Factors affecting Sprint Performance
Sprint physiology is complex, with differing physical qualities more heavily associated with either acceleration or maximum velocity. Having said this, there is strong evidence, especially in field sports, that fast is fast, regardless of the sprint phase. Clark et al. (2019) demonstrated that athletes with greater acceleration qualities tended to also display higher maximum sprint velocities and vice-versa, perhaps dispelling the myth that an athlete is either good at accelerating or at top speed and that these qualities are independent.
Furthermore, Gabbett (2012) highlights the importance of all aspects of sprint performance in team and field sport athletes by identifying how more than 20% of all sprints in professional rugby league matches are over 20-metres in length. This suggests that not only is acceleration critical to sports performance, but top speed may also play a crucial role, particularly when the nature of these longer sprints is considered (match deciding plays, line-breaks, long chases, etc.). Given this knowledge, an appropriate speed program for team sports should aim to develop sprint ability at various phases—both short accelerations and maximum velocity, upright sprints.
One of the key factors associated with high level sprinting, particularly during acceleration, is the efficiency of force application during the stance phase (Morin et al. 2011). Horizontal GRF relative to bodyweight is likely a key to improved speed during the initial strides of a sprint. One method proposed for improving horizontal force production is through horizontally orientated strength training. Exercises such as the barbell hip thrust have been hypothesized to generate greater transfer for sprinting (Contreras et al. 2017) due to the force-vector theory which classifies sports skills on the basis of the direction of force expression relative to the global coordinate frame (Fitzpatrick et al. 2019).
However, some coaches and more recent research suggest this may not be the case, particularly in acceleration, whereby the untrained eye may ignore the trunk/shank angles displayed by athletes resulting in vertical force production intra-individually, while global force production is horizontal in nature (Jarvis et al. 2019). This is not to say an exercise like the hip thrust cannot contribute to improved sprint performance, merely that transfer of training is a complicated topic and is multi-factorial in nature.
Another heavily researched training method for increasing horizontal force application during sprinting is resisted sprints. Resisted sprints come in several forms; resistance band sprints, sled sprints, prowler pushes and although not resisted, incline sprints All these methods work through the same principle of overloading the horizontal component of force application by artificially slowing the athlete down. These methods allow athletes to maintain acceleration posture for far longer than traditional free sprints and can result in far more training density being directed towards peak power production along with reinforcing horizontal GRF orientation (Weyand et al. 2000).
To most effectively improve acceleration performance using resisted sprints, an athlete’s horizontal velocity should be reduced to ~50% of top speed (Cross et al. 2017; Cross et al. 2018). The loads when applied to a sled to create such a decrement in velocity appear to be far greater than what was traditionally believed to be acceptable by coaches who were concerned about alterations to sprint mechanics when using heavy resistance. However, recent literature has demonstrated these concerns are likely misplaced and that a range of light, moderate and heavy sled pushed may be useful at various stages of a properly periodized speed development program (Morin et al. 2017).
Weyand et al. (2000) and Nagahara et al. (2018) demonstrated that larger GRFs produced during each step were particularly important to sprinting speed during the maximal velocity phase. Athletes wanting to run at higher maximum velocities are therefore required to express more force, in a shorter period. Furthermore, lower body strength has also been shown to correlate with sprint performance. McBride et al. (2009) (r = -0.61, p = 0.01), Seitz, Trajano, et al. (2014) (r = -0.57, p = 0.04) and Baker et al. (1999) (r = -0.66, p<0.05) have all demonstrated strong correlations between 1-RM back squat strength and sprint performance in elite athletes.
This should come as little surprise given the gluteal muscles, quadriceps, hamstrings and calves are the prime movers of the force-producing actions shown in sprinting. The strong relationship between lower-body strength and sprint speed may be attributed to the fact that those athletes demonstrating greater force-producing capabilities are able to produce a higher peak GRFs, impulse, and increased rate of force development (Seitz, Reyes, et al. 2014).
Perhaps more important than strength, muscle-tendon unit (MTU) stiffness has been shown to correlate strongly with sprint performance in athletes (Cunningham et al. 2013). MTU stiffness describes the efficiency with which energy can be transferred from the force-producing muscles into force receiving surfaces (i.e. the ground). For example, a stiffer ankle complex will reduce energy leakages between the calves and the foot when striking the ground. MTU stiffness can be assessed through tests such as the incremental drop jump test, where jump height or flight time and ground contact times are used to generate a reactive strength index (RSI).
The reactive strength index helps athletes and coaches better understand the quality of forces and speed of application when assessing vertically orientated interactions with the ground. RSI could be considered a key performance indicator for sprint performance due to its high correlation with sprint speed in both acceleration and at maximum velocity (Cunningham et al. 2013) and thus can serve as a useful assessment tool for coaches and athletes.