Correct movement performance requires balance in muscle length and strength between the antagonistic muscles (joint muscles) and the muscles surrounding a given joint. In the proper conditions, the distribution of strength between the joint muscles creates the optimal mechanical conditions that are necessary to support the bones in the joint during movement; this is called proper muscle balance. Muscle imbalance occurs when there is an imbalance between the joint muscles, e.g. because of low or high tension.
Muscle imbalances can be practically characterised for each area. They can involve opposite sides (the right and left) or the front and back (agonistic and antagonistic).
When it comes to deformities connected with muscle imbalance, we look at the difference in length and strength of the muscles. Most often, pain of the musculoskeletal system is caused by disproportions in the performance of the agonistic muscles and the antagonistic muscles, or imbalances in the muscles surrounding the joints.
The body always chooses the option with less resistance and if there is too much muscle tension or significantly more pressure on a joint in comparison with its antagonistic, then the elements of this joint tend to move in this direction, and movement limitations occur in the opposite direction.
Below is an example of the quadriceps and the hamstring that have an effect on the knee joint by performing opposite movements in the knee. If there is an imbalance between them, this can cause excess pressure on the joint. A severely cramped hamstring (link to article about the hamstring) will make it difficult for the joint bones to glide against each other, which will most likely lead to too much pressure on the kneecap by the hamstring.
There are two known causes of muscle imbalances. The first is connected with biomechanic imbalances. Repeating movements in the same direction or a long-term static position can cause muscle imbalances.
The biomechanic causes of muscle imbalances were popularised by Kendall and Sahrmann.
The second cause is nerve-muscle imbalance, which is explained by the predispositions of some muscle groups to tension or weakening.
This concept was popularised by Jadna and it is based on movement patterns that develop and evolve from birth. It is claimed that the group of tonic muscles is more susceptible to tension but the phase muscles are more susceptible to weakening.