Antagonistic muscle action refers to pairs of opposite muscles working to create movement in opposite directions. Examples include the bicep-tricep pair in arms, and the quadricep-hamstring pair in legs. Confusingly (?!), the term forelimb is used to refer to any limb (leg, arm) in terrestrial vertebrates.
Only one muscle in the antagonistic pair can contract at a given time. The contraction causes either flexion i.e. bending of the limb, or extension i.e. straightening of the limb.
Antagonistic muscle pairs exist because muscles contract but cannot extend. Therefore, to achieve both directions of limb movement, muscles must exist on both sides.
Tendons are strong bands of fibrous connective tissue that attach muscles to the bone. They have special properties that enable efficient use of the muscle action to generate movement. Their elasticity and viscosity provide a store of energy that is released e.g. during strides in walking. Muscles work better if they do not have to elongate, so the ability of tendons to stretch optimises movement.
This is exemplified by the Achilles tendon connecting the calf muscle to the bone of the heel. As the foot is laid on the ground, the tendon stretches, and before the foot is lifted again the tendon goes back to its initial state, releasing energy. The calf muscle experiences minimal stretching itself, and the motion during walking is optimised.
The mechanical properties of different kinds of tendon are determined by the diameter and orientation of the collagen fibres that make it up, as well as the content of proteoglycans (proteins with many sugar chains). The former determine its response to tensile stress, while the latter act against compression force.
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