UPDATE: I have a six year old now on her third lesson with the collpasing first joint problem. I try to get her to play with "beautiful hand shape" but she seems unable. I don't know how much I should stress it, I don't want to turn her off so soon.

ORIGINAL POST:
I have an adult student with long "double jointed" fingers (finger joint laxity). The first joint of each finger collapses back almost 45 degrees. I have asked him to practice a C scale and to keep the joints from collapsing. When he plays the scale slowly and he can just barely manage to keep the joints from collapsing, it seems to requires a lot of effort and control. His fingers want to collapse flatly onto the keys at the first joint. Any thoughts?

He can learn to activate the muscles that will keep his joints from collapsing, although it will take some time and effort. It will be well worth it. One thing to check is whether he is playing the notes only from the fingers or if he is dropping the full weight of his forearm onto each note. If he dropping, try to help him learn how to. Playing the notes with his arm weight will allow his finger muscles to focus on maintaining the arch. If he plays notes from the fingers alone all of his finger muscles will be taxed from trying to depress the keys with enough speed to make the notes sound and he won't be able to maintain any sort of arch.

This should all be practiced slowly at first, but as speed increases the armweight is still very important. Even in very fast passages armweight really helps out the fingers so that they can focus on not collapsing.

Ryan

I have some violin students who have this problem also. I think the method is applicable to piano students as well. Make them lift, seperate and strike their fingers 'inwards' to get a good bite and shape on the piano cover first. Usually I will want them to produce the 'thump' sound for each finger, this will produce some great articulation later on also. If the movement, shape and reflex seems ok, then get him to play the same manner: emphasising on the lifting and striking with a good shape for each finger on the piano. U may try really simple ex like Hanon ex 1. I think it's more appropriate to use really simple ex that allows him to focus more on the finger action rather than notes.
So, lift, and strike inwards.

I use a similar finger motion when playing the piano, although I should note that my "lifts" are actually stretching my fingers out rather than lifting them straight up. The problem with the piano is the mass and distance of travel of the keys. It is really crucial when playing piano to learn how to drop the armweight onto the keys to assist each and every note.

I have a 6 year old now who has had three lessons and seems to make no progress on keeping the first joint of each finger from inverting (collapsing) almost 45 degrees.

As DW suggested, keep the exercise simple. Rather than Hanon I would suggest just a basic 5-finger position. The collapse of the nail joint is prevalent in many pianists even after years of lessons. The knuckle bridge probably collapses as well. While I'm not double jointed, there are some 'exercises' one can do while away from the piano too. To strengthen the nail joints I do something like an isometric exercise. I 'lock' the nail joint of one finger with the corresponding finger on the other hand and pull for a few seconds maintaining the force and making sure the joint is firm and secure. This can be done almost anywhere and anytime. Several times a day helps develop the muscle tissue and certainly helps. For the knuckle bridge one can put the hand on a flat surface and with the hand in a basic hand position depress the knuckle joints all the way but keep the fingers in position--no sliding, slipping, etc.--and then raise it as high as possible (not the wrist!!)while still maintaining the fingers without movement. It's sort of a hand 'pushup'. Each finger needs a firm, positive stroke, but observation of the nail joint is absolutely necessary and conscious effort to see that it becomes firm and stable with no collapse possible.

It's a difficult thing for some and natural for others. Double-jointed people have an additional chore to do I think.