In many courses from Gray Institute there is a lot of time spent “preaching” the importance of understanding real motion and relative motion. REAL motion refers to the motion of bones in three-dimensional space. Each bone can go through 3 rotations and 3 translations (6 degrees of freedom). RELATIVE motion refers to joint motion. The relative motion between adjacent bones determines what is happening at a joint. In many biomechanics laboratories multiple cameras are used to measure the REAL movement of bones. Once the three-dimensional bone movements are measured, then the RELATIVE joint motions are calculated (not directly measured).
The importance of understanding how relative joint motion is created by real bone motion becomes apparent if the movement practitioner is trying to assess or train a joint motion as part of a specific activity or sport. For any single relative joint motion (at any joint in any plane), there are 5 different real bone movement combinations that can produce the “same” motion.
Let’s use the example of hip internal rotation. We need to consider the real bone motion of the pelvis and the real bone motion of the femur. We need to be aware that extremity joint (relative) motion is named for the position of the distal (bottom) bone of the joint (femur in this example).
Relative internal rotation of the right hip joint is produced by these 5 real bone motion combinations:
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Pelvis stationary, femur internally rotates
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Distal bone motion and relative joint motion are the same.
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Pelvis rotates to the right, femur is stationary
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Proximal bone motion is opposite the direction of internal rotation in the transverse plane, but results in relative internal rotation.
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Pelvis rotates to the left, femur internally rotates faster than the pelvis
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Both bones moving in the same direction, and if they move at the same rate, there will be no relative joint motion. With the distal bone moving faster, the real femur motion and the relative hip joint motion are the same.
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Pelvis rotates to the right, femur rotates externally slower than the pelvis
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The bones are moving in the same direction. The proximal bone (pelvis) is moving faster than the femur. Although the femur is externally rotating, the relative hip joint motion is internal rotation.
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Pelvis rotates to the right, femur internally rotates
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The bones are rotating in opposite directions. The real bone motion of the femur and the relative hip motion are the same, but the amount of hip internal rotation is increased by the pelvis rotation to the right.
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For a specific activity, it is also important to determine what forces are driving the bones to move. These forces can physical (e.g gravity) and biological (e.g muscles). This is particularly important when both bones are moving in the same direction. The direction and strength of the driver will determine which bone moves faster. Forces from below the joint (bottom-up driver) will usually make the distal bone move faster. Forces from above the joint (top-down driver) usually will make the proximal bone move faster.
The 3D Movement Analysis and Performance System uses the feet, hands, and eyes as drivers (both top-down and bottom-up) in combination with gravity, ground reaction force, and momentum to analyze global movements. Then the movements can be tweaked in the Performance System to emphasize one of the drivers, thereby creating any one of the 5 different real bone motion combinations that produce the same relative joint motion.