In the description under the Anterior Interior Chain (AIC) in the Postural Respiration manual, descriptor #6 mentions that “As a result of ilium anterior rotation, the femur is biomechanically oriented inward and torsional demands on the psoas as an ineffective femoral external rotator and the vastus lateralis and biceps femoris as antagonistic hip stabilizers, increases. I am having difficulty understanding how the vastus lateralis is involved with hip stabilization. Could you please clarify?
Great Question!
The vastus lateralis and biceps femoris act on the femur (hip) as antagonistic muscles to each other. One externally rotates the femur (the biceps femoris) and one internally rotates the femur (the vastus lateralis) when the foot is planted on the ground. When the VL contracts, the proximal attachment of the vastus lateralis pulls or rotates the femur inwardly therefore producing internal rotational movement that is countered by external rotational movement from the biceps femoris with the foot planted. Therefore, these two muscles act on and under the femur antagonistically to each.
When the innominate (ilium) anteriorly rotates, these two muscles become challenged because of poor position to counteract on each other, for femoral stabilization. Their effectiveness in keeping the pelvis or acetabulum positioned correctly on heel strike is lost, because the psoas has to work harder as an external rotator to keep the femur lined up with the foot at mid-stance. Internal rotation need from the vastus lateralis, anterior glute med/min, and medial hamstrings is now not necessary – since the femur is already directed inwardly by the forward anteriorly oriented innominate and acetabulum. The psoas (ER) and tensor fascia latae (IR) now become your primary femoral and hip stabilizers through anterior innominate rotation. Effective femoral stabilization at mid-stance occurs only when the innominate can move posterior at or upon the “heel off” phase of push off. Without posterior innominate rotation, effective femoral stability from the vastus lateralis and biceps femoris is lost.
