We all know exercise is good for us. We remind ourselves of it whenever we’re tempted to sleep in instead of work out or go home after work instead of to the gym. But what’s actually happening to our bodies during exercise? Does it really give us more energy or is that all in our heads? And does carbo-loading actually do anything for my workouts?

We looked behind the scenes at what’s really going on with our muscles when we exercise to address some of these questions and more.

You really can’t control your heart

We have three kinds of muscles: skeletal, cardiac, and smooth. Our brains control our heart muscles and smooth muscles, but only subconsciously.

When we talk about exercising, we’re referring to our skeletal muscles, the ones we can control. And those muscles use different energy sources depending on how long and how intense your workouts are.

But monitoring your heart rate can help

A higher heart rate means more blood is pumping through our bodies. The whole reason our heart rate increases during workouts in the first place is to get more oxygen to our mitochondria (and remove CO2 waste). Monitoring heart rate during exercise can help track our exercise intensity, and whether we’re getting the most out of our workouts.

Plan time to digest

Trying to digest food while working out is hard. Plus, working out actually moves more blood to your muscles and less to your digestive organs. If you’re trying to use certain nutrients to give your workouts a boost, plan ahead by eating two hours before exercise.

Exercise and energy go hand in hand

Every exercise draws and uses energy in different ways. Aerobic workouts such as walking briskly, running, jogging, dancing, swimming, biking, or playing tennis, basketball, soccer, or racquetball, require more energy over a longer time. Your body uses oxygen to “burn” fats and sugars stored in muscle, liver, and fat tissue. It’s a process that takes longer to generate energy, but ultimately provides a long-lasting source of power.

Anaerobic workouts, such as weight lifting or sprinting, require short, intense bursts of energy.  This energy comes straight from the sugars and creatine phosphate stored in muscles themselves.

If you want to give your body the energy it needs, it helps to know where that energy is coming from in the first place.

Weight training doesn’t burn fat

Fats aren’t metabolized during short burst, high-intensity workouts (anaerobic). That said, how much skeletal muscle mass we have is a big factor in how much “resting” energy we use. Which is why a combination of strength training and aerobic exercise is recommended, especially when trying to lose weight.

Aerobic exercises give you a better ROI

With aerobic exercises, you can draw energy from both fats and carbohydrates. And for every sugar molecule you metabolize, you can produce more cellular energy compared to anaerobic exercise.

Carbo-loading is real…sort of

Carbo-loading doesn’t help with high-intensity, short-term anaerobic workouts. That’s because carbo-loading is a process of saving up sugars to use later. By nature, anaerobic workouts are short-term, and you simply never get to tap into those reserves.

But regardless of how you feel about this trend, aerobic activities still rely on carbohydrates as fuel. So, carbo-loading before long endurance activities is helpful.

This molecule can help

Transforming this energy from food into something we can use only works if we have certain vital resources, like NAD (nicotinamide adenine dinucleotide). Without NAD, our bodies wouldn’t be able to make ATP, and ATP is cellular energy. When it comes to generating energy, NAD is just as important as the foods we eat.

But our supply of NAD can also change depending on our needs and lifestyle habits. So, in addition to monitoring our heart rate, and knowing where our muscles draw energy from during different kinds of workouts, it’s also worth figuring out a way to increase NAD. Whether you’re an occasional yogi or an every day runner, these elements can work together to help us maximize every kind of exercise.

How fast you run doesn’t depend on your muscles

Okay it does, but not as much as you’d think. No matter how much lifting or training we do, the main energy source our muscles need to move is a molecule called ATP. How quickly we can regenerate ATP determines how much energy we can use and how long we have before it runs out. This ultimately limits how long we can run at a particular pace.

You never “make” energy

Energy can’t be created or destroyed (first law of thermodynamics). There’s a constant amount of energy in the universe. All we’re doing when we work out is transforming one kind of energy (food) into another kind (cellular) which then powers everything we do, including moving our muscles.