Across the board, researchers and coaches agree resisted sleds can benefit speed and sprint performance. Where the controversy lies is in the amount of load to prescribe and the specific quality that each load is targeting within a sprint (i.e. acceleration or max velocity). Much of the previous research is split on whether a lighter load (i.e. <20% body weight) or heavier load (i.e. >20% body weight) increases acceleration or maximal velocity.
What the Research Says
In a 2015 review of resisted sled sprinting, researchers found that loads <20% body weight showed more improvement in maximal velocity and loads >20% body weight improved acceleration. Recently, much heavier loads have been looked at for their potential benefits and found that peak power was achieved at loads between 69-90% body weight. This is important because training at peak power may induce greater adaptation when applied during a power movement such as sprinting. In another study, two loads were compared, one that achieved optimal power (heavy load) and the other that achieved a 10% speed decrement, which correlates to 10% body weight (light load). Results showed that the 10% speed decrement group increased their initial acceleration while the optimal power group increased max velocity. This is almost completely opposite of the previous findings, in that lighter correlates to improved velocity and heavy improves acceleration. Prior to the recent research on very heavy loads of 60% and greater, there has been virtually no research on loads above 43%. With all the conflicting studies and little research, the verdict remains split as to what loads produce what adaptation.
The two ways to prescribe sled load are by a percentage of body weight or by a percent decrement of speed. For simplicity purposes, we will refer to the load in terms of percentage of body weight. Practical application for coaches and athletes are not well established like those of resistance training qualities, but we will follow what some of the current research is reporting.
When targeting initial acceleration, a heavier load (>20% body weight) may help improve this quality. This makes for a wide range of possibilities. As previously mentioned, loads between 69-90% achieved peak power which may be an optimal spot to train acceleration. When targeting maximal velocity, a lighter load (<20% body weight ) seems to be more optimal according to much of the research. This could be because an athlete is able to achieve higher velocity and keep very sprint specific mechanics, which will have a high degree of transferability.
In short, light loads (<20% body weight) targets velocity, and heavier loads (>20% body weight) targets intial acceleration within sprint performance.
Though there is a fair amount of research done on resisted sled sprinting, research regarding load prescription and heavy loads are scarce within the research. Much of the conflicting results in the research may be due to individual force velocity profiles of athletes. It may be that the best prescription of load for the athlete will be based on their individual qualities and needs, which can be found by assessing them with a force velocity profile. (For instance, an athlete who already possesses a high amount of force will not see as much benefit from the heavy load as they would from a light load)
All in all, resisted sled sprinting can produce many adaptations from improved kinematics, sprint specific strength, rate of force development, increases in maximal velocity and acceleration which are all beneficial for increasing sprint performance in athletes.