Comparison of two coupling methods regarding coupling patterns, sensitivity to footwear and potential future injury applications.
Researchers have quantified the effect of footwear conditions on movement patterns and injury risk for runners using discrete kinematic variables and/or relative coupling variables. Coupling is typically assessed using the transfer coefficient (TC) or the vector coding (VC) approach. However, a thorough comparison of both methods regarding their interpretation, sensitivity to testing conditions and information regarding coupling strength as one overall coupling score is missing. This study aimed to compare both methods regarding a) their coupling patterns, b) their sensitivity to footwear conditions and c) to discuss both coupling approaches regarding an overall coupling score. 3D motion capture data was collected of 10 males running on a treadmill with and without shoes. Rearfoot frontal and tibia transverse plane motion was analyzed. Discrete kinematic variables and relative coupling variables were calculated via the TC and VC approach for the landing and push-off phase. A novel variable, the coupling score, was developed and calculated using both coupling methods. Coupling variables calculated with both methods showed differences in coupling patterns, especially for the landing phase (TC ≈ 0.5/in-phase towards rearfoot-phase, VC ≈ 300°/anti-phase). VC offers further details, such as coupling frequencies, compared to the TC. Moreover, both methods were unable to distinguish between footwear conditions regarding their coupling patterns or coupling scores. Strong correlations (r ≤ 0.7, p < 0.0125) between coupling scores assessed with either method suggest that both methods contain similar information regarding coupling strength. The coupling score represents a first attempt to quantify a weighted coupling pattern quantitatively. Future studies have to validate whether the coupling score might be linked to injury risks.