Primary Energy System used for Strength Training

We have 3 energy stores ATP (consumed carbs turned into energy), stored glycogen (stored carbs used when ATP runs out) and our aerobic system, when oxygen is available and we are mostly burning fat.

You will burn more calories in a cardio session than one strength training session, but the post training effects of strength training burns more fat in the long run and speeds up your metabolism. Weight lifting uses primarily the anaerobic  energy system, which relies primarily on ATP and then glycogen for energy (when ATP has run out). When we use glycogen stores, we are using stored energy in the muscles and liver.  The tear on muscles from strength training causes damage to the muscle (microscopic tears).

The body then continues to work by repairing the muscles for several hours after your workout, sometimes even up to two days, which is essentially your metabolism working  faster to repair and build bigger muscles. A larger muscle mass means you can store more glycogen, which will inturn provide more energy for your workouts, and the cycle continues.

Endurance workouts eg long distance running will use primarily the aerobic system where the  body uses fat as a main energy source and spare the use of glycogen and blood glucose, so it is available for use if exercise intensity increases and oxygen availability is decreased (like a heavy strength workout). So, while running is using some stored fat for energy, it does not build the muscle, making it less effective for fat loss in the long run.

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