I don't use the side reins as a 'gadget' to build muscle. The muscle building on a horse in light work is only a positive side effect of their use. For me, as stated in my original post, is to give the horse a steady contact onto which they can come onto, using their back to reach into the rein.
So for those who boo side reins or any other lunging 'gadgets'... do you care to explain the way that you lunge which apparently does a better job of getting your horse to reach for the rein, swing their back, stay straight on their line and remain in self carriage without dropping the shoulders or swinging the hind quarters?
Because without an outside rein at least, where does the energy from the hind end go???? Straight out the shoulders, that's where. How is this benificial????