I didnt really know how to title this entry. I have two students who are brothers. Incidently, they both are double jointed in their thumbs and when they play they play back on the "ball" of the thumb joint instead of the pad/tip. They also buckle their first knuckle on their fingers. This is only on their left hand. I've FINALLY cleaned up the right hand. They both have large hands and no troubles that way. However, its not hurting them and I cant seem to fix it. They are extremely uncomfortable when I fix it. Should I just let them go? Their intonation is better than 90% of my students.
I get my students to practice not buckling their fingers by having them hold a glass of water with just their finger tips. My first teacher had me do that and it hasn't been a problem for me or my students ever since. Also, make sure that they are "pulling" the bass into them by the finger tips. Other than that, you got me. Intonation might be good, but the focus of power needs to be at the tip. When they buckle, they transfer the power to the knuckle. Hope that helps.
I have provided some of my weaker handed students with "stress balls", soft foam centered squeeze balls that they can get their whole hand around to crush inwards, including their thumb. Another trick is to change the sensation of fingering from "pressing" to "hooking" the fingers down. Form the letter "C" with bent fingers and thumb. Hold the right hand comfortably in front at about eye level and hook the left hand fingertips over it to support the hanging weight of the left arm. You can liken it to doing chin-ups with your fingertips hooked over the door architrave. Then, seated behind the bass, take the hooked shape and sensation to the bass neck using back muscles to hook the strings down to the fingerboard without using the thumb yet. Finally place the left thumb in position and squeeze thumb and hooked fingers together. Ask the student if this feels very different. Finally, keeping the fingers in this shape, raise and lower the fingers by "flapping" from the big knuckles to drop onto notes. Collapsing fingers tend to be slow and not articulate clearly. Arched and rising/falling like a pianist's are simply the best for strength, speed and clear articulation, and are worth your perseverance. DP