Swimming is an acceptable alternative to cycling in the winter. It allows me to work on muscle groups that don’t see as much use on the bike, and it allows me to retain some base level endurance fitness. Going back and forth in a lap pool isn’t amazing, but when faced with -20F windchill it’s just about the only alternative. I do not do well on treadmills or stationary bikes, because they feel like such poor approximations of the actual thing. I am extremely thankful Chicago has high quality indoor pools through the parks district and very cheap lap swim passes.