Stretching the hamstrings: Do you really know what you are doing?

I'm not a big advocate of stretching (in particular, passive stretching).  One of the reasons I'm not a fan is because most often, people don't have clue what they are stretching.  In fact, most people aren't even stretching what they are intending to stretch.

Lets take one of the most common examples, the hamstring stretch:

Question #1: What are we trying to stretch?

Of course we are trying to stretch our hamstring muscle.  We know that the muscle belly is the most extensible part of a complex that includes muscles, tendons and joints.  In fact, a muscle belly can stretch up to 10% of its resting length without damaging the muscle tissue.  With this being the case, if our hamstring muscle belly was approximately 40cm long, we would only get 4cm stretch out of the actual muscle we are trying to stretch!  This is assuming that we are actually able to ONLY stretch the muscle belly.

Question #2: What are we actually stretching?

As mentioned in the answer to question 1, we are stretching a complex of structures including tendons and joints (just to name a few).  Not only that, but while we are trying to stretch our hamstrings, we are also stretching a number of structures in our low back as we bend forward.  We may be stretching some muscles of the low back, we would also be stretching important ligaments and joint capsules that help with low back stability.  Though this may help with overall flexibility, it is detrimental to stability of the low back forcing back and core muscles to work harder to maintain stability.  As a side note, we know that people with greater low back flexibility are more prone to pain as they age.

Question #3:  What is the limiting factor?

As mentioned, we are stretching a number of structures that have varied extensible properties.  Muscles are clearly the most extensible of the bunch with joint capsules, ligaments, and tendons being less extensible.  One structure that has gone unmentioned (and frankly is often forgotten) is nerves.  Nerve tissue is very tough and not very extensible.  So when we stretch our hamstrings, as we pull our toes towards us, we end up stretching nerves (the sciatic nerve and associated branches).  In most cases, this is in fact the limiting factor during traditional stretching of hamstrings.  An analogy would be like taking an elastic (hamstring) and a rope (nerve) and stretching them as far as they can go.  Your stretch would be limited by the rope and not the elastic.  

Question #4:  Why are we stretching the hamstrings?

In most instances, I understand people are trying to stretch hamstrings to improve flexibility.  Though you may feel that you have improved your flexibility during (passive) stretching, as soon as you stand or alter your position, you nerves tell your muscles to return to their regular length.  To gain any long term flexibility, you would have to stretch numerous times a day.  Of course, my question to you would be why do you want to improve your hamstring flexibility?  Have a look at this for a better understanding of Stability, Mobility and Flexibility

I will follow up with how you can safely stretch your hamstrings should you need to improve your flexibility.