How does over stretching hamstring injury work?

Question

I have recently started doing Yoga, and I think I over stretched my hamstring. (I wanted quick result so I pushed pretty hard at a lot of poses, e.g. forward fold, wide leg forward fold etc) initially it started as a uncomfortable sensation at where the hamstring attaches to the sit bone, I thought it would go away, but it gradually got worse, I searched online and apparently there is this thing called “Yoga butt” or “proximal hamstring tendinopathy”.

I tried to do less intense stretches since then but seems like the discomfort is not going away. My questions are:

1. From my understanding, the flexibility is really controlled by our nervous system instead of the muscle, e.g. a person in a coma can exhibit remarkable flexibility while not as much when awake. And the injury is not a muscle torn like people normally imagined, it’s more like a spring not recoil back properly. But since it’s the nervous system restricting my range of motion, why would me overstretching hamstring injure the muscle? Shouldn’t it only injure the nerve? If so, what does it even mean for a nerve being injured?
2. I heard it takes long time to heal, why is it so? Is it because the injury occurs at the tendon where not a lot of blood flows? Is there any way to actually aid the healing process?

EDIT

maybe let me try to rephrase my main confusion. People that are not conscious (coma or full anesthesia) can move their body with much greater flexibility, because there is no nervous system at play. So I imagine they could be put into a very deep forward fold or leg behind their head, and my assumption is they don’t get tendon injured from it, unlike if they were to over stretch consciously (like what I did). So why is it that they don’t get hamstring “torn” when unconscious but muscle gets injured when there is nerve active?

Answer 1

In stretching, there are two reflexes at work, which are actually the fastest reflexes there are because they are relayed over the fastest (thickest) nerve fibres in our body (Type I a/b sensory and delta-motor fibres, with 50-100 m/s):

1. The muscle spindle reflex: Triggered by proprioceptors embedded drectly into the muscle, they react to force within the muscle. When a muscle is stretched (relatively fast), it reflexively contracts in order to protect itself from overstretching/taring apart. This part is the differential proprioception, ie. it measures a velocity. There is also a slower type (going over sensory Type II and gamma-motor neurons, 20-50 m/s) that measures the length (via proportional proprioceptors) and hightens the tone according to the length, ie. the longer the muscle becomes, the stronger it will tense up against further lengthening. This is what you mainly work against when stretching.
2. The Golgi-Tendon-Apparatus: This reflex works antagonistically and depends on proprioceptors directly embedded into the tendon and is supposed to prevent tendon injuries. When the tendon is experiencing too much tension, two things happen: Through one path, the stretched (but contracting, see above) muscle is neurally inhibited, ie. the tone is lowered, allowing for further lengthening. At the same time, the antagonistic muscle is excited, ie. you start to activate the counter-movement in order to take tension from the muscle + tendon.

Now, why tendinopathy and why near the butt, exactly? There is an intricate interplay between the two (or four, depending on the perspective) reflex mechanisms above, constantly regulating your muscle tone. What you do in yoga is stretching the ischios (back of the thighs) almost exclusively from the proximal lever, ie. you bend the hip with straight legs. This means that you got an extremely long lever (the legs) and extremely strong muscles (hip flexors), often supported by gravity, pulling mainly from the proximal tendon, ie. from your butt bones, with the muscle spindle reflex(es) constantly working against the stretch by pulling the muscle short. It is only natural that the tendon, with one side (bone) being pulled away by strong muscles and the other side (muscle) pulling away as well, is the first place where tension-loosening-further stretch will take a toll. The Golgi Tendon Reflex will try lower the load, basically by sending inhibitory potentials into the motor neurons but this cannot fully compensate conscious effort to stretch further, which excites those neurons.

Basically, the take-away message should be never to stretch into actual pain. It triggers the contraction reflex and makes you ultimately work against your sinews, not your muscle. There should be a sensation and it does not have to be pleasant, yes, but not actual pain.

Also, it is often advised to stretch the ischios by active extension of the knee (ie. both hips and knee are extended from an appropriate pre-tension with bent knee), exactly because it has a lower toll on the sinews due to antagonistic inhibition as a moderator and the tension being dispersed more evenly over all sinews.

As of what to do against the injury: moderate, isometric loading in neutral joint positions is generally the gold standard.

Regarding the comatose/full anaesthesia part

What you describe is used in a surgical procedure called arthrolysis. Since the muscle fibres do not get any signal and they need a depolarisation of their membranes in order to contract, the muscles only give their structural elasticity (titine within, and mainly connective tissue outside of the muscle fibres proper) as resistance against elongation. No base tone nor any reflexes work here. You get the wrong idea, though:

1. The procedure produces immense post-surgical pain. Any existing adherences are literally torn apart, producing structural damage and inflammation around them.
2. It often leads to bad outcomes since the muscles still "lock" in their old ROM when awake.

What stretching routines do is desensitiving the muscle spindle reflex(es) (first and foremost) and only in extreme, pathological conditions they also rip actual significant adherences apart (either between connective tissue or surplus titine in the fibres). Your joints as such have a great ROM, it's just that your muscles are no taught that it is safe to go there. Another fact that is often forgotten is that it is not even physiologically wanted to be overly flexible since the muscles are not able to stabilise the joint in extreme positions since their strength is significantly reduced (because not nearly as many myosin-actin connections can be established due to smaller overlapping zones in the sarcomeres). Also, ligaments once slacking due to repeated, extreme joint positions lose function (stabilisation and arthrokinematics) as well.

Answer 2

I don not really know much about the possible nerve damage, but depending the tendon itself you could perform some isometric contractions activating that hamstring muscle part. Why activating/loading tissue? Since it might be that the tendon-tissue is damaged, and immobilization will enhance scar tissue and this will not be beneficial for your recovery. So, in my opinion isometric contraction will not create any harm and will at the end strengthen your tendon (and maybe make them more resistent to your future stretches).

Note that there is form of stretching based upon isometric contraction followed by stretching, called PNF-stretching.