The most critical part of speed in most sports is the acceleration phase that takes place over the first few yards of a sprint. Athletes in nearly every sport spend most of their sprinting time in the acceleration phase, so improving this critical part of sprinting is a necessity. One way of developing acceleration is by the addition of an external load, which was a topic covered in a recent Journal of Strength and Conditioning Research study.
There are various ways to add a load to sprinting, but the researchers in this study focused on sleds. The participants did multiple trials of sprinting in three conditions:
- Unloaded sprinting
- Sprinting with a sled weighted to ten percent of bodyweight
- Sprinting with a sled weighted to twenty percent of bodyweight
The focus was on the start, and specifically on the force against the ground. More force on the ground means the muscles are working harder, and in theory, should also mean greater improvements over time.
The researchers found that the twenty-percent-of-bodyweight condition increased the ground reaction forces in both legs. However, at twenty percent of bodyweight, the ground reaction forces were only greater than the ten-percent condition in the front leg. This suggests that, in the staggered stance of a sprinting start, the back leg was more involved while carrying the ten-percent load.
The researchers concluded that a twenty-percent load was thus the superior load to work with, since it resulted in greater force in both legs, whereas the ten-percent load only worked on the back leg. The idea is that greater adaptations will result from working with a load that increases ground reaction force.
I would like to see an application of these results in a follow-up study. It’s great that the propulsive forces were greater in the ten-percent-of-bodyweight sled pulls, and it may well be a safe assumption that this will result in greater acceleration for athletes, but it’s still an assumption. Consider a related exercise, vest-loaded sprinting, which does not seem to be as effective for sprinters as sled-loaded sprinting due to its effects on sprint mechanics. Consider also that in many sports, an athlete doesn’t begin a sprint from a staggered stance. So, while it’s nice that this study suggests an approach, the results call for further testing.
Additionally, the results should be applied to both general athletes – such as the participants in this study – and to sprinters specifically. Sprinters are taught to drive equally from each leg when they start, which may alter the results found in the ten-percent condition. If an athlete is trained to start properly, the ten-percent condition could improve force production in both legs, instead of just the back leg.
So while we are at a good starting point, loaded sprinting is still a topic of some controversy. Some coaches shy away from it and some ignore it altogether, while others embrace multiple methods of loaded sprints. Either way, methods involving resistance from behind, like sleds, parachutes, and partner-assisted bungee cords seem to be superior to adding the load directly to the athlete. Just remember to keep your body in the correct position at the point of attachment for the resistance.
References:
1. Casey Cottle, et. al., “Effects of Sled Towing on Sprint Starts,” Journal of Strength and Conditioning Research, DOI: 10.1519/JSC.0000000000000396
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