8.4D: Stability and Range of Motion at Synovial Joints
Tendons provide stability at joints.
- Explain the roles of tendons in movement and flexibility
Key Points
- Although tendons have long been considered just a way to attach muscles to bones, research has shown that their springy properties also allow them to provide stability during locomotion with no active work.
- The elasticity of tendons enables them to release stored energy during walking, allowing the muscles to generate greater force without changing length.
- Many factors influence joint stability and range of motion.
Key Terms
- pronation : The action of rotating the forearm so that the palm of the hand is turned down or back.
- supination : The action of rotating the forearm so that the palm of the hand is turned up or forward
- eversion : The condition of being turned outward.
- plantarflexion : The movement that increases the approximate 90 degree angle between the front part of the foot and the shin.
- dorsiflexion : The movement which decreases the angle between the dorsum (superior surface) of the foot and the leg, so that the toes are brought closer to the shin.
A tendon is a mechanism by which muscles connect to bone and that transmits force. However, over the past two decades, research has also characterized the elastic properties of tendons and their ability to function as springs. This characteristic allows tendons to passively modulate forces during locomotion, thus providing additional stability with no active work. It also allows tendons to store and recover energy with high efficiency.
Effect of Tendon Elasticity
Achilles Tendon : The Achilles tendon, also called the calcaneus, provides stability and limits the range of motion at the ankle joint. It’s depicted in this diagram in relation to the tendo calcaneus.
During a human stride, the Achilles (calcaneal) tendon stretches as the ankle joint undergoes dorsiflexion. During the last portion of the stride, as the foot undergoes plantar flexion (pointing the toes downward), the stored elastic energy is released. Because the tendon stretches, the muscle is able to function with less or even no change in length, allowing it to generate greater force.
Joint Stability
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:
- 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 vice versa. Aging is another factor that influences motion due to decreased fluid, thinning of cartilage, shortening of ligaments, and loss of flexibility.