Understanding the Lactate Threshold
The lactate threshold is often misunderstood and underutilized. The purpose of this article is to discuss the source of lactate, the metabolic consequences of lactate accumulation, and the utility of the lactate threshold in practice.
A discussion of the lactate threshold has to begin with the source of lactate. Lactate is a by-product of carbohydrate (CHO) metabolism, called glycolysis. Glycolysis starts with a molecule of glucose that comes from the blood or muscle glycogen . Blood glucose is derived from either liver glycogen or from the diet. At the onset of exercise the most likely source of glucose is muscle glycogen, but as the reserves become depleted blood glucose becomes more important. During glycolysis, the glucose molecule is broken down in a series of chemical reactions to produce 2 ATP (3 ATP if the source of glucose is muscle glycogen) and pyruvate. Pyruvate has two potential fates; 1) complete oxidation in mitochondria or 2) conversion to lactic acid. The fate of pyruvate is dependent on a number of factors, the most important is the demand for ATP.
If the demand for ATP is high, the conversion of pyruvate to lactate accelerates glycolysis and ATP production. Under conditions where the demand for ATP isn’t as high the pyruvate will be oxidized in the mitochondria. The demand for ATP is dictated by the intensity of the exercise. Complete oxidation of pyruvate produces a greater quantity of ATP; however it is produced at a much slower rate. The amount of ATP produced per unit of time during anaerobic glycolysis is almost 2x faster than aerobic glycolysis. Anaerobic glycolysis has a high rate of ATP production but it has a limited capacity, it fatigues quickly.
Metabolic Consequences of Lactate Accumulation
Even at rest, some pyruvate is converted to lactate, but it is cleared at a rate that equals production. As a result lactate levels stay low in the blood. As the exercise intensity increases so does lactate production and its removal, but at some point lactate production surpasses removal and the lactate begins to accumulate. There is a lot of misinformation about the metabolic consequences of lactate accumulation, one of which is the association with fatigue. Lactate, in itself, does not cause fatigue; rather it is the acidosis that accompanies lactate production that does. Whether or not the acidosis is a direct result of lactate production or another step associated with the high turnover of ATP is currently under debate. But what we do know is this; when lactate levels accumulate – fatigue will follow. Also, we can assume that under exercise conditions when there is an accumulation of lactate that there is a high rate of glycogen usage and depletion of this important, but limited, fuel source will also result in fatigue.
During a graded exercise test (GXT) (see figure) lactate levels stay close to resting values during low to moderate intensity work. As the exercise intensity increases from moderate to high lactate accumulates above resting levels. During high to very high intensity exercise, lactate levels will increase exponentially. During high intensity exercise slow twitch muscle fibers cannot produce the force necessary to meet the demands of the activity and fast twitch muscle fibers are called into action. Fast twitch muscle fibers are more reliant on glycolysis for energy production and therefore produce greater amounts of lactate. This rapid rise in lactate indicates a high rate of ATP production, a high rate of glycolysis and the onset of acidosis in the muscle. This intensity cannot be maintained for much longer than 45 minutes – maybe an hour in well trained athletes who have a high tolerance to muscle acidosis.
Some athletes mistakenly believe that the rise in lactate, called the lactate threshold (LT), indicates a switch from aerobic to anaerobic metabolism. This is not true, the muscles are still very aerobic, yet there is a greater contribution of anaerobic metabolism. The anaerobic contribution comes from the activation of fast twitch muscle fibers. The fuel for this activity may be coming from 99% aerobic and 1% anaerobic sources.
What is the significance of this? Why do we care? Many endurance sports (Triathlons and marathons) are raced for durations that are greater than an hour. Any race that lasts longer than an hour would require the athlete to race at intensities below the LT. Events of shorter distances (10k runs) are race at or above the LT. As you can see, knowing the LT can provide important information for the athlete attempting to determine their race pace and predicting performance. Even more importantly, the LT can be a guide for determining training intensities. Training sessions that are intended stress the slow twitch fibers and fat metabolism should be conducted well below the LT and for training sessions intended to improve lactate tolerance VO2max, and stress the fast twitch muscle fibers should be conducted at and above the LT. The lactate threshold can also be an accurate gauge of which fuel sources are providing the energy during training sessions. For example, as the intensity approaches LT there is greater reliance on CHO metabolism. At first, the CHO will be oxidized aerobically, but once the intensity gets closer to the LT more of the CHO is broken down anaerobically. When a molecule of glucose is broken down aerobically we get 32 ATP, if it is broken down anaerobically we get 2 or 3 ATP. Thus when exercising at an intensity at or above the LT we are using 16X more glucose to produce the same amount of ATP and the glycogen stores will be depleted faster than if the glucose was broken down aerobically. This information would be relevant for any athlete who competes in endurance events where glycogen depletion causes fatigue.
The lactate threshold can be correlated with heart rate, running speed, or power and training zones can be established based on those variables depending on the athlete’s needs. Some coaches use time trials to estimate the LT based on average heart rate or power achieved during a maximal effort over a predetermined distance. The advantage of this method is that it doesn’t require blood sampling and can be conducted under “real life” conditions. However, time trial estimates are also more prone to error and dependent on the motivation of the athlete. Lactate measurements precisely measure the metabolism of the muscle which is the best gauge of intensity not perceived effort.
To summarize, lactate measurement is a measure of the metabolic activity of skeletal muscle. During lower intensity efforts blood lactate levels are relatively low indicating less of a reliance on CHO metabolism and low metabolic stress. As intensity increases, lactate levels increase indicating an increase in CHO metabolism and muscle acidosis. This information is important to understand pacing during endurance events and to precisely determine training intensities that elicit the appropriate response and adaptations during training. The lactate threshold is measured during a graded exercise test. The information provided by a lactate threshold test is helpful for any athlete who wishes to train and perform better.
Jeff received his Doctorate in Kinesiology from the University of Connecticut and is certified by ACSM, NSCA, and ISSN. He is currently Chair of the Departmental of Exercise and Sport Science at Fitchburg State University and the Director of Spartan Coaching. Spartan Coaching will go nationwide in November 2012. www.SpartanCoaches.com
Tags: WOD Fundamentals