A scientific explanation of stretching

This explanation is from the 2nd edition of Overcoming Gravity.  hope you enjoy some of the content from it, as I’ve been working hard to get clear(er) explanations for everything.

How flexibility training works physiologically

The nervous system has afferent and efferent fibers. Afferent fibers are sensor fibers that provide feedback to the nervous system via touch, pressure, or other changes within the body. Efferent fibers provide controlling feedback after the brain and/or nervous system has processed it. In conjunction with this are alpha, beta, and gamma afferent and efferent fibers. The Greek-letter-names of the fibers simply tell you their speed of transmission. In the case with muscle spindles there is gamma afferent feedback to the nervous system and brain when a muscle lengthens. After the afferent feedback has been processed there is alpha efferent and gamma efferent control to the muscles. The alpha efferent fiber control increases tension in the muscles and the gamma efferent control ensures that the muscle spindle reacts appropriately to the change in length.

  • Gamma afferents → brain → alpha efferent + gamma efferent co-activation = alpha efferent contracting muscle + gamma efferent modulating muscle spindle length.

What occurs in flexibility and plyometric training is fusimotor system activation and modulation. The fusimotor system is composed of gamma and beta afferent and efferent fibers. The same process occurs above with the gamma afferents and processing, but the efferent feedback is slightly different. Stretching sends the gamma afferent feedback. The nervous system via the spinal cord and brain processes it. Alpha, beta, and gamma efferents are then sent back to the muscles.

  • * Flexibility training or plyometric training → gamma afferents → nervous system via spinal cord or brain → alpha, beta, and gamma efferents → alpha efferent contraction of the muscles + gamma efferent modulating muscle spindle length + beta efferents modulating muscle spindle sensitivity

Flexibility training aims to induce decreased muscle spindle sensitivity. In other words: when you train and your muscles start to lengthen and move to end range, your muscle spindles decrease sensitivity so that they respond later than in a previous stretch. You move further in your range of motion before the passive tension increases. This leads to gains in range of motion.

There is a division of static and dynamic control for beta efferent feedback to muscle spindles.

  • Static[1] = Muscle spindle sensitivity in static positions = muscle stretch at end range
  • Dynamic[1] = Muscle spindle sensitivity in dynamic movement = stretch shorten cycle = effectiveness in plyometrics

Flexibility training aims to decrease muscle spindle sensitivity to end range in static or slow movements to allow for increases in range of motion, which pushes back the range of passive tension[2]. Plyometric training is targeted at increasing muscle spindle sensitivity in dynamic movements—those used in running and jumping.

It is vital to remember that pain inhibits decreasing muscle spindle sensitivity in flexibility training, and pain also inhibits increasing muscle spindle sensitivity in dynamic or plyometric training. This means that performing flexibility training into the pain range is extremely counterproductive.

[1]: The static and dynamic factors in muscle spindles are nuclear bag fibers if you want to look them up.
[2]: passive tension is basically the nervous system telling the body to contract the muscle when you lengthen it, presumably to stretch.

Therefore, generally stretch into the range of discomfort (not pain) and aim to relax in that range. This can be applied with various techniques such as regular static stretching, proprioceptive neuromuscular facilitation, active isolated stretching, and loaded progressive stretching. This is also similar to movement and position based disciplines such as yoga and whatnot.

Original posts on /r/bodyweightfitness and the other post on /r/flexibility. Cover image from shapefit.

Author: Steven Low

Steven Low, author of Overcoming Gravity: A Systematic Approach to Gymnastics and Bodyweight Strength (Second Edition), is a former gymnast who has performed with and coached the exhibitional gymnastics troupe, Gymkana. Steven has a Bachelor of Science in Biochemistry from the University of Maryland College Park, and his Doctorate of Physical Therapy from the University of Maryland Baltimore. Steven is a Senior trainer for Dragon Door’s Progressive Calisthenics Certification (PCC). He has also spent thousands of hours independently researching the scientific foundations of health, fitness and nutrition and is able to provide many insights into practical care for injuries. His training is varied and intense with a focus on gymnastics, parkour, rock climbing, and sprinting.