Velocity-Matched Resisted and Unresisted Sprinting Across Broad Loads Show Minor Biomechanical Differences.

Many comparisons of unresisted and resisted sprinting do not match running velocity. As such, differences may partly reflect velocity rather than resistance. This cross-sectional study examined kinetic and kinematic differences between unresisted and resisted sprinting accelerations at matched running velocities. Fourteen amateur sportsmen performed unresisted and resisted maximal overground sprints. Motorized resistance was applied via the waist and set at 20%, 40%, and 60% decreases in maximal unresisted velocity (VDEC). Ground reaction forces and segment/joint angles were compared across velocity-matched steps between the conditions using ANOVA and statistical parametric mapping (SPM). Longer contact times were observed for resisted versus the unresisted condition, specifically 6.4% and 16.8% at 40% and 60% VDEC (ES: -0.423; -0.851, respectively). Vertical impulse was 8.0%, 7.3%, and 14.6% greater in the resisted condition at 20%, 40%, and 60% VDEC, (ES: -0.788; -0.551; and -0.862, respectively). Ankle range of motion was 5.8° greater at 60% VDEC (ES: -1.064), and hip angle at toe-off was 2.8° and 8.3° greater at 40% and 60% VDEC, (ES: -0.385; -0.987, respectively), compared to unresisted sprinting. No other statistically significant differences were observed. SPM analyses indicated differences only between kinetics at 20% VDEC and ankle and trunk angles at 20% and 60% VDEC. Our findings contrast the larger and consistent changes typically reported in resisted sprinting, indicating movement velocity may be a confounder. The comparatively modest remaining differences detected may reflect load-specific mechanisms that deserve future study.