You are currently exploring the Fundamentals Library, which is designed to provide a basic overview of the topics that are covered in other longer articles. This article is a part of the Anatomy section.
Human Movement
Every single movement we perform or attempt to perform is done through an interaction of the muscular and skeletal systems. The muscular system is controlled by the nervous system, which generates the motor command and transmits it to individual motor units.
By pulling on the bones, skeletal muscles generate torques in the synovial joints. Torque, like force, has two possible directions, referred to in mechanics as clockwise and counterclockwise. In the context of anatomy, we tend to use the anatomical terms of motion such as flexion, extension, adduction, abduction, and so on.
When we perform an exercise or a particular movement, it is resisted by an external force, whether it is gravity acting on our own body, gravity acting on external objects such as dumbbells, inertia, resistance bands, or any other type of resistance.
Agonists & Prime Movers
During such a movement, some muscles act directly against the resistance (producing torque in the opposite direction). These muscles are typically the ones aimed to be targeted by an exercise and are called agonists.
The example of such a muscle will be the pectoralis major in the context of push-ups. Gravity is "trying" to extend our shoulder, and the pectoralis muscle is flexing it.
Multiple agonists can be responsible for the same motion in a given joint. For example, in addition to the pectoralis muscle, the anterior deltoid muscle is also responsible for this motion in the shoulder.
Agonists that have a high influence on the joint movements that appear to be the most mechanically resisted in a given exercise are often referred to as prime movers. In the context of a push-up, because the shoulder and elbow joints are the most loaded, the prime movers are the pectoralis major, the anterior deltoid, and the triceps brachii.
We haven't mentioned the abdominal muscles because we don't consider our posture to be a limiting factor in an exercise. We did not mention the coracobrachialis or anconeus muscles because they are small muscles that do not contribute much to torque production. We did not mention the medial deltoid because although it is a large muscle region that is also responsible for movement in the shoulder, it is unlikely to be activated as much as pecs or anterior deltoid in the push-up.
Synergists & Fixators
There are muscles that are not prime movers, but which assist the prime movers in a given exercise. These muscles are called synergists. The assistance they provide may be by means of providing stability in a particular region and thus increasing the effectiveness of an agonist, either by increasing its activation or by making its action more efficient. These muscles are often referred to as fixators.
In case of the push-up, these muscles might be the pectoralis minor and serratus anterior, which stabilize the position of the scapula, the rotator cuff muscles, which stabilize the shoulder joint or the superior trapezius, which stabilizes the acromioclavicular joint.
They can also be agonists on their own, but not fundamental from the standpoint of torque production in a given exercise. In the case of push-ups, we could say that the coracobrachialis and the anconeus are synergists because they also contribute to overcoming resistance (while not having as huge of an impact mechanically).
Antagonists
On the other hand, in such movements, there are muscle groups that produce torque in the joints in the same direction as the resistance and, conversely, in the opposite direction to the agonist muscles. These muscles are called antagonists.
From the standpoint of exercise performance, antagonists, when active, negatively affect the external outcome of an exercise.
In the case of push-ups, for example, the antagonist muscles in our shoulders are lats and posterior deltoids.
It Is Contractual
When analyzing the muscles and breaking them down into their functions in an exercise, we may find ourselves going down a rabbit hole. The terms above are helpful, but it is not recommended to be too specific when it comes to classification.
The assessment of agonists and antagonists can be reduced to the mechanics of the exercise, and we can always identify these muscles. On the other hand, synergists, fixators and prime movers are very context dependent and responsive to many variables.
Our intention in execution, slight alterations to the technique or mechanics can greatly alter the demand (activity) and role of certain muscles. Similarly, individual strengths and weaknesses can further alter these relationships.
Fun Fact - Flexing
It is well known in bodybuilding circles that practicing poses that require a lot of flexing is an exhausting task. You've probably tried to flex your biceps more than once (who hasn't?). So you may have wondered, how is it possible to flex a muscle and not cause any motion in the joint?
Well, as we know, there MUST be something causing the opposing torque that our biceps produces. But, it is not gravity, nor any other external resistance. That thing is actually the antagonist muscle - triceps.
Yes, when you flex your biceps, you flex your elbow flexors and extensors at the same time. And the more you flex your biceps, the more your triceps have to flex. Try to flex your biceps in the standard pose and not flex your triceps at the same time. Impossible!
This little experiment is a nice visualization of a complex interaction of different parts of our body and how opposite in function yet interconnected they can be.
