Cardiorespiratory endurance (CRE), also called aerobic endurance, is the ability to perform aerobic activities for a long period of time. It involves two body systems: the circulatory system, including the heart (cardio) and blood vessels; and the respiratory system, which involves the lungs. CRE is usually measured in the military with a run (1.5-, 2-, or 3-mile) for distance.
CRE is an important component of physical fitness. It not only is closely related to your risk of developing preventable illnesses like heart disease, but also your ability to perform on runs, hikes, and other long-duration activities. CRE is also linked to injury risk, so those with poor CRE sustain injuries at higher rates than those with higher levels of fitness. There’s no established “cutoff” for when you become high-risk for injury, but you can figure out roughly where you fall by how well you do on your PFT run. Generally, the closer to the front of the pack you are, the lower your injury risk.
Assess your CRE
Before you start building your CRE training program, you’ll need to know where you stand. If it’s been a while since your last PFT, start off with the run component and make note of your time.
Follow the FITT principle
When you’re designing your CRE training program, follow the FITT principle—consider the Frequency, Intensity, Time, and Type of your workouts. For CRE training, most of the variables will be tied to the type of workout you’re doing. You won’t usually see a high-intensity workout done for a long period of time.
Frequency is the number of sessions in a week that you train. CRE training is usually scheduled for 3–4 days a week. However, you’ll need to take into account any resistance training you’re doing and what type. For example, if you’re in a muscular power phase of your program, you can use sprint workouts for both CRE and power. You don’t need to do 3 dedicated days of CRE in addition to power training. It might just take 1 day for a longer run, or 1–2 easy recovery runs.
Intensity for CRE training is determined as a percentage of your maximum heart rate (max HR). The easiest formula for calculating your max HR is 220 – your age. There are more accurate formulas out there, but this one is close enough for most people.
Time is how long each workout is done for, and it will depend on the type of workout you’re doing.
Type is the kind of CRE training you do. It’s not just running, rowing, or swimming, for example, but specifically how you do each type. Here are common CRE training types and their FITT variables.
Long, slow distance (LSD) is exactly what it sounds like: long distances at a relatively slow pace. What defines “long” and “slow” is specific to your individual fitness level. LSD training is usually done 1–2 times a week, at about 80% of your max heart rate. Depending on your level of fitness, LSD workouts should last about 30 minutes—shorter if you’re out of shape.
Pace/Tempo training is where you run—or bike, swim, etc.—for a set time or distance, similar to LSD training, except you do it at a higher intensity. Tempo runs are incorporated into a CRE training plan 1–2 times each week, and the intensity and time are typically structured around training for a race event. If you’re not an avid 5K or 10K racer, then your PFT run would work for your “race.” There are a few ways to design your tempo-run workouts.
The first way is less scientific and easy to figure out: You run your race distance at a “pace that you could run for an hour” without stopping. It should be fairly difficult, but not exhausting. As your CRE improves, your pace will get faster over time.
The other way to design tempo-workout structures requires you to break out your calculator. You take your known race time, and set your training pace at a certain percentage of that time. For example, if you run 2 miles in 14 minutes, you can set a 4-week tempo plan that runs at 75%, 85%, 95%, and 105% for each week. The lower percentages correspond to easier runs. To calculate the training pace for the 75% week, you take your race time (14 minutes) and add 25% (3 minutes, 30 seconds). This means that you should aim to complete a 2-mile run in 17 minutes, 30 seconds. During the 85% week, you add 15% (2 minutes, 6 seconds), so you complete 2 miles in 16 minutes, 6 seconds. And finally for the 105% week, you subtract 5% (about 4 seconds) from your race time, so you complete 2 miles in 13 minutes, 56 seconds.
If the only “race” you’re training for is your PFT, it’s still recommended to increase the volume (time or distance) of your runs. If you have a 1.5- or 2-mile PFT, training for a 5K would be good, and if you have a 3-mile run, try training for 5 miles.
Sprint repeats are a form of interval training where you sprint for a short distance, no more than 200 yards or 30 seconds, at 100% effort. Rest periods should allow for near-maximal recovery, or 1–3 minutes depending on the sprint duration, to best promote CRE improvement. They’re incorporated into a CRE training plan 1–2 days per week. The “time” variable for sprint training is a range from 5–20 sprints, depending on how long each one is.
High-intensity interval training (HIIT) is similar to sprint-repeat training, except it can be any form of training, not necessarily sprints. Since HIIT taxes the body, it should only be done 1 day a week when incorporated into a full CRE training program. Work periods can range from under 30 seconds to a max of about 90 seconds, and work:rest ratios are about 1:5. The rest is very important for HIIT training. If it’s too short, you can overwork your body, which can decrease performance and increase injury risk. If it’s too long, you won’t see as much benefit from this type of workout.
Fartlek training is the last common training technique to improve CRE. For a Fartlek workout, you build in short bouts of higher-intensity activity into an LSD workout. So if you’re out for a run, you can occasionally sprint between a couple landmarks on your route—for instance, from light pole to light pole. The intensity should be on the higher end of the spectrum, at close to max HR for the short bursts.
Planning your CRE program can be difficult and requires some advance prep. Using a block-periodized structure is very effective in organizing your workouts and ensuring you make progress over time. In general, for the first few weeks of a new program, you’ll gradually increase volume following the 10% rule. The weekly CRE training volume should increase by no more than 10% each week. So if you run 5 miles the first week, you should run no more than 5.5 miles over week 2. After the first 4 weeks, you can start to increase the running intensity through the next training block.
For those starting a new CRE training program, high-intensity workouts need to be balanced with low intensity ones to prevent overtraining. The general rule is that for every single high-stress workout, there should be 2 low-stress workouts in the same week, or a 1:2 ratio. More highly trained people can incorporate more high-stress workouts, eventually progressing to a ratio of no more than 2:1.
Next, it’s crucial to balance your CRE workouts with any resistance training you might be doing. Remember that speed workouts can also help build muscular power, so they can be incorporated well into that type of resistance-training block. LSD training at lower volumes can also work well as an active-recovery run during muscular-strength- or muscular–endurance-building resistance training.
Finally, rest is also important for CRE training programs. For every 12 weeks of continuous training, there should always be one active rest week at the end where you avoid an active exercise program. Staying physically active is important, but it should be easy, and high-intensity workouts should be avoided.