Grouse Grind

During the grouse grind i went past my zone 4. I was breathing hard, my body temperature was rising. As I past the half way mark my legs started to feel fatigue and it became much harder to go up. After passing the last quarter my heart was at it highest and my muscles were starting to give up. Also after half i went from a,light jog to walking up the hill. I believe the energy system used during the grind was Aerobic because it was high endurance and my heart rate was increasing. My fat was used as fuel to give my body energy, so I could keep going and than was burnt off as sweat

 and cellular respiration.