High-intensity interval training (HIIT) is recognized as one of the most effective means of improving cardio-respiratory and metabolic function in athletes (Buchheit and Laursen, 2013). A typical HIIT session involves the repetition of periods of high-intensity exercise (i.e., the intervals) interspersed with periods of lower intensity (i.e., recovery periods) (Buchheit and Laursen, 2013). Long intervals (i.e., >3 min) are typically completed close to the speed/power associated with maximal oxygen consumption (V[O.sub.2max]), because it is believed that this is optimal stimulus for eliciting maximal cardiovascular and peripheral adaptations (Buchheit and Laursen, 2013). However, because V[O.sub.2] and cardiac output (Qc) can be dissociated during intense exercise (Lepretre et al., 2004), and attaining and maintaining an elevated stroke volume (SV) is likely important for improving maximal cardiac function (Cooper, 1997), increasing time spent at maximal Qc (Qcmax) and/or training at an intensity associated with maximal SV may also be important (Lepretre et al., 2004).
The intensity of exercise that maximizes the time at maximal SV is difficult to predict (Gonzalez-Alonso, 2008; Mortensen et al., 2005; Warburton and Gledhill, 2008). The most appropriate HIIT format inducing increased time at Qcmax in well-trained and elite athletes remains to be determined. In untrained males, compared with peak exercise during a graded exercise test, 30-s all out sprints (typical of sprint interval training sessions (Buchheit and Laursen, 2013)), might allow attainment of similar Qc (effect size; ES = -0.1) and even largely higher SV (ES = +1.3) (Fontana et al., 2011). The recovery period is another key factor of HIIT with respect to cardiopulmonary responses (Buchheit and Laursen, 2013). Conjecture remains as to whether maximal SV is reached during the recovery period or during work periods--and whether this response is recovery-intensity dependent (Buchheit and Laursen, 2013; Cumming, 1972; Gonzalez Alonso, 2008; Warburton and Gledhill, 2008). Therefore, the aim of the current study was to examine the effect of recovery intensity on cardiovascular parameters during HIIT.
Subjects Fourteen endurance-trained male cyclists participated in the study (age, 25 [+ or -] 4 years; body mass, 69.6 [+ or -] 4.9 kg; height, 1.77 [+ or -] 0.04 m; V[O.sub.2max], 66.6 [+ or -] 4.2 mL-[kg.sup.-1] x [min.sup.-1]; peak power output, 405 [+ or -] 28 W). The experimental procedure was approved by the Human Research Ethics Committee at The University of Queensland.
The complete methodology has been reported elsewhere (Stanley et al., 2014). Briefly, the cyclists completed an incremental cycling test to determine the power at V[O.sub.2max] (pV[O.sub.2max]) and maximal Qc (Qcmax), and two cycling (Wattbike[R], Wattbike Ltd., UK) HIIT sessions (12-min warm-up, three 3-min work periods (Ex) at 90% pV[O.sub.2max]) 5-7 days apart. The 2-min recovery periods were completed at either 30% pV[O.sub.2max] (30%) or 60% pV[O.sub.2max] (60%) in a randomized order with passive (PAS) recovery always following the final interval. Respiratory gas exchange (ParvoMedics TrueOne[R] 2400, Utah, USA) and HR, SV and Qc (PhysioFlow, Manatec Biomedical, France) (Charloux et al., 2000) were measured continuously. Oxygenation of vastus lateralis (tissue saturation index; TSI) was determined using near-infrared spectroscopy (PortaMon, Artinis Medical Systems BV, The...