Final answer:
Lactic acid fermentation enables muscle cells to continue contracting during physical activity when there is not enough oxygen available. Pyruvate is converted into lactic acid, regenerating NAD+ and allowing glycolysis to produce ATP.
Step-by-step explanation:
Lactic acid fermentation enables our muscle cells to continue to contract during physical activity by providing an alternative pathway for energy production when there is not enough oxygen available.
In this process, pyruvate produced during glycolysis is converted into lactic acid, regenerating NAD+ so that glycolysis can continue to produce ATP.
This allows the muscles to continue contracting even in the absence of sufficient oxygen.