I have to say, I'm kind of surprised at the negative reaction saying smiling while exercising has gotten in this subthread. It really is something a lot of folks do. It's even something that top-tier athletes do on purpose: https://www.cnn.com/2018/02/12/health/smile-running-energy-p...

Even if it isn't something one does without being prompted, frame of mind is such an important part of performance. Anything that can lower perceived effort is worth taking seriously.