Posted by: daw (daw@bellevue-law.com) on Mon Jan 28 13:22:19 2008

Hips "rotate" because the femur in each is turned, by specific (and different) muscles being contracted.

In hip rotation of a swing, the "back" femur rotates medially (front edge of femur rotates inward) and the "front" femur rotates laterally (front eage outward). Stand in a batting stance, then drop your hands so they're palm down on your hips. Then rotate your hips as if swinging. Do it slowly. Your back hand will move towards your belly (medially), your front hand will rotate away from the belly (laterally).

Now rotate your back hip, keeping your "front" hand from moving outward (in other words, without rotating the front femur/hip). Any problem with that? Nope. Merely rotating the back femur does nothing to rotate the front femur.

See if you can get the front hand to turn outward, solely by rotating your back hip as hard as you can, to "transfer force" to the front femur and cause it to turn. Nope.

It is anatomically impossible to transfer rotational energy from the back hip to the front hip. Complete rotation requires coordinated use of muscles in both legs.

The gluteous maximus muscle, one of the biggest in our entire body, is the primmary force driving lateral rotation (front hip), but there are several others involved.

The gluteous medius, another large muscle (but not so large as gluteous maximus) powers medial rotation.