The back line muscle train in a riders body are where a lot of power pack muscles are housed, with the calves, hamstrings and glutes all being connected ted within this train, this is an important line of muscles to understand and to work with as they can cause a lot of positional issues when they are the brains biggest "go to" muscles.
These muscles are responsible for basic movement, walking, running and climbing stairs so it is easy to see why the neuro-pathways to these muscles would be quick and easily accessible for the brain. Just saying to someone to release these muscles, or lengthen them isn't normally going to cut it, which is why you may have struggled for years with he issue of your heals raising or loosing stirrups. You may have been able to sustain a "correct" leg position when you really really focused but as soon as your attention is focused elsewhere, steering lets say you leg heels lift and your stirrups slide, or your calf grips and your leg gets shorter, or your horse slows and your leg aid becomes less affective.
In order to really unpick what's happening with this we need to understand how the brain and body work together: Muscles that are used all the time have very clear neuro pathways that signals can pass down quickly and easily, like traveling down a clear motorway, where as muscles that aren't used very often tend to have dull, overgrown weedy and bumpy country lanes type neuro pathways, that take a while for signals to pass down and so are less easily activated when the brain needs something in that instance and doesn't want to put too much though effort in it happening.
Don't worry though, this can easily be changed, the euro-pathways can become clearer with more use, and that weedy bumpy windy country lane can gradually clear and straighten until it becomes as easy to travel down as the other neropathways, but in order for this to happen these muscles have to be used, have to have signals heading for them regularly and this means we MUST move differently, we have to ask different thongs of our body regularly rather than just the same.
Just trying to fix this on the horse is not enough, think how many steps you walk a day compared to how many steps you ride, unless you are a professional your steps riding are wildly outnumbered by your walking steps, and if you run for a hobby or fitness or cycle you can add these into the side of motor way signals.
In steps conscious movement like Equistretch, Yoga, Pilates, Dance and anything that gets your body moving in weird and funky ways where you brain has to send the signals to the muscles that you want to start to use more as a rider. Equistretch or rider focused movement is obviously the A class standard of this as you are pinpointing movements that help you ride too, but any movement that means you are not just activating the same muscles is better than no movement at all.
This is why I'm an advocate of 5 minutes a couple of times a day rather than 1 hour a week, the more regularly you can send a signal down those clunky country lanes the quicker they will clear, the easier you'll access the muscles.
Also, when a muscle is used over and over again it gets stronger, and shorter! Unless you consciously lengthen it whilst you build strength, think of the body builders at a gym, they don't look long and lean, their range of movement is minimal and their muscles are bulky but short. As a rider we don't want that in any of our muscles we want long and strong muscles, think ballet dancers and gymnasts, we want as much range of movement as possible but with a a stability created by the muscles being strong.
Because the back line of your muscles get used a lot they will have a tendency to get stronger but shorten to what they are needed, muscles don't hang around being long and lean waiting for the moment you need to drop your weight into your heel on your horse, they will get to the length they need for what you are doing, and so if you wear shoes with any sort of heel, even a tiny one, you run on your toes, or upstairs on your toes, you will be unconsciously shortening this back train of muscles.
So just stretch it out and all will be fine?
I see a lot of comments under my social media posts, Squats will do it, do lots of this and you are sorted, but if you "stretch" with no awareness of how the whole of the back line works you can do yourself some damage.
If the back of you legs are short and you aren't aware of how your spine should aline, what your deep core feels like, or how you don't want to round your back or flatten it against the floor, all that you will do is stretch and stretch and stretch your back and not your legs or glutes, at a minimum this will just give you back pain, at worse this will give you back issues, slipped discs etc. You also won't hit the issue you are struggling with as a rider.
So what to do?
Conscious movement that purposefully builds long and strong muscles that gives you correct biomechanics movement a couple of times a day for 5-10minutes,
Chuck out your shoes with heals on them, where good supportive flat shoes, not ones that make you grip or crunch up your foot, you want to be able to read your foot broadly as you walk, and ride!
This class, and then pick some exercises from it to do each day
Change how you sit at work, move more
If you do other exercises that use the back line even more, make sure you balance this out with good quality stretches too
Rinse and repeat
But your first task is this week do this class , and see where you are struggling, just knowing that will motivate you to do the rest.