Our muscles rely on an agonist-antagonist relationship to function properly. For example in a dumbbell bicep curl, the bicep serves as the agonist and while the tricep serves as the antagonist. The agonist provides the force necessary to overcome a load (gravity + dumbbell), while the tricep serves as a brake to decelerate the load if necessary. Additionally, secondary muscles are utilized to take care of some of the other details. Stabilizers ensure the load is kept stable, allowing us to safely increase the amount of force put into the movement. Fixators keep our joints aligned properly, ensuring they're used in the safest and most efficient fashion. Neutralizers act against an unwanted line of force, ensuring we're able to move the load as desired, for example holding a hammer grip during a bicep curl instead of allowing the forearm to rotate. We need all of these participants in order to perform efficient movement, and when one isn't pulling it's weight others are forced to make up for it in an effort to prevent injury. This isn't a big deal in the short-term, but over a longer period of time it can lead to overuse injuries and cause breakdowns in other area of the body. Furthermore, when we apply a load to our muscles in the gym we need to make sure they are operating properly other wise we're increasing the chance of a significant acute injury.
Every muscle has the ability to perform three different types of contractions:
Our body is constantly monitoring every movement and waiting to employ safeguards if we exceed safety precautions. Proprioceptive sensory receptors in tendons known as Golgi Tendon Organs monitor the amount of tension in a muscle, usually signalling the antagonist to relax in order for an antagonist to produce movement. They also restrict the agonist in the event that muscle tension is reaching dangerous levels, acting as an emergency valve to avoid structural damage. When we experience breakdowns in posture or develop compensation patterns, our body may apply tension in certain areas in an effort to complete desired movements and prevent injury. This is where "knots" or "trigger points" come from.
We may be able to release these tension build-ups through techniques such as SMR or traditional stretching, but until we address the underlying weaknesses or motor pattern breakdowns that cause them it's likely that they will continue to occur. Furthermore, if we keep trying to apply stress to these areas (resistance training and other stressful activities) the problem will probably get worse. When we attempt to treat pain or dysfunction, we must always keep in mind that the site where symptoms manifest is almost never the cause of the problem given the synergistic nature of the body.