Do you have any stretches that specifically target the biceps?
The biceps brachii crosses the shoulder and the elbow. Actions include flexion of the shoulder, flexion of the elbow and supination of the forearm. So if you think of it this way, to stretch this muscle you could:
- Stand with your arm at your side.
- Pronate your forearm.
- Extend your elbow (straighten out your arm).
- Extend your shoulder (pull your arm back behind you).
If you are working with a client, you can gently assist in the extension by pulling his arm behind him. You can't really assist at the elbow since it can't extend more than being straight.
You may also want to try this stretch along a horizontal axis, abducting the arm at the shoulder to about 90 degrees (shoulder height) then pulling it back.
Most people's biceps are not very tight/short, which is probably why many references don't include this stretch. The biceps are rarely considered a major contributor to musculoskeletal imbalance either, which is another a reason why we seldom see stretches focused on lengthening the biceps. Your client may only feel this after a heavy set of bicep curls!
I hope that helps.