A muscle has a certain length. When a muscle contracts it generally shortens and when it relaxes it lets go - and with the help of gravity or an external force it will lengthen.

If a muscle is contracting concentrically and you start out in a fully lengthened position then the muscle starts in its lengthened range, moves through its middle range and finally into its shortened range. As it relaxes the reverse happens

there are a few distinctive features about the range a muscle is working in. For example a muscle is usually strongest in its middle range and weakest a the very end of the shortened or lengthened range