It’s Sunday morning. You get out of bed and get ready to go out for a run. You put on your moisture-wicking socks and shirt, lace your shoes, and head out the door. Before you even take your first step, the cortex of your brain stimulates your autonomic nervous system, which causes your blood vessels to constrict and your blood pressure to rise.
Within a few strides of your run, you start to breathe faster and deeper. The number of times your heart beats each minute (heart rate) and the volume of blood your heart ejects with each of those beats (stroke volume) both rise to match the greater demand of your muscles for oxygen. More blood flows through your vessels—15 to 20 times more than when you’re sitting on your couch.
Feeling a bit frisky, you decide to pick up the pace. You start to breathe heavier. Heart rate and stroke volume rise, causing a rise in cardiac output (the volume of blood your heart pumps each minute). Blood flow to your muscles increases. The volume of oxygen you consume increases to keep up with the demand of the run.
You keep picking up the pace until you’re revving your aerobic engine as fast as it can go. Cardiac output is maxed out. You’re huffing and puffing like you’re going to blow Grandma’s house down. Your muscles consume even more oxygen. The pace you’re running when you’ve reached the maximum volume of oxygen that your muscles consume corresponds to a popular physiological marker called VO2max—the maximum volume of oxygen your muscles consume per minute.
- To understand what VO2max is.
- To describe the factors that influence VO2max.
- To give recommendations for specific VO2max workouts.
Together, your maximum cardiac output and the maximum amount of oxygen your muscles extract and use (reflected by muscle mitochondrial volume and enzyme activity) determine VO2max. First measured in humans in the 1920s, VO2max is considered the single best indicator of your aerobic fitness. Think of VO2max as the size of your aerobic engine.
The VO2max of unfit people is limited by both cardiac and muscular/metabolic factors (they lack both a high blood flow to the muscles and abundant metabolic machinery), while the VO2max of aerobically-trained people is more cardiac limited. Training shifts that limitation—the more fit your clients become, the more they move away from a muscular/metabolic limitation to VO2max and the closer they move to an oxygen supply limitation because muscle mitochondrial content can increase a lot more than stroke volume and cardiac output can.
VO2max can be measured either in absolute terms (liters per minute, L/min) or, to compare individuals of different sizes, in relative terms (milliliters per kilogram of body weight per minute, ml/kg/min). The average male has a VO2max around 40 ml/kg/min, and the average female’s VO2max is about ten percent less because she has a smaller heart to pump blood and less hemoglobin in her blood to transport oxygen. By comparison, the VO2max of elite male and female endurance athletes is greater than 75 and 65 ml/kg/min, respectively.
Putting the Concepts to Work: VO2max Workouts
You can improve VO2max a couple of different ways:
- By increasing weekly aerobic exercise volume, which increases blood volume, oxygen transport capacity of the blood, and the muscles’ metabolic machinery (i.e., mitochondria) to use oxygen.
- By interval training at maximum heart rate, which enhances the power of the cardiovascular system to send more blood and oxygen to the muscles.
The more aerobically fit your client is, the more important the intensity of training becomes to improve VO2max. Repeatedly exercising at maximum heart rate and cardiac output is threatening to the cardiovascular system. When the body is threatened, it makes adaptations that assuage the threat and ultimately increase its capability. One of the most elegant adaptations to interval training is hypertrophy of the left ventricle of the heart, causing an increase in maximum stroke volume. The larger the left ventricle, the more blood it can hold; the more blood it can hold, the more it can pump.
VO2max interval workouts should consist of work periods 3 to 5 minutes long, with recovery intervals equal to, or slightly less than equal to, the duration of the work periods. The goal is to reach VO2max during the reps, so the workout intensity should yield 95 to 100 percent max heart rate and feel hard but manageable, about a 9 on a scale of 1 to 10.
Interval workouts are very demanding, but they’re the best types of workouts to improve cardiovascular conditioning. If training runners, VO2max pace corresponds to 1- to 1.5-mile race pace for beginner and recreational runners, and 3K (2-mile) race pace for highly-trained and competitive runners.
All workouts should be preceded by a warm-up that starts at a low intensity and gradually increases in intensity to create a smooth transition from warm-up to workout. Examples of VO2max workouts:
- 6 x 3-minute reps at VO2max intensity with 2 to 3 minutes of active recovery between reps
- 4 x 4-minute reps at VO2max intensity with 3 to 4 minutes of active recovery between reps
- 3 x 5-minute reps at VO2max intensity with 4 to 5 minutes of active recovery between reps
- 3-, 4-, 3-, 4-, and 3-minute reps at VO2max intensity with equal time active recovery between reps
- 2 sets of 3-, 4-, and 5-minute reps at VO2max intensity with equal time active recovery between reps
- 3-, 3½-, 4-, 4½-, 4-, 3½-, and 3-minute reps at VO2max intensity with equal time active recovery between reps
The shorter the reps, the more reps can be done at VO2max intensity. Although it is tempting to exercise at a higher intensity (or, for runners, to run faster) when the reps are shorter, the intensity should be the same for all VO2max workouts because the purpose is the same—to improve VO2max. Exercising at an intensity higher than VO2max is not any better for improving VO2max than exercising at VO2max.
As your clients progress, make the workouts more challenging by adding more reps, making the reps longer, or by decreasing the duration of the recovery intervals rather than by increasing the intensity.
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