Do your knees ache after a ride? Make sure you're notbracing- putting too much weight forward and down into your stirrups. This undesirable position can cause soreness, as stiff joints can't absorb your horse's motion. Stiff legs may also push your saddle back from its usual position, causing your horse to experience wither and shoulder pain.
To fix your position, concentrate on sitting deep in the saddle (rather than deep in your feet), with your seat bones balanced over your horse's spine and your pelvis relaxed enough to absorb your horse's strides. The relaxed posture will allow your legs to swing a bit more than usual. That's fine. Your knees will relax, and you'll have a better balanced seat.