Tons of stuff improves! When I first moved to Germany, I practised six hard hours each day except the Sabbath (Sunday in Calvin/Luther southern Germany). That day I took off allowed my chops and head to recover, and muscles recovering is an improvement in itself! This applies to any work we do, even low impact stuff. A day spent lying in bed will cost us about five percent of our strength, but if we worked all our muscle groups well the day before we'll end up stronger.