If you do any sport, your body builds up the neccessary muscles over time. Horse riding is no different.
In the case of horse riders, these are mainly the calve, thigh, upper arm ones and in the back.
If you have ever ridden before then without doubt you will remember the painful "John Wayne" walk the next day.
What actually happens is that in corner between the crotch and the inside of your upper leg you use specific muscles and tendoms which will grow exponentially and are stretching if you ride regularly.
Your crotch / leg area changes essentially as follows:
From this
|#########|
|#########|
|##+---+##|
|##| . . . . .|##|
|##| . . . . .|##|
|##| . . . . .|##|
to this
. .|#########|
. /#########\
/####/--\ ####\
|###/ . . . . \ ###|
|###| . . . . . |###|
|###| . . . . . |###|
If you have no thigh bands "off horse" and your front shield
is not to wide you are fine.
You just will keep you legs father apart for comfort. And while sitting in the saddle you are spreading your legs anyway.
If you have thigh bands, then you will eventually have to replace them as they become too small.
Also you are going to need a longer chain between the legs as otherwise the growing "corner" muscles will rub against the front shield.
And this is why it is important to have a front shield which is as narrow as possible.