Muscles are capable of functioning in several different ways depending on the motion being performed, the direction of the motion, and how much resistance it must overcome. Muscles must work together to produce different bodily movements with a particular muscle's role changing with each movement.
The five primary roles muscle play are:
1. Agonist (prime mover) - the strongest of a group of muscles performing a similar action.
2. Synergists (helper) - act as accessory muscles in assisting the agonist with an action.
3. Antagonists - oppose the actions of the agonist and synergists, lengthening to accommodate for the shortening (contraction) of the agonist. They are typically located on opposite sides of a joint or bone from the agonist.
4. Stabilizers (fixator) - muscles that contract to hold a body part immobile while another body part is moving.
5. Neutralizers - prevent unwanted motions a muscle can perform so a specific motion can occur.
Proprioception and stretching
Proprioceptors are nerve endings that sense and relay information about the musculoskeletal system to the central nervous system. They detect any changes in physical displacement (position and movement), tension, or force within the body. That information is sent to the brain which interprets the increase or decrease in muscle tension determined by the lengthening or stretching of muscle fibers.
When an agonist and synergists contract, in order to cause the desired motion, it usually forces the antagonists to relax.
This phenomenon is called reciprocal inhibition because the antagonists are inhibited from contracting. The inhibitory response works through the central nervous system, which cannot allow the prime movers and the antagonists to tighten at the same time.
Muscle spindles are proprioceptors (stretch receptors) located in the fibers of a muscle. When muscles are stretched, the muscle spindles measure and record the change in length (including how fast) and sends signals to the spine which convey this information.
They trigger the stretch reflex (myotatic reflex) which attempts to resist the change in muscle length by causing the stretched muscle to contract. Since the impulse only has to go to the spinal cord and back (not all the way to the brain), it is a very quick impulse occurring in 1-2 milliseconds. At this point, the muscle spindles have been activated and are telling the muscle to contract to prevent any further stretching.
The more sudden the change in muscle length, the stronger the muscle contractions will be. Muscle spindles fire faster when the muscle fiber lengthens (stretches), and report changes relative to some preset length so that the brain can rapidly adjust this preset value. This basic function helps to maintain muscle tone, protect the muscles from injury, and prevent overstretching of a joint.
Golgi tendon organs are a proprioceptive sensory nerve endings embedded among the fibers of a tendon (often near the musculotendinous junction) that oppose the muscle spindles and produce an opposite effect. They differ from the muscle spindle in that, when activated, they inhibit (relax) the muscle.
An isometric contraction occurs when muscle tension is increased, but the muscle is not shortened because the resistance cannot be overcome. While the muscle generates force, there is no significant movement or change in joint angle, and muscle length remains constant. Imagine trying to push a concrete wall down. You would be contracting muscles while pushing, without necessarily changing their length.
A typical session of isometric stretching consists of a series of muscle contractions followed by periods of muscle relaxation. The muscle group is passively stretched, then contracts isometrically against resistance while in the stretched position, and then is passively stretched again through the resulting increased range of motion. This triggers the lengthening reaction by the Golgi tendon organ inhibiting the stretched fibers from contracting by the muscle spindles.
Muscle functionality, proprioception, and isometric stretching (along with continued learning and hands on instruction) provide the knowledge to stretch your muscles! Keep learning.