Acting on muscles is the only way one moves his or her skeleton. Therefore, protecting the spine, joints, nerves and blood vessels, requires acting on muscles, while still ensuring that no muscular tissue gets harmed. This equation is complex to solve.
It is important to remember that a muscle can only pull on a joint placed between its two extremities (origin and insertion): The biceps flexes the elbow, but it cannot extend it. Extension of the elbow requires the intervention of the antagonist muscle, the triceps.
Balance vs. work muscles
A useful approach (Samama, 2003) distinguishes between:
- Balance (or posture) muscles, usually in the feet, legs, buttocks, back, neck, back of the head and abdomen. They help standing, sitting or assuming any other posture
- Work muscles, in the arms, hands, fingers and chest. They help to perform diverse tasks (typing on a keyboard, playing the violin, combing your hair) and are sub-divided into two categories: Active muscles, and passive muscles. The latter are not needed to carry out the current task and therefore need to stay relaxed. For instance, when using a computer, the upper shoulders and neck muscles are passive work muscles.
This is a classification used to understand how muscles should be used. There are also other classification schemes, for other purposes (e.g., for treatment or training).
It is not rare, in particular while sitting at a computer, that a lack of control on or strength of balance muscles, or both, leads to overuse of some work muscles, which do not have the ability to sustain such strain during long enough and therefore become painful.
Muscle activation
Muscles can be activated in 3 different manners:
- Concentric: The origin and the insertion of the muscle come closer to each other (e.g. using the biceps flexes the elbow);
- Isometric: The origin and the insertion of the muscle remain at the same distance. The effort produced by the muscle is meant to overcome a resistance (e.g., when statically holding a weight with the elbow flexed, the biceps is active to overcome the effect of gravity, which would tend to extend the elbow);
- Exocentric: The muscle is used to resist another force (e.g., in the previous example, one lets the elbow extend, but at reduced speed). The distance between origin and the insertion increases.
As a rule of thumb, one can say that in daily life activities, one's focus should be on using muscles concentrically. Only when carrying a load, there is a need to "rigidify" the skeleton by mobilizing the lower trapezius, latissimus dorsi and lower abdominals (belly muscles) isometrically. These aspects are discussed in detail in a further section.
A good counter-example is what happens under the effect of stress: Muscles contract without reasonable proportion between the displacement of the bones and the intensity of the contraction (e.g., in the neck or in the forearms). This creates muscle overuse, and compression of other soft tissues (nerves, blood vessels, etc.), which create pain in the long run.
Muscle strength vs. length
The graph below schematically illustrates the relationship between the length of a muscle fiber (horizontal axis) and the force that it can produce (vertical axis).
What should be noticed is the fact that there is an optimum length for each muscle, i.e. a length at which it can deliver maximum strength.
This shades an interesting light on the concept of muscular overuse: An overuse is an unbalance between what is required from a muscle and what it can deliver. The graph above shows that muscular overuse can occur when a muscle is excessively mobilized in a shortened position, or when excessively mobilized in a lengthened posture.
For example, a round shouldered posture overuses the pectoralis minor muscles because they are kept shortened (the shoulder blades are far apart from each other), but it also overuses the rhomboids because they are kept stretched (therefore, the effort required to prevent further lateral displacement of the shoulder blades is excessive compared to what they can deliver).
