Your muscle cells fuel source is ATP (adenosine triphosphate). Food (calories) is broken down by the body to create ATP which is further broken down to be used by the cells. ATP is generated and replenished by different systems in the body, depending on the stressor and the rate at which the ATP is needed.

Phosphocreatine System (Alactic Anaerobic)

Lower rep lifts use what’s called the phosphocreatine system to generate fuel, which is extremely quick & powerful, but only for very short periods (10-20 seconds) while requiring a longer period of time to fully replenish (5 minutes). [Fast glycolytic muscles].

Anaerobic System (Lactic)

Around the 20 second mark is when this system kicks in and begins to generate ATP.  It does this without oxygen, hence the name “an”- or without, “aerobic” – oxygen, which allows it to generate ATP at a faster rate than the aerobic system, but for shorter time periods.  This system is also called the glycolytic system because it uses stored muscle glycogen and blood glucose (broken down carbohydrates) to produce ATP.  While this system will get you around the 2 minute mark or 8-12 reps for performance, it also creates a lot of lactic acid that builds up and causes muscle fatigue.  This system takes 1-2 minutes to fully replenish. [Fast Oxidative Muscles]

Aerobic System

Also known as the oxidative system because it requires oxygen to function, it starts working after that 1-2 minute mark and on into any extended time period.  This system is unique in that it uses both glycogen and fats to fuel production of ATP.  This is where some common misconceptions come into play about the “fat burning zones” of cardio.  While lower intensity steady state cardio 5/10 does in fact utilize more fat for fuel, you’re technically burning LESS CALORIES overall, which is what ultimately matters.    This is also the reason your body NEEDS CARBOHYDRATES, despite your compositional goals.  This system is rarely utilized while doing the actual weight lifting, unless you are doing some sort of metabolic conditioning or extended AMRAP work, but it is doing the recovery work in between sets as your other systems replenish, so there is some very strong argument for the fact that the better aerobic capacity you have, the faster you can recover and the stronger your lifts will be.  Regardless, it should absolutely be incorporated in all training plans.[Slow Oxidative Muscles]