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The elastic energy stored in the Achilles tendon aids humans in bipedal ambulation. This property is absent in the other great apes (orangutans, gorillas, chimpanzees, and bonobos) who, although they can walk bipedally, primarily move on all four limbs.
Tendons traditionally have been considered a mechanism by which muscles connect to bone, functioning to transmit forces. However, over the past two decades, much research has focused on the elastic properties of tendons and their ability to function as springs. This allows tendons to passively modulate forces during locomotion, providing additional stability with no active work. It also allows tendons to store and recover energy with high efficiency.
For example, during a human stride, the Achilles tendon stretches as the ankle joint undergoes dorsiflexion . During the last portion of the stride, as the foot undergoes plantarflexion (pointing the toes downward), the stored elastic energy is released. Furthermore, because the tendon stretches, the muscle is able to function with less or even no change in length, allowing the muscle to generate greater force.
The Achilles tendon provides stability and limits the range of motion at the ankle joint.
Certain joints exhibit special movements including elevation, depression, protraction, retraction, inversion, eversion, dorsiflexion, plantarflexion, supination, pronation, and opposition. A number of factors influence joint stability. These include the shape of articular surfaces (how close they fit); strength and tension of capsule and ligaments (dependent on position); arrangement and tension of muscles; contact with soft parts such as adipose tissue; hormones; disuse causing decrease in synovial fluid, flexibility of ligaments and tendons, and muscle atrophy; gravity; and atmospheric pressure. Typically, the more stable the joint is, the less is its range of motion, and vise versa. Aging is another factor that influences motion due to decreased fluid, thinning of cartilage, shortening of ligaments, and loss of flexibility.
tendons are elastic and function as springs to store and release energy efficiently during movement, tendons passively modulate locomotive forces to provide stability, tendons can lose flexibility through disuse and aging, and tendons stretch to pull the muscle, greatly increasing muscle length, to generate more force
Source: Boundless. “Stability and Range of Motion at Synovial Joints.” Boundless Anatomy and Physiology. Boundless, 01 Jul. 2015. Retrieved 02 Jul. 2015 from https://www.boundless.com/physiology/textbooks/boundless-anatomy-and-physiology-textbook/joints-8/synovial-joints-92/stability-and-range-of-motion-at-synovial-joints-519-323/