Experimental Subjects
Twenty-four young adults (13 women and 11 men) were recruited from the Moncton campus of the University of Moncton. To do so, we invited by posting announcement to voluntarily participate students of the University of Moncton who applied the inclusion criteria. The study protocol was approved by the University’s Human Research Ethics Committee (UHRC), and all procedures followed were in accordance with the Helsinki Declaration of 1975, as revised in 2008. Informed consent was obtained from all patients for being included in the study. In addition to being obese, the inclusion criteria for participation were as follows: participants had to be physically active fewer than 60 min.week−1 as assessed by the International Physical Activity Questionnaire [14] and have no history of metabolic, cardiovascular, or chronic health problems, no history of drug consumption before the study, and no history of smoking. Before entering our protocol, each of the participants was thoroughly familiarized with all testing equipment and procedures. Indeed, each subject cycled for an extended period of time on the same cycle ergometer used throughout the study. Additionally, each participant was asked to determine the height of seat at which they are able to pedal comfortably. Unfortunately, we do not have any objective positioning (knee angle, hip angle, etc.) and this may be considered as limitation of the study. But, it is important to note that the position of each participant, e.g., the seat height, was the same throughout the study.
The protocol then began with three sessions of preliminary testing in order to determine certain key variables. The testing was conducted on three different days (D1, D2, and D3) after an overnight fast. Each day was separated by a minimum of 48 h, and all subjects were asked to avoid physical activity for 48 h prior to each session. In addition, subjects were asked to be well hydrated before each test. The same protocol that has been previously developed by our laboratory [15] seems suitable to delay any early fatigue or discomfort among participants which were all sedentary.
Following the determination of body composition, obese participants (BMI >30 kg/m2) were selected based on the Canadian guidelines for body weight classification in adults [16] and were separated in the following two groups: a control group (without any intervention; n = 12) and a training group (n = 12). Fat-free mass was calculated by subtracting fat mass from body mass.
Anthropometric Measurements
Body mass was measured to the nearest 0.1 kg with the subject in light clothing, without shoes, using an electronic scale (Kern, MFB 150 K100). Height was determined to the nearest 0.5 cm with a measuring tape fixed to the wall. Body mass index (BMI) was calculated as the ratio of mass (kg) to height2 (m2). Body fat percentage was estimated using a bioimpedance machine (Vacumed, Bodystat1500).
Physiology Assessment
On day 1 (D1), subjects performed a maximal test on an upright cycle ergometer (Monark ergomedic 839E electronic test cycle, USA) to determine their peak oxygen consumption (VO2peak). Before beginning the test, adults remained seated for 5 min on the bicycle ergometer in the same position used in subsequent exercise. Resting oxygen consumption was measured based on the mean oxygen consumption of the last 30 s of minutes 3, 4, and 5. No proper warm-up was performed. The test started at an initial power of 25 W and was progressively increased by 25 W every 2 min until exhaustion. In the present study, all participants reach exhaustion at 125 W. In fact, those who could not achieve a power output greater than or equal to 125 W were excluded.
A breath-by-breath automated metabolic system (CPX, Medical Graphics, St. Paul, Minnesota, USA) was used to determine the VO2peak of each participant. Calibration prior to each test was performed with standard gases of known oxygen and carbon dioxide concentration for gas composition and a calibration syringe for volume. The data were averaged on a 30-s interval, and oxygen uptake and respiratory exchange ratio, which is the ratio of carbon dioxide produced to oxygen consumed, were obtained.
Peak oxygen consumption was achieved when a subject fulfilled at least three of the following criteria: a plateau in VO2 in spite of an increase in exercise intensity, a respiratory exchange ratio greater than 1.1, a maximal HR above 90 % of the predicted maximal theoretical HR (220—age in years) or apparent exhaustion [17].
On day 2 (D2), we measured steady-state VO2 uptake at a constant submaximal power (below the VO2peak). After a 3-min resting baseline period, subjects started pedaling at the established power, which was maintained for 10 min at a pedaling rate of 60 revolutions/min (rpm). The 10-min exercise bouts were completed 6 times for each subject at powers of 30, 40, 50, 60, 70, and 80 % of their individual VO2peak power and were separated by a 5-min resting period, and the measurements for each subject were plotted separately and visually checked for linearity as described elsewhere [18]. This demonstrates that our measured VO2 uptake for a 10-min exercise was a steady-state value equaling VO2 demand (total rate of energy release). We therefore conclude that VO2 demand increased linearly with power for all subjects in the examined range. This analysis allowed for the calculation of the accumulated oxygen deficit (AOD) (measured in ml O2 equivalents per kg) for each cycling interval by calculating the difference between VO2 demand for the respective power (from extrapolation of the calculated relationship) and VO2 uptake. The linear relationship between steady-state VO2 values and cycling power was extrapolated and used to estimate energy demand during supramaximal cycling exercise (SCE) [19].
On day 3 (D3), following 10 min of warm-up, subjects performed a force-velocity test on a cycle ergometer using a technique adapted from the study performed by Vandewalle et al. [20]. This test consists of a succession of supramaximal bouts of approximately 6 s, with exercise loads increasing by 1 kg after each bout until the subject is unable to perform the test. A period of passive recovery (5 min) was allowed between successive bouts. The peak velocity for each bout was recorded, and the power output was calculated by multiplying the load and speed. The optimal load corresponded to the load at which maximal power (POmax) was achieved. This load was then used for the training protocol that followed. The force-velocity test was also performed every 2 weeks to adjust the individual power level of SCE. These 3 days of testing were completed before high-intensity training (HIT) (visit 1), and also at the end of training (visit 2).
Training Session
Once participants completed preliminary testing, they were instructed to complete a total of 18 training sessions (three sessions per week for 6 weeks). Each of the prescribed sessions began with a 5-min warm-up of continuous cycling at moderate intensity (40 % of their individual VO2peak power), followed by 6 repetitions of SCE intervals with 2 min of passive recovery between each repetition. Each SCE repetition lasted 6 s, and participants were asked to pedal at maximal velocity against the resistance determined during D3. The repeat sprint cycling test was conducted under the supervision of a member of the research team, and velocities (in RPM) were recorded for each second of the bout in order to ensure that said velocities were constant. Based on the linear regression and the individual VO2max, the workload approximately corresponded to ~350 % of VO2max [12].
In fact, we have chosen in the present study the HIT for many reasons. Firstly, very brief high-intensity repeated exercise comprised of 6- to 10-s sprints induces substantial improvements in both performance and health-related outcomes [21]. For example, 2 to 15 weeks of this type of training results in significant increases in anaerobic capacity, ranging between 5 and 28 % in untrained males [21]. Moreover, it represents a time-efficient approach to obtain health benefits from exercise, as sessions involve a total of only 2–3 min of high-intensity exercise and typically last for less than 15 min [22, 23].
Training sessions were conducted under the supervision of a member of the research team, and velocities (in RPM) were recorded for each second of the bout in order to ensure that said velocities were constant. Additional testing was also completed during the first prescribed training session, as well as during the training session at the end of the sixth week. Indeed, during the first and the final sessions, participants were asked to perform one of their regular training sessions while being analyzed by our breath-by-breath automated metabolic system during only four repetitions. This was done in order to determine the contribution of each energy system. In this study, results are given only from four supramaximal cycling exercise bout given that some of participants felt discomfort by wearing mask destined for gas analysis. Then, the mask was removed to allow participants to complete the entire session bouts. Lactate concentrations were obtained at rest for all experimental subjects, and immediately following the fourth repetition, via capillary blood samples using the LactatePro analyzer. This technique was previously utilized in our laboratory to monitor athletes. In the present study, lactate concentrations served only to complete the profiles of the control and training groups, before and after HIT.
After completing 6 weeks of training, participants were asked to return for a final day of testing (visit 2), during which the procedures of D1, D2, and D3 were repeated, and post-training data was collected.
Calculation of Relative Energy Expenditure: Supramaximal Cycling Exercise Bout
The VO2 demand values of SCE were estimated individually by extrapolating the linear relationship between the power and the VO2 demand values established during the constant submaximal power exercises. The accumulated VO2 uptake and the accumulated VO2 demand were taken as the VO2 uptake and the VO2 demand integrated over the entire supramaximal cycling exercise bout [18]. The AOD was equal to the accumulated VO2 demand minus the accumulated VO2 uptake. This allowed for a measurement of anaerobic (AOD) and aerobic (VO2) energy contributions throughout the four SCE repetitions [24].
Statistical Analyses
After testing for normality (Kolmogorov–Smirnov test), statistical comparisons were made between the control group and the training group on two separate occasions (before and after training). Two-way repeated measures ANOVA was used to determine whether significant changes in energy system contributions emerged between the two groups, and if energy system contributions differed between the two groups. Furthermore, Bonferroni’s post hoc test was performed. Pearson correlations were used to assess the relationship among changes in muscle power, aerobic capacity, and energy contribution modification. A value of p < 0.05 was statistically significant. Analyses were performed using IBM SPSS Statistics 19 software.