When creating or following a training plan, a common term to hear is "threshold training." Run your workout at threshold pace, keep your heart rate at threshold, etc. But what does threshold training really mean?
Lactate Threshold and Blood Lactate
Threshold training is running at the workload equivalent to your measured lactate threshold. When you run, your body utilizes blood glucose or muscle glycogen as an energy source. Both glucose and glycogen are broken down into pyruvate which either enters the mitochondria in muscles for cellular respiration or is converted to lactate, through a process called glycolysis (Dalleck & Kravitz, 2015). This creates energy in the form of adenosine triphosphate (ATP), an important product to know for later. At running intensities below the lactate threshold, pyruvate is shuttled into the mitochondria for cellular respiration. At intensities above the lactate threshold, pyruvate is converted into lactate, as the capacity for mitochondrial respiration has been exceeded. Lactate can then be shuttled back into the mitochondria to be used as a recycled energy source as the muscles continue to work. The lactate threshold occurs when the exercise intensity reaches a workload at which the rate of lactate production exceeds the rate of lactate clearance, resulting in a quick buildup of blood lactate.
Though athletes tend to associate the characteristic muscle burn with lactate buildup, this is not actually what causes this burning sensation. Hydrogen ions are released into circulation as lactate is produced, resulting in acidosis (lowered blood pH, making blood more acidic). Lactate typically joins with a hydrogen ion to produce lactic acid, acting as a buffer to the increase in acidity, but it is the increase in concentration of free hydrogen ions in the blood without the necessary buffering capacity that causes that burning sensation at the end of a hard workout.
Lactate Threshold and Endurance Runners
Endurance runners tend to have a higher lactate threshold than more anaerobic-based athletes. This is due primarily to two adaptations: High concentrations of both lactate and hydrogen ion gated channels in the muscles combined with high muscle vasculature. To start with, the increase in lactate and hydrogen ion channels function to shuttle these byproducts from the blood back into the mitochondria. More channels mean more lactate and hydrogen ion clearance, resulting in a greater capacity to use lactate as an energy source while ridding the blood of the acidic effect of the hydrogen ion build-up. The increase in vasculature adds to this adaptation by allowing for greater blood circulation throughout the body, further aiding in the clearance of lactate and hydrogen ions. This makes the muscles of an an aerobically trained athlete more efficient in recycling the waste products of glycolysis and consequently more resistant to fatigue.
Muscular Fatigue in Endurance Running
**This section is a little nitty gritty, but is still important to really understand the full scope of this topic :) **
What exactly is fatigue, and how do byproducts of glycolysis affect muscle function? Simply put, muscle fatigue is the decreased ability of skeletal muscles to continue to produce the same amount of force over a period of time. Every runner has experienced muscle fatigue. Your form falls apart, every step burns, and you feel like you are recruiting every ounce of energy you have to continue to propel yourself forward. Without getting too complicated, running requires muscle contraction, and muscle contraction occurs via the the interaction of microscopic filaments in your muscle fibers called actin and myosin. Calcium in your muscles bind to a specific site on the actin filament to cause a slight shift, exposing a binding site for myosin heads. Myosin heads then bind to these sites creating what is called a cross-bridge. Myosin heads contain stored energy from the splitting of ATP (energy source - produced from glycolysis, for example) into adenosine-diphosphate (ADP) and an inorganic phosphate (Pi), which allows them to produce a powerstroke, or muscle contraction. During this process, both the ADP and the Pi are dropped from the myosin head, allowing for another ATP to attach at the end of the powerstroke. This allows the filaments to return to their original rested position before repeating the cycle. Muscular fatigue occurs during exercise when the muscle fibers are unable to contract (or produce a powerstroke) as forcefully as they were at the beginning of the exercise bout. Fatigue can be attributed to a variety of physiological reasons, but for the purposes of this article we are focusing on the metabolic factors associated with lactate threshold.
To tie this all together: Glycolysis breaks down glucose into either pyruvate (low intensities) or lactate (higher intensities). Both pyruvate and lactate can be shuttled back into the mitochondria yielding ATP and, in the case of lactate, free hydrogen ions. ATP is needed for muscular contraction while running. Calcium inflow to the muscle fibers begins each muscle contraction, ending with the splitting of ATP into ADP and Pi. Muscular fatigue begins to occur when both hydrogen ions and Pi build up in concentration, as each have been shown to be inversely proportional to skeletal muscle force production (Stackhouse, et al., 2001; Wilkie, 1986). This means that as both hydrogen ion and Pi concentrations increase, skeletal muscle contractile power decreases. Improving an athlete's lactate threshold will increase the ability to use lactate to create energy while clearing the muscles of fatiguing byproducts.
How to Increase Lactate Threshold
An individual's lactate threshold is not a fixed measure and can be increased with training. Learning how to appropriately train the body with this stimulus is key to building aerobic fitness, endurance, and enhancing racing performance.
The most accurate way to measure an athlete's lactate threshold is in a laboratory setting. The athlete is typically on a treadmill and attached to a metabolic cart - a machine that measures the respiration of the athlete via mask and a long tube connected to an air chamber in the machine - as well as a heart rate monitor. The protocol includes a warm up period followed by a progressive work period in which the runner increases in speed and/or percent grade in specific time intervals. At the end of each stage, the athlete's finger is pricked for a blood sample to determine blood lactate concentration. The lactate threshold is determined by the speed, heart rate, or work rate at which there is a sudden steep increase in blood lactate concentration.
Once the athlete's lactate threshold is known, workouts can be individualized to the appropriate work rate. This means that during threshold sessions, the athlete will know exactly what pace or HR to run at. There are two ways to increase lactate threshold: Steady state training and high intensity interval training (HIIT). These two modalities are best incorporated into a training plan by periodization. After a few months of running base mileage to build up a strong aerobic base, steady state training at lactate threshold can be incorporated. This type of training is usually done in the form of a tempo run, and the duration of time is dependent on the athlete's goals and race distances. It is incredibly important to perform these steady state workouts at your true lactate threshold. Running too hard will cause your body to shift into an anaerobic training stimulus, and will not have the desired effect on your fitness. After about 4 weeks of steady state workouts 1-2 times per week, the athlete will need to begin incorporating HIIT to continue to increase their lactate threshold (Dalleck & Kravitz, 2015). This phase of training will last about 4 weeks as well, and fits into a training plan about a month prior to the athlete's goal race. HIIT consists of high repetitions of relatively short and fast intervals of a few minutes each, with either equal or half the time for rest between each rep. These are to be performed at a 95-100% of the athlete's maximal heart rate, and is associated with the work rate of the athlete's VO2max.
Why Should I Improve my Lactate Threshold?
Improving your lactate threshold is a scientifically proven way to improve your running performance. Your lactate threshold can act as a baseline for your training and racing. The higher the baseline, the longer and faster you can run aerobically before running into oxygen debt and entering the anaerobic state. From a performance standpoint this means that during a race you will be clearing lactate from the blood to use as an energy source much more efficiently and for much longer than someone with a lower lactate threshold, allowing you to run the same pace at a lower energy cost. Once it comes down to the kick, you will have an edge on your competitor who has been building up force-decreasing "waste" product in the blood for a longer period of time than you.
The Wrap Up
Lactate threshold training is an often used but not often understood term in many endurance training plans. Understanding how your body physiologically responds to specific training stimuli will enhance your ability as an athlete to train smart, and consequently, race smart. Whether you want to create a training plan for someone else, improve and specify your own training, or are just looking for an edge on your competition, learning to maximize your lactate threshold is a great place to start. Now let's get out there and run.
Dalleck, L. & Kravitz, L. (2015). How to design a lactate threshold training program. ACE Prosource. https://www.acefitness.org/continuing-education/prosource/february-2015/5243/how-to-design-a-lactate-threshold-training-program/
Midgley, A.W., McNaughton, L.R. & Jones, A.M. (2007). Training to enhance the physiological determinants of long-distance running performance. Sports Med. 37, 857–880 https://doi.org/10.2165/00007256-200737100-00003
Stackhouse, S., Reisman, D., & Binder-Macleod, S. (2001) Challenging the role of pH in skeletal muscle fatigue, Physical Therapy. 81(12), 1897–1903. https://doi.org/10.1093/ptj/81.12.1897
Wilkie D. (1986) Muscular fatigue: effects of hydrogen ions and inorganic phosphate. Fed Proc. 45(13), 2921-3. PMID: 3536590.