Excerpted from:  (Page, Frank, Lardner.  Assessment and Treatment of Muscle Imbalance, The Janda Approach.  2010, Human Kinetics, Champagne IL.)

Muscle balance is the relative equality of muscle length or strength between an agonist and antagonist.  This balance is necessary for normal movement and function as it relates to the reciprocal nature of human movement, which requires opposing muscle groups to be coordinated.  Muscle balance may also refer to the strength of contralateral (left vs. right) muscle groups.  Muscle imbalance occurs when the length or strength of agonist and antagonist muscles prevents normal function.

Muscles may become unbalanced as a result of adaptation or dysfunction.  Such muscle imbalances can be either functional or pathological.

Functional imbalances are more common in athletic/active people and are necessary for function. They occur in response to adaptation for complex movement patterns, and include imbalances in strength or flexibility of antagonistic muscle groups.  Because such imbalances are required, they must be managed before they become pathological.  Clinicians must recognize when to treat muscle imbalances, given the pathology and demands of the sport.

When muscle imbalance impairs function, it is considered to be pathological.  Pathological muscle imbalance is associated with dysfunction and pain, although the cause may not result from an initial traumatic event. Pathological imbalance may also be insidious; many people have muscle imbalances without pain. However, if left unaddressed, pathological muscle imbalance will lead to joint dysfunction, altered movement patterns, and pain.

Some injuries cause muscle imbalance, while others result from muscle imbalance:

•  ACL-reconstructed athletes with anterior knee pain have weak hip abductors and tight iliotibial (IT) bands.
• Shoulder impingement is associated with muscle imbalance of the rotator cuff and scapular stabilizers.