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PNF stretching, or proprioceptive neuromuscular facilitation stretching, is a set of stretching techniques commonly used in clinical environments to enhance both active and passive range of motion with the ultimate goal being to optimize motor performance and rehabilitation. The literature regarding PNF has made the technique the optimal stretching method when the aim is to increase range of motion, especially in short-term changes. Generally an active PNF stretch involves a shortening contraction of the opposing muscle to place the target muscle on stretch. This is followed by an isometric contraction of the target muscle. PNF can be used to supplement daily stretching and is employed to make quick gains in range of motion to help athletes improve performance. Aside from being safe and time efficient, the dramatic gains in range of motion seen in a short period of time may also promote compliance with the exercise and rehabilitation program.
In the early to mid 1900s physiologist Charles Sherrington popularized a model for neuromuscular facilitation and inhibition. This subsequently led Herman Kabat, a neurophysiologist, to develop the clinical PNF stretching technique using natural movement patterns. He knew of the myotatic stretch reflex which causes a muscle to contract when lengthened too quickly, and of the inverse stretch reflex, which causes a muscle to relax when its tendon is pulled with too much force. He believed combinations of movement would be better than the traditional moving of one joint at a time. Initial PNF techniques were used to aid the rehabilitation of clients with spasticity and weakness by facilitating muscle elongation. This is theorized to be accomplished through enhanced inhibitory mechanisms affecting the spastic muscle, and improving the muscle strength through improved excitation mechanisms in the weakened muscle.
Kabat started an institute in Washington, DC and by 1951 had two offices in California as well. His assistants Margaret Knott and Dorothy Voss in California applied PNF to all types of therapeutic exercise and began presenting the techniques in workshops in 1952. During the 1960s, the physical therapy departments of several universities began offering courses in PNF and by the late 1970s PNF stretching began to be used by athletes and other healthy people for more flexibility and range of motion. Terms about muscle contraction are commonly used when discussing PNF: Eccentric Isotonic contraction is when the muscle lengthens while resisting an applied force, and isometric muscle contraction is when the muscle remains the same length while contracting.
Proposed mechanisms underlying the PNF stretching response: Autogenic Inhibition and Reciprocal Inhibition have traditionally been accepted as the neurophysiological explanations for the superior ROM gains that PNF stretching achieves over static and ballistic alternatives.
The patterns of movement associated with PNF are composed of multijoint, multiplanar, diagonal, and rotational movements of the extremities, trunk & neck. There are 2 pairs of foundational movements for the upper extremities; UE D1 flexion & extension, UE D2 flexion & extension. There are also 2 pairs of foundational movements for the lower extremities; LE D1 flexion & extension, LE D2 flexion & extension. Various PNF stretching techniques based on Kabat’s concept are: Hold Relax, Contract Relax, and Contract Relax Antagonist Contract (CRAC) etc.
Contract Relax: Passive placement of the restricted muscle into a position of stretch followed by an isometric contraction of the restricted muscle. Most isometric contractions in PNF stretching techniques should be held for a minimum of 3 seconds at a sub maximal effort (20-50% of maximal effort) to avoid muscle fatigue and injury. After the contraction period the patient is instructed to relax the restricted muscle that was just contracting and activate the opposing muscle to move the limb into a greater position of stretch. Through Golgi tendon organ, the tight muscle is relaxed, and allowed to lengthen.
Hold Relax: Very similar to the Contract Relax technique. This is utilized when the agonist is too weak to activate properly. The patient's restricted muscle is put in a position of stretch followed by an isometric contraction of the restricted muscle. After the allotted time the restricted muscle is passively moved to a position of greater stretch. Contraction times and efforts will remain the same as the Contract Relax technique. This technique utilizes the autogenic inhibition, which relaxes a muscle after a sustained contraction has been applied to it for longer than 6 seconds.
Contract Relax Agonist(Antagonist) Contract (CRAC) is usually performed by a passive or active stretch of the target muscle(s) to move the limb into a starting position at first, followed by a sub-maximal isometric contraction of the target muscle and finally an active stretch is used to move the limb into a new greater position. This technique use autogenic and reciprocal inhibition. using reciprocal inhibition is the main cause of the greatest effect of this technique versus the other PNF techniques. 
Rhythmic Initiation: Developed to help patients with Parkinsons overcome their rigidity. Begins with the therapist moving the patient through the desired movement using passive range of motion, followed by active-assistive, active-resisted range of motion, and finally active range of motion.
Rhythmic Stabilization: and Alternating Isometrics are very similar in that they both encourage stability of the trunk, hip, and shoulder girdle. With this technique, the patient holds a weight-bearing position while the therapist applies manual resistance. No motion should occur from the patient. The patient should simply resist the therapist's movements. For example, the patient can be in a sitting, kneeling, half-kneeling, or standing position when the therapist applies manual resistance to the shoulders. Usually, the therapist applies simultaneous resistance to the anterior left shoulder and posterior right shoulder for 2–3 seconds before switching the resistance to the posterior left shoulder and the anterior right shoulder. The therapist's movements should be smooth, fluid, and continuous. In AI, resistance is applied on the same side of the joint. In RS, resistance is applied on opposite sides of the joint. Note this is not a stretching technique, but instead a technique used to strengthen joint musculature and improve proprioception.
Slow reversals: This technique is based on Sherrington's principle of successive induction, i.e. that immediately after the flexor reflex is elicited the excitability of the extensor reflex is increased.This technique is used to strengthen and buildup endurance of weaker muscles and develop co-ordination and establish the normal reversal of antagonistic muscles in the performance of movement.
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