Midstance instant identification methods in sprinting: comparative assessment and biomechanical implications in able-bodied athletes and athletes with lower limb-amputation.

INTRODUCTION: Midstance is a key instant in running, marking the transition from braking phase to propulsion phase. Despite its significance for stability and force transmission, a standard identification method of midstance instant is lacking, leading to inconsistent data interpretations. This study aimed to systematically compare different midstance instant identification methods and to highlight how they affect trunk and lower-limb kinematics in both able-bodied sprinters and sprinters with lower limb amputation. METHODS: 22 able-bodied athletes, 4 athletes with unilateral transtibial and 4 athletes with unilateral transfemoral amputation performed two maximal 60-m sprints on an instrumented track. Kinematic and kinetic data were recorded using 10 infrared cameras and 9 embedded force platforms. Seven methods were considered, including kinematic and kinetic approaches. Sagittal absolute angles of trunk, thigh, and shank segments, as well as the hip, knee and ankle joints were extracted at the instants identified by each method. Both temporal differences and corresponding body postures at the detected instants were evaluated across methods and against the theoretical midpoint of stance (50%). Differences were assessed using either one-way repeated measures ANOVA or Friedman tests, depending on data normality, followed by Bonferroni post-hoc comparisons when appropriate. RESULTS: Instants identified as midstance consistently preceded the temporal midpoint of the stance phase across methods considering both able-bodied and athletes with lower-limb amputation. Kinematic methods showed limited consistency, identifying different instants within the stance phase. Their agreement with kinetic-based methods was only partial and depended on both the identification method and the athlete group. Consequently, midstance identified with different methods reflected distinct body postures within the stance phase. This resulted in significant variations in segment and joint angles, potentially leading to different biomechanical interpretations of sprinting. DISCUSSION: Since the present findings suggest that method performance may differ across able-bodied athletes, athletes with transtibial amputation, and athletes with transfemoral amputation, researchers and practitioners should carefully consider the choice of method, clearly disclose it and ensure its consistent application.