Tendons, whose primary function is to anchor muscle to bone, can also serve as essential locomotion tools. Some tendons act as springs to aid terrestrial animals that bounce and run along the hard ground. These tendons increase force and power and help to recover energy that would otherwise be lost during movement. Even some animals that move within a fluid system, such as pigeons, have been shown to benefit from elastic tendons during flight. But not all tendons are elastic. Many animals have rigid tendons that act more like cords. These tendons do not confer any elastic benefits to the muscle. This is especially true for small animals, whose tendons are relatively thick for their size.
Bat wings have rather long bones with suitably long tendons. Nicolai Konow and his collaborators at Brown University, USA, wanted to understand what these long tendons were doing during bat flight – were they simple rigid cords acting to leverage the wing (flap, flap), or were they springy enough to confer some sort of mechanical advantage to the bat during flight?
To answer this question, the team at Brown filmed four Seba's short-tailed bats (Carollia perspicillata) with a high-speed X-ray system to track the movement of the triceps muscle, its tendon, and the elbow joint during ascending flight as the bats flew through a tube. They also implanted radio-opaque spheres in the bone and at the junction of each bat's triceps muscle and tendon. This allowed them to track the length of the muscle as the elbow joint flexed and extended during each wing beat.
They found that the changes in joint angle and muscle length during flexion and extension were not synchronized, as they would be if the tendon was stiff. They also found that the triceps muscle contracted almost 25% less than would be expected with a stiff system. So not only were the bats’ wing tendons elastic, but they were also reducing the amount of work the muscle was doing – the tendon was doing it instead.
Konow and colleagues state that because wing movement is cyclic, the elasticity of the triceps tendon allows for a recycling of mechanical energy during bat flight. The energy stored between the downstroke and the upstroke is then released during the late stage of the upstroke. The authors say that this timing is key, as the late stage of the upstroke is when the wing is positioned for force production during the next downstroke. The next step in understanding the importance of tendon elasticity in the bat wing, they say, is to estimate the actual amount of energy stored and released during this cycle.
Previous work with pigeons has shown similar elasticity in the tendons powering flight, but not in the case of other tendons in the same birds and not in tendons of other small animals – even though these tendons are crucial for locomotion. So which tendons aid locomotion by virtue of their elasticity is still being explored in different animals and types of movement. Although some mysteries of tendon elasticity remain unresolved, it now appears clear that elasticity helps power bat flaps.