Stretching Tendons: What can we do and why we should challenge our biases. Part 2
In a previous post we looked at how long term static stretching might influence the structural properties of the connective and muscular tissue that crosses joints. The question was whether static stretching changes passive stiffness. Interestingly, we saw good research showing different results. One point of that post was to continually challenge our own biases by looking to other research to try to disprove what you think you know. This post will continue in that vein and will also blow you away with the most interesting thing about long term static stretching that never gets any press Static stretching has been vilified for more than decade. Like most things we've probably run too far in our practical applications of the research. Briefly, static stretching for more than 60 seconds will result in very tiny strength and power losses during simple activities between 1-5%. (Kay and Blazevich 2012, Simic 2013) But these losses in performance don't really translate to more complicated sport performances and other researchers have argued that this muscle inhibition found in a lab setting might not even occur during sport performance (McHugh 2010).
Recent systematic and narrative reviews suggest pre-participation static stretching does not reduce injuries but again there might be a catch. Both McHugh (2010), Witvrouw (2007) and Jamtvedt (2010) have argued that pre-participation static stretching might be injury preventive if you segregate injuries related to muscular strains and the stretch shortening cycle. This post can't go into the gory details of all of that research but the point is to be aware of these dissenting views.
OK. Now here is the research that I find amazing. And again, it strong challenges what we think static stretching does.
How Long Term Static Stretching Could Increase Performance
Again, my previous bias was to not stretch with the exception of sports that needed that range of motion. I still think there are more important things to do with your time but that is another topic. We have some research that long term static stretching actually improves performance in things like strength, power and speed (Kokkonen 2007). To me, those findings are just counter-intuitive and I had difficulty reconciling them with my views of stretching in general.
But there is one possible explanation for changes in performance after long term static stretching and that is related to Tendon Viscoelasticity and this is where shit gets amazing.
Biological materials (e.g tendon and connective tissue) exhibit viscoelastic behaviour. Meaning they have a hysteresis loop when you load and then unload them. Like they have a memory. Their resistance to stretch or stiffness is influenced by previous loading.
How researchers measures tendon stiffness is to take an ultrasound transducer and measure achilles tendon length and then the subject contracts their calf muscle. The tendon will then lengthen. To measure stiffness you just keep contracting harder and harder and the tendon will lengthen more. You then plot force (muscle contraction which is then converted to a joint torque/moment) versus tendon length. The subject then slowly applies less contraction and you keep measuring tendon length. This creates a hysteresis loop. You have a stress-strain curve going up and then coming back down...Load and then Unload. Below is a graph just showing the loading part of the curve and how that curve shifts to the left after strength training. Meaning it gets stiffer. Acute stretching creates a shift in the curve to the right and the tendon is less stiff or more compliant (see Kubo) -
I think I'll do another post on how acute stretching or different contractions influence tendon stiffness but here is another good paper in the meantime (Kay et al 2015) but we here to talk about long term stretching.
What long term stretching does is amazing and counterintuitive.
I have shown graphically below in a slide which is a complete knock-off schematic of Kubo's research.
What do you see above?
- there is no change in the loading part of the curve.
- thus stretching did not change the stiffness part of that curve
- but there is a change in the slope of the curve when unloading (kind of like a concentric contraction after an eccentric contraction).
- notice how there is a loop between the loading and unloading curves? This means the unloading curve is less stiff meaning there was some loss in energy. This is a quality of viscoelastic materials. That energy was lost as heat.
- But what happens after long term stretching to that curve? What happens to the heat loss? Yup. Less energy gets lost as heat after stretching!
- do you see the implications?
Crudely, the unloading part of the curve is STIFFER. There was less energy lost as heat! Might this tendon be more efficient? Might this small change be the mechanism for some of the research suggesting that long term stretching improves strength and performance!
What we can definitely say is that it doesn't appear harmful to performance. We can't say that if you stretch your tendons are going to get loose and you will be sloppy and less energy efficient. We don't have that evidence and that was something that I would have suggested in the past. It seems pretty wrong to me.
You know what the other implications are? FASCIA. Seriously. If you can't decrease tendon stiffness with long term stretching then I would wonder if it is really possible to change stiffer connective tissue like some fascia. Tendons seem to be like springs that store and release energy. And fascia (especially the ITB as argued by Carolyn Eng) seems to play a similar role for energy efficiency. So the idea that our hands via manual therapy change this tissue in the long term should be soundly questioned. You certainly can't foam roll it into a greater length. That's just silly.
OK. Enough. The point is to challenge our biases and that tendons are neat-o. Things are often much simpler and more complicated then they seem.