Understanding the Difference Between Fast and Slow Twitch Muscle Fibres

Running a marathon and sprinting the hundred metre dash use the same muscles—but not in the same way.

There are big differences beneath the surface. One difference is in fibre types: slow twitch and fast twitch. These fibres are both muscle, but that’s all they have in common. Indeed, they both look and act different. This matters for sports training.

Slow twitch muscle fibres, also known as “Type I,” are red and have a small diameter. What’s important to know is these fibres contract slowly and efficiently. They perform at their best during endurance-based activities such as long distance running, cycling, and swimming.

On the downside, slow twitch fibres are unable to produce great amounts of force, which limits the strength and power of the muscle.

Fast twitch muscle fibres, or “Type II,” are white and larger. These bad boys contract rapidly, producing big force in short order. This is great for jumping high, lifting heavy weights, and running super fast—for a few seconds, anyway. You see, Type II fibres fatigue quickly when called upon. And in the process they can produce lactic acid, which in excess will inhibit physical performance.

What does all this mean for athletes? Well, each sport uses one more than the other—even when the mechanics are similar. For example, a Tour de France rider is apt to race well utilizing almost exclusively slow twitch as he averages 150 kilometres per day for three weeks. Meanwhile, a track sprinter will rely solely on fast twitch fibres when he zooms around the velodrome at peak speed for a fleeting stretch of time. Both athletes race bikes, yet utilize entirely different muscle fibre types to perform.

Now, most studies suggest our ability to substantially alter our ratio of slow and fast twitch fibres is modest. However, some level of conversion appears possible, which makes it worth trying for athletes to improve at their craft.

So how do we change our fibre types? The simple answer is that you train them.

Hockey players, for example, leverage mostly fast twitch fibres during typical plays, which switch between low and high action in the blink of an eye. Converting slow twitch fibres could improve a player’s abilities both to explode from still positions and to recover during lulls for the next detonation. An athlete can train for this through heavy strength work (think five reps or fewer) and power work, such as sprinting and plyometric training—maximum efforts that last just 10 to 20 seconds.

Pushing a specific fibre type to exhaustion can force adaptation from the body: when our system recognizes one type is more beneficial it may work to change the ratio. Most of us are born with a roughly even split of Type I and Type II; training and time can shift the odds in your favour, studies show.

Recognize where your sport lies on the spectrum of endurance to power, and train accordingly. Prioritize your more useful fibres in training. And let the other down gently. It’s nothing personal, they’re just not your type.

A version of this content was originally printed in The Progress.