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Energy Systems and VO2 Max

Energy Systems and VO2 Max

For the triathlon, a well trained athlete will rely upon a combination of endurance and speed. Two particular energy systems the body uses for fuel play a key role in designing specific training programs to enhance performance.

The anaerobic adenosine triphosphate – lactic acid (ATP-LA) system must be stimulated to improve motor unit recruitment, elevate lactate threshold and access the fast twitch muscle fibers.  In addition, the aerobic ATP-oxidation system must be trained to increase the cellular mitochondria levels, capillary density, and other adaptations particular to the slow twitch, slow fatiguing endurance muscle fibers.

By tailoring the training to optimize the production of each energy system, the athlete can take advantage of being able perform at a faster velocity for a significantly longer period of time.

There are two ways to stimulate the ATP-LA system.

One is by training long slow distances until the ST muscle fibers fatigue, forcing the body to recruit FT muscle fibers.  This method can take up to 3 hours.

A more time efficient method uses interval training.

Intervals can be broken down into different categories as determined by the intensity, work interval, and rest period between repetitions.  The intensity is based on the lactate curve (lactic acid accumulation versus percentage of maximal oxygen consumption [VO2 max]).  For a moderately trained individual, lactate threshold will occur around 70% VO2max.   Interval training occurs at intensities above the lactate threshold, generally ranging from 80% to 120% VO2 max.

At the lower interval intensities, both the aerobic and anaerobic systems are trained.   The ST and FT fibers are recruited, lactic acid levels rise then fall due to long rest periods.   These interval last for 20 to 40 minutes and are termed “aerobic” because the lactic acid does not accumulate.

International distance triathletes should also include some higher intensity workouts, ranging from 10-30 min, especially during the competitive season. A llowing the lactic acid levels to accumulate during the workout will force certain body adaptations beyond the ATP-LA system.   It could lead to an increased tolerance of lactic acid by producing more chemical buffers in the bloodstream; it could increase the capillary density around the working muscle to remove muscle lactic acid faster; and it could increase the lactate threshold causing a shift from 70% VO2 max to 80% VO2 max. A ll of these adaptations can translate into improved swimming, cycling and running performance.

The aerobic ATP-oxidation system provides the majority of the energy used on a daily basis as well as the energy used for exercise performed below the lactate threshold.

A person can maintain a jogging pace at 50% VO2 max max for over 4 hours before fatiguing. The run might continue on indefinitely if dehydration, glycogen depletion, mental exhaustion, etc., weren’t also factors affecting fatigue.  It is important to train this long-term energy source to improve not only the efficiency of the muscles, but also the nervous system that stimulates the muscles to contract and the cardiovascular system that transports the oxygen to the working muscles.

Distance work is the traditional method used for training.  Over time, the capillary density increases around the working muscle, the oxidative enzyme levels increase, the heart size increases, and the body learns to use its abundant fat storage as fuel for energy.

Interval training can also be used to train the aerobic system.  The lower intensity interval repetitions are performed at 80-100% VO2 max, which uses a combination of aerobic and anaerobic energy systems.  The exposure to the higher than usual blood lactate levels will lead to a new lactate threshold thus the person will be able to perform aerobically at a higher VO2 max.

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