Muscle endurance

Sports performance is influenced by multiple factors, with genetic factors determining between 20-80% of the variation in different traits such as cardiac performance, oxygen consumption and muscle fiber composition. It is the proportion in the two types of muscle fibers that determines whether we can have a greater or lesser facility to perform power or endurance physical activities.

Muscle performance is the ability of a muscle or group of muscles to perform a repetition effort against resistance. Performing several repetitions of an exercise is a form of muscular performance as is running or swimming. If your muscles have to contract in a similar manner more than once you are using muscle performance.

Among the factors that determine muscle performance are strength, fiber type, training and diet. A larger, stronger muscle can perform the same task under load more times than a weaker muscle. A large muscle also has more glycogen (sugar store used for energy) than a small one so it can sustain a series of contractions or perform a job for a longer period of time.

The composition of muscle fibers is strongly predetermined by the individual's genetics and, therefore, the facility to perform one sporting activity or another. In a sedentary middle-aged person, the percentage of type I or slow fibers is 45-55% (in women this percentage increases slightly).

Sprinters tend to have more type II or fast twitch fibers in the legs while endurance athletes show a predominance of slow twitch fibers. Biopsies in the vastus medialis muscle of the leg reveal that the proportion of fast fibers in throwers and jumpers in track and field, as well as in weightlifters, can be up to 3 times higher than in sprinters.The proportion of fast fibers in the medial vastus medialis muscle can be up to 3 times higher (60% fast fibers) than that of long-distance runners (17% fast fibers) and 50% higher than that of bodybuilders, for example (40% fast fibers).

There are two types of athletes according to the predominant type of muscle fiber: power and sprint athletes (fast fibers) and endurance athletes (slow fibers).

Fiber types can determine the ease with which one physical activity or another is performed, and although there may be other associated polymorphisms, one marker has been identified that correlates with the different fiber types.

This genetic marker is found in the ACTN3 gene, and is expressed only in fast twitch muscle fibers and not in slow twitch muscle fibers. The fact that this gene is affected causes a lower amount of functional alpha-actinin-3 protein to be produced, thus hindering the ease with which speed exercises are performed, so they will have greater ease when performing endurance exercises.

- Power and sprint athletes: have greater muscle size and strength, faster sprint times and a greater proportion of fast twitch muscle fibers. They have the allele that produces functional alpha-actinin-3.

- Endurance athletes and recreational athletes: present two copies of the allele that causes functional alpha-actinin-3 not to be produced and therefore their muscles are dominated by slow twitch fibers. Some studies support the hypothesis that individuals with this deficiency, although they have reduced muscle strength values, have certain advantages such as: lower body mass, lower fat-free mass and greater efficiency during endurance sports such as marathon running.

Number of observed variants

13.5 million variants

Number of variants analyzed in the study

1 variant


Del Coso J, Moreno V, Gutiérrez-Hellín J, Baltazar-Martins G, et al. ACTN3 R577X Genotype and Exercise Phenotypes in Recreational Marathon Runners. Genes (Basel). 2019 May 29;10(6):413.

Flück M, Kramer M, Fitze DP, Kasper S, Franchi MV, Valdivieso P. Cellular Aspects of Muscle Specialization Demonstrate Genotype - Phenotype Interaction Effects in Athletes. Front Physiol. 2019;10:526.

Tharabenjasin P, Pabalan N, Jarjanazi H. Association of the ACTN3 R577X (rs1815739) polymorphism with elite power sports: A meta-analysis. PLoS One. 2019 May 30;14(5):e0217390.

The DNA test you were looking for