What is Muscle Imbalance?
This is a situation where two groups of muscles that are designed to work with or against each other enter a state where one set becomes stronger (or tighter), or weaker ( or more unstable); leading to compensation which creates unequal forces at an associated joint or a pain syndrome in the muscles themselves. For example, a muscle having to work against another stronger muscle may be limited in ROM (range of motion), or might have to work harder as a result, leading to tissue breakdown and increased fatigue. Concurrently, a muscle that has to compensate, or do the work, for another muscle that is unstable could lead to excessive use, therefore leading to tissue breakdown and increased fatigue.
A specific example of muscle imbalance would be when one develops tight hamstrings muscles in the back of their leg.
As the hamstrings tighten, the quadriceps (muscles in the front of your thigh) must compensate as a result. This causes more friction in the knee joint and will ultimately lead to anterior knee pain.