When one muscle in a pair contracts the other expands.
contract
Because muscle cells can only contract, not extend, skeletal muscles most work in pairs. While one muscle contracts, the other muscle in the pair relaxes to its original length.
The word "contract" is referring to a muscle doing work, and the opposite is relax. Skeletal muscles work in pairs, and often one of the pair will contract while the other relaxes.
When one muscle in a pair contracts the other expands.
muscles
When one muscle of a pair contracts, the other muscle of the pair relaxes to allow movement of the body part.
When one muscle of a pair contracts, the other muscle of the pair relaxes to allow movement of the body part.
I think what your after is called an antagonistic pair of muscles, for example the bicep and the tricep. When the bicep is contracting the tricep relaxes and when the tricep is contracting the bicep relaxes.
A muscle that opposes the actions of another. =] A muscle that opposes the actions of another. =]
Joints don't have the ability to move. Joints are where two or more bones meet. One end of muscle attaches to the bone and the other end of the muscle stretches cross the joint and attaches to the bone on the other side of the joint. Muscles work in pairs, so that when one muscle contracts (the only movement that muscle can make), the other of the pair relaxes which causes movement at the joint when the bone is pulled by the muscle.
deltoids
Perhaps you mean antagonist and agonist muscles often occur in pairs, called antagonistic pairs. As one muscle contracts, the other relaxes. An example of an antagonisic pair is the biceps and triceps; to contract - the triceps relaxes while the biceps contracts to lift the arm.