RBR Reader Maude asks, “Over the past several years, there has been considerable discussion that lactic acid buildup is NOT responsible for cramping and after-exercise muscle pain. Yet you and other publications continue to use “lactate threshold” as some sort of line in the sand. I’d like to see an article discussing this area.”
Maude, you are asking excellent questions. Different physiologists and different coaches have different definitions of lactate threshold and use the LT differently. Here’s a simplified explanation.
Your lactate threshold (LT) is important because your body uses different metabolic systems to produce energy and which system(s) it uses depends on how hard you are riding relative to your LT.
When you ride your muscles’ energy comes from glucose, which is stored as glycogen (from carbs) and fat (either from your diet or your body) in three different ways.
- Aerobic fat burning—When you are riding at a comfortable pace chatting with friends you are burning primarily fat along with some glucose. Lactate is a fuel and even at this pace you are producing and metabolizing a little lactate.
- Aerobic glucose burning—When you start riding harder like climbing a long hill or into a headwind you burn more glucose and produce more lactate, which your muscles burn. You also keep burning fat.
- Anaerobic glucose burning—When you ride even harder, for example racing, you start burning glucose as your primary fuel using a different metabolic system. Your metabolism also produces more lactate, which your muscles use as fuel. And you keep burning fat.
“Aerobic” means “with oxygen”, i.e., you have enough oxygen to metabolize the fat, the glucose and lactate efficiently. “Anaerobic” means “without oxygen”, i.e., you don’t have enough oxygen to metabolize fat, glucose and lactate efficiently so you metabolize the fuel differently creating lactic acid, which produces that familiar burning sensation in your legs.
Muscle Fiber Types
The energy systems also related to your muscle fiber types:
Slow-twitch fibers, refer to how fast the muscle fibers contract (not how fast your cadence is). Slow-twitch fibers have great endurance but low power. Slow-twitch fibers burn primarily fat and also some glucose, which is stored as glycogen. The fat can be either body fat or food. (Burning fat doesn’t mean you’ll lose weight — losing weight depends on consuming fewer calories.) The glucose comes from the carbs you eat. The more briskly you ride the greater the proportion of energy that comes from glucose.
At endurance pace you produce a little lactate.
Fast-twitch fibers are of two types:
Fast-twitch IIa (moderate power and endurance) muscle fibers burn more glucose and produce and metabolize more lactate. Your slow-twitch fibers continue to work and burn a mix of glucose and fat.
Fast-twitch IIb (high power and shorter endurance) muscle fibers burn even more glucose and produce even more lactate. Your slow-twitch and fast-twitch IIa fibers continue to fire and metabolize fat and glucose.
During exercise, lactate is mainly produced in fast twitch muscle fibers, which use lots of glucose for energy. The lactate is processed (cleared) mainly by slow twitch muscle fibers. This is a complex process involving different lactate-specific transporters and enzymes.
Lactate threshold is the level of exertion at which your body can’t clear all of the lactate and it starts to accumulate.
The LT isn’t like a switch, a point at which you suddenly start accumulating lactate. The LT is more a dimmer switch on a light. You keep turning the dimmer up and the light keeps getting brighter. You keep riding harder and harder and the lactate accumulates faster.
Lactic acid is a byproduct of this metabolism. It’s mainly produced in muscle cells and red blood cells. It forms when the body breaks down carbohydrates to use for energy when oxygen levels are low, i.e., you’re riding anaerobically.
Your body makes lactic acid to fuel cells when they’re working harder than usual. Your liver and kidneys filter lactic acid out of your blood after it’s created in other tissue. They break it down and convert it into new glucose that your body can use for energy.
Why LT is Important
Your LT is important for three reasons:
- Burning glucose aerobically produces a lot more ATP (the actual molecule your muscle uses for energy) than burning glucose anaerobically, i.e., you get many more miles to the gallon!
- Your body can only store enough glycogen for a few hours of hard riding. If you run out, you bonk! When you ride above your LT you’re burning a lot more of this precious fuel a lot faster!
- Training at different intensities relative to your lactate threshold produces different physiological adaptations.
Put simply, at an easy to moderate pace relative to your LT you are increasing your endurance and your capacity to burn fat. Everyone — even the skinniest racer — has enough body fat for an endurance ride. By increasing your capacity to burn fat you then burn less precious glycogen. Riding at an endurance pace you also increase your capacity for storing glycogen.
Riding harder, but still below your LT you are building your sustained power. Glucose becomes the dominant fuel source and you’re still using fat. You’re still riding aerobically and producing more ATP per molecule of glucose and fat.
Riding even harder, above your LT, trains you to go anaerobic for relatively short efforts. How short? If you’re not too far above your LT you could hang with a fast group for 30 minutes or so. If you try to stay with them on a steep climb well above your LT you might blow up after five minutes. Riding anaerobically your metabolism produces significantly less ATP per molecule of glucose, so you burn through your glycogen stores much faster.
Finding your LT
In a sports physiology lab the protocol is to keep increasing your intensity minute by minute. Each minute the tech does a minute blood draw and measures the lactic acid. Your lactate threshold is when the lactic acid accumulates beyond a certain level.
Your LT is the maximum average heart rate you could sustain for a one hour time trial — suffering for an hour! I use a less protracted but more painful test with my riders. My clients warm up for 15 to 20 minutes. Then they ride a 20 minute time trial going as hard as possible. The TT can be an out-n-back course or a loop. The TT could also be a hill climb. The maximum average heart rate a rider can sustain for 20 minutes is about 105% of the max average HR for an hour. If a rider’s HR is 150 beats per minute, then the rider’s LT is about 150 / 1.05 = 143 bpm.
Your ventilatory threshold (VT) is the point at which you start breathing very hard, almost gasping for air. Your heart rate at your VT is usually about the same as your heart rate at your LT.
Perceived exertion means paying attention to the signals that your body is sending you: how fast and deeply you are breathing, how hard your heart is beating, how powerfully your muscles are contracting, etc. According to the American College of Sports Medicine there is a reasonably linear relationship between Rate of Perceived Exertion (RPE) and oxygen uptake and heart rate so RPE provides meaningful feedback, which you could use instead of a heart rate monitor. You can read more in this column: Enjoy Your Riding More: Training by Perceived Exertion.
Some physiologist suggest that perceived exertion is the limiter on how hard you can ride, not actual physiological changes. You can read more in this column: Anti-Aging: New Research on Fatigue.
You are correct that lactic acid is not responsible for cramps and for the after-ride muscle pain, which is called Delayed Onset Muscle Soreness (DOMS). Lactic acid is actually a form of fuel and your muscles burn it rather quickly—it’s gone within an hour unless your muscles produce more. DOMS is caused by micro-tears in your muscles from hard riding and the chemicals that your body produces to help heal the tears.
Cramps are caused by one of two factors:
- Fatigue of the nerve spindles that activate the muscle fibers, making them more excitable.
- Sodium depletion from sweating, which also makes the nerves more excitable.
You can read more in these columns:
- Cramping: A Case Study – The Perfect Storm, Part 1
- Cramping: A Case Study – The Perfect Storm, Part 2
- Why Increasing Intensity is Good for All Road Cyclists
- 6 Kinds of Intensity Training: Which One Is Best for You?
- Intensity Done Correctly Produces Results
- Anti-Aging – Benefits of Training with Intensity
My eArticle Intensity Training: Using Perceived Exertion, a Heart Rate Monitor or Power Meter to Maximize Training Effectiveness explains in more detail your physiology, which type(s) of intensity training is right for you and how to do intensity training including whether perceived exertion, heart rate or power is best for you. The 41-page Intensity Training is only $4.99.
My eBook Cycling Past 50, 60 and Beyond: Training with Intensity explains what happens to your body as you age and the physiological benefits of riding with intensity. I give you five progressively harder levels of training and give three to five examples each of structured and unstructured workouts for each level of training, a total of almost 40 workouts. The 27-page Cycling Past 50, 60 and Beyond: Training with Intensityis $4.99.
My 10-page eArticle Preventing and Treating Cramps explains the leading theories of what causes cramps including fatigue of the nerves to your muscles and sodium depletion. The article describes what you can do to prevent cramps including sufficient conditioning to prevent fatigue of the nerves, stretching regularly, effective pacing and proper pedaling technique as well as the roles of hydration and electrolytes. The article illustrates with photos how to break the most common cramps by stretching gently and then how to flush the affected muscles to prevent them from cramping again. Preventing and Treating Cramps is $4.99
Coach John Hughes earned coaching certifications from USA Cycling and the National Strength and Conditioning Association. John’s cycling career includes course records in the Boston-Montreal-Boston 1200-km randonnée and the Furnace Creek 508, a Race Across AMerica (RAAM) qualifier. He has ridden solo RAAM twice and is a 5-time finisher of the 1200-km Paris-Brest-Paris. He has written over 40 eBooks and eArticles on cycling training and nutrition, available in RBR’s eBookstore at Coach John Hughes. Click to read John’s full bio.