What does stretching do to a joint? We really have no idea. Part I.
How's that title for a proclamation of ignorance? For someone who rarely stretches I sure love talking about it. I've been lecturing on stretching since the late 1990s and always loved to challenge my student's beliefs about what they thought stretching did. The problem was, I didn't challenge my own biases for too long a time.
Ages ago I looked for research to confirm my bias that stretching wasn't necessary for injury prevention, performance or "postural corrections". It was easy to find. Ian Shrier was publishing work suggesting that stretching before competition did not reduce injury (Link here) and my favourite research discussed the mechanisms of stretching, which we can now elaborate on below.
Does stretching change stiffness
For more than a decade and a half I loved challenging students who said that stretching made a muscle less stiff. While this is true immediately after a stretching bout we had some great research by Magnusson more than 18 years ago saying there was no change in the stiffness of the joint with long term stretching. The implications were huge for postural correction folks. It means you can't "loosen" up a muscle and effect some change in static posture because passive tension is unchanged. Below is a simple graph that looks at how stiffness can change about a joint. We create a stress-strain curve by moving a limb through its range of motion passively while measuring the force it takes to move the limb and the range of motion at the same time. This creates a stress-strain curve. The slope of the curve is its stiffness. Some researchers also call this the Passive Resistance to Torque. We can't say what structures contribute to the resistance to torque because we are just measuring joint ROM. It could be capsular, muscle, tendon etc. Researchers try to control for muscle activity by ensuring that EMG is silent.
In the curve below you can see that if a joint is less stiff the curve shifts to the right and if it is stiffer the curve shifts to the left (a left shift consistently happens with strength training).
The assumption for a long time was that long term static stretching shifted the curve to the right. Because I was looking for ways to challenge the utility of stretching I found Magnusson's research showing that this just wasn't true. What Magnusson showed as early as 1996/7 was that long term static stretching did not change muscle stiffness and therefore we could infer that there was no structural changes to the joint/muscle structures. I loved this contrarian shit.
Instead, what Magnusson described was an increase in muscle extensibility (joint ROM) that was proposed to be due to an increase in stretch tolerance. Meaning if you stretch a lot, you get used to it and you can handle the discomfort and push yourself more. Plenty of other researchers supported this finding and I was content for years to run with this idea. I certainly did not go out of my way to challenge my own biases. Below is a graph describing an increase in stretch tolerance.
I'd read whispers of papers challenging this idea but I didn't fully explore them...until this idea became so bloody POPULAR in 2010 with a review paper by Wepler and Magnusson. Now everybody knew/thought that stretching created sensation changes rather than structural changes. I'd lost my edge and had nothing special to add. :) So it was time to really check my biases. And sure enough there was plenty of research showing that long term static stretching did change the stress-strain curve. What a blow to my scientific ego. Kubo (2002) showed this forever ago. I remember reading his abstract and he wrote that stiffness did not change so I thought it was a done deal. The problem was he described stiffness as related to a tendon not the entire joint complex. What many called stiffness he referred to as Passive Resistance to Torque. And guess what? He showed that static stretching changed this - it changes joint stiffness in the long term. And so did other research. Take a look at the graph below. This is what a shift in the passive resistance to torque looks like. There are no less than 5 papers that document this shift.
Confused? You should be. Science sucks sometimes. We have well conducted research with very similar experimental protocols showing different results. One group arguing that increases in ROM are purely neurally driven and the other suggesting there are actual tissue property changes. I've switched sides of the fence personally. I am not in the sensation only camp. Both seem plausible and some work documents this change in the curve. I don't know why there are differences in the outcomes across studies.
How is this relevant to anything?
I don't think this argues that stretching can change posture. We aren't really seeing dramatic changes in the force that a muscle in its neutral position pulls on a bone to change a joint position. It might change the kinematics we see when a joint gets away from the neutral zone though. This would have relevance for the ease movement during end range positions.
In terms of performance many would say that we shouldn't stretch if we want to have a stiff and bouncy muscle. This is often the argument for running. We want a lot of passive energy return and stiffness is a good thing. Well, I'm going to address that idea in Part II of this article and the potential conclusions will be SHOCKING!
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