Final answer:
Burst muscle work is brief in humans because it relies on a quickly depleted supply of creatine phosphate for ATP and is limited by lactic acid buildup, which leads to muscle fatigue and soreness.
Step-by-step explanation:
The two major reasons that the duration of 'burst' muscle work is very brief in humans are due to the limited supply of creatine phosphate and the buildup of lactic acid.
Firstly, creatine phosphate provides a rapid but short supply of ATP, the energy currency of the cell, which is crucial for muscle contractions. This supply can fuel maximum muscle effort only for about 8-10 seconds. After creatine phosphate stores are depleted, the muscle cells switch to glycolysis, a slower process of ATP production, which relies on the breakdown of glycogen into glucose.
Secondly, glycolysis leads to the accumulation of lactic acid as a by-product, causing muscle fatigue and soreness. This makes sustained 'burst' muscle effort unsustainable for long periods, as the muscles experience pain and a reduction in functionality due to the acidic environment created by lactic acid accumulation.