eli5: Why can we bend our index finger’s first joint freely but we can’t bend our other fingers first joint(thumb excluded) without the others slightly retracting?

Question

eli5: Why can we bend our index finger’s first joint freely but we can’t bend our other fingers first joint(thumb excluded) without the others slightly retracting?

In: Biology

Answer 1

Actually I play guitar so in my left hand (and a little with my right) I can do those things. It’s about practice and muscle memory.