The influence of motion capture method, sex, and body mass index on lower extremity kinematics and intersession reliability during a drop-vertical jump.

The purpose was to examine the effects of sex and body size on validity and intersession reliability of lower extremity kinematics from marker-less (ML) and marker-based (MB) during a drop vertical jump (DVJ). Forty participants were grouped by sex (male/female) and body size (high/low-BMI). Participants completed two sessions and lower extremity kinematics were recorded with 3D motion capture. Coefficient of multiple correlation (CMC) evaluated intersession reliability. A 2 (sex) by 2 (body size) by 2 (method) repeated measured ANOVA compared kinematic waveforms via statistical parametric mapping. In the sagittal plane, both methods showed very good to excellent reliability (CMC: 0.82-0.99). In the frontal and transverse planes, ML had moderate to very good reliability (CMC: 0.65-0.88), while MB had lower reliability that varied by group (CMC: 0.51-0.88). Discrepancies in sex-based comparisons occurred in the sagittal plane of the hip between methods, and body size comparisons in the frontal and transverse plane of the knee and ankle. Overall, ML had superior intersession reliability. However, motion capture method influences DVJ kinematics and may be due to differences in segment pose estimation between methods, and unique effects of body size and sex. Future studies should consider participant characteristics when implementing ML and interpreting findings.