My easy pace seems too slow. Surely it should be faster to be of benefit?

This is very common concern amongst both beginners and experienced runners alike.

Despite people often reading about polarised and pyramidal training and ‘understanding’ that the majority of runs should be easy, the common-sense thought of ‘train hard to race hard’ is such an overriding belief / urge that their easy runs become unnecessarily fast.

The answer to this question lies within the aerobic system, which consists of metabolic pathways that allow you to run in the presence of oxygen. The important two points being that the aerobic system is significantly more important for running that the anaerobic system. For a 1 mile race it contributes around 80% of the energy production, and nearly 98% for a marathon race. And to promote most efficient enhancements to the aerobic system you need to spend a lot of time training aerobically, which means running slowly.

There are three physiological processes observed with development of the aerobic system. These are:

1. Capillary development.
2. Mitochondria development.
3. and Myoglobin production.

And studies examining promotion of all these facets indicate that running very slowly is more beneficial that running fast, with most benefits being gained when running in the 50% to 75% of 5km race pace range. As you begin to run faster than 75% of your 5km race pace the benefits progressively drop.

So if you are a 20 minute 5km runner (4:00 min/km pace), you can quite happily run your easy runs at 8:00 min/km and still realise near optimal physiological aerobic adaptation. And similarly, for a 30 minute 5km runner, they can run as slow as 12:00 min/km.

So always remember the adage:

Train slow to race fast.

References and Further Reading

1. Gastin PB. Energy system interaction and relative contribution during maximal exercise. Sports Med. 2001;31(10):725-41.
2. Fitts RH, Widrick JJ. Muscle mechanics: adaptations with exercise-training. Exerc Sport Sci Rev. 1996;24:427-73.
3. Dudley GA, Abraham WM, Terjung RL. Influence of exercise intensity and duration on biochemical adaptations in skeletal muscle. J Appl Physiol Respir Environ Exerc Physiol. 1982 Oct;53(4):844-50. doi: 10.1152/jappl.1982.53.4.844.
4. Terjung RL. Muscle fiber involvement during training of different intensities and durations. Am J Physiol. 1976 Apr;230(4):946-50. doi: 10.1152/ajplegacy.1976.230.4.946.
5. Holloszy JO. Biochemical adaptations in muscle. Effects of exercise on mitochondrial oxygen uptake and respiratory enzyme activity in skeletal muscle. J Biol Chem. 1967 May 10;242(9):2278-82.