Effectiveness of Landmark and Continuous Registrations in Reducing Inter- and Intrasubject Phase Variability

This study aimed to compare the effectiveness of applying landmark and continuous registrations in functional data analysis in reducing inter- and intrasubject phase variability of kinematic data, particularly lower extremity joint angles during the American kettlebell swing (AKS). Twenty healthy male subjects volunteered to perform the AKS test. Three different registration approaches; landmark registration, continuous registration used as an additional method to landmark registration, and continuous registration used as an alternative method to landmark registration were applied, and their effectiveness in aligning the curves was analyzed using functional permutation $t$ -tests. All registration methods showed an improved mean curve than that of the method without registration. The root mean square error (RMSE) values between the mean unregistered curves and the individual unregistered curves were significantly higher than those of the landmark-registered curves ( $p< 0.001$ ) and the continuously registered curves ( $p< 0.001$ ). Continuous registration (as additional method) is the best registration method than landmark and continuous registrations (as alternative method), as it produced the highest mean percent decrease in the RMSE difference between the unregistered and continuously registered (as additional method) curves. Continuous registration (as additional method) appears to be the best method in reducing inter- and intrasubject phase variability than landmark and continuous registrations (as alternative method). Thus, it is recommended to implement continuous registration as an additional method to landmark registration prior to any AKS analysis. Both landmark and continuous registrations (as additional and alternative methods) enhance the likelihood of identifying significant differences between the right and left joint angles.