Abstract
This study aimed to compare the components of force-velocity (F-V) and power-velocity (P-V) profiles and the mechanical effectiveness of force application (or force ratio–RF) among various sled-towing loads during the entire acceleration phase of a weighted sled sprint. Eighteen sprinters performed four 50-m sprints in various conditions: unloaded; with a load corresponding to 20% of the athlete’s body mass (BM); with a load of 30% BM; and with a load of 40% BM. Data were collected with five video cameras, and the images were digitised to obtain velocity from the derivation of the centre-of-mass position. F-V and P-V components and RF were estimated from sprinting velocity-time data for each load using a validated method that is based on an inverse dynamic approach applied to the sprinter’s centre-of-mass (it models the horizontal antero-posterior and vertical ground reaction force components) and requires only measurement of anthropometric and spatiotemporal variables (body mass, stature and instantaneous position or velocity during the acceleration phase). The theoretical maximal velocity decreased with load compared with the unloaded condition (for 20% BM: -6%, effect size (ES) = 0,38; for 30% BM: -15%, ES = 1.02; for 40% BM: -18%, ES = 1.10). The theoretical maximal horizontal force (F0) and maximal power were not different among conditions. However, power at the end of the acceleration phase increased with load (40% BM vs 0%: 72%; ES = 2.73) as well as the maximal mechanical effectiveness (12%; ES = 0.85). The linear decrease in RF was different between 30 or 40% BM and the unloaded condition (-23%; ES = 0.74 and 0.66). Better effectiveness may be developed with 40% BM load at the beginning of the acceleration and with the various load-induced changes in the components of the F-V and P-V relationships, allowing a more accurate determination of optimal loading conditions for maximizing power.
Highlights
Acceleration is a determinant factor for success in sprinting events
The maximal force ratio increased with increasing sled load (ES = 0.57– 0.87, small to moderate effect), and the decrease in the force ratio during the acceleration phase was greater with increasing sled load (ES = 0.74 and 0.66 for 30% and 40% load, moderate effect)
The main findings of this study are as follows: (1) maximal mechanical effectiveness of force application increased with load, and the ability to limit the drop in effectiveness during the sprinting acceleration phase decreased when towing a sled with 30% or 40% body mass (BM) compared with the unloaded condition; and (2) power computed at the end of the acceleration phase was greater with increasing load
Summary
Better sprint acceleration performance is associated with the athlete being able to exert a greater force in the horizontal. Weighted sled towing and sprinting mechanics direction, rather than applying a greater resultant force [1,2,3]. The orientation of the force applied by the athlete on the ground is evaluated with the effectiveness or ‘force ratio’, which is defined as the ratio of the horizontal force to the resultant force averaged over the stance phase [1]. Studies have shown that an attenuated decrease in the force ratio during the sprint is highly correlated with a better performance [1, 2]. A sprinter needs to direct the resultant force onto the ground horizontally and to limit the decrease in the force ratio along the acceleration phase of the sprint race
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