Our bodies switch between THREE energy systems depending on the type of exercise, intensity and duration.
The AEROBIC energy system is the most prominent system used during cardio (such as running and cycling).
Adequate oxygen in the working muscles needs to be present for this system to fully kick into gear and functional optimally...
HOWEVER there is a 'lag' time between when we start exercise and when adequate amounts of oxygen reach the working muscles (approximately 3 - 5 minutes)
This 'lag' time may manifest as fatigue and pain in the muscles which then resolves after 3 - 5 minutes leaving you feeling re-energized and better able to smash your work out!