Stretch Tolerance

Jules Mitchell delves into the science behind stretching in her book Yoga Biomechanics: Stretching Redefined .  Conventional approaches to modern yoga are examined through a biomechanist’s lens, highlighting emerging perspectives in both the rehabilitation and sport science literature. This article is an excerpt from Yoga Biomechanics: Stretching Redefined by Jules Mitchell, copyright of Handspring publishing (2019).

Stretch tolerance describes the limit to which an individual can “tolerate” the discomfort associated with a deepening stretch. A purely “sensory” theory, its basis lies in an individual adapting to and becoming less sensitized to what most consider the painful experience associated with stretching, or finding the sensation becoming less offensive, or more tolerable, after repeated exposure. The theory emerged because all human trials measuring end ROM will always stop at an individual’s tolerance. Unlike animal studies where tissues can be extracted and mechanical limits tested ex vivo, ROM studies on humans are limited by the subject’s request to stop the stretch in the presence of pain and discomfort. The main premise of the sensory theory is that changes in ROM are not due to alterations in tissue properties, but in sensory tolerance.

In the early years of flexibility research, Magnusson, published a classic paper. He looked at the effects of a 3-week stretching intervention on tissue properties as opposed to those immediately following a stretch. His testing protocols were twofold. One measured PRT at a predetermined range assessed by the sensation of tightness. The other was similar but progressed into a painful range. Yoga teachers may understand this distinction via the popular cue – to enter the pose “to the point of discomfort but not to the point of pain.” After the completion of the intervention, the investigators found no alterations in tissue properties, concluding tolerance to be the mechanism of change in ROM (Magnusson, Simonsen, Aagaard, Sørensen et al., 1996).

These papers launched a debate among the mechanical and sensory theorists, and the subsequent publication of multiple papers defending either position or both. Magnusson continued to publish, and 14 years later co-authored a perspective paper proposing that the sensory theory explains the inconsistencies across the literature resulting from the challenges posed in attempts to control for all variables (Weppler and Magnusson, 2010). In spite of these methodological challenges, today, the evidence is compelling enough for us to validate both theories concurrently, but there is still much we do not know.

For example, we do not understand the neurophysiology behind tolerance and how it is regulated in the nervous system. We do have some limited research on the topic of anaesthesia and ROM. Subjects undergoing knee surgery were tested on the “healthy leg” (i.e. the leg not operated on). ROM during a passive hamstring stretch was tested pre-, intra-, and post-operatively. The aim was to compare four variables: spinal anaesthesia, general anaesthesia, a nerve blocker, and an epidural. In all cases, the intra-operative ROM was significantly greater than the pre-and post-operative measurements, which did not change significantly. The spinal anaesthesia resulted in the greatest increase in ROM, suggesting neural regulation of stretching may occur at the level of the spinal cord (Krabak et al., 2001). In another study focusing on the role of pain in ROM, subjects undergoing total knee arthroscopic surgery as a treatment for osteoarthritis were tested. The operative knee was measured for maximal knee flexion and extension prior to surgery and during surgery after a spinal anaesthesia followed by a femoral and sciatic nerve blocker (which blocks nerve impulses, not feeling). Average passive ROM across 141 subjects was greater by 13.4 degrees in flexion and 3 degrees in extension under anaesthesia (Bennett et al., 2009). Whether stretch tolerance or other painful symptoms are the limiting factor in ROM, we have support for a sensory theory that warrants further research.

At this point, we are treading dangerously close to the field of neuromechanics, which is not the title of this book. The neuromechanical aspects of stretching and ROM would require an in-depth study of the nervous system, stretch reflexes, electromyograph amplitudes, and even neurological disorders such as cerebral palsy, stroke, and spinal injuries. The neurophysiology of pain is also beyond our scope here, although we will return to the topic of pain and its relationship to injury later. Since the biomechanical aspects of stretching focuses on load, tissue structure, and mechanical behaviours, we will stay within those boundaries. Any improvements in ROM as a result of loading are incidental in this context. It is my position that influences in ROM are likely a function of both sensory and mechanical mechanisms, but exactly how, when, or why each is a factor, we don’t yet understand. Thus, where my interest lies is in the question of how, when and why tissues adapt their mechanical properties when loaded in tension.

We will delve into the topic of muscle length to elucidate this point. The  full treatise is available in Yoga Biomechanics: Stretching Redefined .

Thought Provoker: Tightness

How do you explain the feeling of tightness that compels people to stretch?

Do you think tightness is a function of mechanical tissue properties or a sensory experience?

How can you determine if a muscle is tight; would it require a laboratory setting? Does this method of measurement support your explanation of tight?

This article is an excerpt from Yoga Biomechanics: Stretching Redefined by Jules Mitchell, copyright of Handspring publishing (2019).

References

Bennett, D., Hanratty, B., Thompson, N. and Beverland, D.E., 2009. The influence of pain on knee motion in patients with osteoarthritis undergoing total knee arthroplasty. Orthopedics, 32(4).

Krabak, B.J., Laskowski, E.R., Smith, J., Stuart, M.J. and Wong, G.Y., 2001. Neurophysiologic influences on hamstring flexibility: a pilot study. Clinical Journal of Sport Medicine, 11(4), pp.241-246.

Magnusson, S.P., Simonsen, E.B., Aagaard, P., Sørensen, H. and Kjaer, M., 1996. A mechanism for altered flexibility in human skeletal muscle. The Journal of physiology, 497(1), pp.291-298.

Weppler, C.H. and Magnusson, S.P., 2010. Increasing muscle extensibility: a matter of increasing length or modifying sensation?. Physical therapy, 90(3), pp.438-449.