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1st Feb


Stabilizing Muscles and Joint Instability

It is now common, even in the health club arenas, to hear the terms “stabilizing exercise”, and “core muscles”. But what do these terms really mean, what are such muscles, and why are such exercises and muscle groups so important?


When we look at our skeletal muscle system, we know that not all muscles are the same in their function and physiological makeup. Some muscles are located deep and close to our joints, have predominantly slow twitch type muscle fibers, are often short muscles, and can contract for prolonged periods and have some tension most of the time. These are endurance type muscles, whose primary function is to ensure that the joints can move in a mechanically normal fashion. They thereby provide stability to the joints. These are the stabilizing, or true “core muscles” in our body.

An authority on the muscular system, Vladimir Janda, M.D. referred to these muscles as the “tonic muscles”. They are there to provide support, not to move our limbs or spine.

Other muscles are more superficial, typically longer, with greater percentage of fast twitch fibers, which therefore have greater potential to generate strength, but have less endurance capacity. These muscles are the “prime movers”, or “phasic muscles” according to Dr. Janda, whose primary role is to generate motion.


In a perfectly operating human body, we would have an optimal balance between the stabilizing and the prime mover muscles. However, much can happen to upset this balance, especially after injuries, but also contributing to causing injuries and pain.

The muscles that are affected negatively the most with injuries and painful conditions, and with increasing age and loss of conditioning, are the stabilizing muscles. They tend to atrophy and get weaker, have inhibited activity, or with pain also go into guarding or spasm, further increasing our pain and abnormal movement patterns. There is a specific neurological basis for this mechanism.

Therefore, in physical therapy we almost always first evaluate and address these stabilizing muscles, before engaging the longer movers, to restore normal mechanics and function. This is as true for the elderly, as for the elite athlete.

This philosophy of training is based on our definition of joint instability as “a joint which mechanically moves abnormally, thereby increasing the likelihood of tissues
in and around the joint getting compromised”.


The term “core muscle training” is often used and applied loosely in fitness circles. If there is a muscle imbalance, and especially during or after an injury or painful condition, it is essential to isolate the stabilizing muscles and apply training based on knowledge in anatomy, physiology and biomechanics.

The stabilizers need to be activated, initially without significantly engaging the larger muscles, and the exercise intensity at first must be low, and movements performed with many repetitions or a position held for a longer period of time, to build endurance, and restore normal coordination.

It should also be added that normal joint stability cannot be achieved if the joint is too stiff (which prevents normal mechanics and muscle activation), is pathologically unstable (severely excessive joint motion e.g. due to major trauma), or in some joints if there is a complete tear of stabilizing muscles (e.g. rotator cuff tear). Joint mobility restrictions can be treated in physical therapy by an orthopedic manual therapist, whereas the two latter scenarios usually would require the help of a surgeon. A thorough evaluation is always the first prudent step.

Enjoy our gorgeous winter (how lucky we are!), and please pass this letter on to anybody you think would be interested.

Best wishes,


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