Muscles in the human body are made of a mixture of different muscle fiber types. The composition of these muscles varies depending on the activities they are used for. Postural muscles for instance (e.g. calves and spinal muscles) are mainly made up of slow twitch fibers as they do not need to generate much power and are considerably resistant to fatigue. Furthermore, during muscle contraction, only the required fibers will contract. Only slow-twitch fibers will contract if there is a weak contraction. Fast oxidative and fast glycolytic fibers will contract if powerful effort is made (heavyweight lift) along with the slow-twitch fibers, with the former fibers contracting last.
The 3 types of muscle fibers are present in every human being’s body in set percentages. The properties of our muscle fibers can be changed because of the type of exercises being done. An athlete with an initial amount of fast and slow twitch fibers can have slow twitch fibers demonstrating characteristics of fast twitch fibers through specific training like sprint or heavy weightlift.
Although only a select few are blessed with the type I muscle structure of an Olympic marathon runner, or the type II construction of Usain Bolt’s legs and calves, performance can be improved through correct and adequate hard work and training.