I've been studying about coordination and its development for a while now have a tip you guys can try out. In the same vein from the internal hip rotation thread, each leg rotates quite a bit and finding the optimal angle of leg rotation for each motion has big implication in effective and efficient use of the lower body which affects the balance and mobility. A very common tendency for rec tennis players is their feet are pointing the same direction too much while playing. A better dynamic form or posture for movement around the court and stroke execution is to point the feet outward not parallel. To see the difference try shuffling with this form and feel the difference. you can shuffle to make some shapes to make it more challenging. like in a circle and figure 8 and vary direction and orientation. unless you already have this form you'll feel soreness in some muscles and it's normal. once this form becomes habitualized you'll see the improvement in mobility and balance. try to incorporate into your other tennis stroke forms. Parallel feet are effective mostly for explosive forward motion and any other motions in tennis should use this feet outward form. Would love to hear what you think of this if you try.