Our body uses food (chemical energy) to create movement (mechanical energy)
Unlike a car that burns the same fuel source (gas); our muscles require different energy sources depending on the intensity and duration.
Endurance athletes need to care about three energy sources: fat, lactate, and carbohydrates.
Fat is the largest energy system in our body. For example, a 150lb male who is 10% body fat has over 50,000 calories of energy stored in their body.
Fat can be thought of as the ‘easy intensity fuel’ and is the primary energy source for endurance events (Ironman, marathon, Tour de France).
Carbohydrates are reserved for ‘hard intensity’. Most people only have 2,000-2,500 calories (or about 30 minutes) of this energy stored in their body.
Sandwiched between fat and carbohydrates is lactate. Lactate is an energy source but is inefficiently used by the body. Training at this intensity is typically known as ‘junk miles’ because it’s too hard to allow for optimal fat usage and too easy for the body to effectively use carbohydrates.
Most athletes that spend a large majority of time training their lactate energy system experience a constant circle: they never get faster and are often injured. Training above and below the lactate energy system is critical for performance increases!