Using Lactate Threshold Data
Information provided by a lactate threshold test has a number of purposes. By understanding the role that lactate plays in exercise metabolism, the exercise physiologist can use the information from lactate threshold tests to predict proper racing and training paces, and assess the fitness of a subject or the efficacy of the training program. Although lactate production does not contribute to acidosis and lactate itself does not appear to cause fatigue, blood lactate accumulation does indicate that the body is relying on substantial contributions from anaerobic glycolysis to meet exercising energy requirements. Knowing the exercise intensity at which this occurs is valuable for two reasons: When glucose and glycogen are metabolized to lactate, only two or three ATP moleculesare generated per molecule of carbohydrate consumed compared to the 36 to 39 ATP moleculesthat are generated when pyruvate is produced and consumed through oxidative phosphorylation. Thus, the advent of lactate threshold signals that the body is consuming glucose and glycogen at an increased rate in respect to ATP production, which, ultimately, can lead to premature carbohydrate depletion and exhaustion. Therefore, athletes who partake in events that challenge their glycogen storage capacity should take into consideration the need to preserve carbohydrate stores when planning their pacing strategies.
Increases in blood lactate concentrations also indicate that the subject’s ATP consumption rate is beginning to exceed the ability to provide ATP through the oxidative pathway. The increase in blood lactate levels seen at this transitional intensity indicates that the body has to rely on glycolysis to provide adequate ATP supplies for the exercising muscle. Though lactate production does not result in acidosis and has a questionable role in causing fatigue, the accumulation of lactate in the blood indicates that maximal sustainable rates of exercise and ATP production are close at hand (Morris and Shafer 2010).
The relationship between lactate threshold and the rate of consumption of carbohydrate stores, and correlations between lactate threshold and maximal sustainable work rate, make lactate threshold a good predictor of endurance exercise performance. Previous studies (Foxdal et al. 1994; Tanaka 1990) have demonstrated close agreements between running paces at lactate threshold and average paces during competitive running events in distances ranging from 10,000 meters to the marathon. In studies using cycling ergometry, power outputs that elicited lactate threshold were similar to average power outputs during time trials ranging from 60 to 90 minutes (Bentley et al. 2001; Bishop, Jenkins, and Mackinnon 1998). However, in time trials ranging from 25 to 35 minutes, subjects typically maintain significantly higher power outputs than those that elicited lactate threshold (Bentley et al. 2001; Kenefick et al. 2002). Despite these discrepancies, correlations between power outputs at lactate threshold and average power outputs during the shorter time trials remained remarkably high, suggesting that performance in these events can be predicted from lactate threshold data with reasonable accuracy.
As in many physiological and anatomical systems, the mechanisms that influence lactate threshold are responsive to exercise training. Properly designed training programs can increase the capacity of the oxidative pathway by increasing oxygen delivery to the working muscle (Schmidt et al. 1988), mitochondrial numbers (Holloszy and Coyle 1984), and oxidative enzyme levels (Henriksson and Reitman 1976). These improvements in oxidative capacity increase the muscle’s ability to produce ATP, consume pyruvate, and regenerate NAD resulting in a reduced reliance on lactate production and an increase in work rates that are required to elicit lactate threshold.
Unlike maximal oxygen consumption, which can be significantly influenced by genetic factors (Bouchard et al. 1986), the exhibition of lactate threshold when expressed as a percentage of maximal oxygen consumption is primarily influenced by the level of conditioning (Henritze et al. 1985). This sensitivity to exercise training makes lactate threshold useful for assessing aerobic fitness and the efficacy of training programs. Well-trained endurance athletes tend to exhibit lactate threshold when exercising at 80% or more of their maximal oxygen consumption, whereas untrained people experience lactate threshold at substantially lower intensities (Joyner and Coyle 2008). Continued training at or above the work rate that elicits lactate threshold also results in increases in the power outputs that cause increased rates of lactate production and accumulation (Henritze et al. 1985). Therefore, the efficacy of a training program can be assessed by measuring lactate threshold prior to, and following, program implementation. A rightward shift, as seen in figure 6.10, suggests that the training program has been successful in increasing the work rate that elicits lactate threshold and maximal sustainable work rates.
The ability of lactate threshold to respond to training and predict competitive performance also makes it useful in prescribing proper training intensities. Scientific evidence supports the overload principle of training (Weltman et al. 1992), which suggests that the most effective way to improve physiological capacity is to train at an intensity that exceeds current ability. Thus, effective training strategies involve assessing athletes’ current performance capacities and using work intervals that exceed their current maximal sustainable work rates. Undoubtedly, the most accurate way of measuring an athlete’s performance capacity in a particular event is to measure performance during that event. Unfortunately, lengthy endurance events such as the marathon are physically taxing, which makes performing them simply to test performance capacity impractical. However, the relatively short and low-stress nature of a lactate threshold test makes it ideal for frequently assessing an athlete’s ability.