Providing movement in the Human Movement System requires the contraction of muscle fibers that are innervated by a signal to the central nervous system stimulating muscles that attach two bones forcing them to shorten the distance between them in a one of three classes of levers found in the body. The four characteristics of a muscle are 1)extensibility 2)contractility 3)excitability 4)elasticity. Being that the main component of the muscle belly provides the dominant characteristic of elasticity, simply meaning that it can regain it’s original shape, the characteristic I will discuss in greater detail is the extensibility. When looking at the properties of muscles and tendons, both fibers have the ability to be extended, and the benefits of static stretching to the myofascial tissue to improve Range of Motion (ROM) throughout a joint.
When a muscle is resisted upon, as in a stretch or opposing a force that the muscle is acting upon to move or prevent from moving, our CNS sends feedback to our muscle to cause the filaments of actin and myosin to begin their process of contracting. During this moment of contraction, the fibers of the muscle slide past one another creating the muscle contraction as the fascia of the tendons which are attached to the muscle, act as an anchor to pull the two bones closer together. As our bodies continue to provide thousands of contractions throughout the day to move our bodies in all planes of motion, the viscoelastic component(fascia, or hardened tissue) that also surrounds the muscle belly which house the actin and myosin, can become shortened creating a greater strain on the arthrokinematics(joint movement) of the joint in motion. Progressive activation of the muscles around the joint that are not provided a stretching component, can begin to alter the kinematics of the joint leading to altered length-tension relationships, altered force couple relationships, and finally altered arthrokinematics of the joint, leading to potential injury(Clark and Lucett, 2010). Simply stated, some muscles with begin to shorten and become overactive, while the opposing muscles will lengthen and become weakened, altering the motion of the specific joint, leading to future muscular injuries).
Static stretching, which is passively holding the muscle in an eccentric muscle contraction, is believed to produce both mechanical and neural adaptations that lead to an increase in joint ROM(Clark and Lucett, 2010). During a static stretch, our muscles sense a stretch reflex and automatically attempt to minimize the stretch of the muscle by the activation of our muscle spindles, which are sensory neurons inside the muscle that are activated when a stretch to a muscle is occurring too rapidly. Static stretching which fosters the stretching of a muscle in a slow, controlled manner, limits the activation of the muscle spindles and appears to decrease the motor neuron excitability, thus inhibiting the effects of Golgi tendon organs(force receptors). Clark and Lucett state that over time, the increases to the possible ROM a joint exhibits might be an increased tolerance to the stretch and not necessarily the changes to the viscoelastic properties of the myofascial tissue. Clark and Lucett define the characteristics of static stretching:
- “The elongation of neuromyofascial tissue to an end-range and statically holding that position for a period of time.”
- “Maximal control of structural alignment.”
- “Minimal acceleration into and out of the elongated position.”
Looking at the benefits and risks with static stretching, questions arise as to when static stretching should be performed and the frequency of sessions one should engage in. Numerous studies have determined that stretching prior to physical exercise or activity have demonstrated that force production has decreased and that injury risk were not necessarily decreased. While the questions of when to static stretch still exist, it has shown that by decreasing the stiffness in a muscle allowing for greater ROM , decreases the overall work necessary for the muscle to move against resistance, thus increasing overall performance.
In a study by Jonathan Anning, Phd, courtesy of the NSCA, he studied the effects of pre-exercise static stretching in comparison to dynamic stretching, which is defined as slow, controlled, accentuated movements that increase tissue temperature, and increases neural stimulation, as well as Proprioceptive Nueromuscular Facilitation(PNF), which is a partner assisted stretch that incorporates both passive and active muscle stretch. Anning concluded that preceding an activity that required optimum force output, static stretching decreased the total amount of force that the muscle was able to transmit. He believed that during static stretching, there is a loss of muscle stiffness, increase in muscle compliance and a reduction in neural stimulation. These neural reflexes (myotatic reflex) were inhibited during a session of static stretching that increased muscle compliance, which limited the force output of a muscle. This reduced neural activation coupled with muscle compliance was Annings conclusion as why activities requiring optimal force output should not be preceded by static stretching. On the contrary, dynamic stretching which are common body movements that are completed in a slower, more controlled manner with greater ROM than typically the activity would require, helps increase the neural activity, and “prepares the body” for what actions it will be performing. In addition to the neuromuscular patterns, dynamic stretching increases tissue temperature and begins the process of muscle contraction in which neurotransmitters are being released to the muscle causing the actin and myosin filaments to begin their process of sliding past one another, but must be completed in a non-fatiguing manner. Anning reported in his study that the concept of PNF stretching, which requires holding the muscle in an extended position during a passive stretch, as well as a contracted phase against the partner assisting, also showed that this either resulted in a decreased force output or no change at all.
Looking at the three forms of stretching the muscle, dynamic stretching showed to be the most beneficial to perform prior to engaging in maximum output activities, while static and PNF showed to either decrease maximal output or show no changes.
There are some common misconceptions about Static Stretching. Here is a great video that addresses those(courtesy of NASM)
Clark, Micheal, and Scott Lucett. NASM Essentials of Corrective Exercise Training.
Philadelphia, PA: Lippincott Williams & Wilkins, 2010. Pr