MY opinion, is to always start very easy work up to a bit more action, whatever your action might be, then do a brief yet thorough stretch, start back into your action and increase, as stated in an above post, based on your class.
For an easier class focus on form as they work at their level, learn or recover…from their current status. For more bootie kickin’ classes begin strong and start build at about 15 minutes climbing in effort level keep going to 45 minutes, for an hour class. At 45 minutes I begin to slowly decline effort, exact opposite of the build. Again it depends on the class description.
This is a basic idea. During higher efforts, if you’re doing more anaerobic, you have to, based on physiology, ease up for recovery. The decrease at the end is to let lactic acid begin to flush…then have a good stretch. People who want to go harder the entire time can, especially if it’s only one part of their workout.
Hope this helps!
Marcum Healthy 🙂