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)?
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).