Swimming performance is dependent upon swimmer's capacity to generate a high mechanical power output to overcome the hydrodynamic resistance from the water. An increase in swimming speed requires a corresponding increase in the applied muscle force, becoming clear that muscle strength is determinant of success in swimming performance (Formosa et al., 2011; Sharp et al., 1982). Most studies approaching the relationship between strength and swimming performance have focused on upper extremity strength and sprint performance, demonstrating a strong relationship (Loturco et al., 2016; Morouco et al., 2014; 2015a), whereas some have reported a much weaker association (Costill et al., 1986; Formosa et al., 2013). It is apparent from the literature that land-based strength measures may not be as closely associated to swimming performance as strength measures obtained during swimming or swimming-like activities (Vorontsov, 2011). In addition, different force measurement systems may present distinct results. For instance, 55% of variation in magnitude of active drag analyzed by the MAD system and assisted towing method may be explained by different swimming technique required in each system (Formosa et al., 2012).
In order to increase swimming performance, athletes have incorporated strength and power exercises into their training programs, much of which are performed on land-based exercises using swim bench and free weights. Despite requiring access to certain testing equipment (hardware and software), the measure of the swimmer's capacity to generate force is a very important practice (Loturco et al., 2016), being useful to monitor training progress and the efficacy of the swimmer's land- and water-based training programs (Vorontsov, 2011). Hence, the ability of swimmers to generate force has been assessed using land-based isometric and isokinetic tests, which are designed to determine the contribution of isolated muscle actions around a particular joint (e.g., shoulder girdle muscles). Isometric tests on land-based is associated with swimming performance (Loturco et al., 2016). In addition, it is used to establish the swimmers' strength profile, clinically assess shoulder pain (McLaine et al., 2017) and monitor fatigue (Matthews et al., 2017). However, such tests may not fully reproduce the neuromuscular and biomechanical conditions involved in the stroke during swimming (Marinho and Junior, 2004). In fact, more studies are needed to verify the correlation between isometric land-based test and swimming performance. In addition, the easy evaluation of the forces generated by a swimmer, under conditions that closely replicate the swimming demands (e.g. tethered swimming that involves multi-joint movements), is inevitably of interest and value to swimming coaches, sport scientists and other practitioners.
Tethered swimming is performed attaching a swimmer to an inelastic cable and the other end is connected to a load cell mounted on the end wall of the pool. This approach involves muscle activation patterns very similar to those observed in free swimming (Bollens et al., 1988) and it has excellent test-retest reliability (Nagle et al., 2016; Kjendlie and Thorsvald, 2006). Although arm stroke kinematics in free swimming may differ slightly from tethered swimming, in which the body does not displace relative to the water (Maglischo and Maglischo, 1984; Yeater et al., 1981) and may affect the force applied, tethered swimming has been considered a specific method to evaluate force in water (Dos Santos et al., 2013; Lee et al., 2014; Morouco et al., 2011; Santos et al., 2016). In addition, tethered swimming provides an attractive possibility to quantify propulsive forces produced by each side of the body and, therefore, to determine asymmetries in propulsive forces. Thus, tethered swimming involves a more ecologically valid approach when compared to other testing methods evaluating in land-based tests (Dos Santos et al., 2013).
Swimming performance can be viewed as a function of the propulsive forces generated by the left and right sides of the body. Although similar contributions to propulsion might be anticipated from both body sides, a number of studies have shown differences in technique coordination (Barden et al., 2011; Chollet et al., 2000; Seifert et al., 2005a) and propulsive forces between left and right sides (Dos Santos et al., 2013). Dos Santos et al. (2013) reported that elite front crawl swimmers are more symmetrical than their sub-elite counterparts (13 N vs. 18 N of peak force difference between right and left sides in elite and sub-elite swimmers, respectively). It means that the elite swimmers present more similar propulsive forces between body sides when compared to their sub-elite counterparts. Thus, the ability of each side of the body to produce propulsive forces may be highly related to the capacity to generate large torques around the relevant joints. Some studies have showed a positive relationship between the force generated during land-based testing and swimming velocity in able-bodied (Hawley and Williams, 1991; Sharp et al., 1982) and disabled swimmers (Dingley et al., 2014). To date, no studies have determined whether differences in the ability to generate force with the left and right upper extremities in land-based strength test (using a maximal voluntary isometric contraction test) are related to propulsive force asymmetries measured during a tethered swimming.
Therefore, the aims of this study were to investigate whether land-based and tethered swimming strength tests can explain swimming performance in 200-meter front crawl and whether these tests were able to identify bilateral symmetry in force production. It was hypothesized that forces measured by land-based and tethered swimming tests would be correlated and these tests would also correlate with swimming performance and, that both tests (land-based strength and tethered swimming tests) would be able to identify possible asymmetries.
Eighteen male swimmers (age 21.3 [+ or -] 4.6 years; stature 1.77 [+ or -] 0.06 m; mass 69.6 [+ or -] 6.6 kg) of local competitive level (competitive experience greater than two years) provided informed consent to participate in the study which was approved by the University Ethics Committee. Their mean best 200 m freestye performance was 139.1 [+ or -] 8.3 s and training frequency was at least three times per week.
The participants attended two sessions, separated by a minimum of one day and maximum of four days between them. They performed a maximal effort during 200 m front crawl swim and after resting, participants performed 15 seconds maximal effort tethered front crawl swim. The land-based isometric strength test was completed in other session. The order of these sessions was randomised.
A brief non-controlled low-intensity warm-up was performed for 10 minutes in a 25 m swimming pool with a water temperature of 29[degrees]C. Then, participants completed the 200 m swim with maximal effort. An experienced experimenter recorded the performance time manually...