Am I right in thinking it is just in one quad/leg.
If so, working out why that is might be the best thing - both legs have run the same miles, so why is one suffering sooo much more?
Can't be the running - the other leg is fine - that leg can cope with the running mileage.
There is an imbalance somewhere.
There is something is your life - probably completely unrelated to running that is stressing one leg more than the other.
Like when Man U finally got to the root cause of Giggs's persistent left hamstring problems – the overly stiff clutch on his new Ferrari.
Most people’s jobs, or something else in their lifestyle, involve an unbalanced use of their muscles.
Usually this doesn’t matter – until that person starts over-stressing all those muscles with, for e.g., distance running.
I once treated a runner, whose one sided injuries we put down to the fact that he spent most of his working day kneeling – on one knee – and always the same knee - for years. This resulted in a (relatively) chronically shortened hip flexor and calf on the on the non-kneeling knee.
If you find there might be something like that out there – that might be the start to getting to the route of the problem.
With the "natural" method, the customer relies on, and has to trust, the salespersons spiel about what they've seen, what that means and what shoe they then need etc.. the fact that they were, for example, rolling in in the neutral shoe and the stability shoe that they were fitted with sorted it etc...
With the "video" method, they get to see themselves rolling in, and then see themselves not rolling in in the stability shoe.
The visual confirmation and the subsequent placebo effect of that should not be underestimated.
The customer will end up in the same shoe either way - but are almost certainly more likely to go away and "run happy" in the shoe they believe in more by their own visual affirmation that it does the job.