The correct answer is:
As the cells run out of oxygen they switch to anaerobic respiration, which allows the cell to make small amounts of ATP in the absence of oxygen.
During intense physical activity, such as weightlifting, if the muscle cells do not have enough oxygen to support aerobic respiration, they switch to anaerobic respiration. This process allows for the production of ATP without oxygen, although it is much less efficient and leads to the production of lactic acid, which can contribute to muscle fatigue.