Interest in optimizing the magnitude of adaptation resulting from physical training, while minimizing the time and effort devoted to training, is a topic of considerable interest within the exercise community. Including classical studies of interval training for athletic performance (Astrand et al., 1960; Muller, 1953) the substantial body of evidence regarding the effects and side effects of variations in the Frequency, Intensity, Time and Type (FITT) of training are effectively codified in ACSM's Guidelines for Exercise Testing and Prescription (Pescatello et al., 2014). This evidence is further addressed in the broad public health recommendation that healthy adults should accumulate 30 min of moderate intensity exercise on most if not all days of the week (Haskell et al., 2007), and that individuals interested in enhanced outcomes (including competitive performance) should regularly do both a larger volume of training and higher intensity training (Billat, 2001; Selier et al., 2013). Active research continues designed to determine how specific variations of FITT might further optimize adaptations to exercise training. The literature, particularly with reference to high intensity interval training (HIIT), has recently been reviewed (Buchiet and Laursen, 2013a; 2013b; Kessler et al. 2012; Weston et al., 2014). Since one of the chief barriers to broad public participation in exercise programs is a perceived lack of time (Salmon et al. 2003), one of the appeals of HIIT training has been that it potentially represents a more time efficient way to accomplish the adaptive goals of exercise training. Indeed, Gillen et al (2014) have shown that as little as three 10 min sessions weekly, with only 3 x 20s high intensity, could effect both muscle oxidative capacity and several markers of cardiometabolic health. Beyond the importance of time efficiency, there are a number of known motivations for participation in exercise programs (extrinsic motivators generally associated with changes in the body) and sport (intrinsic motivators related to pleasure and mastery) (Kilpatrick et al., 2002, 2005). These motivators can be contextualized within the concept of self-determination theory, which suggests that human activity can be understood within the context of seeking autonomy, competence and relatedness (Kilpatrick et al. 2002). Amongst the predictors of continuing an exercise program is recognition of the importance of enjoyment to long-term adherence with exercise programs (Dishman et al., 2005). There are relatively little data available regarding how different types of exercise programs are perceived by exercisers. Early evidence suggests that high-intensity interval running might be more enjoyable than moderate-intensity continuous exercise (Bartlett et al., 2011 Jung et al. 2014), although everyday experience suggests that higher intensity exercise is inherently less comfortable (i.e. enjoyable). This is important because even if exercise programs can be constructed in a very effective and time efficient format, if they are not perceived as enjoyable there is little likelihood that the program will be sustained for long enough to achieve reasonable health and fitness outcomes.
Beginning with studies demonstrating the value of interval training in clinical populations (Smodlaka, 1963; Meyer et al., 1990), and inspired by evidence that very high-intensity training can simultaneously produce adaptations in both aerobic and anaerobic exercise capacity (Tabata et al., 1996), interest in the potential value of HIIT, as an alternative to conventional training, has been considerable during the past 20 years. Studies from a number of laboratories, with protocols designed more to demonstrate the rapidity of molecular signaling events following high-intensity training (Babraj et al., 2009; Burgomaster et al., 2005; Gibala and McGee, 2008; Gibala et al., 2012; Helgerud et al., 2007, Whyte et al., 2010) than to have practical use (Bayati et al., 2011; Guiraud et al., 2010; Little et al., 2009; Matsuo et al., 2014; Nybo et al., 2010; Osawa et al., 2014; Rognmo et al., 2004) have demonstrated the ability of HIIT to produce large gains in both aerobic and anaerobic exercise ability, often with a remarkably reduced direct exercise time requirement. However, since these protocols have widely different levels of experimental control (sedentary vs aerobic exercise), there is still debate over the relative value of HIIT training relative to steady--state training. Further, since many of the HIIT protocols can present significant discomfort to the exerciser, the likelihood that long term adherence to HIIT training will be high enough to promote long term beneficial outcomes is of concern. However, we have little direct evidence about how different training programs are perceived. Accordingly, the purpose of this study was to compare physiologic responses of two basic HIIT variants against a steady-state training control in previously inactive young adults, as evidenced by changes in both aerobic and anaerobic exercise capacity. Additionally, we sought to evaluate how training was perceived in these groups, from the perspective of factors that might influence long term adherence.
Sixty-five (23 male, 42 female) relatively-sedentary subjects volunteered for the study. Their ages ranged from 18-28 years. The protocol, purpose, and risks of the study were explained to all interested participants. The Physical Activity Readiness Questionnaire (PAR-Q) was administered to the subjects prior to participation to rule out contraindications to participation. In order to be eligible for the study, subjects could not have been exercising more than twice per week at low-to-moderate intensity during the preceding three months (e.g.
An incremental exercise test, performed on an electrically braked cycle ergometer (Lode Excalibur, Groningen, NL), was used to assess aerobic capacity. The subjects were instructed to abstain from caffeine for 6 hours before the test, which was conducted (within subject) in a period of [+ or -] 2 hours of each day. A practice test was not administered. The test began with a 5-min resting period to allow measurement of resting HR, followed by a 3-minute warm up at 25 W. After 3 minutes, the load was increased by 25 W per minute. Subjects pedaled at a cadence ~80 rpm. The test was terminated when subjects were too fatigued to continue, or when the cadence fell below 60 rpm. Maximal HR was measured using radiot-elemetry (Polar Electro-Oy, Kempele, Finland). The Rating of Perceived Exertion (RPE) was measured during the test using the Category Ratio (0-10) RPE scale (Borg, 1998). Respiratory metabolism was measured using open-circuit spirometry, with a mixing chamber based metabolic cart (Parvo Medics, Sandy, Utah). Calibration was completed before each test using a reference gas (16% [O.sub.2] & 4% C[O.sub.2]) and room air. A 3-L syringe was used to calibrate the pneumotach. V[O.sub.2] was summated every 30s, and the highest 30s value during the test was accepted as V[O.sub.2]max. A verification trial was not performed as we have previously found that there is no systematic change in VO2max during a second exercise effort at higher muscular power output (Foster et al., 2007). The peak aerobic power, expressed per kg BW ([P.sub.aer]PO) was accepted as the PO for the highest stage completed plus proportional credit for incomplete stages.
As a measure of anaerobic power-capacity, the subjects performed the Wingate Anaerobic Test (Bar-Or, 1987). The test was performed, on a different day, on an electronically braked cycle ergometer (Lode Excalibur, Groningen, NL), in the constant torque mode. The subjects warmed up for 5-min at 25W. In the last 5-s of the warm-up period, the subject increased their pedaling rate to >100 rpm (with no resistance on the flywheel). At the beginning of the test the resistance was increased to 0.075 kg x [kg.sup.-1] BW and the subject attempted to maximize their pedaling rate for the next 30s. Peak power output (PPO) (the highest PO observed during 1s during the test) and the mean power output (MPO)(the average PO over the 30s duration of the test) were recorded from the ergometer software. The PPO and MPO were expressed relative to BW. As an additional marker of exercise capacity, the Combined Exercise Capacity (CEC) was calculated as the mean of [P.sub.aer]PO + PPO + MPO, and expressed as W x [kg.sup.-1] BW.
One day during each week of the 8-week training program the subjects completed the Exercise Enjoyment Scale (EES) (Stanley et al., 2010). The ESS was administered pre-, during- and post-training to determine the subject's perceived level of enjoyment. A rank of zero indicated the absence of enjoyment, while a rank of seven indicated high enjoyment. All subjects were directed to rank their perceived level of enjoyment at the exact moment in time that the scale was administered. Within subjects, the ESS was admininstered on the same day of each week. For logistic reasons, between subjects the administration of the ESS was distributed throughout the week.
Following pre-testing, the subject's exercise capacity was ranked based on the CEC. Males and females were ranked separately. From these...