Kermit maintains that it’s not easy being green. I argue that actually, being a frog carries certain perks; athletic prowess being one of them. Frogs have the impressive Olympian ability of jumping huge distances. Other perks include being amazingly cute (desert rain frog anyone?), and in one impressive but sadly extinct case, the ability to convert their stomach into a uterus. Apparently they can also devour everything in their path.

Let me give an example. A bullfrog can comfortably jump a distance of about 1 metre. That’s huge. That’s like me (being moderately unfit) doing a long jump the length of a bus without a run up. And when we look at the bullfrog athletes of the world, at one of the many frog jumping contests, those individuals can jump over 2 metres.

When you look at the nuts and bolts mechanics of it, this seems impossible. A frog just doesn’t have enough muscle power available to propel it 2 metres. In fact the muscle power required is about 1.5 times the amount that their muscles can supply.

Another problem is that muscle tends to contract either slowly and forcefully, or quickly and weakly. It can’t have it both ways. Jumping frogs however do need to extend their back legs quickly and with high enough force to launch them into the air.

Cleverly, frogs have worked out how to hoard energy in their legs, and then release it quickly in one go. Why do we think this is true? Read on.

Much of this post refers to a study from a research group in Boston. The study uses a combination of research on bullfrogs, computer models and comparisons between the two.

Computer models are abstract ideas that aim to explain a real event. We perform simulations of movement, and then compare the results from the simulation to the actual results seen in real animals. At the fundamental level, the model that agrees the most with reality is deemed to be the best fit. It’s the most representative until another model comes by that outperforms it. So computer modelling isn’t a doorway to truth, but a tool that we can use to piece together how something works.

Forget about frogs and modelling for a second. Look down at your calf. If wearing trousers, roll up a leg. Stretch out your foot so your toes point away from your body. Do you see your calf muscle bunching up? That’s your gastrocnemius muscle contracting. And the tight band running from this muscle to your heel is your Achilles tendon, which is a continuation of your gastrocnemius. The muscle is the bit that contracts, and the tendon transmits this force onto a distant object. In this case the tendon transmits the force onto your heel bone, or calcaneous.

Some tendons do much more than just transmit energy, they can also store it. A good analogy for these tendons is an elastic band. An elastic band stores energy as it stretches. Stretch an elastic band between your thumb and forefinger; the energy is stored in the elastic. Let go with your thumb, and the energy is explosively released; the effect is the elastic band is propelled across the room.

frog
Photo by Tony Alter, via Flickr

Frogs make good use of elastic storage. Take a look at the frog below:

The muscle shown is an ankle extensor muscle. When the muscle (in red) contracts, it pulls on the tendon, which in turn pulls the ankle straight. This is one of the muscles that powers the jump. Because the tendon is elastic, it gets stretched and so gets filled with elastic energy, before actually pulling the ankle straight. This part is similar to you stretching the elastic band with your fingers.

Something else is needed for explosive elastic propulsion – a trigger. When you shoot an elastic band, the trigger is you letting go with your thumb, allowing the rubber band to ping. So really all the trigger is doing is allowing the elastic elements to suddenly offload their stored energy. Different sorts of triggers exist in the animal world; the frog’s is a subtle one.

The frog’s crouched starting position is a good one to encourage elastic energy storage. A lot of energy is required to move the legs from this into a straighter position (try it yourself). Until the muscle has produced enough energy to move the joints, the power output is stored in the tendon.

This trigger is created by the change in posture, from crouched to straight leg. As the tendon gets filled with energy, the muscle continues to slowly contract. This slow, forceful contraction is enough to drag the leg into a less crouched position. The shift in posture encourages the tendon to give up its stored energy. The elastic component, having been stretched by the contracting muscle, suddenly goes #PING#, explosively pulling the back legs into full extension, which catapults the frog’s body upwards and forwards.

The picture below illustrates this.

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Sequence of events in a jump. Red arrows indicate the ground’s equal and opposite reactive force to the frog’s. From Naewelaerts and Aerts 2005.

This sequence of pictures shows a frog’s body posture during the jump. For almost half of the jump, the frog doesn’t seem to move much. During this period of non-movement, the extensor muscle is working. The energy from this work is stored in the tendon, which is eventually released during the latter part of the jump.

The key thing here is that elastic tendons allow us to increase our performance, beyond what our muscles can do on their own. Without elastic tendons, running would be very difficult, if not impossible.

But why do we care in the first place? Firstly, frogs are ace. But this aside, working out how frog legs work helps us to pick apart how biological locomotion works as a whole. This can inform ideas about how we can train athletes, treat injuries or even build better robots.

For me though, the coolest thing is to understand how things around us work, full stop.

PS. Happy holidays! We will definitely be a tidal wave of slithering, slimey horror – devouring and destroying everything in our path.

Chris, Ellen, Tom.

#EvoBiomech

Roberts TJ, Marsh RL.  J Exp Biol. 2003 Aug;206(Pt 15):2567-80 Probing the limits to muscle-powered accelerations: lessons from jumping bullfrogs.

Title photo courtesy of Brian Gratwicke, via Flickr

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