Originally Posted by slowandsteady

Unless your spin classes lasted less than 60 seconds there is no way you spent the entire time high in the anaerobic zone. If your max HR is 189, then 160 bpm is just aerobic. High anaerobic would have been 180bpm+

Zone 1 65% of MHR (recovery rides) <150bpm
Zone 2 65-72% of MHR (endurance events) 150-159bpm
Zone 3 73-80% of MHR (high level aerobic activity) 160-167bpm
Zone 4 84-90% of MHR (lactate threshold(LT,AT); time trialing)172 - 177bpm
Zone 5 91-100% of MHR (sprints and anaerobic training) 178-189bpm

Remember that you need to factor in your resting HR. The HR above are for me with a max of 188 and a resting of 76.