I use (or used) my very basic Polar HRM in spinning class. It helped me stay in my target zone. Interestingly, I found that I was able to slow down because I was often over my target zone.

Unfortunately, I inadvertently left the wrist part on the spin bike at the gym a few weeks back, and so I only have the strap. Anyone know if I can just get another wrist part, or do I have to buy them together?