Weak muscles can be the bane of many movement problems and the cause of much dysfunction in athletes and patients alike.
Punctum Fixum refers to the ability of a muscle to have a solid attachment. In reading through Rehabilitation of the Spine, by Dr. Craig Liebenson, the example given is the forearm flexors. The forearm flexors must be stabilized by the muscles that stabilize the elbow, and the elbow in sequential fashion by the shoulder girdle. So if that is weak, you may have weak wrist flexors.
In my own practice here in Grand Rapids, I often see the VMO (vastus medialis oblique) not have the strength it may need. Checking the attachment at the adductor magnus revealed the adductor was short/tight. Release the adductor, the VMO magically contracts strongly again.
Take home message, just because a muscle is weak, doesn't mean there is something wrong with the muscle in and of itself. Check the chain.