How do muscles work?

Basically speaking, human evolution has enabled us to deal with two different types of activities. Activities like walking, talking, lifting a spoon to your mouth or typing on the computer require our muscles to contract for a longer period of time but with little force.So these “daily” activities solicit the use of endurance fibers. Others such as reacting quickly, runnung after the bus, jumping a hurdle or lifting a heavy load require a short and intense muscle contraction. These “survival” activities, therefore, solicit the use of strength fibers. To move from one activity to another or to combine the two types, different energy sources will become available.

Slow twitch Fiber = Endurance
Fast Twitch Fiber = Strength

On a scale of 0 to 100% (100% being our maximum muscle capacity), the first 50% represent Slow twitch fibers (STF). They are also known as Type II, aerobic or red fibers. The latter name refers to the fact that they carry red blood cells and need oxygen for their development. The second 50% are called Fast twitch fibers (FTF), also known as Type !, anaerobic (without oxygen) or white fibers. The average human body is composed of 50% STF and 50% FTF, but it is possible to change this ratio depending on how we exercise on a daily basis.

Sprinters such as Usain Bolt or Carl Lewis reached a ratio of 65% FTF to 35% STF, while marathon runners such as Patrick Makau or Tasame Dame have about the same ratio, only reversed. Any exercise requiring less than 60% of our muscle capacity uses the endurance fibersș beyond 60% we will use the strength fibers.

The questions I often hear in the gym are: At what intensity should I train? and Should I lift heavy or light weights?

To this, I answer that it all depends on your goal. If you are aiming mainly to lose weight and burn fat, you should target the area below 60% (endurance zone). That’s because slow twitch fibers are the type burning more fat. If your goal is to gain muscle mass, you should focus your training between 60 and 100% of your muscle capacity, as FTF is the only type of fiber for growth and muscle development.

Let’s look at an example: if your maximum capacity for a bicep curl is 20kg, working with a weight of 10-12 kg will solicit more slow twitch fiber and therefore burn fat. By working with a weight of 16kg, however, you are building more muscle mass.