Science Investigates Optimal Loads for Sled Pulls
Weighted sprinting has become a controversial topic of late. It’s a method that has pros and cons, with research to support both. In a recent study in the Journal of Strength and Conditioning Research, investigators examined the effectiveness of loaded sleds.
The researcher’s primary concern was the change in form that can occur during sled training. The researchers hypothesized that if adding resistance to an athlete reduces his or her top speed by more than ten percent, then it also must change his or her form substantially. Changes in form can make an athlete slower and weaker when it counts.
In the case of sprinting, the added weight changes an athlete’s technique just a little bit for every bit of resistance. For example, adding heavy resistance via a chest harness can cause the athlete to run more upright because of the pull, which is poor form in sprinting. Adding the resistance to the waist, another common method, can pull the hips back. Again, that’s going to reduce an athlete’s ability to maintain or develop proper form for maximal acceleration.
To test the effects of loaded sprinting, the researchers in the Journal study took nineteen physically active students and divided them into three groups. Each group used a different load, which was added to a sled that they pulled. The load corresponded to 5%, 12.5%, and 20% of their bodyweight for each group. They each trained twice per week, using the assigned sprinting load for a total of seven weeks.
Before and after the seven-week period, each participant was tested on sprinting speed, squat performance, and jumping ability (both loaded and unloaded). They didn’t work on these exercises during the training period.
All of the loads improved speed, but they did so in different ways. The twenty-percent load was best at developing acceleration up to thirty meters. The other two loads were better at improving top speed up to the forty-meter distance. The two heavier loads also improved jumping and squatting performance.
Although the researchers noted that form can be altered by the addition of loads, in this study they only measured changes in speed. As a result, there were numerous confounding factors that would affect more experienced athletes who are serious about maximizing, rather than just improving, their sprinting ability.
For example, practicing unloaded sprinting probably would have improved the speed of the participants. Unfortunately, unloaded sprinting was not tested at all, making it impossible to determine how much of a role the weight of the sled played. Furthermore, the improvements in speed might have been also due to increases in leg power, as demonstrated by improvements in jumping ability. Despite increases in leg power, form may have been poorly reinforced. This might reduce speed in the long run, even though the athletes improved in the short term.
For improving speed in moderately fit runners without considering form, it seems that a variety of loads can be used effectively. However, for more experienced athletes, this research may not apply.
1. Beatriz Bachero-Mena, et. al., “Effects of resisted sprint training on acceleration with three different loads accounting for 5%, 12.5% and 20% of body mass,” DOI: 10.1519/JSC.0000000000000492
Photo courtesy of CrossFit Impulse.