Fast-Twitch Muscle or Slow-Twitch Muscle: Which One Are You?

Not all of our muscles are created equal. Case in point, how I feel when I lift weights for my upper body versus when I weight train for my lower body.

With my upper body muscles, it feels like I could go forever. It takes a long time for my muscles to fatigue. Yet, when I train my legs, after just a few reps they start to fatigue. “Why is that?”, you wonder. Well, it seems my upper body is built for endurance and my lower body is built for speed, or more scientifically, I carry more slow-twitch muscle fibres in my arms and more fast-twitch muscle fibres in my quads and hamstrings.

Muscle Fibres Explained

Our skeletal muscles, those wonderful muscles that connect the bones together and that we train in the gym, comprise about 40% of our body weight. Within each muscle are microscopic proteins bundled together to form a fibre. These fibres are then bundled together to form what is called a fascicle (rhymes with popsicle), and the fascicles are bundled together to form a whole muscle.

Within these bundled fascicles lie three different types of muscle fibres (as well as gradations between them all), which dictate how much weight, or force, it can produce and how long it can generate this force. We all have these fibres, whether we exercise or not, and they vary from muscle to muscle.

The first type of fibre is called slow-twitch fibres, or Type I fibre.

These are our Energizer Bunny muscle fibres. They are recruited for endurance work because they can go on and on and on. However, while they can carry us the distance, they sure can’t press a lot of weight. Marathon racers would have a high percentage of these fibres in their legs, while body builders wouldn’t.

The second fibre type are our fast-twitch, or type IIa fibres.

These are recruited for activities that require speed, strength and power. They contract quickly, but fatigue quickly which is why I nick-named these fibres our Arnie-fibres. Sprinters would have a lot of these muscle fibres in their legs, whereas long-distance bicyclists wouldn’t.

The third type of fibre is the fast-twitch B (type IIb) fibres.

These are our super-charged fast-twitch muscle fibres, and are only recruited for short, intense activities such as jumping, sprinting at full speed and lifting very heavy weights.

Actions of Each Muscle Fibre

All of our actions are dictated by these fibres. That is why some people can lift heavy weights, but can only run for 5 minutes on the treadmill.

Of course, all muscles can be trained for any activity, so don’t use this as an excuse not to get your cardio in, but our muscle fibres do dictate what action will be easier for the body to perform.

In an anatomy class that I took years ago, I remember the instructor telling us that in some Eastern block countries they actually used to take biopsies of children’s legs to see what percentage of fast and slow-twitch muscle fibres they had.

This test then decided what sport the child will be sent into. Because let’s face it, if someone has a lot of slow-twitch fibres (our Energizer muscle fibres) in their quadriceps, they won’t be bringing home the Gold medal in the sport of power lifting.

To determine which muscle fibre type you have ask yourself the following questions (Idea, May 2010):

Are you able to do lots of repetitions when lifting weights, or do you fatigue after a few?
If you fatigue, you probably have a more fast-twitch fibres – like I found out in my legs. If this is the case wind sprints, HIIT workouts and low rep/high weight workouts are best suited for your body type.

Are you better at sprint and power activities or at endurance activities?
If you love to sprint, then you are like me again and have more fast-twitch muscle fibres. If you are a marathoner, then you carry more slow-twitch fibres, but if you enjoy 5km & 10 km then you possess a bit more fast-twitch. I use this excuse whenever someone tried to get me to run a marathon. “Sorry, I’d would love to, but I’m fast-twitch in the quads.”

Which type of workouts feel easier and more natural: a) long, aerobic workouts and light weights with lots of reps or (b) sprints and heavy weights with few reps?
If you answered (a), you have more slow-twitch fibres. If you answered (b), you     have more fast-twitch fibres.

Which workouts do you look forward more to: (a) aerobic/endurance workouts or (b) anaerobic/strength workouts?
If you answered (a), you have more slow-twitch fibres and if you answered (b) you have more fast-twitch fibres.

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