Heavy sleds and the death of the 10% rule

Where the 10% rule came from

Ask most sprint coaches how heavy a sled should be and you will get some version of the same answer: not heavy enough to slow the athlete by more than about ten per cent, because anything more wrecks the mechanics. It is one of the most durable pieces of received wisdom in speed training. It is worth knowing where it came from.

The 10% figure traces back to a 1998 article by Kevin Jakalski in Track Coach, weighing up the pros and cons of parachutes, tubing and towing with high-school sprinters. He suggested athletes should not be slowed by more than a tenth because of the changes it produced in ground-contact dynamics. That was a considered coaching judgement published in a coaching periodical. It was not the conclusion of a controlled trial. Over the following two decades it was cited forward into the peer-reviewed literature until it acquired the authority of a finding.

The kinematic work that came afterwards did give it partial support. Lockie and colleagues towed field-sport athletes at 12.6% and 32.2% of body mass and found stride length cut by roughly 10% and 24% respectively, with stride frequency also falling but by less, alongside increased ground contact time, trunk lean and hip flexion. Maulder's group later looked at block starts under roughly 10% and 20% body mass and found that performance fell as load rose, but that the lighter condition left start technique and step kinematics essentially intact.

The rule was never really about ten per cent. It was about specificity — keep it looking like sprinting.

That logic holds up if the training effect depends on the movement resembling the competition movement. And for one purpose it clearly does. Petrakos, Morin and Egan's systematic review found that light loads, under 10% body mass, produced small decrements in acceleration but moderate improvements in maximum velocity in sprint-trained athletes. If the session's target is top-end mechanics, the old rule is still the right rule. The mistake was applying it to everything.

The heavy-sled studies

The paper that broke the consensus was Morin's 2017 pilot. Sixteen amateur soccer players were split between a sled loaded at 80% of body mass and an unresisted control, over eight weeks and sixteen sessions of ten twenty-metre sprints. The very heavy group increased maximal horizontal force production, with an effect size of 0.80 against 0.20 for the controls, and improved the mechanical effectiveness of force application — how much of the total force they could direct horizontally — with an effect size of 0.95 against −0.11.

That reads as a rout, and it has been cited as one. It should not be. The authors called it a pilot study for a reason: sixteen participants, amateur players, and a between-group difference in horizontal force that was itself unclear. What it demonstrates is that eighty per cent of body mass does not destroy an athlete and may develop something worth having. It does not demonstrate that heavy is better than light.

The wider picture is messier and more honest. Petrakos and colleagues found that moderate through very heavy loads produced anything from trivial to extremely large improvements in acceleration — a range of 0.5% to 9.1% — in strength-trained and team-sport athletes, while concluding that whether resisted sprinting beats unresisted sprinting at all remains unresolved. Alcaraz's meta-analysis reached a similar place: sled work improves acceleration rather than maximum velocity, with continuing disagreement about load and training status.

More recent work has started to map the dose to the phase. An eight-week study in adolescent sprinters found that 40% and 50% body mass improved 30m performance, while 25% improved 60m, with the heavier groups also showing changes in trunk angle and push-off angle through the acceleration. Choose the load from the phase you are training, not from a rule of thumb — heavy for early acceleration and force orientation, light for the transition to top speed.

Force-velocity profiling, in plain terms

The theoretical engine behind the shift is straightforward once the jargon is stripped out. Acceleration is limited by how much force you can put into the ground horizontally and how well you can keep orienting it that way as you get faster. Run an athlete under a series of increasing loads, measure velocity in each, and you can fit a line through the results to estimate a theoretical maximum force, a theoretical maximum velocity, and the peak power that falls out between them.

Cross and colleagues then asked which load maximises horizontal power output. The answer landed at roughly a 50% decrement in maximum velocity, which in recreational athletes and sprinters meant somewhere between 69% and 96% of body mass. That single result is what finished off the 10% rule in practice. It put a number on heavy that was seven times heavier than the number coaches had been using.

The assumptions underneath the numbers

Three of them deserve more scepticism than they usually get.

Friction. A load expressed as a percentage of body mass is not the load the athlete experiences. Cross's work on determining friction and effective loading showed that the coefficient has to be established for the specific sled and surface. Thirty per cent of body mass on a dry mondo track and the same sled on wet grass are different sessions. If you prescribe in %BM and train on more than one surface, you are not prescribing anything.

Between-athlete variation. Cahill's group profiled sled pulling in young athletes and found the method itself reliable — the slope of the load-velocity relationship varied by around 3% between sessions, and the loads producing a given velocity decrement varied by under 5%. But the load required to produce the same decrement in different athletes ranged from 71% to 107% of body mass for a 50% drop. A squad-wide percentage prescription is therefore giving different athletes entirely different sessions.

Reliability of the profile itself. This is the uncomfortable one. Work on the KiSprint system found the slope of the force-velocity relationship and the rate of decrease in force ratio — the two variables coaches most want to act on — showed unacceptable reliability, leading the authors to question the use of sprint profiling for individualised prescription. The same pattern has appeared in vertical jump profiling, where the slope varied by 14–30% between sessions. Use profiling to track change across a block, where the training effect is usually larger than the noise. Do not use one test to label an athlete force-deficient and rebuild their programme around it.

Optimal power is not optimal adaptation

The most common misreading of this literature is subtle. Cross identified the load that maximises horizontal power during a sprint. That is an acute mechanical observation about what happens in the moment. It is not evidence about what produces the best long-term adaptation, and no one has yet shown the two coincide. In every other area of strength training we accept that the load which maximises power output is not the load that best develops the athlete.

Morin's own framing is more careful than most of the coaching content built on top of his work: heavy sleds are a candidate answer to a specific problem, either an inability to produce horizontal force or an inability to orient it, rather than a load everyone should be using. Treat the heavy sled as a treatment for a diagnosed deficit, not a staple of every acceleration session.

How I load a sled without a force plate

Almost none of us have radar. Here is what the evidence supports for coaches who do not.

Prescribe in velocity decrement, not body mass, and measure it on your surface with your sled. An unresisted baseline over 20m at the start of every sled session takes ninety seconds and makes every load after it meaningful. I work in three bands: around a 10% decrement when the target is top-speed mechanics, around 25% for late acceleration, and 40–50% when I want early acceleration, shin angle and horizontal force orientation. Re-baseline whenever the surface or the sled changes, because the friction assumption underneath everything has just changed with it.

Then judge the rep with your eyes as well as the clock. The reason the 10% rule survived so long is that it encoded something true — that a session should still resemble the thing it is training. The rule was wrong in its universality, not in its instinct. Heavy sleds earned their place by showing that resemblance matters less in early acceleration than we assumed. They did not earn the right to be the only thing on the track.