How does stretching work?
In general, stretching works via reflex mechanisms that are augmented by special mechanoreceptors located in muscles and tendons
Mechanoreceptors are specialized structures that respond to mechanical pressure within tissues, such as touch, pressure, stretching, sound waves, motion and distortions in body tissues. They then transmit signals through sensory nerves.
Muscle Spindles are sensory receptors that run parallel to muscle fibers, which are sensitive to changes in muscle length and rate of length change.
Spindles help regulate the contraction of muscles via the stretch reflex mechanism aka the myotatic stretch reflex.
This mechanism is a normal response to the body to a stretch stimulus in the muscle and is designed to protect and prevent overstretching and muscle damage.
As a side note, if a muscle is tight, it may be due to muscle spindle activation of the stretch reflex as a protective mechanism.
This neurological tension is serving a purpose- providing stability.
Under these circumstances, would you want to stretch the involved muscle and could stretching the involved muscle result in adverse consequences with respect to injury, stability and pain? (More on this later)
Gogli Tendon Organs(GTOs) are specialized sensory receptors located where the skeletal muscle fibers attach to the tendons, which are sensitive to changes in muscular tension and rate of tension change.
Activation of the Gogli tendon organ causes the muscle to relax, which prevents the muscle from excessive stress or injury, thereby serving as “safety devices”.
This is known as the inverse stretch reflex, which results in decreased muscle tension.
This reflex can be mediated by a vigorous muscle contraction, pressure or by passive stretching.
Techniques like Self-myofascial release, stretching, fascial manipulation, joint manipulation and dry needling can all affect the GTO.
Joint receptors, such as Pacinian corpuscles are located around joint capsule and respond to pressure, acceleration, and deceleration of the joint. These signal extreme joint positions, thus helping to prevent injury.
Some proposed mechanisms of action with respect to mechano receptors and stretching
Autogenic inhibition- The process by which neural impulses that sense tension are greater than the impulses that cause muscles to contract, providing an inhibitory effect to the muscle spindles.
Gentle pressure will stimulate the Golgi tendon organ and create autogenic inhibition, decreasing muscle spindle excitation and releasing the hypertonicity(tension) of the underlying musculature.
Decreased muscle tension results in elongation of muscle fibers, taking advantage of viscoelastic properties of muscles and tendons thereby allowing the muscle to relax and lengthen.
Reciprocal inhibition– The simultaneous relaxation of one muscle and the contraction of its antagonist (opposite muscle) to allow movement to take place.
In this reflex, the contraction of an agonist results in a decrease in the neural drive to it’s antagonist.
In other words, you can’t contract two opposite muscles simultaneously.
Stress Relaxation- The stress relaxation phenomenon occurs when a muscle is under constant stress.
When stress (e.g. a constant stretch) is applied, the viscous material in muscles loses its ability to resist the stretch over time, resulting in a slow increase in length, or “creep”.
The Gate Control Theory
Melzack and Wall proposed that a gating mechanism exists within the spinal cord where Small nerve fibers carrying pain messages and large nerve fibers which are pressure sensitive come together.
Because the pressure fibers are larger and myelinated, they make it to the spine before the pain signals do when they are stimulated simultaneously, resulting in the inhibition of pain signals..
With respect to stretching, as the muscle is stretched beyond its active motion and then the participant is instructed to resist the stretch, followed by the target muscle being stretched further, there is a large force produced in the targeted muscle.
This large force stimulates pain receptors and the aforementioned Golgi Tendon Organs (GTO).
Repetition of this process results in GTO adaptation and decreased inhibition, allowing the muscle to produce a greater amount of force.
Ultimately, there is increased muscle length and the potential for greater force production due to the increase in ROM and decreased GTO inhibition.
By manipulating mechanoreceptors through various stretching techniques, pressure, vigorous muscle contraction and either passive or active movements, the proposed mechanisms of stretching: autogenic inhibition, reciprocal inhibition, stress relaxation and the gate control theory can help explain how the nervous system responds and can aid in bridging theory with evidence based practice.
Don’t stretch the truth
People generally stretch for four reasons:
To increase range of motion
To reduce pain from overactive muscles (and sometimes erroneously for underactive muscles- more on this below)
Despite all the research surrounding stretching, this topic remains controversial.
Some research show that stretching done prior to strength activity can decrease strength for up to two hours post stretch.
There is also evidence that stretching negatively influences performance based on reports of impaired balance, reaction times, movement times and jump height.
Research does support pre-activity active isolated or dynamic stretches as long as muscle imbalances are not present to impede proper movement as well as post-activity stretching to increase range of motion and performance,
So, should you stretch?
Yes, but based upon an assessment to determine what muscles are tight/overactive and need to be lengthened and which are weak/under active and need to be strengthened.
Janda, a Czech physiatrist was instrumental in discovering predictable patterns of muscle imbalance that can help a trainer or health professional in the clinical decision making as to what muscles to stretch and which to strengthen. He labeled these:
1. The Upper crossed syndrome
2. The Lower crossed syndrome
3. The Lower extremity postural distortion pattern
By using a movement assessment, such as the Overhead Squat, a trainer or health professional can observe dynamic movement impairment syndromes and postural distortion patterns characterized by over and underactive muscles and then design an appropriate program consisting of flexibility and strengthening exercises.
For example, in the upper crossed syndrome scenario (Arms fall forward in the Overhead squat assessment), the pectoral group would be short and therefore require lengthening, whereas the rhomboids, while feeling tight, are “locked long”, or neurologically tight. Stretching these muscles would exacerbate the problem. These muscles need to be strengthened.
A common scenario in the lower crossed syndrome (forward lean in the overhead squat assessment) is overactive hip flexors (TFL, rectus, psoas) and under active hip extensors.
The overactive hip flexors reciprocally inhibit their functional antagonist (glute max) resulting in synergistic dominance of the erector spinae and hamstrings (Biceps femoris).
The hamstrings feel tight, but are “locked long” (also neurologically tight) and likely providing stability to the lumbopelvic hip complex. Stretching the hamstrings under these circumstances would compromise core stability and potentially result in an injury.
When the assessment guides the intervention, in this case what to stretch and what to strengthen, the result is optimum length-tension relationships, joint alignment and force-couple movements, leading to optimum neuromuscular efficiency:
The ability of the nervous system to properly recruit the correct muscles (agonists, antagonists, synergists and stabilizers) to produce force concentrically, reduce force eccentrically and dynamically stabilize isometrically in all planes of motion.
There are three categories of stretching
Corrective flexibility is used to address postural dysfunction, muscle imbalance and joint dysfunction. It incorporates self-myofascial release (e.g. foam roller) and static stretching. Self-myofascial release and static stretching use the principle of autogenic inhibition through GTO stimulation to cause muscle relaxation.
Active flexibility is used to improve soft tissue extensibility in all planes of motion by employing the neurophysiological principle of reciprocal inhibition.
Active flexibility uses agonists and synergists to actively move a limb through a range of motion, while the functional antagonist is being stretched, e.g. actively contracting the hip flexors, to stretch the hip extensors, or actively extending the leg by contracting the quadriceps to stretch the hamstrings.
Because you cannot co-contract opposing muscles simultaneously, the muscles opposite to those contracting are reciprocally inhibited, allowing for a greater stretch.
Neuromuscular stretching (e.g. CRAC- contract, relax. antagonist contract) also known as Proprioceptive Neuromuscular Facilitation, is based on the neurophysiological mechanisms of autogenic inhibition and reciprocal inhibition.
A trainer or health care professional passively moves the limb until the first resistance barrier and then has the person actively contract the agonist (muscle opposite to the one being stretched) with 25% maximal resistance isometrically (against the tester) for 7-15 seconds.
After relaxation of this brief contraction, the limb is then actively moved by the individual to the next resistance barrier and held for 20-30 seconds. This is repeated three times.
Functional Flexibility is dynamic stretching used to improve multiplanar soft tissue extensibility and optimum neuromuscular control at full ranges of motion, while performing movements that require the body’s muscles to control the speed, direction and intensity of the stretch.
Examples of Functional Flexibility include: Lunge with rotation, prisoner squat and tube walking. This type of flexibility is ideally done prior to an event and with movements that closely simulate the event.
Understanding the mechanisms behind how mechanoreceptors influence muscle length and tension is helpful when designing a treatment or rehabilitation plan involving procedures such as manual therapy and stretching.
By stimulating mechanoreceptors such as GTO and Spindles, reflex mechanisms such as autogenic and reciprocal inhibition can result in enhanced neuromuscular efficiency.
This equates to pain reduction, prevention of injuries and performance enhancement.