What Happens When We stretch?

Anna Barnsley Physio, Dave Palmer, will tell you!!

Any athlete or sports physician will tell you that being flexible is very important in order to achieve optimal performance and prevent injury.  In order to achieve flexibility many people take part in frequent stretching sessions.  There are numerous forms of stretching out there; static, ballistic, proprioceptive neuromuscular facilitation (PNF) to name a few.  The debate regarding which is the most effective technique is both vast and contradictory.  Most people pick a form that they feel works for them.  But what I am interested in is to find out what stretching actually does on a physiological level, and how flexibility actually improves by stretching?

Most sports clinicians will measure “muscle length” by measuring the muscle range over a specific joint. For example; a straight leg raise to test hamstring length.  I will therefore use this term throughout the article.

Reading through the latest literature reviews there appears to be four mechanical theories to explain increased muscle length.  They are:
• viscoelastic deformation
• plastic deformation
• increased sarcomeres in series
• neuromuscular relaxation

Viscoelastic Deformation
Muscles are seen to have properties of both “elasticity” and “viscosity”.  They are elastic because when a muscle is stretched; once the stretch is removed it can return to its original length, a bit like an elastic band.  However, as it is stretched the muscle will also resist the stretch, and how much resistance depends on how quickly the stretch is applied and how long it is held for: it is therefore said to act viscously. 

When a muscle is held in the stretched position for a period of time the muscle’s resistance to the stretch will gradually decline.  With a reduction in the resistance you will get an increase in muscle length, this immediate increase in length is called viscoelastic deformation. 

However studies have shown that this increase in length is minimal and temporary, for example, by 30 seconds following the initial sustained stretch was released the muscle was back to its original length.

Plastic Deformation of Connective Tissue
This is possibly the oldest and most common theory of them all, and suggests that when a muscle is stretched beyond its elastic limit, permanent changes take place in the connective tissue, which increases its length. Connective tissue makes up a variety of physical structures within the body, including tendons, and is the fibrous tissue that forms a connective framework for the whole body.  This is the explanation you will most commonly hear advocates of stretching tell you, however this theory is very poorly supported in the literature and struggles to gain any support from human trials. 

Increased Sarcomeres in series
Sarcomeres are the cells that are responsible for human muscles contracting.  And some animal studies have shown that with prolonged immobilization of a muscle in extreme positions, the number of sarcomeres in the muscle has changed.  It is theorized that in order to adapt to the new length the muscle changes the number and length of the sarcomeres in order to be able to perform a muscular contraction at this new, increased length.

Not only does prolonged immobilization differ massively from every form of stretching, it isn’t actually possible to test this theory on human subjects due to ethical issues.  It’s also important to point out that rat muscle, which is commonly the subject of study, and human muscles differ significantly in terms of their continued growth and maturity.

Neuromuscular Relaxation
This theory suggests that muscle length is limited by involuntary neuromuscular “stretch reflex” contractions, when put under a sustained stretch.  So when a muscle is taken to a length passed what it is capable of we have involuntary muscle contractions to restrict the movement in order to protect the muscle from damage.  So this theory suggests that if we can stimulate the neuromuscular reflexes that oppose this reflex and therefore induce relaxation, we can achieve increased muscle length by overcoming the stretch reflex.  It is also proposed that in order to achieve a longer term increase in length these reflexes can actually be adapted over time with repeated stretching.

However… research has shown that not only do these stretch reflexes not receive significant activation from long, slow static stretches but even struggle to achieve significant stimulation from quick, “bouncy” stretching which are supposed to provoke the highest amount of stimulation of them all.

So… not so good so far! All mechanical theories are certainly lacking in the research department.  For this reason it has been proposed that the increase in muscle length following a stretching regime must be due to a reduction in stretch “sensation”.  

Sensory Theory
If we look at all these studies into stretching they all have one thing in common…. How we judge the ‘end point’ of a stretch. This is when the patient says “OW, stop!”, so the measurement is always based on what the patient feels when we stretch their muscles through a certain range.  This feeling is influenced by many things; such as a person’s perception of what their range should be, or simply how much pain the patient can tolerate.  Either way it is very subjective and described as the patient’s stretch “sensation”.

So, could increase in muscle length purely be because the patient has now got used to that painful feeling you get when a muscle is stretched to its’ limit, and you can therefore take the muscle further before they feel that original pain intensity, rather than the actual muscle physiologically increasing in length?

This theory does have support in the literature following both single stretch sessions and following 3 – 8 weeks of stretching programs to the Hamstrings, Quadriceps and the Calves.  This theory also has support from the multidimensional muscle testing, which has found no physiological change to the muscle, yet an increase in range of movement.

So how does this impact on our practice?
This has particular impact on our treatment of clients with Hypermobility (those who have excessive mobility at their joints).  Using mechanical theories our treatment principles would be to strengthen the local stabiliser muscle groups in order to achieve “passive resistance” to the stretch, therefore restricting excessive movement.  However, if we are to consider the sensory theory and see this excessive movement as partly due to “faulty” sensory feedback; then we must incorporate proprioceptive techniques to work on interpretation of end of range.  Proprioception is the system the body uses to be able to know where a joint is in space without the need to look at it.  By improving this system and therefore improving the neuromuscular control and feedback at the joint, we may be able to improve the muscle length across that joint.

Strengthening local stabilising muscles still play a large role in the treatment of hypermobility in order to assist the neuromuscular system in controlling this excess range of movement. With an improved ability to activate these stabilising structures this may in fact improve the patients’ sensitivity to the feeling of their end of range.

This also has implications for judging the frequency of stretching, with the goals of the stretching program being an improvement in proprioception or that end of range “sensation”.  In order to achieve a significant shift in “proprioceptive control” then the stretch will have to be performed frequently as part of a daily routine.

Whether muscle length is restricted or excessive, it can lead to whole host of injuries; ranging from wear of joint surfaces, muscle imbalances or movement dysfunction.  Achieving optimal flexibility across all joints must be an integral part of our treatment planning in our quest to achieve optimal performance.  The Sensory Theory certainly seems to be the most popular and well supported theory in explaining improvements in muscle length and should therefore be what we base our techniques principles on, until further research can give us more clarity.