Muscle balance, or the relative equality of muscle length or strength between an agonist and its respective antagonist, as well as contralateral sides, is necessary for normal movement. This is due to the reciprocal nature of movement, which requires the muscles to be coordinated. Muscles can become unbalanced due to either adaptation, through improper training or a movement dysfunction, like over-eversion of the foot. The major difference between a functional muscular imbalance and a pathological muscular imbalance, is a pathological imbalance impairs function. This leads to dysfunction, pain, altered movement patterns.
Pathological imbalance can be the result of injury be the cause of injury (shoulder impingement), or a compensation for an injury (people with reconstructed ACLs typically experience hip weakness due to shortened IT bands). On the other hand, functional imbalances are typically found in athletes and they can be beneficial to their performance. They are a result of the complex movement demands of the sport. For example, volley ball players typically have greater shoulder internal rotation, elbow extension, and wrist extensor strength compared to their age-group. It should be the goal of the clinician to manage a functional muscular imbalance in an athlete before it becomes pathological. This can be done by quantifying strength ratios between agonists and antagonists, and then by strengthening the weaker muscle to help lessen the imbalance.
In the biomechanical approach, supported by Sahrmann, muscle imbalance is the result of prolonged postures and repetitive movements, which causes adaptations in muscle length, strength, and stiffness, leading to impaired movement. Sarcomeres increases due to activity lead to muscles becoming longer, while inactivity leads to a decrease in sarcomeres causing shorter muscles. These adaptations lead to altered participation of muscle synergists and antagonists eventually hindering normal movement. The precision of joint motion changes when a particular synergist muscle becomes overly dominant, causing abnormal stress on the joint. To identify a dysfunction in in the biomechanical approach, a clinician should look for overly dominant muscles. Treatment is focused on shortening overly long muscles and strengthening the weaker ones. In the neuromuscular approach, supported by Janda, muscle imbalance is caused by an impaired regulation of the neuromuscular system. Muscles are predisposed towards imbalances due to their role in motor function in responding to stimuli and reacting to changes in the peripheral joints.
Muscle imbalance is developed after injury or from alterations in proprioceptive input from abnormal joint position or motion leading to an impaired relationship between muscles prone to tightness (postural muscles) and muscles prone to inhibition. Assessment is focused on movement quality and evaluating the sequencing of firing of synergists. Treatment is focused on normalizing function of the periphery and restoring balance between strong and weak muscles, and working on coordination, posture and balance. Muscle imbalance can be caused either biomechanically or from the neuromuscular system.
Muscular imbalances causing issues throughout the neuromuscular system makes sense in a tensegrity perspective. In tensegrity, structural homeostasis is maintained by balancing the compression and tension of tissues. If a muscle is unbalanced it will disrupt homeostasis causing dysfunction throughout the person’s whole body. An imbalanced muscle will cause issues for all other tissues at the organ level including tendons, ligaments, bones, fascia, and other muscles. In the dynamic systems theory an imbalanced muscle is resulting in pain, can be an organism constraint that can be treated through manipulation of task constraints and/or environmental constraints. For example, ankle instability can be caused by a strength imbalance between the invertors and evertors. Treatment is focused on reestablishing motor control of these muscles by moving to a less predictable environment, like an unstable surface, to challenge the sensorimotor system or by slightly modifying the task constraints by gradually improving a person’s range of motion as they go through ankle inversion and eversion exercises.