Planning Your Workout | Understanding Muscle Fibre Types

Planning Your Workouts
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The moment you enter the weight room you begin to ask yourself questions: How much weight should I be lifting? How many reps should I perform? How many sets? What am I doing here? Why does this even matter?!

If you are new to the gym, perhaps new to exercise or still trying to understand the whole thing, rep and set selection can play a major part in your progress and training success. It is key to understand there is no perfect rep and set scheme. It is very dependant on the individual’s make up, his or her experience and their specific goals – so it varies greatly.


When planning your workout you must first decide on your desired training outcome, once you have chosen that then you can select the number of repetitions per exercise.

The table below is a rough guide of the relationship between rep selection and the training effect it has.

Training Effect Reps per set % of 1RM (Intensity)
Strength 1-6 80-90
Hypertrophy 6-12 60-80
Endurance 15+ >40

The training outcome above for Hypertrophy (meaning to build muscle) isn’t entirely accurate, and can vary depending on the individual’s fibre make up. As the average gymgoer tends to only want to build muscle, and fibre type playing such an important role, I have chosen to make this the focus of the article.


There are two main types of muscle fibres, slow twitch (Type I) & fast twitch (Type II).

Slow twitch or Type I muscle fibres are smaller in diameter, produce less force and are more resistant to fatigue. Their main activities are maintaining posture – i.e. stabilisation and are required by the body for endurance-based activities such as long distance running, swimming and cycling.

Fast twitch or Type II muscle fibres are far larger in diameter than slow twitch, they produce much more force and are less resistant to fatigue. Their primary function is for rapid, intense movements such as sprinting and power lifts.


The average person has a good mix of both slow and fast twitch fibres, so unless you have spent much of your life sprinting, swimming or routinely taking part in other endurance based activities it is highly unlikely that your body is one fibre type dominant.


Fast twitch fibres respond best to relatively low volume, longer rest intervals and high intensity. Slow twitch fibres are the opposite, they respond best to high volume and shorter rest intervals. Most importantly, fast twitch muscle fibres have a significantly greater growth potential than slow twitch fibres. The hypertrophy rep range listed above is best suited for a fast twitch muscle fibre while slow twitch can differ greatly.

If you are having a tough time building muscle in a particular part of your body it is possible it may be made up of mainly slow twitch muscle fibres. If that is the case I would suggest trying a higher number of reps and shorter rest intervals. An example of a muscle primarily made up of slow twitch fibres would be the forearms, for training these I suggest 5 sets of at least 12 – 20 repetitions and no more than a minutes rest.

The body is not an accurate science, for Hypertrophy training unless it is a muscle group that is really lagging behind or a notably fast or slow twitch muscle then ensure a great variety of rep selection. The 6-12 mentioned in the table above is a great marker to shoot for, try for 6 weeks hitting around the 8 mark, then the following 6 weeks aim for 12. Make sure you to aim for at least 80% intensity, this means roughly choosing a weight based around your 8RM.  You should feel the final few reps of each set challenging, and increase your weight after two to four weeks of training.


Regardless of your exercise goal or the genetic make-up of your muscle fibres, staying on top of your nutrition is key to facilitating the muscle growth and recovery you desire. This means getting your kitchen game spot on by addressing your basic macronutrient requirements (and nutritional flaws), as well as supplementing your diet to ensure you’re getting the nutrients you need, at the times you need them.

This is why supplements like Pure Whey Protein™ and Complete Multivitamin Complex™ are so commonplace because they allow you to increase your daily protein intake with minimal fuss, and plug any gaps in your vitamin and mineral spectrum, as a result of training demand or food nutrient deficiencies, respectively.

About the Author

Dejan and Jonny are both fitness professionals stemming from a vast sporting background. Based in London they are the founders of ‘LetsTrain’ – an online, forward-thinking, fitness and nutrition resource.

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