Picture this:
You've just completed a session with your PT that involved:
4 rounds for time of:
20 weighted squats, 15 pushups, 15 cable rows, 1min plank and a 500m row.
That's a hard session!
Your heart rate, breathing rate, oxygen consumption and metabolic rate (the rate at which your body burns energy) would increase significantly during a session like that. In fact: these markers would all increase during any physical activity you partake in, with the degree of increase being related to the intensity of the session.
A low intensity session results in a small increase in your breathing rate, oxygen consumption and metabolic rate. A moderate session results in a moderate increase in your breathing rate, oxygen consumption and metabolic rate. A high intensity session results in a large increase in your breathing rate, oxygen consumption and metabolic rate. Make sense?
Ie; the more intense the session, the larger the increase in your metabolism.
Question: Do you think that all of these markers would immediately return back to normal, resting levels as soon as you put down the weight after your final rep?
No.
It takes time for your body and all of it's processes to return back to a resting state after your session is complete. We refer this time as EPOC: (Excess Post-Exercise Oxygen Consumption) a time that your body is consuming oxygen and burning energy at a higher rate even when your session is over.
See the diagram below comparing EPOC after a low intensity session compared to a high intensity session:
Pay attention to the Orange section: after the 'heavy' exercise session (diagram b) our oxygen consumption is greater, meaning that we burn a higher amount of total energy and often for longer period of time than what we do after a 'light' or 'moderate' session (diagram a).
The more intense the session, the longer your body remains 'in EPOC' and the longer your body is burning energy faster than it normally does at rest.
'So why is EPOC valuable if I'm trying to lose body fat?'
If your metabolism has increased, as it does during a period of EPOC, your body is looking for more and more energy to burn. So, as long as you aren't filling your body up with high energy density foods, then your body is going to use it's existing energy stores (read; body fat) to fuel that metabolism increase. This means you'll burn more body fat. That's a win for fat loss!
To increase your fat-burning potential; challenge yourself to increase your EPOC and get yourself burning more energy for hours after your session is over.