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In this blog post/podcast episode we want to delve into a few more components of understanding how your body ticks by discussing heart rate variability (HRV), V02 max and anaerobic threshold (AT).
I received a question from a listener related to our podcast about heart rate training. He says,
“The only thing you did not discuss and maybe there is no answer to this, but I’d like to know more about monitoring and tracking recovery heart rate after a workout. I “appear” to recover very quickly after a long or hard workout, as my heart rate comes back to normal quickly. Do you have any data or thoughts on this?”
This excellent question leads into the topic of heart rate variability (HRV).
Heart Rate Variability
When we’re dealing with monitoring heart rate it’s important to understand that our heart rate is not a fixed number and heart rate variability (HRV) would be a better way to look at heart rate values.
HRV is the variation in time intervals between heart beats and is a reflection of the fact that your heart rate changes slightly from beat to beat. For example, when you inhale your heart rate speeds up and it slows down when you exhale. The heart rate is controlled by the autonomic nervous system (ANS) which means it happens without you thinking about it. This is a good thing because it would be horrible to have to think about keeping your heart going. The ANS also controls the nervous system, blood pressure regulation and respiration.
A healthy cardiovascular system is indicated by a heart rate that returns to the normal baseline quickly. The more relaxed and free from fatigue the body is, the more variable the time between heartbeats. HRV data can reflect the impact of fatigue due to prior exercise sessions, hydration levels, allergies, and all types of stress. On the other hand a low HRV has been shown as a predictor of myocardial infarction (or heart attacks) and chronic health conditions, which is something we all want to avoid.
How to measure HRV:
You will need one of the following products:
To Get Started:
- Get the items needed to monitor HRV.
- Measure some baseline readings during a light training week.
- Consistently measure HRV in the same position each day. It takes about a minute to get a reading and then the app will give you the all clear for hard training or suggest a light training or rest day.
Understanding VO2 Max
Endurance running is aerobic exercise, which means it depends upon the use of oxygen. If oxygen is in limited supply then the reaction pathway slows and less ATP (adenosine triphosphate) is produced resulting in slower running. The faster you run the more oxygen must be available to your muscles. This is where V02 max fits in. Think of your muscle fibers as what drives your marathon endurance potential. Vo2 max is what provides constant assistance to these muscle fibers.
VO2 max (stands for maximal rate of oxygen consumption) and is the measurement of the uppermost rate of oxygen consumption in an individual runner and is widely accepted as the single best measure of cardiovascular fitness and maximal aerobic power. It is an indicator of the rate oxygen can be brought in and used by the exercising muscles. Vo2 max values are read this way: 50mL/kg/min which translates to 50 milliliters of oxygen per kilogram of body weight per minute. Basically the higher your Vo2 max number the better your athletic potential, but the variation can be quite large.
Absolute values of VO2 max are typically about 40-60% higher in men than in women. Men typically have higher measurements due to the increased size of their thoracic (chest) cavity and greater muscle mass.
Here are some examples of what Vo2 max could look like:
- Post-heart attack/pulmonary or respiratory disease patient = 22mL/kg/min
- Sedentary adult = 32mL/kg/min
- Male runner with a 5k PR of 18 min = 60mL/kg/min
- Elite endurance runners = 75-85mL/kg/min
- Oscar Svendson- male Norwegian cyclist (highest recorded number)= 97.5mL/kg/min
- Matt Carpenter- Pikes Peak Marathon record holder =92mL/kg/min
- Killian Jornet (ultrarunner)=89.5mL/kg/min
- Steve Prefontaine =84.4mL/kg/min
- Joan Benoit-Samuelson (highest VO2 max for a female runner) =78.6mL/kg/min
- Greta Waitz (marathoner) =73.5mL/kg/min
- Thoroughbred racing horse = 180mL/kg/min
- Siberian Husky running in the Iditarod = 240mL/kg/min
Tim Noakes, a professor of exercise and sports science at the University of Cape Town, describes a number of variables that may affect VO2 max: age, gender, fitness and training, changes in altitude, and the action of the ventilatory (breathing) muscles. Even though a person has a high Vo2 max it doesn’t automatically mean that they’ll have the fastest race time. Some runners might not reach their V02 max before the leg muscles start working anaerobically (they run out of oxygen) which leads to the release of high amounts of lactic acid. If the lactic acid builds up it interferes with muscle contraction, causing fatigue and leads to a slower pace.
If a couch potato does a program of 3 months of endurance running they may raise their VO2 max by about 15-20%. You can also see a small drop after 2 weeks of not training. Building up your ability to run longer and faster in the aerobic zone will cause some improvement in performance. Then once you have a good aerobic base built it’s actually higher intensity interval training that will boost V02 max the most.
How to Test Your Vo2 Max:
- The best way to get your VO2 max tested accurately is to go to an exercise science lab.
- The self-test I did is called the Cooper Test. It was developed by Kenneth Cooper, a well known physician and researcher who introduced the concept of aerobics in the 1960’s. He conducted a study in the Air Force which resulted in the Cooper Test to determine VO2 max.
- To perform this test start by warming up with 10-15 minutes of easy running.
- Then run around a track for 12 minutes at a sustained moderately hard pace and record how many meters you covered.
- In this formula V02 max is the measure of distance covered running in 12 minutes (in meters) minus 505, divided by 45. My formula looks like this: 2494.5meters minus 505 divided by 45= 44.2mL/kg/min
You can also do an estimate V02 max though a number of tests. Brian Mac has several tests to choose from on his website.
Understanding Anaerobic Threshold
The anaerobic system functions primarily without oxygen. It can provide a powerful boost to your running but it can only provide short bursts of speed before energy stores are depleted and lactic acid builds up in the muscles. Too much lactic acid or lactate in your system isn’t productive for marathon running but it can actually serve as a temporary energy source for the muscles (think of it as the last crumbs of a meal). Lactate is a product of glycosis (the conversion of carbohydrates into glycogen). It’s a natural by-product of the body produced by muscle contraction.
During exercise your body relies on the aerobic pathways to meet oxygen demand and when it can no longer keep up the anaerobic pathways make up the difference. After you go beyond your aerobic threshold (the point where your body clears lactic acid out at a steady pace) you hit the lactate threshold (or anaerobic threshold (AT) where lactic acid starts to build up in your bloodstream. AT is one of the best predictors of endurance performance and usually occurs at between 60-90% of your Vo2 max. If you take a group of runners with similar Vo2 max values the person with the higher AT will have the racing advantage.
Fueling the Systems
Another component of the aerobic/anaerobic balance is glycogen depletion. The energy to fuel both of these systems comes from fats and carbohydrates (and a small amount of protein and phosphates). Our bodies can store limited amounts of carbohydrates in the liver and muscles in the form of glycogen. However it can also convert our fat stores into energy.
Fat is very high in energy as it provides nearly twice as many calories per gram as carbohydrates. However the oxidation (or converting) of fat to energy is slower. Fat can only be oxidized or burned in the presence of oxygen and your body will happily burn fat until approximately 50% of your Vo2 max is reached. When the intensity of your running increases above this point the body will turn to carbohydrate stores.
The point where carb stores are being burned is at the aerobic threshold. Your body needs primarily glycogen for faster paced running and the body only stores around 2 hours worth. If you’ve ever “hit the wall” that’s probably the point where your body ran out of glycogen, burned through the glucose in your blood and then left you with low energy.
The Fat Burning Zone
The key for a marathoner is to train the body to burn fat longer by doing most of your training runs under the aerobic threshold. You may hear about Zone 2 training which is in the fat burning zone. As you do endurance training your body adds mitochondria (the powerhouse of the cells) which adds more enzyme activity and oxygen to your system. The enzymes and oxygen allow more fat to be oxidized (burned) for energy in muscle contractions.
The longer you can burn fat during a long run or marathon, the longer you can hold on to your glycogen stores. This is one reason why it’s important to pay attention to pacing during your training. One purpose of your easy training runs is to teach your body to oxidize fat more effectively. If you run every workout at the same pace (usually upper zone 3-4) you never train your body to burn fat more efficiently and instead rely almost completely on glycogen.
You can go your whole running life and be completely happy without totally understanding V02 max, heart rate variability, anaerobic threshold or lactate threshold. However I believe that the more you educate yourself about the workings of your body the more it will empower you to get better results in your training, accomplish your goals and reach your marathon potential.
(1) Running Science by Owen Anderson, Phd
(2) Lore of Running by Tim Noakes
(8) Hanson’s Marathon Method, p.23