against those obtained using conventional method in which motion capture and force plate data are used. Twelve participants performed the controlled hopping task at 2.2 Hz, on a force platform, and six high speed cameras recorded the movement. To calculate the AJS in both methods, the lower extremity was modeled as a three linked rigid segments robot with three joints. In the new method, the contact time and flight time were used to calculate ground reaction force, and inverse kinematic and inverse dynamic approaches were used to calculate the ankle kinematic and kinetic. The AJS calculated using the new method was compared against the results of conventional method as the reference. The calculated AJS using this new method (506.47 \(\pm\) 177.84 N\(\cdot\)m/rad) showed a significant correlation (r = 0.752) with the AJS calculated using conventional method (642.39 \(\pm\) 185.96 N\(\cdot\)m/rad). The validation test showed a mean difference of \(-\)24.76% using Bland-Altman plot. The presented method can be used as a valid, and low-cost tool for assessing AJS in the field in low resource settings. inverse dynamics; joint stiffness; inverse kinematic; hopping; ankle +
Footnote †: journal: Applied Sciences  
19 November 2021 15 December 2021 2021 20 December 2021 202  
calculated using the ground reaction force measured using a force plate through the inverse dynamic method. Using this conventional method, higher reliability in calculating the stiffness in ankle joint compared to knee joint was reported in hopping test [3; 4; 9]. A review of such models that were developed based on motion analyses and the measurement of ground reaction force indicated an appropriate reliability for: leg stiffness, the ratio of maximal ground reaction force to vertical displacement of center of mass during contact time, and AJS but an inappropriate reliability for knee joint stiffness while hopping [10]. Additionally, strong correlation was reported between the leg stiffness and ankle joint stiffness in hopping [9] when using the conventional method of calculating ankle joint stiffness. Stiffness of the ankle joint calculated using the conventional method has been considered in wide range of studies such as: comparison of AJS in healthy vs. pathological participants [11; 12], the influence of AJS on sport performance [10; 13; 14; 15; 16], the role of AJS on the assessment of risk of lower extremity injury [16; 17], the reliability of measuring AJS [10; 17; 18], comparison of the different methods of measuring AJS [8; 19], and comparison of leg stiffness against AJS [17]. In addition, AJS calculated using conventional method was used to: compare the variation in AJS with increase in running speed [20], compare the AJS between sprint and endurance athletes during a variety of hopping and drop jump tasks [21], assess the performance between endurance trained athletes and participants from the general population during a maximal hopping test [22], investigate the effect of foot strike and shoe features on ankle joint stiffness [23], and investigate the effects of training on AJS [24]. Hence the previous studies have expressed that AJS could be helpful in the assessment of human locomotion. Despite the wide range of applications for the leg stiffness, its calculation through conventional methods needs access to motion analyses and the measurement of ground reaction force using force plate that are not commonly available in low-resource settings.