The 4th Hamstring
A piece of anatomy that I think escapes from our brain pretty quickly after school is the '4th hamstring' / pseudo adductor that we get from the fascial tension sharing between the short head of the biceps femoris, through the linea aspera on the posterior femur to some of the adductors.
Because of the strong connections between these structures and their respective lines of tension, we can end up seeing symptoms that we normally would not necessarily associate across these otherwise, very separate tension tracks. Especially with the prevalence of hypermobility, I see a lot of these issues originating from fibular head instability, leading to a range of pelvic and hip problems. That said, even for those who do not have genetic or injurious hypermobility, neurologically derived aversion to tension of the nerves wrapping around the fibular head is often enough to change tone and hold a state of hypermobility, and then problems in these lines.
Dysfunction Presentation and Assessment
Disruption of this line can lead to issues in multiple directions, with a slightly misaligned pubic symphysis affecting bladder/uterus issues, the actions of the hamstring and anything else which attaches to the fibula, or any of our hip/pelvis stability and function that the adductors attach to. However, I most frequently see this interchange coming from and leading to one specific dysfunction combination: Fibular head irritation or instability inhibiting normal adductor function, leading to reduced abduction range as well, and commonly an irritated obturator nerve that just doesn't want to go away despite dealing with the usual 'upstream' issues we find or hunt for, in addition to lateral hamstring inhibition. This will also typically affect pubic symphysis alignment and bladder/uterus issues.
I've had this affect peoples feet, jaw issues, and a lot of other things, so don't be limited by this one common pattern; think of the anatomical/tension lines if you're finding yourself scratching your head, and see if this is contributing.
To Assess
Manually muscle test lateral hamstring function, and the ipsilateral adductors. You can get more specific about which adductors and different fibers of the hamstring, though I find when this is positive, testing from neutral position shows me all I need to see, with inhibition usually being present without any need to stress the fibular head. If it's a more minor issue, you may need to manipulate the fibular head and see its effects on function.
Now check the fibular heads movement in sagittal translation to see which direction is 'easy' to move the head into. Hold into that direction and retest the hamstring and adductors. If this improves function, (or makes it significantly worse), this is a confirmed functional relationship.
If you're uncertain, move the fibular head towards the direction of restriction, and retest. Unless something is weird going on (and there are some options for that which take more explanation than I'll put here today), that should make the tests worse (though if so bad already, it may be hard to see any additional worsening of the response).
I do also find it helpful to manually check the sagittal mobility of the opposite fibular head for context. I find many people to have one that's hypermobile, though especially in an H-EDS population or other conditions of hypermobility, that may not give you much useful context.
Treating
If you have infrared tools available, I do really like using that on the overall fibular head first, with timing set to get the ligamentous connections between the fibula and the tibia, which helps calm down irritation and support any local necessary tissue recovery.
I like to work the nerves which travel around the fibular head first for restrictions and irritations, as they will often be the biggest local perpetuator of dysfunctional tension/stability for the fibular head.
I then do visceral style (such as Barral) induction based treatment to the LCL, followed by sagittal translations (first into ease, then into tension, and lastly with a light lateral distraction), each with parasympathetic 'stims' (which I've spoken about before but I know is not very descript, if you're not already familiar).
I will next do the same with a lateral coronal traction.
This is usually this simple, and I will retest, getting a few activations of the inhibited muscles to help integrate the change. I find the fibular stability is usually notably improved immediately, and will continue to improve to some degree over the next few days.
After this, I will turn to other things which were fighting treatment and approach those, such as the pubic symphysis and bladder/uterus, the obturator nerve, or other affected relevant targets and 'clean up' after the contribution this had.
In severe cases of ligamentous instability, such as if there has been a fairly severe injury to the ligaments here which I cannot remedy adequately by improving the functional stability of the joint through a restored level of local neuromuscular function (allowing ligaments to tighten up like a rubber band over-stretched long term finally being left to elastically recoil), I may refer out to prolotherapy or other similar interventions. An unstable fibular head can cause substantial issues and we don't want someone to have to suffer through that.