This article is from the Recreational Figure Skating FAQ, by Karen Bryden with numerous contributions by others.
The quadriceps (quads for short) are four muscle sheets running along
the outer and inner thigh. Although one would think that skating is an
excellent exercise for the quads, with all that knee bending, this is
only true for the outer quads. The inner quads are not used to the
same extent. This could result in a strength unbalance between the
muscles which can slowly pull the knee cap out of track and cause or
aggravate chronic knee pain. Here are a couple of exercises which are
useful to prevent and treat this condition:
--- Slide your back down a wall until you reach a sitting position,
without letting your knees pass beyond your ankles (the knee joint
will be at about 90 degrees). Sit unsupported for as long as you can
while squeezing a cushion or pillow between your thighs.
--- Sit near the edge of a chair or low table with your feet resting
on the floor. Raise one leg so that it is extended forwards (it does
not have to be totally straight) and as turned out at the hip as you
can manage (ie, the inside of your leg will be facing the ceiling).
Don't slouch. If necessary, use a wall to prop up your back. Stay like
that for 30 seconds, then do the other leg, rest and repeat again.