The main finding of the current study is that while the ACTN3 R577X polymorphism was largely associated with a distinction between runners specializing in sprint events compared to runners specializing in long-distance events, this difference was not found among swimmers specializing in sprint events compared to swimmers specializing in long-distance events.
Athletic events can be divided by the distance or time of the activity. Other parameters, like power, speed, and endurance, are also used to characterize specific sports. Track and field is a good example of the use of these descriptors and their activity time frames in differentiating between events based on energetic resources. Endurance-type athletic events are characterized by relatively low-intensity, long-lasting exercise that relies primarily on the aerobic energy-generating process [18]. The term aerobic refers to the use of oxidative phosphorylation to adequately meet energy demands during exercise [19]. In contrast, strength, power, and speed-type athletic events are characterized by intense efforts lasting a short time and by the use of anaerobic metabolism as the energy source.
The ACTN3 R577X polymorphism is a well-documented genetic marker that enables sport scientists to distinguish between a genetic predisposition towards excellence in power-type events or in endurance-type events [14]. Indeed, in the present study we found a significant difference in ACTN3 R577X polymorphism prevalence among LDR compared to SDR, with a high prevalence of 40.3 and 61.8 % for RR genotype and R allele frequency in SDR compared to 20.0 and 42.3 % for RR genotype and R allele frequency in LDR. This finding may suggest that the ACTN3 R577X polymorphism enables to distinguish between two types of track and field events—a “pure power event” and a “pure endurance event.” Yet, for a higher resolution distinction like the one needed to distinguish between middle-distance runners (MDR) and LDR performance, or between MDR and SDR performance, other genetic markers or profiles may be needed. This assumption should be examined in future studies. Since swimming events are also divided by the distance or time of the activity, one would assume that swimming events can also be classified into “endurance-type events” and “power-type events.” Subsequently, the genetic background that influences a track and field athlete’s capability to excel in one sport discipline (e.g., sprint running) rather than another (e.g., long-distance running) would, presumably, be similar to the genetic background that influences a swimmer’s capability to excel in a specific sport discipline. However, in contrast to our hypothesis, no significant association was found in the present study between the ACTN3 R577X polymorphism and swimming performance. This finding is consistent with previous reports that failed to find an association between the ACTN3 R577X polymorphism and swimming performance in Caucasians, East Asians [16], and Spanish swimmers [17]. In contrast, R allele was significantly higher in Taiwanese female international sprint swimmers compared to national-level sprint swimmers and the general population [5].
ACTN3 plays a pivotal role in muscle metabolism, structure, and fiber-type distribution [2, 20], and therefore it has a direct effect on the ability to perform in elite power events. However, based on the present findings, it may be that none of these are detrimental to swimming. It may be that the aspect of power performance affected by the polymorphism is less important in swimming relative to other sports, possibly because of the relatively lower stress put on muscles supported in water and the lack of eccentric contractions [21]. Moreover, in swimming the produced power is lower compared to land activities of similar duration [22]. Efficiency is a critical factor in this sport, which includes several aspects of technique. Swimming is a highly skilled sport, where the neural and biomechanical skills are the greatest contributor for force production [22–24]. Overall, in swimming, the athlete’s technique and body physique have a greater impact on performance than in other sports such as running.
In line with this, we have previously found a strong significant correlation (r = 0.74) between 100 m and the 2000 m swim times suggesting that swimming times are largely affected by swimming technique and by the swimmers’ size (particularly limb length) [25]. These may, at least partially, mask metabolic differences between swimmers, enabling technically skilled and/or tall swimmers to excel at all swimming distances. These relationships are unique for swimming, and this assumption can be supported by the past records of top world-class swimmers, such as Ian Thorpe (world record holder in the 100-m relay and individual 200, 400, and 800 m) and Grant Hackett (world record holder in 200-m relays and individual 400, 800, and 1500 m), as well as others who excel in both short and long swimming distances. A similar phenomenon is very uncommon among runners. To the best of our knowledge, this relationship between short- and long-distance performances in swimming has not been reported previously in other sport types.
Lastly, there is the possibility that a type II error accounts for the fact that we do not see an association between the ACTN3 R577X polymorphism and swimming performance. Large studies with the power to detect significant associations at genome-wide level have not yet been conducted. Although a meta-analysis of the association between ACTN3 and sprint/power athlete status demonstrated evidence for a real association [13, 26], many studies—mainly with small sample sizes—have failed to observe any association between ACTN3 variants and sporting performance.
It should be noted that both LDS and SDS had a higher RR genotype frequency (38.0 and 36.4 %, respectively) compared to controls (22.1 %). This result implies that both LDS and SDS may benefit from ACTN3 R allele existence, strengthening the notion of no clear ACTN3 polymorphism differences between power and endurance swimmers. Moreover, RR genotype and R allele frequency were higher among LDS (38.0 and 57.0 % for RR genotype and R allele frequency, respectively) compared to LDR (20.0 and 42.3 % for RR genotype and R allele frequency, respectively). This may be explained by differences in the specific activity duration of competitive swimming and running. The longest Olympic swim (1500 m ~ 15 min duration) is much shorter than the longest Olympic running race (marathon ~2 h 10-min duration), and therefore ACTN3 power characteristics are needed for long-distance swimmers. It is possible that if open-water swimmers (Olympic race—10K) had been included among the long-distance swimmers in the present study, the differences in the R allele frequency between LDS and LDR would disappear. When comparing the short-duration events, there is also a difference in the specific activity duration of competitive swimming and running. The 100-m distance covered in swimming at approximately 50 s, while the same distance covered in running at approximately 10 s. As a result, a sprint runner relies mostly on anaerobic energy sources, while a sprint swimmer uses also aerobic energy components. Therefore, the SDS and the LDS share some common features, and the ACTN3 RR genotype is not suitable as a distinguishing genetic marker between them, as it does for SDR and LDR.