Typically, moderate intensity continuous exercise has been recommended for improving aerobic fitness (Garber et al., 2011), however high-intensity intermittent exercise (HIIE) has gained popularity due to its benefits in improving aerobic fitness (Helgerud et al., 2007), while being executed in a short period of time (Gillen and Gibala, 2014). Although HIIE has been studied and is currently recommended (Garber et al., 2011) there are still some questions about its prescription due to the characterization of some acute physiological and mechanical responses which are not fully elucidated as the total work done, intensity relative to maximum indexes (percent of maximum power output and maximal oxygen uptake). This is a result of the large number of variables that can be manipulated during HIIE (Buchheit and Laursen, 2013). With respect to intensity, a practical mode for prescribing HIIE is through all out efforts, as this type of exercise does not require a prior test to identify exercise intensity. In this kind of exercise the maximal intensity is employed and the participant can self-regulate the intensity of the efforts according to their knowledge of the total volume of the session (Billaut et al., 2011).
Another important aspect that needs to be considered in the prescription of all out HIIE is that the physiological and mechanical responses to HIIE are affected by sexual dimorphism (Billaut et al., 2012). Studies investigating differences between men and women in all out efforts utilized protocols composed of a single effort (Esbjomsson-Liljedahl and Jansson, 1999; Leicht et al., 2011; Lovell et al., 2011; Perez-Gomez, 2008), or brief repeated efforts (Billaut et al., 2003; 2009; 2012; Laurent et al., 2010; Townsend et al., 2014). In general, men achieved higher peak and mean power, while women demonstrated lower performance decrements (Billaut et al., 2009; Leicht et al., 2011; Perez-Gomez et al., 2008), even when performance was relativized by body mass (Perez-Gomez et al., 2008). Physiological mechanisms responsible for sex-related differences may involve differences in energy metabolism (Hunter, 2014) being that greater power performed by men can be explained by their superior anaerobic potential (Esbjornsson-Liljedahl and Jansson, 1999) and a greater proportion of carbohydrate utilization (Carter et al., 2001).
However, little is known concerning more prolonged high-intensity intermittent all-out exercise (those featuring an HIIE session), predominantly protocols that are effective for decreasing fat mass and increasing long-term aerobic fitness. Thus, knowledge of acute responses to this type of exercise could help to create inferences about long-term adaptations (Trapp et al., 2008).
Performance relative to maximal aerobic power (MAP), oxygen uptake ([VO.sub.2]) and heart rate (HR) relative to its maximum values ([VO.sub.2]peak and HRmax, respectively) are indices relevant for training prescription, and aerobic and anaerobic adaptation to training depends on these parameters (Buccheit and Laursen, 2013). However, these responses during self-paced all out HIIE are not understood, especially with regard to sex differences.
Thus, the objective of the present study was to compare the performance (total work and power decrement), utilization of anaerobic reserve, blood lactate (peak and delta), respiratory exchange rate, [VO.sub.2] and HR relative to its maximum index in response to a high-intensity intermittent all-out exercise protocol between men and women. Thus, the main hypotheses of the present study were that the men would perform HIIE in higher intensities in terms of power than women, with same cardiovascular responses (oxygen uptake and heart rate) indicating a greater utilization of anaerobic pathways.
Participants completed three experimental sessions separated by at least 72 hours. During the first session, anthropometric measurements and [VO.sub.2]peak test on a cycle ergometer were taken. In the second session, the participants were submitted to a familiarization of high-intensity intermittent all out exercise protocol (60 sprints of 8s effort and 12s pause), and in the third session they performed the exercise per se. This protocol was chosen because it was shown to improve aerobic fitness and reduce the fat mass (Heydari et al., 2012; Martins et al., 2015; Trapp et al., 2008).
Ten men and nine women, physically active, participated in this study. Participants were included if they did not report any health problems and/or neuromuscular disorders that could affect their ability to complete the study protocol. Furthermore, all were free of any drug or ingestion of nutritional supplements during the period of the study. Participants took part voluntarily in the study after being informed about the procedures, risks, and benefits and signed an informed consent form. This study was approved by the local ethics committee. The women were tested in the follicular phase (1-10 days after the onset of menstruation) of the menstrual cycle.
Height was measured using a stadiometer with a metric scale affixed to the floor, and body mass were measured using an electronic scale (precision 0.01 kg). Body fat percentage was estimated via skinfold, circumferences and bone diameter measurement (Drinkwater and Ross, 1980). Skinfold thickness measurements were carried out using a Harpender plicometer (John Bull British Indicators, England), three times at each point in a rotation system (the median value was used), as described by Drinkwater and Ross (1980).
The participants performed an incremental test to volitional exhaustion on an electronically braked cycle ergometer (Lode, Netherlands). The initial load was set at 30W and it was increased by 25W x [min.sup.-1] for men and 15 W.min-1 for women. Cadence was set at 70rpm, and subjects were instructed to perform the test until they could no longer continue. The test was finished when subjects could not maintain the load for 5s in the fixed cadence. Strong verbal encouragement was given during the test. The [VO.sub.2] was measured (MetaMax[R]3B, Cortex, Germany) throughout the test and the average of the last 30s was defined as [VO.sub.2peak]. The maximal load reached in the test was defined as the maximal aerobic power (MAP). When the subject was not able to finish the 1-minute stage, the power was expressed according to the permanence time in the last stage, determined as the following: maximal aerobic power = power of last stage completed + [(time, in seconds, remaining in the last stage multiplied by 25W or 15W)/60s]. The respiratory compensation point (RCP) was determined by point of a nonlinear rise in the ventilation (VE) related to carbon dioxide volume (VC[O.sub.2]) (Meyer et al., 2005).
High-intensity intermittent all out exercise
Participants performed a warm-up at 50% MAP for 5min, and after a 2-min rest interval they started the all out exercise. Participants cycled (electronically braked cycle...