Not
only are you working the inner thigh (hip adductors) in the leg
that is being raised and lowered, you are isometrically working the hip
adductors on the leg that is holding you on the ball. In fact, you may
feel more fatigue in the inner thigh that is not moving.
Your
hip adductors are working as pelvic stabilizers along with your
other trunk muscles. Your hip muscles often work this way during
functional movements to maintain a stable pelvis because it is only
with a stable pelvis that we can create effective and efficient limb
movements.
If
you are having difficulty with this ball exercise you can make it a
little less difficult by
stretching both arms
forward to increase your base of support,
softening your ball
to increase the ball's area of contact with the floor,
propping it up
against a wall until you get used to the movements involved,
reduce your lever arm
by placing the ball under your knee instead of your foot.