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For many people, stretching repeatedly over a period of time improves their flexibility. I want to know whether this improvement commonly seen is due to a change in the nerves or the muscle’s material properties.

Are the changes in flexibility seen on the order of minutes/hours to weeks/months due to changes in the nervous system's reaction (e.g. reflex habituation) or changes in the viscoelasticy properties of the muscles (e.g. muscle length via serial sarcomere number)?


Excerpts:

To help answerers, I want to provide a few excerpts from two sources. The first source is from 1998.

"This article reviews recent findings regarding passive properties of the hamstring muscle group during stretch based on a model that was developed which could synchronously and continuously measure passive hamstring resistance and electromyographic activity, while the velocity and angle of a stretch was controlled. Resistance to stretch was defined as passive torque (Nm) offered by the hamstring muscle group during passive knee extension using an isokinetic dynamometer with a modified thigh pad. To simulate a clinical static stretch, the knee was passively extended to a pre-determined final position (0.0875 rad/s, dynamic phase) where it remained stationary for 90s (static phase)."

“A significant decrease was observed for energy (30%) and stiffness (13%) in the dynamic phase, and passive torque in the static phase. However, the observed decline in the variables measured was transient and they returned to baseline within 1h.”

“Long-term stretching (3 weeks) increased joint range of motion as a result of a change in stretch tolerance rather than in passive properties.”

Source: Magnusson, Stig Peter. "Passive properties of human skeletal muscle during stretch maneuvers." Scandinavian journal of medicine & science in sports 8.2 (1998): 65-77.

The second source I want to cite is The Complete Guide to Stretching by Christopher Norris. Paraphrasing pages 56-58, he says stretching can affect the following:

  • Habituation – when the stretch reflex becomes desensitized and the reflex doesn’t activate as easily when stretched
  • Muscle stiffness – a muscle has an elastic recoil when stretched. The degree of this recoil is the “muscle stiffness”
  • Stretch tolerance – the amount of discomfort induced by a stretch that a person can endure
  • Serial Sarcomere Number (SSN) – a muscle can lengthen or shorten in length when the number of sarcomeres change

Norris also notes that the first 3 criteria usually change on the order of months (called “acute” or short-term) while the SSN tends to change on the order of years (called “chronic” or long-term).

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