If a part of the body is outstretched, it is stretched out to its full length. Think of outstretched arms, waiting for a hug, or an outstretched hand, offering help or asking for it. A cat may sleep with outstretched legs. It is almost always used this way, as a describing word, not as a verb. People also say stretched out, which is the same picture in looser talk. Though actually you will hear outstretched far more with arms and hands than with other things.