There are from 640 to 850 muscles in a human's body depending on the source. Each one has a specific role that it fulfils perfectly. Following is a brief overview of some of the common muscular functions.
According to Vladimir Janda, a Czech neurologist and physiatrist, muscles can be classified as postural and phasic.
Postural muscles mainly resist gravity and sustain posture. They are composed mostly of slow-twitch muscle fibres and have good endurance. They are prone to becoming hypertonic as they need to maintain some contraction for most of the day and sometimes at night. Postural muscles tend to get short and tight due to trauma, over- or under-use.
Phasic muscles are better suited for movement and contain mostly fast-twitch muscle fibres. They fatigue more easily and are prone to inhibition. Phasic muscles usually get long and weak in cases of improper use.
Understanding the balance and relationship between these is helpful for understanding postural abnormalities including but not limited to upper- and lower-cross syndrome.
Different sources propose various lists of postural muscles. It is also important to acknowledge that hardly any muscle is exclusively one or the other type. In reality, the distinctions are much more nuanced and can vary in different activities and between people.
A possible list of postural muscle can look like this:
Prime mover muscles
Prime movers, also called agonists, are the main muscles generating a particular movement through their contraction. The fact that agonists do most of the work during a movement does not mean they will tire first. Often the prime mover is also the largest muscle in the group creating motion in the joint and also is most enduring.
An example of an agonist is biceps brachii during an arm curl. The biceps, however, is not the only muscle capable of performing elbow flexion. The brachialis lies deep to the biceps and inserts into the ulna, which cannot rotate. So, it is actually the only pure elbow flexor, as the biceps can also supinate the forearm by inserting into the radius.
It is important to emphasise that the same muscle would be the prime mover during both the lifting and the lowering phase. The only difference is that during the lifting phase, the biceps contracts concentrically (shortens) and during the lowering phase it contracts eccentrically (lengthens) to control the descent.
The term agonist is only meaningful when talking about a muscle’s role in a particular movement.
The antagonists, or opposing muscle, as the name implies, generates torque in the joint opposing the agonist. This helps in controlling the movement and can be especially important during ballistic (very quick) movements. Usually, the antagonist would lie on the opposite side of the agonist. A process called reciprocal inhibition neutrally blocks the motor units of the antagonist to allow it to relax and lengthen during the agonist’s contraction.
When throwing a dart, for example, the triceps (agonist) will contract briefly and rapidly to accelerate the elbow extension. This will be followed almost immediately by a brief activation of the elbow flexors (antagonists) to produce a quick stop of the movement.
During slower movements, like the arm curl described above, the antagonists will act in exactly the opposite way of the prime mover. So, the triceps brachii (antagonist) will passively lengthen during the lifting phase simultaneously with the biceps contraction. Then, during the lowering phase, it will passively shorten as the biceps lengthens.
Antagonism is an emergent property of the muscle. So, it only arises in relation to the prime mover during a specific joint action.
Synergist muscles work together with other muscles to create movement. They can support either the agonist or prime mover or with the antagonist the opposing muscle. Most common movements performed in everyday life or in sports require the action of synergists. There are two types of synergists:
Also called 'neutralisers', synergists balance the motion generated by the prime mover to direct the generated force within the desired space and direction. For example, during a chin up the deltoids, rhomboids, trapezii are key synergists supporting the lats and biceps.
Another important role of synergists is to facilitate fixation of certain parts during complex across multiple joints. Muscles can contract only to 40% of their full length and even this may not be achievable due to a tight antagonist. For example, synergists facilitate the fixation of the wrist when clenching the fist to allow us to fully flex the fingers.
Fixators eliminate the unwanted movement at the origin of the agonist (prime mover). Thus, they act as stabilisers.
The role of fixators is especially important when they act in conjunction with multiarticulate/multijoint (attaching to more than one boner) muscles. Such muscles would move both bones when contracting, thus making everyday movements impossible, if they were not supported by fixators.
In the arm curl example, for instance, the biceps flexes the elbow. However, the biceps attaches both to the radius and the scapula. The scapula is extremely mobile, so it would move during an arm curl were it not held in place somehow. This is accomplished by the isometric contraction of the traps and rhomboids, which keep the scapula and hence the clavicle in place.