|
MuscleMatrix -> RE: Gluteus Mechanics (May 17, 2008 5:02:31 PM)
|
Gluteus maximus is a fan shaped muscle, therefore it can function in several roles around multiple axes depending on hip joint position. The direction and point of application of the specific external force that is creating a torque challenge for the musculature is key to determining need for an opposing internal torque created by gluteus maximus as well. In other words, we can't begin to discuss the contractile capabilities of a muscle unless we are referring to a specific joint position AND a specific external torque challenge around a specific axis.
One can be actively moving into hip extension with virtually no need for contraction of the gluteus maximus. Example - eccentric portion of supine straight leg raise. Around the longitudinal axis of the femur, one can move into external rotation without active contraction of the gluteus maximus as well. Example - prone knee bent 90 degrees, TB pulling tibia into external rotation against my active eccentric contraction of my hip internal rotators.
The point is that we need to abandon the idea of associating a motion with a muscle like we get from our kinesiology and biomechanics textbooks. The textbooks' list of motions associated with a muscle are usually biased toward concentric actions and from a starting point of anatomical position. The key is the direction of the resistance (external torque challenge around an axis of a joint) which creates the need for a select group of muscle tissue that is strategically positioned on the other side of the axis with a line of force from the fibers that can directly oppose the externally imposed resistance. Forget about the motion...it is the resistance created around a joint axis that's the key. In addition, consider an isometric where there is no motion.
Now, let me get back to your first question. Let's say you are positioned prone with your knee bent 90 degrees. If you extend your hip against gravity, any muscle tissue that falls on the posterior side of the med-lat axis of the hip joint can be involved in hip extension. However, gluteus maximus as a whole has the largest mechanical advantage due to moment arm length. With the hip in neutral around the longitudinal axis the sacral fibers actually have the largest moment arm for hip extension around the med-lat axis. With the hip in external rotation the moment arm size for hip extension begins to shift toward the iliac fibers. With the hip in internal rotation the moment arm size for hip extension begins to shift toward the coccygeal fibers. The largest hip extension mechanical advantage for gluteus maximus as a whole though is when the hip is in neutral around the longitudinal axis.
The magnitude of contraction will primarily be dependent on the external torque created by the resistance (whatever that happens to be: gravity, TB, manual, etc) and the ability of gluteus maximus to contract and resist that external torque. IF g max is inhibited, there are plenty of other hip extensors that can compensate/substitute and try to "pick up the slack". Hamstrings are usually one of the faithful volunteers. However, they will never be as effective as the gluteus maximus properly innervated. Well actually ... in a position of 90+ degrees of hip flexion, hamstrings actually have a bigger moment arm for hip extension than most of the g max though. Here's an example of where another muscle may gain mech advantage for hip extension over g max dependent on hip joint position.
Iliac g max can even act as a hip internal rotator (concentric, isometric) when the hip is in 90 + degrees of hip flexion. Coccygeal g max can adduct (concentric, isometric) and other parts can abduct depending on hip joint position and resistance encountered.
So your question doesn't turn out to be that simple after all.
|
|
|
|