The human body is a unique orchestra.
Talking about movement execution our nervous system will engage the necessary muscles, contributing to the movement, to provide the most efficiënt motor control, within the relationship of mobility vs. stability.
Due to this process we can distinguish different classifications of muscle interaction:
1 – AGONISTS
Muscles acting as prime movers of the action. Prime movers are the leaders of the movement, taking on the primary role of movement execution. Assistant movers play a role in assisting the the prime movers with stabilization. A great example can be found in the assistant movers surrounding the scapula – e.g. shoulder retraction in the pull-up where the upper part of the trapezius will be responsible for the dynamic action, while the lower part has mainly a stabilizing role.
2 – ANTAGONISTS
Muscles who’s action is directed opposite to the agonists. The most important thing to take away here is that antagonistic muscles relax when the agonists contract, but an antagonistic muscle contraction could occur when the agonists contract extremely rapidly to prevent joint damage due to the large momentum in the moving limb. This process, also named reciprocal inhibition, is one of the main causes of musculoskeletal injury – e.g. the hamstring can tear if it contracts simultaneously to the quadriceps while an athlete is sprinting or kicking.
3 – STABILIZERS
Muscles stabilising the body while other muscles carry out the movement. This stabilization can happen static – e.g. during the pushup the spinal erectors provide isometric stabilization, keeping the spine in a neutral position, while the pectoralis, anterior deltoid and triceps execute the pushing movement in the upper body. Or it can occur dynamic – e.g. in basic locomotion – running/walking – the quadriceps induces knee extension, while at the same time stabilizing the knee joint when making ground contact. The medial part of the quadriceps – Vastus Medialis Obliquus (VMO) – provides a very important role in medial stabilization and should be addressed in isolation in a comprehensive training program.
4 – NEUTRALIZERS
Muscles counteracting the intention of other muscles to provide the opposite action of the agonists. The easiest way to understand this is the simultaneous contaction of the internal and external obliques, in the sit-up, to maintain trunk flexion and assisting the rectus abdominis muscles to provide pure trunk flexion.
Although these categories exist, it’s quite inaccurate to divide muscles in specific categories. We can only identify a specific role for a muscles in a given situation.
The most important thing to remember is that our body has neuromuscular processes to make all our muscles work synergistically to produce the desired motion.