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 (Figure 1). 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.
Certain joints exhibit special movements including elevation, depression, protraction, retraction, inversion, eversion, dorsiflexion, plantar flexion, 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.