Side reins aren't made for forcing into an unatural outline, if done up too tightly then yes, it will but if done up correctly with a knowledgeable person lunging the horse can take the side reins forward and allows them to accept the contact, working through their back correctly.
I was a member from a while ago, but forgot my email and password so I created a new one :p
This is Cosmo, when we had a snow day on the 1st, we only had a few cm at the time, and I only rode for about 15 mins, just to keep him ticking over, and he was behaving himself.
Anyways, there is a...