Interval Training for Endurance Running: How Fast is Too Fast?

Running interval training is an effective way to boost stamina for endurance runners. This is especially true for those runners who’ve built up a good base of aerobic endurance but there fitness is starting to plateau.

Today we will look at how to set up your interval training to maximise the benefits whilst minimising fatigue and avoiding unnecessary workouts with little value. Ignore the “sciency” stuff if it’s not your thing and head straight to Step 1.

The intervals we will dive into are critical velocity intervals. Critical velocity (CV) is a pace you can maintain for 40-60 minutes. Basically it is a pace that is slower than VO2max but is slightly faster than your lactate threshold.

Completing intervals at CV helps recruit type IIa muscle fibres which in turn helps build stamina in endurance runners (time to exhaustion at a given pace). At critical velocity, all musculature and motor units are engaged. By recruiting these less used muscle fibre types in training, you are able to maintain speeds for longer before you fatigue on race day.

Now that you know the basics of CV intervals, we’ll take you through 5 steps to help build your CV interval training session

Step 1: Determine your critical velocity

There are multiple ways to do get an approximate estimate of critical velocity:

  • Run for 3 minutes on a flat track at an all out pace. Do not pace yourself, keep pushing as hard as you can for 3 minutes. The average pace of your last minute is a good estimate of your critical velocity.
  • For more experienced runners, your 10km race pace is approximately your critical velocity pace. E.g for a 40 minute 10k, your critical velocity is 4 min/km.
  • Run for 1 hour and try cover as much distance as possible over this time. Your average pace will be a good estimate of your critical velocity.

Step 2: Constructing your interval training workout

First and foremost, the scientific evidence on how to best construct running intervals sessions is flimsy at best. Most studies look at average runners who’s aerobic capacity would improve from almost any type of interval training session. 

Our advice is based on some of this evidence, but more on the advice of exercise physiologists and experienced coaches working in the endurance and ultra running field.

Length of intervals

For moderately trained runners, intervals should be 3-6 minutes in length. For runners new to the sport, intervals of 1-2 minutes are fine, with an aim to build up to longer intervals. 

Length of rest period

The length of the rest interval is not important for longer intervals. For people doing sprint interval training, the rest period is very important (we will discuss sprint intervals in a future article). For CV intervals, a rest period anywhere between 2-5 minutes is fine. The aim of the rest period is to recover enough to be able to complete another interval at CV pace, not to fully recover. 

Rest intervals should be run at a slow jog. Do not stand still for your rest intervals. A slow jog will ensure your aerobic system does not have time to recover, meaning you will continue to challenge your aerobic system on the next interval.

Number of intervals

For a moderately trained runner, the aim is to reach 30-60 minutes of time at critical velocity per 7-10 days.

So, we can construct our interval sessions in many ways to reach this target.

For a beginner runner:

6-8 x 2 minute intervals at CV pace with 3 minute rest intervals, 2 x per week. (Total time at CV= 24-32 minutes

For a well trained runner: 6-7 x 4 minute intervals at CV with 2 minute rest, 2 x per week. (Total time at CV= 48-56 minutes

OR to complete it in one harder session. 2 sets of 6 x 5 minute intervals with 3 minute rests. Divide the 2 sets with a 5-10 minute slow jog. Do this once per week (Total time at CV= 60 minutes)

Step 3: How often should I complete critical velocity intervals

As in the examples above, 1-2 session for every 10 training sessions is the perfect amount. Anymore more than 1-2 sessions does not seem to result in further improvements. We recommend splinting the intervals into 2 seperate sessions of beginners. For well trained runners, doing the CV intervals in one harder session is appropriate. 

Step 4: When to complete interval training

For most runners, including interval training all year round is appropriate. Depending on your goals, you may aim for 60 minutes at CV per week during peak training, whilst looking at 30 minutes at CV per week during recovery periods.

Step 5: Re-measure your critical velocity

Once you have incorporate CV training into your program for 4-8 weeks, we recommend remeasuring your critical velocity using the measure used it step one. By doing this, you can monitor improvements in CV and adjust your interval pace accordingly.

Important tips on critical velocity training

  • If you notice your CV pace is no longer improving despite consistent training, it may be a good idea to rest from CV interval training for 1-2 weeks. This will give your body time to recover and adapt appropriately, as you may be overreaching with your training. 
  • Do not neglect top speed running. Many runners will stop doing top speed work, thinking CV intervals are enough. It is important to include sprint training to ensure all muscle fibres are recruited in training. Even doing 5-6 80-100 meter sprints during a 10k easy run is enough to maintain this top speed.
  • Critical velocity training works best for 800m runners up to ultra marathon runners. For shorter distance runners, short sprint intervals are more important.
  • The reason longer intervals at faster paces aren’t necessary for endurance runners is that at CV pace, all musculature used in endurance running is engaged. Paces faster than CV will lead to increased fatigue and increased recovery time without further benefits.
  • You do not have to complete intervals on the same repeated stretch of road or track. Intervals can easily be completed on a trail run. Just ensure you consider hills when working out your CV pace.

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