I've been practicing in pretty flimsy footwear since I started learning to play.
At some point during my practice session, my back starts to get achy. Nothing major but if I don't stop and stretch, those big muscles running up and down either side of my spine get tighter and tighter. 

Today, by chance, I was still wearing thick-souled, very supportive sports shoes when I started practicing. 

Over an hour later... lo and behold... not a trace of tight back muscles!

Have I magically improved my tension problem over night or is it possible that this is connected to my super sneakers?