the flexor<span>relaxes and the </span>extensor <span>contracts to extend or straighten the limb at the same joint: For example, the </span>biceps muscle<span>, in the front of the upper arm, is a </span>flexor<span>, and the </span>triceps<span>, at the </span>back<span> of the upper arm, is an </span>extensor<span>.</span>