Normal Barrier To Motion

Definition of a Vertebral Motion Segment

A vertebral motion segment consists of an adjacent pair of vertebra, along with the intervening disc and ligamentous structures. Motion of the superior vertebra is described relative to the inferior vertebra, and rotation is defined using the anterior surface of the vertebral body as opposed to the elements of the posterior arch.

Definition of Diagnostic Convention

There are two conventions used to describe dysfunction of a verterbal motion segment:

• Positional Diagnosis -- Position of the restricted segment.
• Motion Restriction Diagnosis -- Motion lost to the restriction.

You should be able to translate easily between these two formats used to describe diagnostic findings. The physician must be able to identify and characterize normal and abnormal range of motion in order to identify and characterize normal and abnormal barriers to motion.

The total range of motion of a joint is limited by the structural integrity of the physical components. External forces that attempt to move a joint beyond anatomical barriers result in fracture, dislocation, and/or soft tissue damage. The position defined as the anatomical midline (Midline Neutral) is located between the extremes of the anatomical barriers.

The total range of motion can be subdivided into a range of passive movement that is defined by an elastic barrier and a range of active movement. The elastic barrier is a normal barrier to motion that results when all tension has been taken up within the joint and its surrounding tissues. The remaining space between the elastic barrier and the anatomical barrier is termed the paraphysiologic space. It is within this space that the high-velocity, low-amplitude thrust appears to generate its characteristic popping sound.

The range of active movement, defined by the physiological barrier, is less than the range of passive motion. The physiological barrier is a consequence of increased tension within the muscle and fascial components. Frequently there is a reduction in the available range of active motion due to myofascial shortening. Active movement of the musculoskeletal system results from voluntary contraction of muscles by the patient. Passive movement of the musculoskeletal system results from the application of external forces by the examiner. Both active and passive efforts may be used by the operator during the diagnostic and treatment processes.

Increase in Muscle Tension as a Result of Muscle Activation

1. Concentric contraction -- When a muscle develops sufficient internal tension to overcome an external resistance so that the muscle visibly shortens and moves a body part, it is said to be in concentric contraction. When you curl a dumbbell, the biceps muscle contracts concentrically.
2. Eccentric contraction -- When an externally applied force is greater than the internal tension generated by the muscle so that the muscle lengthens, it is said to be in eccentric contraction. When you slowly lower a curled weight from the shoulder, the biceps muscle contracts eccentrically.
3. Isotonic contraction -- Contraction of a muscle where the internal forces generated by the muscle remain constant. If externally applied forces are greater than internally generated forces, we would say that an isotonic eccentric contraction had occurred. If externally applied forces are less than internally generated forces, we would say that an isotonic concentric contraction had occurred.
4. Isometric contraction -- Contraction of a muscle that is not accompanied by movement of the joint. The internal forces generated by the muscle are equal and opposite to the external forces that are being applied to the muscle through the joint.

Increase in Muscle Tension as a Result of Passive Movement

When a normal muscle is passively stretched beyond its resting length, tension is developed mainly in the parallel and series elastic components. Tension within the muscle is small at lengths that are close to the muscle's resting length. However, as the length of the muscle approaches the elastic limit, tension within the muscle increases dramatically. Stretching the muscle beyond the elastic limit results in tissue damage caused by tearing of the muscle. It is this sensation of elasticity and the absence of a hard barrier that we are expecting to find in a normal joint.