When a muscle fiber contracts, it contracts completely. There is no such thing as a partially contracted muscle fiber. Muscle fibers are unable to vary the intensity of their contraction relative to the load against which they are acting.
If this is so, then how does the force of a muscle contraction vary in strength from strong to weak? What happens is that more muscle fibers are recruited, as they are needed, to perform the job at hand. The more muscle fibers that are recruited by the central nervous system, the stronger the force generated by the muscular contraction.
The energy which produces the calcium flow in the muscle fibers comes from mitochondria, the part of the muscle cell that converts glucose (blood sugar) into energy. Different types of muscle fibers have different amounts of mitochondria. The more mitochondria in a muscle fiber, the more energy it is able to produce.
Muscle fibers are categorized into slow-twitch fibers and fast-twitch fibers.
Slow-twitch fibers (also called Type 1 muscle fibers) are slow to contract, but they are also very slow to fatigue.
Fast-twitch fibers are very quick to contract and come in two varieties:
- Type 2A muscle fibers which fatigue at an intermediate rate.
- Type 2B muscle fibers which fatigue very quickly.
The main reason the slow-twitch fibers are slow to fatigue is that they contain more mitochondria than fast-twitch fibers and hence are able to produce more energy.
Slow-twitch fibers are also smaller in diameter than fast-twitch fibers and have increased capillary blood flow around them.
Because they have a smaller diameter and an increased blood flow, the slow-twitch fibers are able to deliver more oxygen and remove more waste products from the muscle fibers (which decreases their “fatiguability”).
These three muscle fiber types (Types 1, 2A, and 2B) are contained in all muscles in varying amounts. Muscles that need to be contracted much of the time (like the heart) have a greater number of Type 1 (slow) fibers.
When a muscle first starts to contract, it is primarily Type 1 fibers that are initially activated, then Type 2A and Type 2B fibers are activated (if needed) in that order.
The fact that muscle fibers are recruited in this sequence is what provides the ability to execute brain commands with such fine-tuned muscle responses. It also makes the Type 2B fibers difficult to train because they are not activated until most of the Type 1 and Type 2A fibers have been recruited.
Located all around the muscle and its fibers are connective tissues. Connective tissue is composed of a base substance and two kinds of protein based fiber.
The two types of fiber are:
- Collagenous connective tissue
- Elastic connective tissue
Collagenous connective tissue consists mostly of collagen and provides tensile strength.
Elastic connective tissue consists mostly of elastin and provides elasticity.
The base substance is called mucopolysaccharide and acts as both a lubricant (allowing the fibers to easily slide over one another), and as a glue (holding the fibers of the tissue together into bundles).
The more elastic connective tissue there is around a joint, the greater the range of motion in that joint. Connective tissues are made up of tendons, ligaments, and the fascial sheaths that envelop, or bind down, muscles into separate groups.
These fascial sheaths, or fascia, are named according to where they are located in the muscles:
- endomysium – the innermost fascial sheath that envelops individual muscle fibers
- perimysium – the fascial sheath that binds groups of muscle fibers into individual fascicle
- epimysium – the outermost fascial sheath that binds entire fascicles.
These connective tissues help provide suppleness and tone to the muscles.
Cooperating Muscle Groups
When muscles cause a limb to move through the joint’s range of motion, they usually act in the following cooperating groups:
Agonists – are also referred to as prime movers since they are the muscles that are primarily responsible for generating the movement. They create the normal range of movement in a joint by contracting.
Antagonists – act in opposition to the movement generated by the agonists and are responsible for returning a limb to its initial position. Antagonistic muscles are found in pairs called antagonistic pairs. These consist of an extensor muscle, which “opens” the joint and flexor muscle, which does the opposite to an extensor muscle.
Neutralizers – perform, or assist in performing, the same set of joint motion as the agonists. Neutralizers help cancel out, or neutralize, extra motion from the agonists to make sure that the force generated works within the desired plane of motion. A muscle responsible for eliminating or canceling out an undesired movement.
Stabilizers – provide the necessary support to assist in holding the rest of the body in place while the movement occurs. As an example, when you flex your knee, your hamstring contracts, and, to some extent, so does your gastrocnemius muscle (calf) and lower buttocks.
Meanwhile, your quadriceps are inhibited (relaxed and lengthened somewhat) so as not to resist the flexion. In this example, the hamstring serves as the agonist, or prime mover; the quadriceps serves as the antagonist; and the calf and lower buttocks serve as the synergists.
Agonists and antagonists are usually located on opposite sides of the affected joint (like your hamstrings & quadriceps, or your triceps & biceps), while synergists are usually located on the same side of the joint near the agonists. Larger muscles often call upon their smaller neighbors to function as synergists.
The following is a list of commonly used agonist / antagonist muscle pairs:
pectorals > latissimus dorsi (pecs & lats)
pnterior deltoids > posterior deltoids (front & back shoulder)
trapezius > deltoids (traps & delts)
abdominals > spinal erectors (abs & lower-back)
left > right external obliques (sides)
quadriceps > hamstrings (quads & hams)
shins > calves (lower limbs)
biceps > triceps (upper limbs)
forearm flexors > extensors
Types Of Muscle Contractions
The contraction of a muscle does not necessarily imply that the muscle shortens; it only means that tension has been generated. Muscles can contract in the following ways:
This is a contraction in which movement does take place, because the tension generated by the contracting muscle exceeds the load on the muscle. This occurs when you use your muscles to successfully push or pull an object.
Isotonic contractions are further divided into two types:
Concentric contraction: This is a contraction in which the muscle decreases in length (shortens) against an opposing load, such as lifting a weight up.
Eccentric contraction: This is a contraction in which the muscle increases in length (lengthens) as it resists a load, such as lowering a weight down in a slow, controlled fashion.
During a concentric contraction, the muscles that are shortening serve as the agonists and hence do all of the work.
During an eccentric contraction the muscles that are lengthening serve as the agonists (and do all of the work).