High-intensity exercise describes physical exercise that is characterized by brief, intermittent bursts of vigorous activity, interspersed by periods of rest or low-intensity exercise. It is an effective alternative to traditional endurance training to improve health-related markers in both healthy individuals and diseased populations (Gibala et al., 2012). The success of high-intensity training in sports resides in its efficiency. There is strong evidence that sprint training increases exercise performance (Bangsbo et al., 2009; Iaia et al., 2008; 2009), and that it also maintains (Iaia et al., 2009) or even increases (Bangsbo et al., 2009) muscle oxidative capacity.
By reducing volume and increasing intensity, well-trained runners (Laursen and Jenkins, 2002; Smith et al., 2003), cyclists (Laursen and Jenkins, 2002; Stepto et al., 1999; Weston et al, 1997), and soccer players (Thomassen et al., 2010) increase their performance. However, little is known about swimming. One study shows that well trained swimmers maintain swimming speed after training for four weeks at an intensity around the lactate threshold (Faude et al., 2008). More recently, speed training has been proposed for elite swimmers in order to increase training efficiency (Kilen et al., 2014).
Swimming is performed as an under-water dynamic activity, which may influence exercise adaptations and acute responses. Water immersion induces hydrostatic pressure, which can cause the displacement of fluids within a person from the extremities towards the central cavity. This displacement of fluids increases the translocation of substrates from the muscles, increases cardiac output, reduces peripheral resistance, and increases the ability of the body to transport substrates (Wilcock et al., 2006). Water immersion per se may actually play an important role in muscle recovery after exercise, since hot water immersion is able to restore muscle mechanical function after exercise (Vaile et al., 2008).
In-water (IN) passive recovery increased performance during a set of 6 repetitions of a 50 m swimming sprint every 120 s when compared with out-of-water (OUT) recovery (Buchheit, 2010). This effect was associated with a lower heart rate peak and lower blood lactate in adult swimmers (Buchheit, 2010). Swimmers are often young when they reach elite level competition, with peak freestyle speed achieved between the ages of 21 and 23 (Rust et al., 2014). It must be also highlighted that among the swimming medalists in the 2012 Olympics (n=78), 25 were under 21. Little is known about the incidence of sprint swimming on adolescent swimmers. Adolescents produce lower lactate levels at a given intensity (Tolfrey and Amstrong, 1995), which may alter the effects that IN recovery has on performance and lactate production.
Buchheit et al. (2010) tested 50m sprint bouts. However, final turns during a 100 m and 200 m are performed under fatigue, and sprint swimming training should take care of the specific needs of the swimming race (Maglischo, 2003). Thus, the 100 m swimming distance could be more beneficial for needs of swimming well-trained swimmers in helping fulfill the physiological races. In this regard, cold water (16[degrees]C) immersion does not enhance 100 m swimming sprint performance despite a greater perception of recovery (Parouty et al., 2010). However, swimmers usually train at a temperature of 2628[degrees]C. By doing recovery at this temperature re-heating is not necessary, as shown by Parouty et al. (2010) after immersion in 16[degrees]C water, but it may retain the beneficial effects of the hydrostatic pressure.
The aim of the present study was to test the hypothesis that 5 minutes of IN recovery, in thermoneutral water, will increase performance relative to OUT during a 5 x 100 m sprint swim. The secondary hypothesis is that IN recovery will decrease lactate production and peak heart rate (peak HR).
Twelve well-trained adolescent swimmers participated in the study. Their mean ([+ or -] SD) age, height, and body mass were 15 [+ or -] 0.8 years, 1.75 [+ or -] 0.05 m, and 72.0 [+ or -] 5.5 kg, respectively. All swimmers had competed in regional and national championships in the previous twelve months, and were experienced in 100 m front crawl sets such as the one used in this study. They normally trained ten to twelve hours per week. Swimmers were informed about the purpose of the study and any known risks, and then gave their written consent to participate. The Ethical Advisory Committee of the University of Jaen (Spain) approved the test protocols and all procedures. All experimental work conforms to requirements stipulated in the Declaration of Helsinki.
The study employed a randomized crossover design were each subject completed two 5 x 100 m all-out swimming separated by at least 48 h and a maximum of 5 days. The study was carried out in a 25 m indoor swimming pool, environmental temperature was 30 [+ or -] 0.5[degrees]C and water temperature was 26.7 [+ or -] 0.3[degrees]C. The protocol applied during these two testing days was the same with the exemption of recovery between bouts. The 5 x 100 all-out protocol was carried out by swimming front crawl style interspersed by 5 minutes of in- or out-of-water passive recovery. First day IN or OUT condition was chosen randomly for each swimmer, and the second day the opposite recovery condition was applied. The reason why they were instructed to take 5 minutes of rest is that in order to achieve adaptations in response to sprint training a recovery period of 4 to 6 times the duration of the exercise is needed (Iaia and Bangsbo, 2010). A longer period of rest might have magnified the effect of hydrostatic pressure.
Two weeks before the start of the study fifteen swimmers from local swimming clubs were recruited. In order to avoid any learn effect the swimmers performed the 5 x 100 m test at least once before the start of the study without attending to the recovery condition. Over the five repetitions, the fastest (RSb) and mean (RSm) times were recorded. Fatigue was assessed during the protocol as a percentage of sprint speed decrement which was calculated as follows: 100 - ([Rsm/RSb] x 100). This method is thought to provide a reliable method for assessing fatigue during all-out bouts (Glaister et al., 2004). After this preliminary test 3 subjects were eliminated because their fatigue was higher than 7%. These subjects had not performed regular training...