A very popular conceptual approach to the kinetic chain of global body movements is to consider the bone linkages as alternating joints possessing mobility and stability. Many movement practitioners write about this approach originally attributed to Gray Cook and Mike Boyle. The knee, being a “stable” joint lies between the foot and hip (mobile joints). The lumbar spine (stable) lies between the hip and thoracic spine (mobile). The recognition that dysfunction in the mobile joints leads to excessive motion in the stable joints is important, but increased compensatory motion in adjacent joints is a feature of the kinematic chain, and has nothing to do with the mobile-stabile concept.
The mobile-stable joint sandwich
The “mobile-stable joint sandwich” can be appealing. Structurally, the “mobile” joints appear to have more motion than the “stable” joints, but this isn’t true when considering all three planes of motion. For example:
- The knee while having limited motion in the frontal plane, has more motion in the sagittal plane than the ankle or the hip
- The lumbar spine joints have more sagittal plane motion that a thoracic spine joint
More importantly, this misguided approach to analyzing and training functional movement will lead to improper and ineffective training and rehab programs. Mobility-stability must be trained in all joints in order to maximize performance and prevent injury. There are two principles of Applied Functional Science that apply in this case:
- 3D – all joints and muscles work in each of the three planes
- Mobile-stability – functional movements require motion and control from all joints
The Scapula-Thoracic Joint
One example is the shoulder girdle, which is considered a “stable” joint because the glenohumeral joint is considered “mobile”. Nothing could be further from the truth of function. The scapula-thoracic joint (along with the clavicle) is a hallmark example of the necessary mobile-stability that prevents shoulder and upper extremity injuries.
To understand this, consider the extremes of the mobile-stability spectrum. For maximum stability, the scapula could be screwed to the ribs. Great stability, but the loss of upper extremity function is obvious.
For maximum mobility, cut all the scapula-thoracic muscles. The upper extremity would become useless because the required stability of the scapula on the thorax would be lost.
Three-dimensional knee motion
The “mobile-stable joint sandwich” leads to disastrous results when applied to the knee. Because the knee has only a small amount of motion in the frontal plane and is often injured by excessive motion in the frontal plane, many practitioners suggest that the valgus position should be completely avoided. However, watching any sport will provide all the evidence needed that this motion is essential.
Rather than being eliminated, the knee abduction motion that creates valgus must be promoted utilizing the Principle of Mobile-Stability. The frontal plane motion, as part of the three-dimensional knee motion, is essential to activate the posterior-lateral hip muscles. These muscles are lengthened by frontal plane motion, and they then decelerate the motion preventing injury and providing the power to reverse the motion (loading before exploding).
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The Principles of Applied Functional Science, including Mobile-Stability, 3D, and loading before exploding, provide the foundational basis for true functional assessment, training, rehabilitation, and injury prevention.
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