The Basics: Muscle Fiber Type and Athletic Ability

Human muscle fibers can be classified into two categories. Slow-twitch (red) muscle fibers and fast-twitch (white) muscle fibers. You’ve probably heard of the different muscle fiber types before, but you may not have realized that each muscle type’s predominance is determined by genetics.

The ACTN3 (Alpha Actinin) gene is only active in fast-twitch (white) muscle fibers, and plays an important role in their function. This gene is frequently inactive due to a gene mutation that reduces the function of white muscle fibers and therefore the explosive power produced by the muscles. The red muscle fibers increase the stamina of the muscles.

Each individual has two genes that produce ACTN3, and the following gene combinations are possible:

  • Endurance type: both genes are inactive and don’t produce ACTN3 protein (24% of population)
  • Power type: one of the genes is active and produces ACTN3 protein (44 % of population)
  • Power type: both genes are active and produce ACTN3 protein (31% population)

The second sports gene, ACE (Angiotensin Converting Enzyme), plays an important role in blood pressure regulation.

ACE has two forms: the endurance sports variant of the ACE gene, which has a positive effect on endurance of the muscles (found in elite marathon runners) and the power form of the ACE gene, which makes the muscles more suited to power and sprinting. Every individual has two genes of this type with following possible combinations:

  • Endurance – both genes endurance (25% of population)
  • Endurance – one gene endurance, one power (50% of population)
  • Power – both gene power (25% of population)

If both genes are present, a general genetic predisposition to a particular mix of endurance and strength training, which can vary greatly from person to person, occurs. This knowledge can influence the individual training program, depending on the type of sport performed.