I’ve heard it all my life. Hold the stretch for about 30 seconds or so, but I never asked why. According to our anatomy and physiology, it matters!
Mechanoreceptors (Sensory receptors)
What tells us that a muscle is stretching too much or that it needs to stretch more? Our bodies have muscle spindles and golgi tendon organs that provide that particular service.
Muscle spindles are the major sensory organ of a muscle. They are microscopic fibers that run parallel to a muscle fiber. The main function of a muscle spindle is to alert the brain, via the central nervous system, that a muscle is lengthening too far or too fast (Clark, Lucett, McGill, Montel, & Sutton, 2008). When excess lengthening occurs the muscle spindles contract, resulting in micro muscle spasms or a tight feeling.
Golgi tendon organs are located between a muscle and a tendon, also referred to as the musculotendinous junction. Their job is to to relax a muscle when it is placed under excess tension (Clark, Lucett, McGill, Montel, & Sutton, 2008) For example, if you try to lift a weight that is too heavy, then the GTOs will send a message to the muscles involved to stop the action. The relaxation prevents a muscle from tearing.
Hacking the Body With the 30 Second Stretch
You can hack your body’s system by holding a static stretch for a prolonged period of time. Doing so starts a process called autogenic inhibition. In short, autogenic inhibition is an override of the muscle spindles by the golgi tendon organs, allowing the muscles to relax and lengthen (Clark, Lucett, McGill, Montel, & Sutton, 2008). The 30 second rule is based on this process. Generally speaking, most static stretches should be held long enough for the golgi tendon organs to override the muscle spindles– approximately 30 seconds.
Clark, M. A., Lucett, S. C., McGill, E., Montel, I., & Sutton, B. (2018). NASM essentials of personal fitness training. Burlington: Jones & Bartlett Learning.