Thank you for the replies... it's quite interesting. So I presume that being double-jointed might appear as if people are playing with weak fingers and not fully supporting them with their hand.
I had a student yesterday whose finger 5 on the RH sticks out quite a bit. I tried to get him to do an exercise where he holds finger 5 on G, and tries to hold it as he plays underneath the pattern: C D E F E D with 1-2-3-4-3-2 etc.
Every time he tried, he could play C D & E but the second he tried playing F, his G (5th finger) would release from the key - unable to physically hold it down. To me I thought it was just bad technique from his previous teachers, but he told me that it was a little sore to try and force his finger to do the exercise.
Is this normal for most people who are double-jointed??? I'm FASCINATED by this only because it's something I've never really experience before.
When I started with held-note exercises, I found it impossible to do them gently, i e I would put a lot of pressure on the finger(s) holding the key(s) down. In the instances, where the use of one finger causes an involuntary lift in another finger, it can sometimes be caused by pressure and improper use of the extensors.
The extensor muscles in the forearm, the ones that extend each finger, are interconnected, but when used gently, they can lift each finger, except the 4th, rather independently. However, if you put a lot of pressure on one finger, and it primarily resists that pressure by engaging the extensors, the other fingers will involuntarily lift. Finger movements "pushing away" from you, or pressing down with the arm on a finger like described above, can cause this. It could be possible your student is doing some kind of uncoordinated pushing movement with the 4th finger, rather than a gentle gripping movement.
However, it could also be a much simpler coordination problem: your student has simply not figured out what instructions to send from the brain to the fingers to move the 4th without moving the 5th.