Final answer:
At the anaerobic threshold, as RER increases, it signifies a switch from aerobic to anaerobic metabolism, leading to the accumulation of lactic acid and a higher demand for carbohydrates as a fuel source.
Step-by-step explanation:
When the Respiratory Exchange Ratio (RER) increases at the anaerobic threshold, it causes a shift from aerobic metabolism to anaerobic metabolism. This threshold indicates a point during intense exercise where muscles start to rapidly break down glucose without the use of oxygen – a process known as anaerobic glycolysis. As a result, the by-products of this energy pathway, primarily lactic acid, begin to accumulate in the muscles.
The increase in RER suggests that carbohydrates are becoming the predominant fuel over fats, as represented by a ratio approaching or exceeding 1.0. Higher RER values indicate that less oxygen is being utilized relative to carbon dioxide produced due to the predominance of anaerobic glycolysis. Intense muscle activity leading to oxygen debt ______ which is the amount of oxygen required to restore ATP levels, convert lactic acid to pyruvic acid, and replenish the body’s oxygen reserves after exercise.