Why muscles act in pairs
Contraction generates tension in a skeletal muscle. This force is transferred to bones, allowing parts of the incompressible skeleton to change position.
A tendon forms the non-elastic connection between a muscle and a bone. Because it stretches very little during contraction, force generated by the muscle can be transmitted effectively to the skeleton.
Muscles shorten to produce a pull; they cannot actively push a bone back in the reverse direction. Opposite movements therefore require two muscles with opposing actions, known as an antagonistic pair.
Movement in one direction
One member of the pair contracts while the opposing muscle relaxes. Tension from the contracting muscle moves the attached bone.
Movement in reverse
The roles switch. The second muscle contracts as the first relaxes, producing movement of the bone in the opposite direction.
Biceps and triceps
The biceps and triceps of the upper arm provide a familiar example of antagonistic muscle action.
- Raising the lower arm involves contraction of the biceps and relaxation of the triceps.
- Straightening the arm involves contraction of the triceps and relaxation of the biceps.