The aerobic energy systems produces energy through the aerobic process. It is the primary (but not the only) energy source for any activity lasting more than 2 minutes. Intensity can be very high or very low. At very low intensity, there is no recovery time. Example: a 30 minute run. At very high intensity, there is a 1:1 or less work to rest ratio. Example: 3 minute run followed by a 3 minute rest x 5.