Validity and reliability of a smartphone motion analysis app for lower limb kinematics during treadmill running

Objective: To investigate the validity and reliability of a smartphone application for selected lower-limb kinematics during treadmill running. Design: Validity and reliability study. Setting: Biomechanics laboratory. Participants: Twenty healthy female runners. Main outcome measure(s): Sagittal-plane hip, knee, and ankle angle and rearfoot eversion were assessed using the Coach's Eye Smartphone application and a 3D motion capture system. Paired t-test and intraclass correlation coefficients (ICC) established criterion validity of Coach's Eye; ICC determined test-retest and intrarater/interrater reliability. Standard error of measurement (SEM) and minimal detectable change (MDC) were also reported. Results: Significant differences were found between Coach's Eye and 3D measurements for ankle angle at touchdown and knee angle at toe-off (p < 0.05). ICCs for validity of Coach's Eye were excellent for rearfoot eversion at touchdown (ICC = 0.79) and fair-to-good for the other kinematics (range 0.51–0.74), except for hip at touchdown, which was poor (ICC = 0.36). Test-retest (range 0.80–0.92), intrarater (range 0.95–0.99) and interrater (range 0.87–0.94) ICC results were excellent for all selected kinematics. Conclusion: Coach's Eye can be used as a surrogate for 3D measures of knee and rearfoot in/eversion at touchdown, and hip, ankle, and rearfoot in/eversion at toe-off, but not for hip and ankle at touchdown or knee at toe-off. Reliable running kinematics were obtained using Coach's Eye, making it suitable for repeated measures.