A muscular force's effect on the hip joint surface during unilateral stance
Using the technique of vector resolution, we can analyze the effects that force in the hip abductors (vector A) has on the hip's articular surfaces.
We begin by drawing a red, dashed reference line that approximates the hip joint surface's orientation. The joint surface is curved, so we estimate the orientation of a line that is a tangent to that curved surface.
| The hip abductors (vector A) act during unilateral stance to oppose the hip adductor moment that gravity produces, and to keep the pelvis level.
We translate vector A through space, neither tipping it nor changing its magnitude, until its point of application rests on the reference line. In its new location (vector M), we resolve it into two component vectors. One component of the muscle's force (Vector C) compresses the acetabulum into the femur, while the other shear component (Vector S) causes the acetabulum to slide along the surface of the femoral head.
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This analysis, along with that of gravity's effect on the hip joint, illustrates how both gravity (body weight) and the hip abductor muscles produce forces that compress the hip joint surface. Because the abductor muscles' force exceeds that of the superincumbent body weight, most of the compression that the hip joint surface experiences during single limb stance, walking, or running actually comes from muscle action. This explains the curious finding that compressive forces at the hip can equal 2.4 to 2.6 times body weight in unilateral stance, and 4 to 7 times body weight during gait.
The analysis also explains certain behaviors among people whose hip joints are deteriorating due to a degenerative disease like osteoarthritis. An arthritic hip joint's response to joint compression includes inflammation and pain. Individuals seek to decrease hip pain by decreasing joint surface compression. Because they cannot easily accomplish this by reducing the superincumbent body weight, they reduce instead the abductor muscle activity that produces a large part of the joint compression. They accomplish this in a manner similar to the one people use whey they cannot develop abductor muscle force, by altering their posture and alignment to reduce the moment arm of gravity's force on the hip joint. This reduces force demands on the hip abductors, thereby reducing the net compressive force on the hip joint surface.
Not only do they adapt by using a lateral trunk lean when they walk, but they benefit from using an assistive device, like a cane, on the side opposite to the painful hip joint.