May 2005
Lactic Acid: Friend or Foe
Why Test for it?
by Robert Forster P.T.

There has been a lot of confusion about lactic acid and what it means to the performance athlete and their training.

The confusion seems to be born from two facts. First, science certainly does not have all the answers when it comes to exercise training and the limiting factors in performance. Example: For years scientists thought lactic acid caused fatigue. Now it’s understood that in itself it does not cause fatigue but at the point where it collects in the muscle cell, fatigue is quick to follow.

An example of a well developed
Blood Lactate Test Graph,
with accurate training zones

Secondly, what is known is often taken out of scientific context and expected to be a stand-alone truth. Example: Lactic acid is a terrible thing because training programs are designed to reduce its production. Lactic acid is a natural by-product of anaerobic exercise and, no, it’s not all bad – but we still want to limit it’s production at certain times in the training cycle.

The truth in these statements is only evident with a complete understanding of muscle physiology and the goals of endurance training programs. First, a review of exercise physiology is helpful. In the continuum of exercise intensities there are two definite domains. At the lower intensities the majority of muscle energy is generated by aerobic metabolism of FATS as a food substrate. To metabolize fats, which are virtually unlimited even in the thinnest of athletes, the muscle cells need an adequate supply of oxygen. As exercise intensities increase the muscles begin to work so hard that they outpace the body’s ability to deliver oxygen. Metabolism then shifts into the anaerobic domain. Here the muscle cell must metabolize CARBOHYDRATES to produce muscle energy with the by-product of lactic acid. At first this is no problem as the lactic acid is produced at a rate which can be used by the aerobic muscle fibers as a fuel, buffered by the chemicals released in the cell or cleared from the cell and taken away by the blood vessels. Prolonged exercise at this intensity, however, creates problems as it consumes all available carbohydrates and the body cannot re-synthesize enough to maintain this exercise intensity (the bonk). At this point the athlete is forced to slow down or stop exercise all together as the finite amount of carbohydrate (glycogen) stored in the muscles and liver is totally exhausted.

If exercise intensity increases even more, lactic acid accumulates in the cell to a point where cellular metabolism is slowed and fatigue progresses to muscle failure. Although it’s not clear if lactic acid itself creates changes in the cell that cause fatigue or other unknown events occur simultaneously to cause fatigue - it really matters little to the athlete.

The physiology of the aerobic and anaerobic muscle function identifies a clear path to improved performances in endurance sports. The aerobic metabolism of fats yields the highest return of muscle energy (ATP) and produces the least amount of by- products (no lactic acid) while it spares precious carbohydrate stores. Furthermore, proper training can create improvements in aerobic metabolism up to 12-14% whereas, anaerobic energy production, through higher intensity exercise and intervals, improves only 2-6%. The objective of training endurance athletes is clearly to increase the contribution of aerobic metabolism to the overall energy requirements of a given task. To improve the aerobic fitness of an athlete we must first examine the physiological limitations of aerobic energy production.

As discussed earlier the body’s inability to deliver adequate quantities of oxygen to the muscle cell to burn fats is what shifts muscle metabolism to the anaerobic domain. When studying the critical path here, research has shown it is not heart and lung function that is the limiting factor. These organs strengthen quickly. Instead the vascular network of capillaries which ultimately deliver the oxygen to the working muscle is what needs to be better developed. Furthermore, once the oxygen is delivered in abundant quantities the muscle cell will build the internal aerobic machinery (mitochondria) to metabolize fats and power the muscle. The only way to create a better capillary network and improve mitochondria density in the muscle (aerobic infrastructure) is when workouts create a great demand for oxygen. The body will then respond or adapt to meet those demands. Indications for training: long, slow workouts create a demand for oxygen and the body will respond by creating the aerobic infrastructure necessary to meet these demands. Harder intensity workouts simply cause the body to shift to anaerobic energy production and relieve the demand for oxygen and therefore stimulate no improvements in aerobic infrastructure.

What are the Signs of Inadequate Base?
The most common sign of inadequate aerobic base is inconsistent performances in training and racing. Other signs are frequent illness or injury once the hard work begins.

How Low is Low Intensity?
To know accurately how low the intensity of exercise needs to be to build your aerobic engine one must test lactate levels in the blood while performing a ramped exercise test. Lactic acid that cannot be buffered in the muscle cell or used by the aerobic muscle cells as a fuel is cleared from the muscle and swept away by the capillaries. In the blood this is called lactate. When a progressive exercise test is performed and the intensity of exercise is increased at regular intervals there is a point where lactate concentration approximately doubles its’ resting levels. This is at the lactate threshold and indicates (in heart rate or power wattage) the upper limits of an athlete’s current aerobic ability. To improve this system all exercise must be done below this intensity to create a great demand for oxygen and stimulate bodily adaptations to meet this demand. This is the best return for time spent training with the greatest rewards in performance.

The blood lactate test is the only way to determine this upper limit of your purely aerobic functioning. VO2 testing will determine the anaerobic threshold where metabolism is predominantly anaerobic but gives no indication at lactate threshold to indicate at what level base work should be done. Neither do field tests.

How Much is Enough Base?
This depends on your event, past history of training and years training your aerobic system. But most cyclists need to perform around 1,000 mile of base training to maximize their fitness gains and performances. Only retesting can accurately determine adequate improvements in aerobic function.

Can I do any High Intensity Exercise During the Base Building or Aerobic Development Phase?
Higher intensity efforts will shift metabolism to the anaerobic energy system and diminish the demand for oxygen and relieve the body of its need to develop the aerobic infrastructure needed. According to one of America’s premier cycling coaches and author, Joe Friel (personal communication), only brief high intensity efforts shorter than 15 seconds where no lactic acid (known as alactic training) are permitted during base training. Many athletes who train as hard as possible all the time have developed a bodily preference to anaerobic energy production. The goal of their training should be to atrophy this energy pathway with long slow distances that develop the more efficient aerobic pathway.

What Are the Other Benefits of Aerobic Training?
Aerobic muscle fitness benefits the athlete by meeting a greater percentage of energy needs with a clean burning and abundant fuel source (fats) but it also helps the athlete in anaerobic efforts. Since the anaerobic muscle fibers use lactic acid as an alternative fuel source a well-developed aerobic system helps clear lactic acid produced during harder efforts.

If I’m Already Racing is it too Late to Gain Additional Aerobic Base?
No, it’s never too late in the season to improve aerobic base but it’s more difficult once you have begun anaerobic training. The key here is to do the aerobic work at very low intensities once you know your lactate threshold. As Chris Carmichael says ‘cyclist’s hard workouts are too easy (due to accumulated fatigue) and easy workouts are too hard (due to ego).’

What are the Other Benefits of Blood Lactate Testing?
Blood lactate testing will demonstrate an athletes’ full fitness profile indicating which energy systems are fit and those that are in need of improvement. From this data a coach can design a training program including accurate heart rate training zones for intervals training.



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