There are 4 ‘mechanical theories’ as to why tissues are easier to stretch after isometric contractions….most have been discounted or challenged….and there is a ‘sensory theory’ that is the current favourite… and possibly others?

 Firstly…to clarify what is meant by the word ‘muscle’ ? 
“Skeletal muscles comprise contractile tissue intricately woven together by fibrous connective tissue that gradually blends into tendons.
The tendons are made of fibrous connective tissue and attach the muscle to bone.(Hollinshead & Rosse 1985) These cannot be separated during routine clinical testing and stretching procedures, nor during functional activity.
Both the muscular contractile tissue and tendon exhibit changes in biomechanical properties and cross-sectional area in response to exercise, disuse and ageing. (Magnusson et al 2008)
The word “muscle” [therefore] indicates the entire skeletal muscle, including the contractile tissue and tendon components.”

(Weppler & Magnusson 2010)



This is a summary of theories regarding ‘increased muscle extensibility’/increased range of motion, following isometric contractions and stretching:

Mechanical theory 1:

  • Viscoelastic deformation?
  • This effect is brief at most (Weppler & Magnusson 2010)
  • The magnitude and duration of any transient viscoelastic deformation in muscle length in humans varies depending on nature of stretch.(Ryan et al 2008)

Mechanical theory 2:

  • Plastic (permanent) deformation?  There is no evidence to support this in humans (Weppler & Magnusson 2010)
  • The limited evidence for this relates to Warren et al’s (1971) study on rat-tails.


myofiber connection Huijing p114 copy



Mechanical theory 3:

  • There is an increase in sarcomeres in series?
  • Yes – but as the number of sarcomeres change, the length also changes, so there is no overall length increase. (Weppler & Magnusson 2010).
  • Immobilisation results in increased number of sarcomeres (if immobilised in extension) or decreased number of sarcomeres (if immobilised in flexion), however length overall does not change.
  • It is not known whether therapeutic interventions modify number of sarcomeres in series. (Williams & Goldspink 1978)

Mechanical theory 4:

  • Neuromuscular relaxation?
  • This is not supported by evidence. (Weppler & Magnusson 2010).
  • It has been suggested that involuntary neuromuscular “stretch reflex” can limit muscle elongation during static stretching procedures – and that slowly applied static stretch stimulates neuromuscular reflexes that induces relaxation, enhancing increased extensibility.
  • Experimental evidence does not support these assertions.(Reid & McNair 2004)


Sensory Theory:  Increased tolerance to stretch ? (Weppler & Magnusson 2010)

  • Does extensibility (‘length’) of a muscle depend on sensation limits (discomfort, pain) or on structural mechanical limits?
  • “To what extent this adaptation is a peripheral or central phenomenon or a combination thereof remains to be established.” (Magnusson et al 1996)

Weppler & Magnusson (2010) note:

“Is passive muscle stiffness necessary to stop joint motion, or is it possible that just
the subject’s sensory perception of stiffness or perception of moderate stretch can be a limiting factor? Studies evaluating the biomechanical effects of stretching reveal that in controlled clinical settings under the condition of slowly applied passive stretch, it is subject sensation—not the degree of stiffness—that limits joint motion. Researchers have been able to apply passive torque up to the sensory endpoint of pain or
stretch tolerance without being limited by stiffness. It seems reasonable that subject sensation could both alter and reflect the way the tested muscle is routinely used in function.”


  • Isometric contractions have an analgesic effect (Bement et al 2008, 2011 )
  • Stretching and contractions induce fluid changes (albeit temporary) that result in more pliable connective tissue (Langevin et al 2005, Schleip et al 2012)
  • Stretching and contractions result in analgesic release of endocannabinoids (McPartland 2008)



Cellular effects

Cyclic short-duration stretches (CSDS) such as those resulting from repetitive motion strain increases the risk of musculoskeletal injury. Myofascial Release … applies an acyclic long-duration stretch (ALDS), to muscle fascia, to assist in injury repair.”

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Skeletal myoblasts differentiated to form myotubes :A developing skeletal muscle fiber with a tubular appearance

  • When subjected to mechanical strain, fibroblasts within muscle fascia secrete IL-6, which induces myoblast differentiation – essential for muscle repair
  • ALDS following CSDS increases myotube number by 78% (P < 0.05).”


Doubtless more will emerge, but for now we have modest or no support for mechanical changes, strong support for sensory changes (“increased tolerance to stretch’) and suggestive evidence of ‘other’ mechanisms at work.

For more on stretching see:







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  • Bement M et al 2011 Pain Perception After Isometric Exercise in Women With Fibromyalgia Arch Phys Med Rehabil 92:89-9
  • Bement M et al 2008 Dose response of isometric contractions on pain perception in healthy adults Med Sci Sports Exerc. 40(11):1880-1889
  • Hicks M et al 2012 Mechanical strain applied to human fibroblasts differentially regulates skeletal myoblast differentiation. J. Applied Physiology 113(3)465-472
  • Hollinshead WH, Rosse CM. Textbook of Anatomy. 4th ed. Baltimore, MD: Lippincott Williams & Wilkins; 1985.
  • Langevin, H et al 2005 Dynamic fibroblast cytoskeletal response to subcutaneous tissue stretch ex vivo and in vivo. Am J Physiol Cell Physiol 288: C747–C756, 2005.
  • McPartland J 2008 Expression of the endocannabinoid system in fibroblasts and myofascial tissues. Journal of Bodywork and Movement Therapies 12:169–182
  • Magnusson SP, Simonsen EB, Aagaard P, et al. 1996 A mechanism for altered flexibility in human skeletal muscle. J Physiol. 497(pt 1):291–298.
  • Magnusson SP, et al 2008 Human tendon behaviour and adaptation, in vivo. J Physiol 2008; 586:71–81
  • Reid DA, McNair PJ. Passive force, angle, and stiffness changes after stretching of hamstring muscles. Med Sci Sports Exerc. 2004;36:1944–1918.
  • Ryan E et al 2008 The time course of musculotendinous stiffness responses following different durations of passive stretching. J Orthop Sports Phys Ther. 38:632–639.
  • Schleip et al 2012 Strain hardening of fascia: Static stretching of dense fibrous connective tissues can induce a temporary stiffness increase accompanied by enhanced matrix hydration. Journal of Bodywork & Movement Therapies 16:94-100
  • Warren CG, et al 1971 Elongation of rat tail tendon: effect of load and temperature. Arch Phys Med Rehabil.52:465–474.
  • Weppler C Magnusson S 2010 Increasing Muscle Extensibility: A Matter of Increasing Length or Modifying Sensation? PHYS THER. 90:438-449.
  • Williams PE, Goldspink G. 1978 Changes in sarcomere length and physiological properties in immobilised muscle. J Anat. 127(pt 3):459–468.