By Chris Beardsley
Chris Beardsley graduated from Durham University with a Masters Degree in 2001. He since contributed to the fields of sports science and sports medicine by working alongside researchers from Team GB boxing, the School of Sport and Recreation at Auckland University of Technology, the Faculty of Sport at the University of Ljubljana, the Department of Sport at Staffordshire University, and the College of Health Solutions at Arizona State University. He is currently a Director at Strength and Conditioning Research Limited.
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Resisted sprinting is now a fairly common method for improving sprint running performance, especially in team sports athletes. Depending on the type of resistance, the resistive force is exerted on the athlete either vertically (e.g. weighted vest) or horizontally (e.g. weighted sled).
Since many studies have shown that horizontal force production is closely linked to sprinting ability, most coaches use horizontally-directed resistance for resisted sprinting training, normally by weighted sled towing.
When sled towing was first introduced, sprint coaches expressed concerns that if the loads were too heavy, then this would alter sprinting movement patterns during the resisted sprint. And if sprinting movement patterns were altered during the resisted sprint, then this could affect movement patterns during an unresisted sprint performed later.
Loads that reduced sprinting speed by 10% were thought to be fairly safe, as they only changed sprinting technique slightly. However, researchers soon identified that both light and heavy sled load can improve sprinting speed.
In fact, heavy sled towing seems to improve sprinting ability more than light sled towing in athletes!
This is probably because it is only heavy sleds lead to greater horizontal impulses (force x time) compared to unresisted sprinting. Since horizontal force production is linked to sprinting ability, this make sense.
Changes in movement patterns during resisted sprinting probably do not produce adverse effects on movement patterns during an unresisted sprint performed later.
This is logical, since we do not generally get concerned about the negative impact of back squats on vertical jumping performance!
And this is a good analogy, because both jump squats and heavy back squats can improve vertical jump height. And now, research has found that even very heavy sled towing can improve sprinting ability. In fact, it improves sprinting performance by more than unresisted sprinting in athletes.
Taken together, these studies suggest that just like gym-based resistance training, sled towing can be performed with a range of loads from light (reducing sprinting speed by 10%) to very heavy (80% of bodyweight) and that all of these options can improve sprinting performance in athletes of varying abilities.