Author(s): KURITA, T., MIYAZAKI, T., KUBOTA, H., TAKAI, Y., Institution: NATIONAL INSTITUTE OF FITNESS AND SPORTS IN KANOYA, Country: JAPAN, Abstract-ID: 2797

INTRODUCTION:
Resisted sprinting (RS) suppresses increases in running speed by applying an external load from behind the runner. When a high load is imposed, the movement can be constrained to resemble the initial acceleration phase of unresisted sprinting (US), whereas a lower load can be adjusted to approximate the near maximal speed phase. Similar to US, RS also comprises an acceleration phase and reaches a plateau once a certain running speed is attained (plateau phase). A recent study has reported that, at the same running speed, there are no differences in kinematics or kinetics between US and RS. In contrast, other research has shown that near the plateau phase of RS, horizontal ground reaction forces (GRFs) decrease and vertical GRFs increase, even at matches speeds. The discrepancy may be attributable to differences in running phase (acceleration vs. plateau) during RS. Therefore, this study aimed to clarify how differences in running phase during RS affect sprint kinematics and kinetics.
METHODS:
Seventeen male collegiate soccer players performed maximal 15 m sprints in four conditions: US and RS with the loads corresponding to 5%, 10%, and 15% of body mass (1080Sprint). GRFs were recorded on a 50 m force-plate form (TF-90100), and 3D kinematics were captured with an optical system (Mac3D). Spatiotemporal variables, GRF variables, center of mass (CoM) height, scissor angle, and lower-limb joint kinematics/kinetics were calculated. For each load, the two steps immediately preceding the 15-m mark were defined as representative of the plateau phase. Two steps from unresisted sprinting (US) and other RS conditions matched for running speed were then extracted and compared with the plateau-phase variables at each load. To identify the running phases in US and RS, a saturated exponential model was fitted to the speed–distance relationship to estimate maximal running speed and the distance at which it occurred for each trial, and these values were subsequently used for normalization.
RESULTS:
During the plateau phase, RS exhibited a lower step frequency and a longer step length compared with US. This difference was attributable to an increased vertical impulse. In addition, hip and knee joint angles during stance were more extended, and the scissor angle was greater in RS than in US. The COM was also positioned higher, suggesting a more upright trunk posture in RS relative to US. In contrast, during the acceleration phase, RS showed no significant differences from US in most variables.
CONCLUSION:
RS exhibited sprint mechanics similar to US during the acceleration phase but differed from US during the plateau phase.