EVERY training session should have a specific fitness goal, and subsequent sessions should gradually build towards a desired fitness outcome. Too many ‘professionals’ in the industry have a complete lack of knowledge when it comes to interval training. They prescribe something along the lines of 1km track repeats with 1 minute rest, repeated 5 times, with no understanding of what this session is trying to achieve, other than it being a “solid workout”. Interval training is planned periods of increased intensity, accompanied by planned periods of recovery. The key word here is ‘planned’. An interval training session is therefore made up of several ‘work’ components and several ‘rest’ components. This is referred to as the ‘work:rest ratio’. A work period of 5 minutes, accompanied by 2.5 minutes rest has a work:rest ratio of 2:1.

So, What’s Wrong with 1km Repeats with 1 Minute Rest?

Well, nothing, if you can explain why you’re doing them. Are you trying to improve your aerobic power? Are you trying to improve your anaerobic threshold? Are you trying to improve your base-engine? These are the questions professionals are either:

Getting wrong, or;
Prescribing inappropriate work:rest ratios and training intensity to elicit maximum adaptation.

Another question which must be asked is how long does it take you to complete a 1km interval? If there are 10 athletes all completing the same session, the fitter ones will be running their 1km repeats in around 3 minutes, while others may take 5-6 minutes to complete their efforts. If they are all receiving 1 minute rest in between, their work:rest ratios are significantly varied. The fitter ones have a work:rest ratio of 3:1, whereas the less fit athletes have a 5:1 or 6:1 ratio. This can completely change the focus of the session.

A better alternative is to run to time, rather than distance, thereby controlling the work:rest ratios of multiple athletes. I guarantee an athlete will hold a higher pace when they know they are running for 3 minutes, than if they have to run for 1km if their 1km time is up around 5 minutes. Increasing this running speed will again change the focus of the session, because the intensity has increased, and the work:rest ratio has changed in favour of more rest.

The Next Question That Needs to be Answered is Why Did We Choose 1 Minute of Rest in Between 3 Minutes of Effort?

This again ties back to asking ourselves what the focus of the session is. A threshold session aimed at improving our tolerance and clearance of lactic acid will have shorter rest periods than a VO₂ max session aimed at improving our aerobic power. Why? Threshold efforts need to be completed in the presence of increased lactic acid to achieve the desired adaptation. Reducing the rest period allows an athlete to partially recover, but not so much that lactic acid is completely cleared from the muscle. On the flip side, VO₂ max intervals are all about accumulating “time at VO₂ max”. These sessions must be completed above 95% of velocity at VO₂ max to achieve the desired adaptation. How is this achieved? Significantly longer rest periods than threshold efforts to allow athletes to recover and clear out the majority of lactic acid in order to keep the intensity high. I’d love to find an athlete who can sustain above 95% of velocity at VO₂ max for 5x1km intervals with 1 minute rest, it’s just physiologically improbable.

So What is The Ideal Work:Rest Ratio?

This is where the science of physiology and art of coaching needs to find a balance. Training sessions need to be varied to suit an individual and their training needs, taking into account training status, injury history, motivation, and goals. At the end of the day, there are more ways than one to skin a cat, but by structuring your work:rest ratios around the following fundamental rules, you will be able to begin training with focus:

Base Training with The Goal of Improving Your Aerobic Engine.

Main adaptations: increased aerobic capacity through improvements in mitochondria, capillarisation, myoglobin, size of the heart, strength of slow twitch fibres, muscle glucose uptake.
Ideal work:rest ratio: 1:0 (no rest – continuous training).
Ideal intensity: 56% VO₂ max to aerobic threshold (the point at which you begin producing lactic acid above resting values).

Threshold Intervals with The Goal of Improving Your Functional Capacity (Maximum Speed You Can Hold for An Hour).

Main adaptations: improved ability to buffer and clear hydrogen ions (‘burning’ in the legs).
Ideal work:rest ratio: 2:1, with the work phase being above 5 minutes. Example: 10 minutes work, 5 minutes rest/easy jogging.
Ideal intensity: aerobic threshold to anaerobic threshold (the maximum intensity you can hold for an hour).

VO₂ Max Intervals with The Goal of Improving Your Aerobic Power.

Main adaptations: Improvements in mitochondria, capillarisation, myoglobin, hypertrophy of the heart, muscle glucose uptake, oxygen kinetics.
Ideal work:rest ratio: 1:1, with intervals lasting between 2-4 minutes. Example: 3 minutes work, 3 minutes walking recovery.
Ideal intensity: >95% velocity at VO₂ max.

Anaerobic Intervals with The Goal of Improving Peripheral (Muscular) Adaptations.

Main adaptations: Improved ability to tolerate and clear lactic acid, strengthened fast twitch fibres, improved muscle glucose uptake, some improvement in mitochondria.
Ideal work:rest ratio: 1:2 or more rest. Example: 30 seconds on, 60 seconds off.
Ideal intensity: Above 100% velocity at VO₂ max.

Note that these are the fundamental rules of work:rest ratio that you need to know, but which can be modified to suit you or your athlete’s requirements. If you are currently completing 5x1km intervals with 1 minute recovery as a session, that’s fine, it’s just important to understand why you’re completing this session, which training adaptation you are trying to achieve with this, and whether your work, rest or intensity variables can be altered to get a greater training stimulus. With the number of overuse injuries occurring in endurance sports, it’s important to train smarter, not harder.

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Written by Luke McIlroy – Director of Sport Science at METS Performance Consulting

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