Muscle length is often discussed as the cause of stiffness and immobility. It is spoken about as if it is a given.
It seems perfectly logical:
"Tight/short" hamstrings = I can't touch my toes.
However, this assumption is flawed. An increasing body of research is indicating that the range of motion at our joints (as with many other things) is likely multifactorial. We need to consider regulators of mobility or muscle extensibility outside the length of our tissue. One such new theory addresses factors that have to do with our nervous system.
There is some evidence for the length of tissue increasing after a stretch due to the viscoelastic properties of the tissue. In plain English this means our tissues can bend, stretch, deform and bounce back to their original shape. However, these effects are minimal and transient (Weppler and Magnusson).
Often, what I am finding in practice, is that "tightness" is a manifestation of muscle weakness.
Is this the case for everyone? Of course not.
Some people get more mobile from regular old stretching.
But even in those cases, is it the muscle length that is changing? I honestly don't know. Maybe if it happens slowly over time. That at least seems plausible and is in accordance with tissue healing/remodeling time lines (and I think may of us know this intuitively. If we stop resistance exercise, our muscles atrophy (shrink). This is also true for bone density and other connective tissues. The same idea can also be applied in the reverse direction, with training, tissue density, size, and strength increases).
Definitely not in cases where there are rapid changes in joint motion, such as in a matter of days or even seconds. One such example is a PNF (proprioceptive neuromuscular facilitation) style stretch, where a briefly held resistance is used to facilitate greater range of motion:
The only thing that can adapt this quickly is the nervous system. It is not possible to create a structural change to tissue with a single input (I believe it was Dr. Andreo Spina who I first heard make that point. Apologies if it was someone else and I am misremembering).
Again, one singular input in not capable of changing your soft tissue struture....UNLESS -
1) you have a scalpel, or
2) you use a lot of force. I mean A LOT. Like enough to cause an injury, where there is a loss of tissue integrity.
This example teaches us that there are relevant factors to our active mobility, outside of tissue length.
Are you someone with chronically "tight" hamstrings that just can't seem to get anywhere, no matter how much you stretch them?
Then you're long overdue for changing things up.
One road probably worth going down is to strengthen your hamstrings.
There are a lot of checks and balances in our nervous system, and we haven't even begun to scratch the surface in understanding them. But again, a clinical observation of mine is that in cases of chronically "tight" hamstrings, many times upon assessment the muscles are weak against manual resistance. It is as if the body is protecting itself by restricting motion due to weakness of that area - as greater ranges of motion tend to involve greater magnitudes of force within the tissues.
With great ranges of mobility, comes great responsibility.
These principles can be applied anywhere in the body, not just the hamstrings. If you're constantly stretching a "tight" area of the body and getting no where, try to increase the capacity of that area by strengthening and see how it affects your mobility.
And let me know how it goes!
Thanks for reading,
Dr. Piotr