Muscle Imbalance?

Written by Dr. Siff

It is often fashionable to attribute poor performance or sports injuries very glibly to "muscle imbalance", which is a result of one's regular training or sport leading to one muscle group being stronger or weaker than another. Sometimes, this imbalance is quantified by use of an isokinetic dynamometer, by "manual testing" or on the basis of differences between the isolated actions of "agonist" and "antagonist" muscles.

Despite the fact that substantial differences in strength or joint torque between opposing limbs have not categorically been shown to correlate with an increase in the incidence of injury, this very general notion of "muscle balance" often seems to be applied in a very simplistic manner to address the alleged problem. This diagnosis then guides the therapist or trainer to draw up a list of suitable exercises to increase the strength of the apparently offending muscles. But is this approach necessarily logical and appropriate?

Since strength is a function of musculoskeletal structure and function, any alleged imbalance may be modified by improving the structure or the function, or both, of these features. This is where a therapeutic dilemma arises. Is the "imbalance" due to some sort of deficit in intrinsic strength of the muscle itself or is due to inefficient activation or motor control of the muscles involved in the action? After all, what may be considered as a deficit in muscle hypertrophy or intrinsic strength may really reflect some deficit in neuromuscular control, so that a more appropriate solution would be to improve the motor skills in specific movement patterns.

Part of the problem may even be related to the unwanted intervention of inhibitory processes which limit one's ability to produce maximal strength (or joint torque) in a given action, something which occurs quire reflexively in many muscle groups to prevent further trauma. For example, it is well known that damage to the knee joint commonly causes reflex inhibition of the knee extensor muscles, so that measuring the strength of muscles where some small injury or overtraining is present may be futile. The muscles involved may be in "ideal" balance and very strong, but inhibition associated with some other injury or fear of injury will distort any tests which purport to assess relative muscle strength.

It might even be that a certain combination of structural and functional approaches, a mix of enhancing both hypertrophic and neuromuscular types of therapy would offer the best way of addressing the "imbalance".

In short, is the concept of "muscle balance" being far too simplistically and liberally applied? If so, what suggestions would you make to rectify this flawed approach to the management of the given problem?