Most of our 600 skeletal muscles are composed of a mixed ratio of three different muscle-fibre types that 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 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.
As a distance runner, you may think you only need to train those slow endurance-oriented fibres. But even marathoners 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 IIa fibres. As the race goes on and muscle glycogen is depleted, however, more fast-twitch IIa fibres are needed to maintain muscle force. And as the slow- and fast-twitch IIa fibres become depleted late in the race, the IIb fibres need to pitch in.
So if you fail to train your fast-twitch buddies, 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 5-K or gutting out the last few kays of a marathon, you need to include a mix of faster workouts in your training every week. The workouts below, as numbered from one to six, represent a continuum that progresses 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.
- Build both types of fast-twitch muscle fibres by running two of these workouts per week. Be sure to include at least one of the last four workouts every two weeks.
- Run one or two easy kays before and after each workout to warm up and cool down.
- Don’t let more than two weeks pass without doing at least one of the last four workouts (the highest intensity) to maximise your fast-twitch potential.
1. Standard Tempo Run: 25 to 30 minutes at 15 seconds slower than 10-K pace.
2. Long Intervals: 4 x 1-K at 5-K race pace with 3 to 4 minutes jog recovery.
3. Hill Repeats: 2 sets of 6 to 8 x 150-metre hill repeats. Sprint up, walk down.
4. Speed Repetitions: 8 x 400 metres at kilometre race pace with 2 minutes jog recovery
5. Sprints: 8 x 200 metres at 800-metre race pace with 200-metre slow jog recovery
6. Strides: 10 to 12 x 100-metre strides. Stride the straightaway, jog the turn.