The goal of endurance athletes is to improve endurance capacity. Most muscle adaptations that enhance endurance take place in the mitochondria. Mitochondria, the muscle energy producing factories represent almost 7% of your muscles total volume. Mitochondria are responsible for the metabolism of oxygen and nutrients into CO2 and ATP, the energy currency that drives muscle contraction.
Over 40 years ago researchers observed that an increase in endurance capacity was associated with an increase in the volume of mitochondria. Endurance adaptations involve two key elements, an increase in the size of the mitochondria and an improvement in the efficiency in which the mitochondria convert nutrients and oxygen into energy. The response of the mitochondria to endurance training is significant. Researchers have noted a 40% increase in mitochondrial protein following endurance training. The type of mitochondrial adaptation is a function of your training regimen. With more common endurance training such as 60 minutes at 65% VO2max there is an improved capacity of the mitochondria to utilize fat as the energy source, thereby sparing muscle glycogen. At more intense exercise training such as repeated efforts of high intensity cycling there is an improvement in the respiratory capacity.
One of the key questions often asked by endurance athletes is how long do the mitochondrial adaptations for improved endurance last. A recent study provides insight. Researchers found that mitochondrial adaptations to high volume training are rapidly reversed. In their study the investigators induced a 40-50% increase in mitochondrial volume after three weeks of high intensity interval training. However after three weeks of reduced volume training mitochondrial adaptations returned to baseline levels. The bottom line- you have to maintain training levels if you want sustained improvements in endurance capacity.