Understanding ratio of forces during early acceleration: calculation considerations and implications for practice

This study aimed to: 1) understand how the calculation of step-averaged ratio of forces (RF; horizontal component as a percentage of total ground reaction force (GRF) magnitude) affects the RF-horizontal velocity (vH) profile; 2) establish the importance of RF to early acceleration (i.e. up to the end of step 4) performance; 3) assess how well the RF-vH profile obtained from a simple macroscopic model over the entire acceleration phase corresponds to the true external kinetics during just early acceleration. Twenty-four trained male sprinters completed four maximal effort 60 m sprints (two standing starts, two block starts), and GRFs from the block exit/initial push off and first 50 m on the track were analysed. The use of step-averaged force data to obtain step-averaged RF consistently led to a greater (systematic bias: +8.6 to +9.2%) theoretical maximal RF (RF0) and a steeper (systematic bias = -1.7%∙s/m) rate of decline in RF (DRF) than the use of instantaneous RF data, and the use of step-averaged force data was proposed as yielding values with a more appropriate assessment of the “mechanical effectiveness” over the entire step. Average RF during early acceleration was of greater relative importance for early acceleration performance than average resultant GRF magnitude during early acceleration, particularly from block starts (linear regression standardised β-coefficient for average RF = 0.82, for resultant GRF magnitude = 0.10). Direct measures of DRF and RF0 obtained from the block exit/initial push off and first four steps combined to explain 93-95% of the variance in early acceleration performance, with RF0 being of greater relative importance for performance than DRF. There was a near perfect correlation (r = 0.94 to 0.95) between the macroscopic modelled RFMAX value (RF at 0.3 s based on a linear fit to modelled RF-vH from 0.3 s onwards) and early acceleration performance. Whilst the ability to achieve high levels of resultant GRF magnitude cannot be ignored, particularly for standing starts, athletes should be encouraged to achieve a high initial ratio of forces during early acceleration before then focussing on arresting the decline in RF as the early acceleration phase progresses. Practitioners can use RFMAX from a simple macroscopic model fitted to the entire acceleration phase in field-based testing to provide a highly appropriate assessment of early acceleration performance.