Slow movements give your muscles time to adjust to a stretch.
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When you understand the science of muscle stretching, you can understand why your muscles resist stretching and how to overcome that natural tendency. Skeletal muscles attach to bone with tendons. When you stretch a muscle, the tendon sensory organs -- called stretch receptors -- signal your spinal cord, causing the muscle to contract. However, if you maintain the stretch, the reflex action releases after 10 to 30 seconds, according to the McGraw Hill Higher Education website. This reflex response is why it is important to hold stretches until you feel the muscle relax.
Why Slow Movements Yield Greater Stretch
Your muscle sensory feedback system is comprised of Golgi tendon organs -- that record pressure -- and muscle spindle apparatus -- that measure length. Spindle apparatus align themselves in a parallel pattern, which allows length sensitivity. Stretching the spindles sends signals to primary and secondary sensory endings, according to McGraw Hill Higher Education. The sensory organs respond in sequence during a slow stretch, with the primary organs responding first and the secondary organs responding later in the stretch. However, sudden movements cause both primary and secondary endings to respond at the same time, resulting in a strong contraction or even a spasm.
Assisted Muscle Stretches
Proprioceptive neuromuscular facilitation, PNF, provides greater gains in muscle stretching through the use of muscle isolation and an assistant. To achieve a PNF stretch, you deliberately contract the muscle you want to stretch and wait for the relaxation response. Once the muscle relaxes, an assistant can apply passive resistance and stretch the muscle. For example, lying on a bench, you raise one leg straight up, with your toes pointing at the ceiling, and contract your quad muscles. A trained assistant places a hand between your knee and your quads while you press against the hand to increase the stretch provided by raising your leg. Through the application of force greater than the force created by the contraction, your muscle lengthens.
Multiple Sets of Stretches
Muscle lengthening provides greater range of motion, ROM, which is important to athletes and individuals who have decreased ROM due to injury, a sedentary lifestyle or aging. The greatest muscle resistance occurs during the first muscle stretch. The majority of resistance occurs within the first 15 seconds of a stretch, with little if any change after 30 seconds. Although the tension period is about the same length during the second and subsequent stretches of the same muscle, the resistance is lower from the beginning, decreasing with each successive stretch.
How Stretching Realigns Scarred Tissue
During contraction, thick and thin muscle myofilaments increase in their areas of overlap. During a stretch, overlapping decreases as muscle fibers elongate. When you stretch a muscle to its maximum resting length, additional stretching impacts nearby connective tissue. During this action, the collagen fibers of the connective tissue align themselves along the same line of force as the tension, according to "What Happens When You Stretch" on the Massachusetts Institute of Technology website. This type of stretching can rehabilitate scarred tissue.