The finger flexors cross the elbow joint, the superficial finger flexors are weak elbow flexors (Palastanga 4th ed. p151), so a flexed (locked off) elbow will slightly shorten them.
Wrist position has a direct and considerable influence on the length of the finger flexors. Elbow and shoulder position will have an indirect influence on the wrist position in climbing and hence length of the finger flexors. The obvious example of this is the notorious lifting elbows as forearm pump becomes terminal. This seems to be a mechanism to allow some wrist extension hence lengthen the finger flexors and eek out a little more force from them by shifting the point on the length-tension curve.