Questions still persist as to what happens to a muscle when it is stretched - is there an actual mechanical increase in the length of the stretched muscle or is the increase in extensibility due to a sensory modification? A recent paper by Ben and Harvey1 summarizes a randomized, controlled trial designed to determine whether regular stretch increases hamstring muscle extensibility. The authors concluded, "Six weeks of sustained 30-min daily stretch does not increase the extensibility of the hamstring muscle of healthy individuals. It does, however, improve stretch tolerance leading to increased joint range of motion without any actual improvements in muscle extensibility."
Weppler and Magnusson2 concluded that a sensory theory was more valid in improving muscle extensibility than proposed mechanical theories. Regarding mechanical theories, a prominent rationale used to explain increased extensibility is that because the muscles are viscoelastic structures, elastic change occurs. Viscoelastic stress relaxation occurs as the muscle's resistance to stretch gradually decreases due to various types of stretches, such as contract/relax, constant load and repeated cyclic stretches.
However, studies on humans have showed viscoelastic deformation to be transient in nature and that the muscles have already recovered from the relaxation by the next stretch.3 Weppler and Magnusson2 also dismissed the concepts of plastic deformation or increased sarcomeres in series creating a permanent stretch with ordinary stretching. Others feel that plastic deformation will increase length of tissue only if the load is past the elastic limits of the tissue; otherwise, the tissue will gradually return to original resting length after the load is removed.
For an increase in connective tissue elongation, an actual microfailure must be produced for permanent elongation. This may necessitate a low level of connective tissue damage or collagen breakdown, followed by the classical cycle of tissue inflammation, repair and remodeling, followed by continued stretching procedures4 to maintain the elongation. Long-term studies are still needed to prove these concepts. And regarding permanent elongation, studies are also needed on the fascial system since "fascia is the only tissue that modifies its consistency when under stress. It is plastic but also malleable and it changes its consistency when manipulated."5
Magnusson, et al.,6 also dismissed proprioceptive neuromuscular facilitation as resulting in permanent changes, since in studies investigating "contract relax," no significant electromyographic activity of the stretched muscles was found,6 meaning the increase in end-range joint angles could not be attributed to neuromuscular relaxation.
So, since these studies do not show a mechanical cause, the proposed sensory theory is that the increased muscle extensibility after stretching was based on the ability of the subject to withstand increased pain, allowing the stretch. Weppler and Magnusson2 feel that further studies are necessary to determine if the adaptation of the subject is a peripheral or a central phenomenon, or a combination of the two. Ben and Harvey reached a similar conclusion, stating that improved participant tolerance to the discomfort associated with a stretch allowed them to tolerate larger stretch torques.1
References
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