Linkages are the key: Research casts fresh light on the role of muscles and tendons

A shows the lower part of a horse's hind limb, displaying the "reciprocal apparatus", parallelogram four-bar linkage or "pantograph" mechanism. B is an annotated sketch, while C represents a reduction of the linkage apparatus, highlighting the parallelogram property. It is, in effect, a four-bar linkage system.
“A” shows the lower part of a horse’s hind limb, displaying the “reciprocal apparatus”, parallelogram four-bar linkage or “pantograph” mechanism. “B” is an annotated sketch, while “C” represents a reduction of the linkage apparatus, highlighting the parallelogram property. It is, in effect, a four-bar linkage system. Image: Usherwood,

The way in which muscles and tendons work together in order to transfer weight without expending unnecessary energy has been revealed in a fresh study in Britain.

The findings of the Royal Veterinary College research represent an important advancement in our understanding of the role of muscles and tendons, enabling different perspectives and approaches to surgery, rehabilitation and prosthetics.

Professor Jim Usherwood, in a commentary just published in the Journal of Experimental Biology, looked at the biomechanics behind the athletic motion of horses in his research.

Muscles and tendons are crucial parts of both human and animal anatomy, yet due to their complexity, there is still much to learn about their roles.

Thinking of muscles and tendons as parts of a bike, muscles might be viewed as a motor, powering movement, or brakes and shock absorbers, which dissipate energy, while tendons could be the transmission, acting as a bicycle chain, or springs, like the tyres.

While significant research has been conducted on this more traditional understanding of their functions, this study ignores these roles and instead highlights the “tricks” by which muscles and tendons allow body weight to be supported during horizontal motion while avoiding mechanical work – effectively acting as the wheel and bicycle spokes.

In his study, Usherwood interpreted animal structures through the lens of classic linkage mechanics, as well as considerations for muscles’ abilities and limitations.

He also worked with Lego and lollipop sticks to build models which revealed the 4-bar and 6-bar linkages, as well as other various linkages at play, in locomotion, and how different muscles become engaged at different times through simple changes in geometry.

Using these methods, the study found that when running, animals such as horses avoid a lot of unnecessary work by sliding their hips and shoulders over their feet.

Muscles, tendons and bones allow this action by forming various linkages where the links – which act as the bicycle spokes – do not change length when loaded by bodyweight.

The findings demonstrate how an intricate muscle structure works together to support complex biomechanics, but also makes the complexity much easier to understand.

Historically, in the context of surgery, rehabilitation and prosthetics, great focus has been placed on the importance of the elastic recoil in tendons. However, better understanding the linkages has the potential to better inform choices regarding surgical approaches, rehabilitation programmes and prosthetic designs.

Where previously “springy” tendons and legs were viewed as the key to efficient locomotion, now the “sliding” and linkage aspects should be viewed more prominently, perhaps as the dominant factor to economical locomotion.

“It has been known for 40 years that running animals manage to ‘slide’ their bodies over their feet, and that this could be an economy trick, just like a wheel or skate allows weight forces to be carried during horizontal motion,” Usherwood says.

“What has been missing is how the muscles manage to achieve this without wasting a huge amount of energy pulling against each other.

“By spending plenty of time with a puppy, lots of Lego, and a computer, I was able to identify some of the linkages involved – and these linkages have been known about since the Industrial Revolution.”

In his paper, Usherwood says it would certainly be premature to suggest that the current spring paradigm of tendon function in running animals should be discarded.

“But the linkage paradigm does begin to bring together observations of gross anatomy, limb mechanics and muscle activation in fast quadrupedal mammals by relating the conditions for low mechanical work at the level of the legs to the geometry of low work demand at the level of the muscles.”

Usherwood, J.R. (2022). Legs as linkages: an alternative paradigm for the role of tendons and isometric muscles in facilitating economical gait.
J Exp Biol (2022) 225 (Suppl_1): jeb243254.

The study, published under a Creative Commons License, can be read here

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