Most of our 600 muscles are composed of a mix of three different muscle fibre types, which vary in their abilities to produce force and energy. Slow-twitch muscle fibres, also known as type I, have low power but high endurance capabilities. Fast-twitch muscle fibres, which come in two varieties (type IIa and type IIb), have low endurance but exert more force than slow-twitch fibres. All of these muscle fibres come along for the ride when you run, but some get a better workout than others, depending on your pace.
All easy running is handled by slow-twitch muscle fibres - no surprise there. As running intensity increases, more slow-twitch fibres are recruited. Once you're running at a moderate intensity, fast-twitch type IIa fibres join their slow brothers in action. And as you progress from moderate to maximum muscle force (think sprinting), fast-twitch type IIb fibres are called upon.
If you're a distance runner, you may think you only need to train those slow, endurance-oriented fibres. But even marathon runners need to develop their fast-twitch fibres for peak performance. Early on in a marathon, runners use mainly slow-twitch and a few fast-twitch type IIa fibres. As the race goes on and the muscle glycogen is depleted, however, more fast-twitch type IIa fibres are needed to maintain muscle force. And as the I and IIa fibres become depleted late in the race, the IIb fibres need to pitch in. So if you fail to train your fast-twitchers, they won't be able to come to the rescue late in a long run.
To make sure your fast-twitch fibres are there for you whether you're pushing the pace in a 5K or slogging through the last few miles of a marathon, you need to include a mix of faster sessions in your training every week. The sessions below represent a continuum progressing from moderate to maximum intensity.
As you move from tempo runs to long intervals to repetitions to sprints, you increase the percentage of fast-twitch fibres used. You recruit more fast-twitch IIa fibres during the tempo runs and intervals, and press the fast-twitch IIb fibres into action during the shorter, faster repeats. Pick any two sessions each week, and do them all at least once a month. But don't let more than two weeks pass without doing at least one of the last four sessions to maximise your fast-twitch potential.
Fast-twitch sessionsDo these twice a week as part of your regular training, to train your fast-twitch muscle fibres.
1 The standard tempo run
After a 10-minute warm-up, run 25-30 minutes at 20 seconds per mile slower than your 10K pace.
2 Long intervals
Warm up, then run four x one mile at 5K pace with three-four minute recovery jogs between.
3 Hill repeats
Warm up, then run two sets of six-eight x 150m hill repeats. Sprint up, walk back down.
4 Speed repetitions
Warm up, then run eight x 400m at mile race pace with two-minute jog recoveries between.
Warm up, then run eight x 200m at 800m race pace with 200m slow jog recoveries between.
Warm up, then run 10-12 x 100m strides. Run on a track - stride the straights and jog the bends.