Tendons Store Elastic Energy

a frog with spots resembling leopard spots sits in algae

Biological Strategy

Northern leopard frog

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Image: Jessica Bolser/USFWS / Flickr / Public Domain - No restrictions

Store Energy

Once a living system captures energy or transforms one energy form into another, it must frequently save that energy for future use. But energy is difficult to store in some forms. So living systems need strategies to either use energy quickly, or to convert it from forms that are difficult to store (such as electrical or kinetic) to more storable forms. For example, grasshoppers store energy as potential energy in an elastic material in their tendons. When they need to jump, that energy converts into kinetic energy, providing the force needed to escape predators.

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Move in/on Solids

To obtain needed resources or escape predators, some living systems must move on solid substances, some must move within them, and others must do both. Solids vary in their form; they can be soft or porous like leaves, sand, skin, and snow, or hard like rock, ice, or tree bark. Movement can involve a whole living system, such as an ostrich running across the ground or an earthworm burrowing through the soil. It can also involve just part of a living system, such as a mosquito poking its mouthparts into skin. Solids vary in smoothness, stickiness, moisture content, density, etc, each of which presents different challenges. As a result, living systems have adaptations to meet one, and sometimes multiple, challenges. For example, some insects must be able to hold onto both rough and slippery leaf surfaces due to the diversity in their environment.

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Amphibians

Class Amphibia (“to live a double life”): Frogs, salamanders, toads

Amphibians live a “double life” by being able to survive both on land and in water. Unlike reptiles with dry, scaly skin, amphibians have smooth skin that needs to be kept moist. Many live in the water through development and start life with gills, and then transition to breathing with lungs. They can get even more oxygen through cutaneous respiration, or breathing through their skin. This class represents the first animal group to be able to efficiently walk on land—aided by the well-defined limbs that gave them a “leg up” on their finned competitors.

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Tendons of the northern leopard frog store elastic energy which is rapidly released during a jump.

“The catapult-like mechanism that has been hypothesized for frog jumping requires pre-storage of elastic energy, followed by the rapid release of this energy during the jump. The pattern of muscle length change and joint motion observed in the plantaris confirms this hypothesis. Early in the jump, the plantaris longus muscle shortened without joint movement (figures 1 and 2), showing that the tendon stretched to store work done by muscle contraction. This was followed by a period of high angular acceleration of the joint and minimal muscle shortening (table 1), indicating a substantial contribution of tendon recoil to powering ankle extension. Although we did not measure tendon length directly, the observed pattern of decoupling of muscle fascicle and joint motion would be difficult to explain by mechanisms other than tendon stretch and recoil…Since elastic energy storage occurs even in frog jumps which do not show exceptional distance, takeoff velocity or power output, it is likely that elastic energy storage is far more common in accelerations than indicated by prior indirect methods of detecting it. The presence of elastic energy storage and recoil in submaximal jumps may also be informative in future investigations into the nature of the catch mechanism in anuran jumping.” (Astley and Roberts 2011:3)

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