Muscles are important, but stiff tendons are the secret to high-speed performance

Sports Performance

31 July

KYDPL KYODO / AP
From: The Conversation, 2 August, 2021, Muscles are important, but stiff tendons are the secret ingredient for high-speed performance (theconversation.com)

You might be surprised to learn that most of the explosive power displayed by the fastest sprint runners on earth, and other elite athletes, doesn’t come from their muscles, or even from their minds – it comes from somewhere else.

Muscles are important, but the real secret is using training and technique to store and reuse elastic energy in the best way possible – and that means making the most of your tendons. By understanding how this power is produced, we can help people walk, run and jump into older age and how to walk again after injury or illness.

Muscles are remarkably powerful. The average human calf muscle weighs less than 1 kilogram but can lift a load of 500 kg. In some cases, our calf muscles have even been shown to handle loads approaching a tonne (1,000 kg)!

But muscles have a major performance issue: they can’t produce much force when they’re shortening at high speed. In fact, when we move at our fastest, muscles can’t theoretically shorten fast enough to help us at all - so how is it that we can move so quickly?

Muscles are strong, but slow

Muscles produce most of their force through the interactions of two proteins: actin and myosin. The rotating, globular “head” region of the long myosin filament attaches to the rod-like actin to pull it along in a sweeping motion, like an oar producing force to pull a boat along the water. So actin and myosin filaments form powerful mini motors.

Trillions of these mini motors together the large forces we need every day to walk upstairs, carry our shopping bags, or take the lid off a jar.

Trillions of actin and myosin proteins work together to make your muscles contract and your body move.

The head region of myosin is only 20 nanometres long. It’s so small that there’s no point comparing its size to a human hair, because it would barely even cross a handful of DNA molecules laid side by side.

Because it’s so short and only pulls actin a small distance in each stroke, a large number of strokes are needed to shorten a muscle by any distance. It’s like using first gear to get up a hill in a car or on a bike – good for force, but not for speed.

At the molecular level, your muscles are a bit like first gear on a bike: great for force, not so good for speed. Ljupco Smokovski / Shutterstock

And the faster the muscle shortens, the less time each myosin is attached to actin, which reduces force even further. At a certain shortening speed, muscles can’t produce any force at all.

We can measure the power athletes produce during running and jumping, and we can estimate the power a muscle should produce by its size and the type of fibres it contains. When we compare these two values, we find that muscles can’t even produce half the power generated in sprinting or vertical jumping. And in overarm throwing, muscles can produce only 15% of the total power.

Energy return systems

So if the muscles aren’t producing the power to move a body at high speed, where is it coming from? Humans, like most other animals on Earth, make use of an “energy return system”: something that can store energy and release it rapidly when needed.

Our energy return systems are made of a relatively long, stretchy tendon attached to a strong muscle. When the muscle produces force it stretches the tendon, storing elastic energy. The subsequent recoil of the tendon then generates a power far superior to our muscles. Our tendons are power amplifiers.