If you’re an advocate of injury-free running, you likely place a high value on warming up before your run and cooling down afterward. However, this alone is not enough. You might also engage in interval training to further improve your abilities, but there’s a chance you’re not executing interval training correctly.

Therefore, it’s worth ensuring a proper understanding of interval training and “recovery intervals.”

It’s well known that the benefits of interval training lie in the recovery intervals. Recovery intervals are defined as recovery periods following high-intensity training, allowing the body to prepare for the next about of high-intensity exercise. Recovery intervals can involve slow jogging, brisk walking, or resting, and their purpose is to help the body recover to maintain or improve performance in subsequent exercises.

When discussing recovery during interval training, three variables need to be optimized:

1. Recovery time
2. Mode of recovery
3. Activities during recovery

To answer these questions, consider the following factors:

• Who is training?
• What type of training is being performed?
• What is the training goal?

Today, we will discuss this important topic of interval training!

Interval Training Structure: Repeats and Recovery

In “interval training,” the “interval” refers to the recovery periods between fast runs, and “repeats” refers to the specific durations of fast runs separated by specific paces.

“Repeats” represent the fast-running parts, while “recovery intervals” represent any distance and time between them.

When designing training schedules, recreational runners are recommended to structure recovery in two ways:

• As a set distance (e.g., 16 x 200m with 200m slow jogging recovery)

Choosing the correct recovery length is a broad topic that covers all possible situations and outlines the fundamental science behind what happens during recovery so you can make more informed decisions in your training.

Generally speaking, interval training can be divided into two categories:

1. Repeats performed at intensities above your lactate threshold or critical speed
2. Repeats performed at intensities close to or slightly below your lactate threshold or critical speed

Recovery Time for Repeats Above Lactate Threshold or Critical Speed

When running speed exceeds the lactate threshold (MLSS) or critical speed (CS), your body is in a state of metabolic instability (e.g., oxygen uptake gradually increases to VO2 max, heart rate gradually rises to max heart rate, and blood lactate steadily rises).

In these exercises, the role of recovery intervals is to allow these metrics to return to a baseline so you can repeat the exercise without reaching exhaustion.

The critical speed model is a useful conceptual tool; it suggests that running above critical speed consumes limited “anaerobic” energy reserves. Once these reserves are depleted, you will be too fatigued to continue. For example, running continuously at a 5K pace for 5 kilometers will deplete your anaerobic energy reserves from 100% to 0%.

pubmed.ncbi.nlm.nih.gov/28332113/

Now, if you divide the same amount of running into 5 x 1000m with N seconds of recovery, the difficulty of the exercise depends entirely on the duration and intensity of N.

The length of recovery intervals follows an exponential decay process with a half-life. Recovery happens quickly in the first 10 seconds and then slows down.

For example, the intramuscular phosphocreatine (PCr) levels, an excellent indicator of anaerobic energy dependence, show that recovery occurs quickly in the early stages and then slows down.

When conducting high-intensity 300-meter repeats at 1500m pace, a recovery time of three times the repeat length is necessary. However, this recovery speed varies with an individual’s ability.

Recovery Time for Repeats at or Below MLSS

When engaging in high-intensity aerobic exercise (e.g., CS-, half-marathon pace, or marathon pace runs), during recovery intervals, you can choose between two goals:

1. Clear residual metabolic by-products to prevent rapid lactate accumulation
2. Stress the body to improve its ability to convert lactate into an energy source, a process known as lactate oxidation

These are very different goals and therefore require different strategies for recovery interval lengths.

1. Maintaining Control in High-Intensity Aerobic Training

For classic Daniels’ cruise intervals (e.g., 10 x 3 minutes at T pace), it’s possible to briefly exceed lactate intensity during the repeats. Short rest intervals can help clear metabolic by-products and bring the metabolic state back below MLSS.

In this case, due to the short half-life of the exponential decay function of recovery intervals, 1 minute of recovery time is sufficient for runners in good condition. Stronger runners might reduce this time to 45 or even 30 seconds without much trouble. Even for relatively long T pace repeats (e.g., 3 x 10 minutes at T pace), only 2-3 minutes of recovery is typically needed.

2. Increasing Muscle Lactate Oxidation

For half-marathon and marathon runners, advanced techniques involve using fast recovery intervals to put the body in a physiological state where lactate produced can be used as an aerobic energy source.

This requires two conditions:

1. Increased lactate production in fast muscle fibers
2. High carbohydrate oxidation rate capacity in slow muscle fibers

This can be achieved by alternating between fast but fully aerobic pace repeats (e.g., marathon or half-marathon pace) and “recovery” intervals that are still quite fast—only 10-15% slower than the repeats.

This fast recovery is necessary to create a favorable metabolic condition for lactate transport and oxidation during high-intensity activity. Therefore, recovery intervals should not be too slow to burn significant amounts of lactate during the intervals.

This is why top marathon and half-marathon runners use “alternating pace training,” such as 10 x 1k (105% MP) / 1k (90% MP).

Completing a kilometer at a fast recovery pace allows them to spend more time in a state of high lactate transport and oxidation, thus gaining greater training stimulus for these abilities.

What to Do During Recovery?

Four options: standing, walking, jogging, or fast running, with speeds ranging from very easy running pace to quite fast recovery intensity.

1. Standing Recovery
   Most coaches do not recommend complete standing still. Being completely still for more than thirty seconds is inadvisable. Walking takes advantage of the “muscle pump action” to maintain blood flow, which standing lacks.
2. Walking Recovery
   Walking recovery is used when you want to recover quickly without gaining any additional benefits. For example, 6 x 300m at 100% 1500m pace with 4 minutes of walking recovery.
3. Jogging Recovery
   For healthy runners, jogging won’t significantly hinder recovery.

Example: 4x1600m at T pace with 1 minute and 30 seconds of jogging recovery. At T pace, lactate accumulation doesn’t exceed the threshold, making the recovery half-life short enough that 1 minute and 30 seconds of jogging is sufficient.

4. Running Recovery: Easy, Moderate, Fast
   To understand the impact of intensity during recovery intervals, refer to the illustration showing black and white circles representing different recovery intensities.

Notice that resting recovery is faster than continuing exercise. Black circles represent true rest (sitting on a bike), showing that:

1. Recovery is slower
2. Recovery values drop below the baseline

The speed of recovery isn’t depicted, but it’s faster initially and slows down later, with lower overall anaerobic capacity regeneration at higher recovery intensities.

Using Easy, Moderate, and Fast Running Recovery

Incorporate low-load Fartlek-style accelerations in normal easy runs to help your legs feel stronger for the next day’s training.

Example:

• 60 minutes easy run: The entire session lasts 60 minutes, mostly at an easy pace.
• 30 minutes acceleration run: Within the 60 minutes, spend 30 minutes alternating accelerations.
• 30 seconds fast run: Accelerations are 30-second fast runs at around 5K pace.
• Every 2 minutes and 30 seconds: Each fast run is followed by 2 minutes and 30 seconds of easy pace recovery.

This method integrates short bursts of high intensity within a long easy run, enhancing training efficiency while avoiding overtraining. It’s suitable for runners who want to maintain aerobic base while adding some anaerobic endurance.

Example:

• 8km easy run + 3 sets (2.4km at 101-103% marathon pace, 800m easy to moderate recovery)

Running 7.2km directly at 103% marathon pace might be challenging, but running 2.4km intervals below T pace (around 106% MP) shouldn’t significantly tax your anaerobic reserves. Easy to moderate recovery prevents 2.4km repeats from being too fast.

For half/full marathon runners, this can serve as a foundation for more structured training with quick recovery intervals.

Fast Recovery Intervals

Fast recovery intervals are beneficial for elite runners to break through training plateaus. Proper pacing skills are crucial to execute such training correctly. Overtraining by running repeats too fast or recovering too quickly can be counterproductive.

Previously discussed principles of fast recovery intervals create a metabolic state in muscles favorable for lactate oxidation. Running intervals at or slightly below MLSS for long distances (e.g., 1km at T pace or 2-3km at marathon pace) elevates blood and muscle lactate levels.

High-intensity recovery during intervals helps the body transport and oxidize lactate for energy.

Example:

• 8 sets (1km at 105% MP, 1km at 90% MP)

Such training can evolve in three directions:

• Higher training volume (e.g., 10 x 1k / 1k)
• Greater stretch (e.g., 5 x 2k / 1k)
• Faster recovery pace (e.g., 8 sets of 1km at 105% MP, 1km at 95% MP)

Summary

Most of the time, using time-based recovery is optimal. Distance-based recovery is useful for team training or when you want more flexibility to adjust recovery by feel.

After repeats faster than MLSS, the goal during recovery intervals is to regenerate some of the anaerobic energy consumed.

For repeats at or below MLSS, the two goals are either to prevent repeats from being too fast or to mobilize the body’s lactate transport and oxidation mechanisms.

In the first case, short slow jogs are best. In the latter, you should run faster for longer (about 3-4 minutes) at 5-15% slower than MLSS.

Recovery is an exponential decay process with a half-life, so it starts very quickly and then slows down. Short recovery for moderate-intensity repeats and longer recovery for very fast sprints or extended moderate repeats are needed.

Recovery speed depends on your aerobic capacity. For sub-2:50 marathoners, fast recovery intervals below MLSS mobilize lactate oxidation as fuel, a particularly useful skill. This is why top marathoners use recovery intervals only 5-15% slower than their repeats in MP and HMP training, a secret to elite marathon training.