At rest and under steady-state exercise conditions, there is a balance between blood lactate production and removal. The lactate threshold refers to the intensity of exercise at which there is an abrupt increase in blood lactate levels. Lactate threshold training refers to specialized training regimens and workouts focused on improving the lactate threshold.
Historically, maximal oxygen uptake has been viewed as the key component to success in prolonged exercise activities. However, more recently it has been recognized that the lactate threshold is the best and most consistent predictor of performance in endurance events. Research studies have repeatedly found high correlations between performance in endurance events such as running, cycling, and race-walking and the maximal steady-state workload at the lactate threshold.
Although the exact physiological factors of the lactate threshold are still being resolved, it is thought to involve the following key mechanisms.
Following adequate build-up in training volume, the next training period to be addressed is steady-state, continuous lactate threshold training. The rating of perceived exertion (RPE) scale may be the most accurate way to determine training intensity during this aspect of training. Research has shown that RPE is strongly related to the blood lactate response to exercise regardless of gender, training status, type of exercise being performed, or training intensity. This is noteworthy, as other methods of monitoring intensity at lactate threshold have been known to have serious flaws in methodology, resulting in under-estimating or over-estimating training intensity. Similar to the time-line increase in training volume, steady-state workout sessions can be increased in duration from a starting point of 10 minutes by 10%–20% per week. Evidence suggests steady-state sessions of 30 minutes in duration are sufficient for optimizing the improvement in lactate threshold during this phase of training. The progression from 10 to 30 minutes steady-state workouts may be accomplished gradually over 6 weeks to 3 months.
Knowledge of the metabolic pathways of energy production provides a basis for understanding the importance of lactate threshold training to endurance performance. All energy transformations that occur in the body are referred to as metabolism. Thus, a metabolic pathway is a series of chemical reactions that result in the formation of adenosine triphosphate (ATP) and waste products (such as carbon dioxide). The three energy systems of the body are the phosphagen system, glycolysis (break down of sugar), and mitochondrial respiration (cellular production of ATP in the mithochondrion). The phosphagen system is the simplest energy system of the body with the shortest capacity (up to 15 seconds) to maintain ATP production. During intense exercise, such as in sprinting, the phosphagen system is the most rapid and available source of ATP.
During submaximal endurance exercise, the energy for muscle contraction comes from ATP regenerated almost exclusively through mitochondrial respiration, which initially has the same pathway as glycolysis. It is a misconception to think that the body's energy systems work independently. In fact, the three energy systems work cooperatively to produce ATP. Through glycolysis, blood glucose or muscle glycogen is converted to pyruvate that, once produced, will either enter the mitochondria or be converted to lactate depending on the intensity of exercise. Pyruvate enters the mitochondria at exercise intensity levels below the lactate threshold, while at exercise intensity levels above the lactate threshold the capacity for mitochondrial respiration is exceeded and pyruvate is converted to lactate. It is at this point that high-intensity exercise is compromised, because the glycolytic and phosphagen energy systems that are sustaining the continued muscle contraction above the lactate threshold can produce ATP at a high rate, yet are only capable of doing so for short durations of time. In summary, the energy for exercise activities requires a blend of all the energy systems.
However, the determinants of the involvement of the particular energy system are highly dependent on the intensity of the exercise.
The lactate threshold is dependent largely on training status of the individual. In a nonexercising individual lactate threshold ranges anywhere between 40% and 55% of maximal oxygen uptake. The lactate threshold for individuals who are recreationally endurance-trained is approximately 65% of maximal oxygen uptake. Elite, endurance trained individuals have lactate threshold values upwards of 75%–80% of maximal oxygen uptake; and in some instances even higher. Frank Shorter, winner of the gold medal in the marathon in the 1972 Olympics, reportedly had a lactate threshold that was at an incredible 92% of maximal oxygen uptake.
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American College of Sports Medicine, 401 W Michigan St., Indianapolis, IN, 46202-3233, (317) 637-9200, Fax: (317) 634-7817, http://www.acsm.org .
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Lance C. Dalleck, BA, MS, PhD