Anaerobic threshold (AT) is a frequently used term that sometimes causes a little confusion. What is the AT and how can you use it to run faster? For starters, the AT is an extremely reliable and powerful predictor of performance in aerobic exercise. To explore further, I will begin with a brief, oversimplified, review of physiology. Muscles can "burn" glucose two ways, aerobically ("with oxygen") and anaerobically ("without oxygen"). Both systems generate a temporary energy store, called ATP, which in turn produces mechanical work. However, there are some major differences.