Your first week of training for the marathon can feel like someone poured hot acid all over your legs. As the weeks progress, however, your running time and distance improve and your thighs don't burn as much. This is because your body is adapting by a process called anaerobic threshold, which is a point where lactate, or lactic acid, begins to accumulate in the bloodstream. For marathoners, anaerobic threshold determines whether they can complete the race or not.
Anaerobic Vs. Aerobic
Your body uses two modes of energy production that influence anaerobic threshold, which is like changing gears when you drive a stick-shift. Aerobic respiration relies on oxygen to produce energy, while anaerobic respiration does not. When the demand for oxygen exceeds the rate in which oxygen is delivered to your muscles during exercise, your body gradually switches to anaerobic respiration, which increases the rate of lactate production. The anaerobic threshold is the tipping point when the rate of lactate production exceeds the rate of lactate removal.
Why It Matters
Even though marathon running is mostly aerobic in nature, marathoners still need some anaerobic training to develop a higher anaerobic threshold. This is necessary to resist lactate buildup in your blood, which causes your muscles to cramp and stop contracting when too much lactate is present. As your body adapts, the rate of lactate removal becomes higher than the rate of lactate buildup. Training at or a little above the anaerobic threshold improves both anaerobic threshold and aerobic capacity -- your heart's and lungs' ability to pump blood and exchange gases between blood.
Improved Oxygen Intake
Experienced marathoners don't get out of breath as easily as beginners because their high aerobic capacity can deliver an adequate supply of oxygen to their muscles during very high-intensity exercise. As your anaerobic threshold increases, so does your VO2 max, which is the rate of how much oxygen your body is consuming while performing high-intensity exercise. With a large supply of oxygen in your blood, lactate accumulation decreases, allowing you to continue to run without getting the burning sensation in your legs. Elite runners attain anaerobic threshold between 80 to 90 percent of VO2 max without "feeling the burn" in their legs, while untrained runners peaks at about 55 percent of VO2 max.
A Balancing Act
Just like a golfer who needs to constantly practice her swings, marathoner runners need to balance their training ratio between anaerobic and aerobic exercise. Although the ratio varies among individuals, the rule of thumb is an 84 to 16 ratio, where 84 percent of your training should be aerobic while the remainder should be anaerobic, says exercise and running coach Roy Benson. For example, if you choose to train for a total of 65 miles a week, 9.1 of those miles should be training for anaerobic threshold.
Nick Ng has been writing fitness articles since 2003, focusing on injury prevention and exercise strategies. He has covered health for "MiaBella" magazine. Ng received his Bachelor of Arts in communications from San Diego State University in 2001 and has been a certified fitness coach with the National Academy of Sports Medicine since 2002.