Muscle Slack: Bas Van Hooren on Speed and Power

Based on Episode 52 of the Just Fly Performance Podcast, a conversation between host Joel Smith and Dutch sport scientist Bas Van Hooren.

Muscle slack is one of those ideas that quietly rearranges how you think about the weight room. Bas Van Hooren, a Dutch sport scientist and national-level middle-distance runner who has published on the topic with Frans Bosch, defines it as the split-second delay between when a muscle fires and when it actually starts moving a load. In a movement that lasts only a few hundred milliseconds, that delay decides whether an athlete is fast or a step late. In this conversation he explains what muscle slack is, why counter-movements and heavy loads are not always enough, and how these ideas change the way we should lift, jump, and train the hamstrings.

Key Takeaways

• Muscle slack is the delay between a muscle contracting and force actually reaching the ground, and it can eat up to 100 of the roughly 300 milliseconds an explosive action allows.
• Pre-tension through co-contractions is the only strategy that reduces slack in real time. Counter-movements and external load can reduce it too, but they are not always feasible in a sporting moment.
• Leaning on a big counter-movement may not teach athletes to produce force rapidly on their own, which is what fast sporting actions demand.
• Judge eccentric, isometric, and concentric at the muscle fiber level, not the whole muscle-tendon unit. In fast running the hamstring shows a mix of a small eccentric action and a significant isometric one, even as the whole unit lengthens.
• Sprinting forces are enormous (hamstrings about five times bodyweight, calves about seven). Very heavy strength training can overload the hamstrings, but it is difficult, so coordination matters as much as plates.

What muscle slack is, and why milliseconds decide it

Van Hooren co-wrote a paper on muscle slack with Frans Bosch, and he frames it in terms of time budgets rather than physiology.

The time to produce force in many movements is only about 300 milliseconds, and muscle slack can take up to 100 milliseconds and even more. So if a muscle needs to produce maximum force as fast as possible, but the delay between contraction and force production is 100 milliseconds, then there is only about 200 milliseconds left to apply force, and this results in decreased performance.

The slack itself is the take-up in the series-elastic elements: before a contracting muscle can transmit force to something external, it first has to pull the slack out of its own tendon. Shrink that delay and the athlete either has more time to produce force or completes the action sooner, both of which matter in sport. Van Hooren lays out three ways to reduce it: pre-tension through co-contractions, a counter-movement, and an external load. Only one of them survives contact with a real game.

Co-contractions do the real work

Of the three strategies, co-contraction is the one an athlete can actually use mid-play. Firing agonist and antagonist together pre-stretches the muscle-tendon unit and takes up the slack before force is needed. The other two come with a catch.

For the acute case, co-contraction is the only strategy that reliably takes up the slack. Counter-movements and external loads can reduce it as well, but they are not always feasible in a fast sporting action, and leaning on them does not teach an athlete to generate that pre-tension independently.

His argument against leaning on counter-movements is practical. A jump with a big dip almost always beats a jump without one, but competition rarely grants the time for that dip. Van Hooren points to the striker who needs a long backswing before kicking, and finds the ball already gone.

The concern is not that the nervous system becomes lazy; it is that relying on a big dip may not train the rapid, self-generated force that sport demands.

He is careful about the other edge of the sword too. Too much co-contraction triggers reciprocal inhibition and a tibialis anterior that will not switch off, both of which bleed force. The fix is training volume: enough plyometric exposure and the body refines the timing until the pre-tension helps without the interference.

Eccentric or isometric? Look at the fiber, not the muscle

Underneath the slack discussion sits a distinction Van Hooren thinks most coaches get wrong. The labels eccentric, isometric, and concentric should describe what the muscle fibers do, not what the whole muscle-tendon unit does.

In many movements classically considered to have an eccentric phase, there may be no eccentric phase, at least not at the muscle fiber level. Lengthening of the whole muscle-tendon unit does not necessarily reflect lengthening of the fascicles or muscle fibers. And since the muscle fibers are what mostly get injured, that’s what we should be looking at.

The running hamstring is his prime example. As the swing leg decelerates, the whole unit lengthens, which looks eccentric on paper. At the fiber level, animal studies suggest something else entirely.

The muscle fibers are activated and they remain at one length, so isometric, and then the tendon stretches and eventually recoils, causing the swing leg to retract. The tendon acts as a sort of spring, and it also protects the muscle fibers from this fast applied stretch.

Van Hooren adds that more recent evidence points to a mix rather than a pure isometric action: a small eccentric action alongside a significant isometric one, with top-level sprinters perhaps able to minimize the eccentric part. That matters because training adaptations are specific to contraction type and to length. If the hamstring works isometrically near a particular length during fast running, then hammering it with eccentric work may miss the point. Van Hooren’s proposed answer is a single-leg Roman chair hold: lift a weight, hold it isometrically for about three seconds, and lower it, with the pelvis left free to rotate so the body can self-organize the length where the hamstring is strongest. Whether it truly transfers to high-speed running is a question he flags as still open, and part of what his hamstring PhD set out to test.

Overloading a sprint is hard, so coordination matters

One reason he keeps returning to coordination is that the forces in sprinting dwarf anything a barbell can add.

The force acting on the hamstrings is about five times bodyweight at maximum-speed running, and for the calf muscles it’s even up to seven times bodyweight. So if you want an overload in a quantitative way, you’d have to put at least seven times bodyweight on your shoulders. Perhaps we should be looking at providing overload in a qualitative way, from a coordination perspective.

Very heavy strength training can overload the hamstrings, but it remains difficult, so much of the overload still has to come from complexity and specificity rather than sheer load, an idea he credits to Frans Bosch. He also warns that isolating biarticular muscles like the hamstrings and gastrocnemius can backfire, because their job is to transport energy across two joints. He cites a classic Bobbert computer model in which increasing a muscle’s strength without updating its timing made the model jump lower, and a later human study in which training a ballistic squat that let the ankle contribute improved vertical jump while isolated single-joint work slightly decreased it. Strength that the nervous system cannot coordinate is not always an asset.

How this changes the weight room

Asked for concrete changes, Van Hooren starts with amplitude. Shrinking the counter-movement forces athletes to create their own pre-tension instead of borrowing it from a dip.

The most important one is to minimize the amplitude of counter-movements in training. So counter-movement jumps, you could largely replace by squat jumps.

The same logic cleans up single-leg hang cleans and snatches, where athletes often sneak in a little dip before the pull, and even medicine ball throws, where the wind-up usually goes uncoached. To make lifts more specific to running he layers constraints: start a power clean from boxes to remove the deep sitting position, go to a single leg because running applies force one leg at a time, and land on a box to blunt the eccentric impact and give an external focus. Two tools, in his experience, force the pre-tension almost automatically.

Two ways that are really good are time pressure and using an unstable load. With a water bag or a barbell with weights hanging from bands, if you don’t have pre-tension, you simply lose balance.

Frequently asked questions

What is muscle slack?
Muscle slack is the delay between a muscle contracting and the recoil of its series-elastic elements, the moment when force actually starts moving a load. Van Hooren says it can take up to about 100 milliseconds, a large bite out of the roughly 300 milliseconds an explosive athletic action allows, so reducing it leaves more time to produce force.

How do you reduce muscle slack?
There are three strategies: pre-tension through co-contractions, a counter-movement, and an external load. In a live sporting moment, co-contraction is the only one an athlete can actually apply. Van Hooren notes that counter-movements and external load can reduce slack too, but they are not always feasible in sport, and leaning on them may not teach athletes to create pre-tension on their own.

Are the hamstrings eccentric or isometric during running?
The whole hamstring muscle-tendon unit lengthens during the swing phase, which looks eccentric, but at the muscle fiber level the evidence points to a mix of a small eccentric action and a significant isometric one, with the tendon stretching and recoiling like a spring. Van Hooren argues that because contraction-type and length adaptations are specific, this should shape how we train the hamstrings.

Should athletes train counter-movement jumps or squat jumps?
Both have a place, but Van Hooren recommends leaning toward squat jumps and other reduced-dip variations to force athletes to build pre-tension rather than depend on a large counter-movement, since that pre-tension is what carries over to fast sporting actions.

How do you overload muscles that see five to seven times bodyweight in sprinting?
Matching those forces with load is hard. Very heavy strength training can overload the hamstrings, but only with difficulty, so Van Hooren leans on coordination as well: more specific and more complex exercises, single-leg and reduced-dip variations, and unstable loads, rather than simply adding plates.

About the authors

Bas Van Hooren is a Dutch sport scientist, strength and conditioning specialist, and national-level middle-distance runner. He is an assistant professor in the Department of Nutrition and Movement Sciences at Maastricht University in the Netherlands, where he earned his PhD; he holds a bachelor’s in applied sport sciences from Fontys University and a master’s in human movement sciences from Maastricht. His research on muscle function, muscle slack, and running mechanics, much of it with Frans Bosch, has appeared in journals including Sports Medicine and the Journal of Sports Sciences and on SimpliFaster.

Joel Smith is the host of the Just Fly Performance Podcast and the founder of Just Fly Sports, a former collegiate strength and track and field coach focused on speed, power, and athletic development. Listen to the full episode with Bas Van Hooren on Just Fly Sports.