Cardiovascular Endurance 101

Bottom Line Up Front: I get a lot of questions about “cardio” or hear someone say how they do “cardio” two times per week, but I don’t hear a lot of explaining about what that really means. This post will clarify the potential outcomes from cardiovascular endurance training and discuss some of the nuance to the variations. It also will have templates and examples for different workouts for different goals.

Image from CG Trader

First, let’s clarify terms.

Cardio = cardiovascular endurance. It is basically the ability to deliver oxygen to muscles. Lungs breath in oxygen, it passes to the blood, red blood cells pick it up, blood delivers it to the muscles, and, finally, the muscles use it to produce energy. A way to measure your cardiovascular endurance capacity is a VO2 max, maximal volume of oxygen uptake (V = volume, O2 = oxygen). It is measured in milliliters of oxygen used in one minute per kilogram of body weight (mL/kg/min). A good VO2 max for a middle aged man is 40. An elite level is above 60 and as high as 90. Heart rate isn’t a good measure to determine your capacity to deliver oxygen, but it can be useful in determining how difficult some activity is (higher heart rate = more difficult). Here are some search results for VO2 max if you want to get into it in greater detail.

Second, let’s talk about why you would do cardio.

If you are doing “cardio” you are working on some form of oxygen delivery, whether it’s increasing how much oxygen can be delivered and/or how long it can be delivered. A third reason may be to sustain your existing cardiovascular endurance capacity (this is much easier). Deep dive into the three goals of cardio training time.

  • Goal 1: Increase total oxygen delivery – If you are trying to improve the total capacity to deliver oxygen, you are trying to increase your VO2 max. This is exactly like getting stronger by lifting heavy things. You want your top end limit to go up. In theory, if you increased your VO2 max, you would perform a cardiovascular endurance event faster. If you had a VO2 max of 40 mL/kg/min and can run a mile in 7 minutes, if you trained to improve it to 45 you should run a mile in less than 7 minutes now. I wouldn’t say it’s an equal increase necessarily, i.e. if you improve your VO2 max by 10%, you would run 10% faster because there are too many variables when factoring in performance in an event other than max potential.
  • Goal 2: Increase length of delivery – If you are trying to perform long bouts of cardio endurance activity, you want to be able to go for a long time at a high percentage of your VO2 max. You don’t need a higher max but to operate at a high percentage of that max for a longer length of time. For example, same guy with a V02 max of 40 is able to run a three miles in 21 minutes by holding at 70% of his VO2 max for the majority of the run (his steady state pace). If he were to not increase his VO2 max but instead train his body to operate at a higher percentage of his max, he would run three miles now in less than 21 minutes.
  • Goal 3: Sustain – In this case, you aren’t improving anything, just trying to sustain capacity either for performance or for health (holding on to a healthy baseline of cardio endurance). This may be for a variety of reasons. One would simply be for a non-athlete who wants to be healthy. Another would be for an athlete who is periodizing training. He or she may be focusing on improving strength and sustaining cardio endurance for a phase of their training. The good news is sustaining requires a lot less time and energy than improving something.

Third, let’s talk about training for each of these goals.

Note there is a lot of bleedover with each, i.e. by training to increase your VO2 max, you may improve both the absolute max and your ability to work at higher percentages of it for longer periods of time and vice versa. Training for cardio endurance is perfectly analogous to training for strength. Getting stronger = increasing VO2 max. Better muscular endurance (ability to do a lot of reps at a given weight) = increasing your steady state speeds. If you got stronger in the bench press, it probably would lead to more push-ups although that wasn’t the goal. Remember that when training, we are trying to get an adaptation from the body. The body is stingy/economical with resources. It won’t allocate resources to improve something unless it has to for survival. Your body doesn’t know you are running on a treadmill in order to run a marathon in the Fall. It thinks you are running away from something or running after something you need to survive, so it adapts. If you don’t challenge it, it won’t adapt either. If you can run a 7-minute mile and you never try to run faster than a 7-minute mile, your body won’t adapt because it doesn’t need to as it can already accomplish the task. It will hold on to what it has, however, as long as you keep asking it to run that speed occasionally.

We will use heart rate now as a measure of effort because it is not possible for almost anyone to know what percentage of their VO2 max they are operating at without being in a lab hooked up to a machine. We will assume max heart rate is equal to VO2 max, i.e. if you are at your theoretical max heart rate, you are at your actual VO2 max. While there are many ways to determine max heart rate, a quick and dirty method is to simply subtract your age from 220 to get a ballpark figure. Max heart rate tends to go down with age so this accounts for that. As any generalized measure, it fails to account for individual variance such as gender, fitness levels, etc. You would be surprised though how close it is in my experience. So for a 40-year old, max heart rate is 180 beats per minute (220-40 = 180 BPM).

Note also that you can do any traditional or non-traditional “cardio” activity to get cardio benefits. Traditional is walking/running, rowing, cycling, jumping rope, or swimming. You can also get your heart rate up very high doing burpees, bodyweight squats, or high-rep kettlebell swings. Heart rate is the key here really no matter how you do it.

  • Goal 1: Increase total oxygen delivery – In order to increase your max, you have to push the limits of your max. Again, just like strength training, if you want to squat more for a one rep max, you have to lift weights at/near/above your max to force the body to adapt. To get an adaptation for increasing VO2 max, you need to train at high heart rates at/near your max. General guidance would be to periodically operate at or above 90% of your theoretical max, which you can obviously only do for a short period of time. If it’s your max, you can’t redline there for 20 minutes. Just like max speed or max strength, the nature of max effort is it can’t be held for other than a short period of time. Interval training is perfect for increasing your VO2 max.
    • Example workout: 10 rounds of row sprints. Each round is 20 seconds of all out effort followed by 40 seconds of rest. This is a 1 to 2 work to rest ratio. As you do these rounds, you will get more and more fatigued if you keep up the intensity. You won’t be able to get a max heart rate most likely on your first few intervals no matter how hard you row. By the end starting at maybe the 5th, 6th or 7th rounds, you should hit at or near your max heart rate a few times. I did this today and got to 160 BPM on my last two which is above 90% of my theoretical max.
    • Variation: You could perform each interval work portion until you reach 90% or more of your theoretical max heart rate, then rest until your heart rate gets down to a lower threshold recovery number, say 60% of max. For the 40-year old (max HR of 220-40 = 180), this would mean rowing until you get to 162 beat per minute (90% of 180) then resting until you heart rate gets to 108 BPM (60% of 180). Repeat 10 times.
    • Notes: You could vary this structure a lot more ways. More total rounds, longer work times, less rest times, etc. There is no magic number that can be applied to everyone. The idea is to hit a high heart rate at least a few times in a given workout. How many times per week you need to do this is also dependent on a lot of factors. If you are a cardiovascular endurance athlete (runner of marathons, ultras, or 5ks; rower; cross country skier; etc.) you will likely do this more than a generalist who wants to be all around fit (Soldier, police, someone into general physical preparedness). As a Soldier, I get my heart rate high in one session per week on average.
  • Goal 2: Increase length of delivery – In order to get better at this, we don’t want to push into our VO2 max but instead want to operate at a slowly increasing percentage of it over time. To do this, we are generally going to settle into long, slow sessions. Some people call these “tempo runs”, which is basically prescribed sub-maximal pace running close to your max steady state pace. Remember we have to challenge ourselves here or the body won’t respond. So if you can run a 7-minute mile, you can’t run at a 15 minute per mile pace and expect to increase your steady state speed. We want to push the body for some length of time (highly variable here based on goals, but generally for 20-40 minutes) at a pace that is at your steady state max, the pace at which you can’t go faster before fatiguing so much that you have to slow down.
    • Example workout: Row for 20 minutes at a pace of 2 minutes per 500 meters. This should be challenging to complete yet a pace you can hold for the entire time.
    • Variation: Just like for goal 1, we can use heart rate instead of distance or pace. We could do 20 minutes at 70% of max heart rate. Adjust percentage based on difficulty, i.e. if 70% for 20 minutes is no problem, try 75% or 80% next time. Either way, you are using heart rate to determine your pace. You can adjust pace inside the workout to keep the heart rate the same. As you get into the end and are fatigued, you may need to slow down to stay at the same heart rate.
  • Goal 3: Sustain – In order to sustain a certain VO2 max or steady state threshold pace, you still need to get there periodically to remind the body you want to keep allocating resources to this physical capability. You just don’t need to train as often or as long when you train. Method of training is similar as outlined above in both goal 1 and 2, but length of each session, intensity (heart rate levels), and how often you do it can be reduced. In some cases, it can be reduced a lot. I have found I could go weeks without any cardio training and maintain my performance. I didn’t just sit on the couch and eat Doritos, but I didn’t do any direct cardio training. This will vary person to person due to health, fitness, etc. Just like any skill (cardio, strength, guitar, dancing, math, foreign language), periodic practice can keep you proficient, but in order to get better, you need to practice harder and/or more often.

There has been a lot of talk recently in health and fitness circles about “zone 2 cardio”. Heart rate zones are just ranges to quantify intensity. Zone 1 is easy (50-60% of max), zone 2 is light (60-70%), zone 3 is moderately challenging (70-80%), zone 4 is difficult (80-90%), and zone 5 is maximum effort (90% and above). For non-athletes, mostly for health, a lot of people I trust/admire/respect have moved to training in zone 2 a few times per week for 20-30 minutes. They have given up the hard cardio sessions as either unproductive, unnecessary, or even unhealthy. I tend to agree with this assessment and would encourage everyone who is not an athlete, i.e. someone who doesn’t depend on cardiovascular endurance during physical performance for their job, to largely follow this method of training 1-3 times per week at a light to moderate intensity to have a well-functioning cardiovascular system. I would couple this with strength training for everyone, which is another article in and of itself. But you would do very well with two light cardio sessions per week using any modality and two strength training sessions (one upper body, one lower). This will cover cardiovascular health, retain/increase muscle mass, and retain/improve bone density.

As far as cardio training, I would term “athletes” in this instance to include those in sports that demand cardiovascular endurance (basketball, soccer, rowing, swimming, or running) and those in jobs where they may need to display cardiovascular endurance such as the military, police, and firefighters. These athletes need to train for both goals 1 and 2 generally.

If you are a football lineman, volleyball player, or powerlifter, you are an athlete but do not have any cardio demands in your sport. You would still benefit from cardio training and could choose a few zone 2 sessions every week to maintain health.

As a Soldier (non-cardio sport athlete and generalist), I have found 1-2 cardio sessions per week is more than enough. I usually do equal amounts of high intensity intervals and steady state cardio work. Lately I have been combining them in the same session although doing one interval session and then the next being a steady state session is common for me. Today I did a combined session of 20 minutes of intervals (30 seconds of effort, 1-2 minutes of rest) on a rower then 10 minutes of steady state walking and jogging on an incline treadmill right afterwards. Note I kept increasing the speed and incline of the treadmill but did not take any breaks, so my heart rate continued to rise as I finished running at the highest incline. I had an average heart rate of 127 BPM for the session with a 173 BPM maximum reached. You can see the results from my session graphically below.

Final Thoughts

Last few comments on this topic, mostly these are pet peeves of mine. First, is that doing cardio every day or more than a few times per week is almost always utterly useless. You just can’t do the same thing over and over without either wasting time or increasing likelihood of injury. I see Soldiers often running, usually long slow running, every single day they do PT. They would get most likely the same results with about 25% of the work. They would be able to get bigger, faster, stronger, or more powerful with all that time back. They also are chronically injured. Knees, hips, back, etc. all from just over-training (really over-running). If you are a career military member who needs a knee or hip replacement from simply doing PT, you are doing it wrong. You are costing the US government a ton of money on your care too. And your quality of life the rest of your life will suffer also. Do you ever see anyone (who knows what they are doing) do the exact same strength training routine every day? Doing the same four-mile run each day is exactly like doing the exact same 5 sets of 5 back squats with the same weight each day. More doesn’t make you better to a point because you have to factor in recovery. It’s not like practicing your times tables every day or playing the piano every day because recovery between bouts is not important.

Second, long, slow cardio capacity is probably the least useful physical performance skill to have. Think about an end of the world scenario (an actual SHTF moment), how often do you think you would need to run at 70% of your max heart rate for a long period of time? Think also about a hunter-gatherer 50,000 years ago too. In both cases, there is a lot of walking and a lot of moving light objects. Occasionally, there would be bursts of high intensity either lifting something heavy or sprinting from or to something. Look at the body of a competitive marathon runner. Do you want to look and perform like that? Unable to deadlift your body weight off the ground, emaciated, weak, and even likely unhealthy? Not me.

Finally, if you do choose to use running as your modality for cardio sessions, avoid pavement, run on inclines (it is impossible to heal strike on an incline), and wear good shoes (see my approved footwear article for an explanation and recommendations).

There is a lot of nuance to this, which is where a qualified trainer can help smooth out the edges as to how long, how often, what types, etc. you should be doing based on your goals. Hopefully you now have a greater depth of understanding of this topic.

Post questions/thoughts to comments.