With the growing prevalence of obesity and metabolic disorders such as T2D (Gastaldelli, 2008), there is increasing interest in determining factors that maximise fat oxidation during exercise training to induce weight loss in obese adults (Corpeleijn, 2009). Aerobic exercise is commonly advocated to improve whole body fat oxidation (Zarins et al., 2009), with the highest rates of fat oxidation repeatedly seen with moderate-intensity aerobic exercise (Melanson et al., 2002; Romijn et al., 2000; Saris et al., 2003). Moderate-intensity exercise training is commonly undertaken as a continuous bout at a constant mechanical workload. However, for some individuals constant load work can be difficult to perform for the duration required to attain an adequate training dose. Therefore, participation in multiple short bouts of 10-15 mins was advocated in sedentary individuals who perceive barriers to continuous exercise (Jakicic et al., 2001; Jakicic et al., 1995; Murphy et al., 2009). Recently, there has been growing interest in using shorter interval stages in the diabetic and obese populations (Boutcher, 2011; Earnest, 2008; Hansen et al., 2010), which involves repeated bouts between 30 secs and 5 mins and interspersed by similar durations of rest or low-intensity bouts (Laursen and Jenkins, 2002). Obese women perceived moderate-intensity interval exercise (i.e. alternated 80 and 120%VT every 2 mins for 32 mins) as being less hard than continuous exercise (i.e. 100%VT for 32 mins) (Coquart et al., 2008). Therefore, obese individuals could use moderate-intensity interval exercise instead of moderate-intensity continuous exercise to improve compliance to training.
The intensity ([FAT.sub.max]) that elicits maximal fat oxidation (MFO) during a graded exercise test (GXT) has been suggested as a reference method to prescribe exercise training where optimising fat oxidation is the goal (Achten et al., 2002). The main advantage of the [FAT.sub.max] test is that the MFO is determined with the use of a single GXT protocol, rather than undertaking several constant load tests performed with different workloads on different days (Meyer et al., 2007). Given the interest in moderate-intensity interval training as a strategy for improving exercise compliance (Coquart et al., 2008), it would be valuable to know if the [FAT.sub.max] test is a valid means of prescribing moderate-intensity interval training. Moderate-intensity interval training may not induce exercise duration-related drift in fat oxidation, seen in continuous exercise (Cheneviere et al., 2009; Meyer et al., 2007). It may be more closely related to the fat oxidation values during 3-5 min stages of GXT protocols.
There is some evidence that the impact of moderate-intensity continuous and interval training on fat oxidation are different. Venables and Jeukendrup (2008) investigated the effect of four weeks of moderate-intensity interval training which consisted of 5-min at 20% above [FAT.sub.max], alternated with 5-min at 20% below [FAT.sub.max] on fat oxidation in obese men, compared with moderateintensity continuous training at the level of [FAT.sub.max]. In this study interval training did not increase fat oxidation during a 30-min constant-load test compared with the baseline although the same participants were able to increase fat oxidation by 44% after continuous training.
The mechanical workload for continuous aerobic training is commonly prescribed by the relationships between physiological variables such as oxygen consumption (V[O.sub.2]), heart rate (HR) and the concentration of blood lactate (BLa) and mechanical work (Hofmann and Tschakert, 2011) and between the rating of perceived exertion (RPE) and mechanical work (Chen et al., 2002) derived during a GXT. There is support for the use of GXT for prescribing workloads for exercise training as the physiological responses (V[O.sub.2], HR, BLa) to constant-load moderate-intensity exercise relate well to the responses in the GXT (Salvadego et al., 2010; Steed et al., 1994). It is important to confirm whether the relationship between mechanical work and physiological variables (V[O.sub.2], HR and BLa) and between mechanical work and RPE during an exercise session of moderate-intensity interval training relate to the physiological-mechanical work and psychological-mechanical work relationships at MFO in the GXT.
The protocol of GXT to determine MFO was modified in terms of initial and incremental workloads among untrained obese individuals to meet their fitness levels (Bircher et al., 2005; Haufe et al., 2010; Perez-Martin et al., 2001). For example, Bircher and Knechtle (2004) started the test with 100 W in the athletes and 40 W in the obese. Roffey (2008) adapted Achten's protocol among obese men to start with 50 W followed by increments of 30 W. Haufe et al. (2010) designed the protocol to start the workload at 25 W, with increments of 25 W every 2 mins. Perez-Martin et al. (2001) suggested a protocol of four 6-min steady-state workloads at 30, 40, 50 and 60% [W.sub.max]-predicted with a warm-up stage at 20% [W.sub.max]. Bircher et al. (2005) compared two protocols; one was defined as 35 W increments for 3 mins and a total of 20 mins and the other increased according to HR and was 26 W increments for 5 mins and a total of 45 mins, and found significant differences in MFO in men.
The current study aimed to compare fat oxidation, physiological variables (V[O.sub.2], HR and BLa) and RPE corresponding with [FAT.sub.max] derived from a GXT and during a 30-min moderate-intensity interval exercise training (MIIT) session consisting of 5-min stages at 20% above then 20% below [FAT.sub.max].
Participants included 12 sedentary overweight/obese men. The characteristics of participants were: age (29 [+ or -] 4.1 years), BMI (29.1 [+ or -] 2.4 kg-m-2), fat mass (31.7 [+ or -] 4.4 %body mass) and V[O.sub.2peak] (31.8 [+ or -] 5.5 ml-kg-1-min-1). Participants were recruited from the staff and student population at the Queensland University of Technology (QUT) and the Brisbane metropolitan region via e-mail and flyers posted on community noticeboards. Consent was obtained and prior to undertaking the study, the participant was required to gain medical clearance to perform a maximal exercise test. The study protocol was approved by the Human Research Ethics Committee at QUT (HREC No. 0900000338).
The study was a cross-over design. Each of the 12 participants completed two sessions: a GXT to determine MFO and V[O.sub.2max] and a moderate-intensity interval exercise session. The tests were performed on a braked cycle ergometer (Monark Bike E234, Monark Exercise AB, Sweden). Seat position was adjusted so that the knee was slightly flexed (about 5[degrees] less than maximal leg extension) with the ball of the foot on the pedal, and the handlebar was adjusted so that the participant was on an upright posture. Cadence was maintained at 70 rpm during all tests. Expired air was collected and heart rate was monitored during tests.
All participants were asked to maintain their normal dietary intake between tests, and to replicate their food intake as closely as possible on the day before the exercise tests. Participants were also asked to abstain from strenuous exercise and the consumption of caffeine and alcohol in the previous 24 h. They were instructed to wear lightweight, comfortable clothing during the tests. All tests were undertaken after an overnight fast and were run in an air-conditioned laboratory with the temperature held constant at 21[degrees]C.
[FAT.sub.max] and V[O.sub.2max] protocol
The [FAT.sub.max] graded cycle ergometry protocol was discontinuous, with participants cycling at 35 W for 4 min followed by a 4-min rest interval. Participants remained seated on the cycle ergometer during the rest interval while finger tip blood lactate samples were immediately collected and perceived effort determined using the Borg Scale 6-20. At the end of the rest interval the work rate was increased by 17.5 W and the participant cycled at the new workload for 4 min. The discontinuous sequence of 4-min work-rest stages with 17.5 W increments in workload continued until the workload at which RER reached 1.0 and remained above 1.0 during the final 2 min of exercise. After a 4-min rest, participants commenced the second phase of the test designed to determine maximal aerobic power. Participants cycled for a minute at a workload two increments lower than the intensity at which an RER of 1.0 was reached, after which the mechanical work was increased by 17.5 W every minute until volitional exhaustion. Finger tip blood lactate samples were collected at the end of this period. [FAT.sub.max] was determined for each participant by examining individual relationships between...