Buckle-in or Shed the Sled?
Resisted sled training can improve some elements of speed, but it has limits.
In sports like football, soccer, and track, athletes gain a competitive advantage by having increased sprint speeds over their opponents. Because of this, athletes expend time, effort, and money training to become faster. Some common methods to do so include weightlifting, running long and short distances, as well as ‘resisted runs.’ These resisted runs can include parachutes or weighted sleds attached to athletes’ waists, as pictured above.
Why exactly would tying a sled to runners help them improve their sprint speeds? Obviously, the sled is heavy and makes it harder for the runner to move, so the idea is that it helps increase sprint speed when an athlete is subsequently not tied to the sled. But in terms of the physics behind the sled’s intended benefits, what is really going on is that the weight of the sled creates friction. That is, it is not directly the weight of the sled that makes it hard to pull, but rather it is the friction caused by the contact between the sled and ground that is the critical training element.
More specifically, a sled’s friction is determined by both the weight of the sled and the ‘coefficient of friction’ of the ground, which is essentially how much friction the ground has. For example, ice has a very low coefficient of friction, whereas carpet has a high coefficient. The equation for this interaction is as follows: FF = FN μ, with FF being the force of friction, FN being the normal force (in this case, simply the weight of the sled), and μ being the coefficient of friction. If either the normal force or coefficient of friction is increased, then the frictional force is increased accordingly.
With the general idea behind how sled pulls work in mind, a key question is whether using them results in positive outcomes for athletes. There have been a lot of real-world studies on this topic. A recent 2019 literature review published in the Sports Medicine journal found that ‘resisted sled training’ (RST) showed some benefits, but there were many conditions that shaped whether these benefits were realized or not – where RST seemed to work only for certain aspects of an athlete’s speed and only for certain individuals. For example, the review’s authors noted that acceleration improved with RST, but maximum velocity did not. For acceleration, RST was shown to be more effective for males than females, and more effective for recreational or moderately trained athletes than highly trained athletes. Still further, loads greater than 20% of an athlete's body mass did not seem to produce greater improvements in acceleration. In some cases, a load this large actually reduced the magnitude of benefits associated with RST and very high loads (e.g., 80% body mass) only improved pushing force.
In contrast to the modest acceleration gains that can be reaped from RST, gains in maximum velocity were not clearly evidenced. The review found that across all groups tested – categorized by age, sex, training level, load, or surface – only negligible velocity benefits were demonstrated.
As mentioned above, the surface on which the sled is used matters for the functioning of the sled itself. Interestingly, the surface also matters for the effectiveness of RST – the 2019 review revealed that rigid surfaces (e.g., gym floors, concrete lots) yielded better results than grass for acceleration improvements, likely due to more consistent friction and force transfer associate with surface rigidity.
What’s the main evidence-based takeaway for athletes here? Well, for one, increasing top sprinting velocity (speed) is not helped by sled pulls. If this is the ultimate goal of training, an athlete would be better off focusing on other exercises, such as plyometrics like box jumps. However, sled pulling can be helpful in developing acceleration, which is still an important training goal for athletes, as long as the total weight stays below 20% of one’s body weight. Sled-pulling with loads that are near one’s body weight hamper acceleration but can be helpful for athletes who want to develop a strong pushing force.
