Hey Ren - sounds like you might be putting pressure on the sciatic nerve in the low back. How's your posture when you're sitting and playing? Are you leaning over the guitar? The reason I ask is generally when you get the heaviness in your legs or tingling, it's not the legs. It's your posture and the pressure you're exerting on your back. You know the quickest way to lessen back pain? Ab work. Get your stomach muscles tight and that will help take pressure from the spine. Watch your posture and stretch. It's kinda like driving. They say you should pull over at least once an hour and get out and walk a little. Same thing with sitting and playing. Get up, walk around a little and stretch your legs. It'll help. Trust me. :)