Stretching Overview

In an extensive comprehensive review of international research papers concerning indications and effectiveness of different stretching techniques, there appears to be no agreement.  There seem to exist a vast amount of stretching techniques, each with its own described precise application. However, the theoretical models attached to these stretching techniques are based on incomplete scientific study or basis.

Most studies show that each individual technique seems to have some value when the goal is to improve mobility. The majority of the studies identify the short-term effects of stretches on the subjects, but none deal with the long-term effects.

Most authors conclude that the application of their technique improve mobility which is dependent on properties of the connective tissue rather than on neurophysiological mechanisms.

International studies were done and compared results of static, post-isometric and dynamic stretching of the hamstrings. All three stretching methods showed an increase in hamstrings mobility.

The post-isometric method was superior to the static model; however, the largest increment of mobility was gained by the dynamic stretch technique.

Research papers tried to identify what factors resulted in the improvements of mobility. Findings showed that improvement is based on an increased tolerance of the muscle-tendon unit to stretch triggers. Stretching did not change the stretch-tension curve.

The stretching probably caused a mechanical elongation to bring about an effective lengthening of connective tissue; however, this was only maintained over a short period of time.

Another aspect of stretching was reviewed in regards to whether it improved muscle regeneration. Three different stretching techniques were studies which were static stretch, intermittent dynamic stretch and post-isometric relaxation.

The findings showed that static stretching consistently resulted in worsening of muscle regeneration. The post-isometric relaxation had an inconsistent influence, where in some subjects it led to improvement, in others worsening of muscle regeneration.

Finally, intermittent dynamic stretching led to an overall improvement in muscle regeneration. One researcher commented that static stretching may actually slow the regeneration process as it may compress the capillary system and interrupt vascularization. Good vascularization is needed to flush out the metabolic intermediate and end products of forceful muscle activity.

Many therapists who do not have access to objective measuring methods rely on Sherrington Principle of Reciprocal Inhibition to explain their stretching techniques. However, researchers using EMG measuring methods have not observed this reciprocal inhibition. The question arises whether the basic models for neurophysiological reflexes really exist.

Other researchers have remarked that the role of the connective tissue as the most important factor in resistive stretching is overestimated. At the same time, the researchers feel that the resistance of muscles to stretch in underestimated. Muscle imbalances of agonist and antagonist can be explained by Sherrington’s Law of Reciprocal Innervation.

This law implies that a strained or tight muscle will inhibit its antagonist. Therefore, the tight muscles will create weak antagonistic muscles which create a muscle imbalance that adversely affects the normal pattern of muscle movement. Strengthening these tight muscles will have the effect of further increasing the tightness and perpetuating the antagonistic weakness.

Therefore, the concept should be applied that before strengthening exercises are prescribed, muscle tension must be released through stretching prior to any strengthening.  Strengthening already tight muscles will promote further weakness and imbalances.

Facilitating full flexibility of both the agonist and antagonist will allow for a more balanced strengthening and elimination of one muscle group dominance.

Proprioneuromuscular facilitation or PNF is a technique commonly associated with contract-relax stretching (hold-relax) and contract-relax agonist contraction methods. At present, researchers have not found evidence for a mechanism behind the effectiveness of PNF techniques.

PNF stretching techniques use volitional contractions in an attempt to achieve increased range of motion by minimizing the active component of resistance attributed to spinal reflex of pathways.

In the hold-relaxed agonist contraction method, the contraction and relaxation of the muscle to be stretched (antagonist) is followed by a concentric contraction of the muscle opposing the muscle to be stretched (agonist) to inhibit further the antagonist through the alpha-gamma co-activation and reciprocal innervation. 

Although PNF techniques have been based on neurophysiological factors resulting in neuroinhibition of the muscle undergoing stretch, researchers have now disproved that. Researchers point that it is the mechanical factors rather than the stretch factors that are responsible for the increased length and stretching after PNF.

The mechanical response is based on the muscle-tendon unit responding to its viceroelastic properties allowing for length increases without stretching. The elastic component related to stretch are directly related to the applied force while the viscus properties are time dependent and late change dependent, such that the rate of deformation is directly proportional to the applied forces.

Studies have also looked at whether slow repetitive speeds or high explosive speeds improve muscle strength and endurance.  Although slow repetitive speeds have a definite place in the development in hypertrophy and focal muscle endurance, they have minimal improvement in developing absolute or speed strength.

High threshold motor neurons are optimally recruited at high tension, and this requires acceleration. As a rule of thumb, lower the weight over two to three second duration and then launch the weight with as much explosive power as possible to improve muscle building and strength.

The slow twitch fibers respond to higher weights and slower speeds whereas the fast twitch fibers respond more quickly with greater gains with acceleration and lower weights.

Should you have any further questions regarding this article, please direct your questions or comments to "Ask the Doctor" section.

Copyright © 2004 - 2009 Taras V. Kochno, M.D.  All Rights Reserved
Board Certified in Physical Medicine and Rehabilitation

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