Caffeine is a popular ergogenic aid among athletes, with investigations into its effect on swimming performance dating back to the 1970's (Makoc and Vorel, 1974). Research into the human response to moderate doses of caffeine (3-6 mg.[kg.sup.-1]) has demonstrated that plasma caffeine levels rise to between ~15-40 [micro]mol x [L.sup.-1] between 40-80 min post-ingestion (Spriet, 2014). Additionally, there is now a large and accepted body of literature which demonstrates that a moderate dose of caffeine is ergogenic for endurance performance in a range of sports, including freestyle swimming (Macintosh and Wright, 1995). The mechanism for endurance performance enhancement was first thought to be an epinephrine-induced increase in free-fatty acid oxidation and glycogen sparing (Davis and Green, 2009). However, it is now thought that a central nervous system (CNS) mechanism, via adenosine receptor antagonism, is more likely, and may also translate into high-intensity performance improvement (Davis et al. 2003).
Sprint swimmers are one group of athletes who could benefit greatly from caffeine's ergogenic potential for high-intensity performance. Events from the 50 m to 400 m freestyle typically last from ~20-240 s at the elite level, and strategies to improve one off sprint performance and repeated sprint training sessions are constantly being sought. However, investigations into the effect of caffeine on high-intensity exercise performance are less researched than for endurance exercise, with an early swimming study finding that 250 mg of caffeine enhanced 2 x 100 m freestyle sprint performance only in trained participants (Collomp et al. 1992), and a later trial (6 mg x [kg.sup.-1]) finding no improvement in repeated 30 s cycle sprint performance (Greer et al. 1998). More recently, some evidence supports the use of caffeine to improve repeat-sprint performance in trained males (Pontifex et al. 2010) and team sport athletes (Schneiker et al. 2006), while other investigations have demonstrated no improvement (Astorino et al. 2012; Lee et al. 2012). It should be noted that these investigations differed in calibre of athlete recruited, gender of participants, mode of caffeine ingestion (capsules, vs pills vs caffeine containing energy drink) and duration of repeat-sprint task (from 2 min-72 min), but very little research exists into the efficacy of caffeine to improve sprint swimming, where efforts are typically longer in duration than for traditional repeat-sprint exercise. Pruscino et al. (2008) reported that 6.2 mg x [kg.sup.-1] caffeine did not improve 2 x 200 m (separated by 20 min) freestyle performance in six elite male swimmers, but recent research involving shorter rest durations and a greater number of repetitions is lacking. Additionally, there is a scarcity of field based investigations utilising elite athletes to determine the efficacy of caffeine to improve high-intensity performance in this highly trained population (Burke, 2008).
Due to the lack of information available on caffeine's ability to improve sprint, and repeated sprint performance in highly trained swimmers, the aim of this investigation was to determine the efficacy of a moderate dose of caffeine to improve 6 x 75 m freestyle performance in elite freestyle sprinters. It was hypothesised that 3 mg.[kg.sup.-1] of caffeine would lead to worthwhile improvement in the first sprint, and mean sprint time throughout the 6 x 75 m repeated sprint trial.
Nine highly trained ([greater than or equal to]8 sessions per week) national level male swimmers (aged 20.8 [+ or -] 2.8 y; stretch stature 1.90 [+ or -] 0.07 m; body mass 83.9 [+ or -] 6.4 kg), were recruited. Prior to participation in the study, swimmers gave informed consent and completed a pre-participation health history form. Ethical approval was granted by the Human Ethics Committee of the University of Western Australia (RA/4/1/4734).
To assess whether CAF ingestion enhanced 6 x 75 m freestyle performance, a three week, single-blind, crossover study was designed. Initially, all participants completed a familiarisation session at the same pool to be used during the investigation (25 m indoor pool, 26.5 [degrees]C). Both sessions throughout the subsequent two week trial were performed at the same time of day, on the same day of the week (to minimise the effects of any possible confounding variables such as diurnal variations in treatment metabolism). The weekly micro cycle of training volume and intensity for each individual was kept consistent and participants were asked not to train in the morning prior to testing. The two sessions were completed under a CAF condition (using commercially available caffeine tablets: NoDoz), with the other utilising an artificial sweetener placebo (Equal). Participants were asked to refrain from consuming any products containing CAF in the 48 h prior to each session and completed a food diary in the 24 h prior to each session and kept their nutrition consumption consistent prior to each testing session. A sleep diary was kept for the week following each trial to assess the effect of each substrate on sleep.
Treatment group allocation
Participants were allocated into a treatment group prior to their first session via simple randomisation. Each week, substrate tablets were ground up and body weight corrected doses were deposited into white gelatine capsules (Capsuln Co). Participants consumed the same number of capsules in both trials. All capsules were consumed with 600 mL of water over 5 min, commencing 60 min prior to the first sprint.
Warm up: All participants completed a standardised 1500 m warm up commencing 45 min prior to the testing session. The warm up consisted of 500 m of easy freestyle swimming followed by two sets of: 3 x 100 m freestyle on a 1:40 min time cycle, 2 x 50 m freestyle at on a 45 s time cycle and 2 x 50 m freestyle as: 15 m explode and 35 m easy.
Experimental Protocol: The experimental trial comprised of 6 x 75...