Participants and study design
Six men and two women (23–54 years of age) met in the morning (07.00–10.00 am) for measurements of RMR. Thereafter, each participant performed four field-cycling tests outdoors and a maximal cardiorespiratory fitness test on a cycle ergometer in the laboratory. All participants were of Caucasian origin, non-smokers and without overt disease or use of medications.
The participants were given oral and written information about the study objectives and methods. Data was stored according to guidelines by the Norwegian Social Science Data Services.
Measurements
Height and body mass were measured (in light clothing, without shoes) using a stadiometer and a physician’s scale (Seca 713, Birmingham, UK).
Field tests
Oxygen uptake (\( \overset{.}{\mathrm{V}}{\mathrm{O}}_2 \)) was measured with a portable oxygen analyser (MetaMax 3B-R2, CORTEX Biophysik GmbH, Leipzig, Germany) and a breathing mask, which were dressed according to the instructions of the manufacturer. MetaMax 3B-R2 has been found reliable and valid (approximately 10% difference between methods) when compared to the “Gold standard” the Douglas bag method and a secondary criterion machine known to be accurate, the Jaeger Oxycon Pro system [26]. Prior to using, the system was turned on for at least 30 min, and then calibrated prior to every test. First indoor calibrating of the gas analysers by using a reference gas (16% O2, 4% CO2), and then verifying the calibration against ambient air. Secondly, a volume calibration was performed using a standardised 3-L syringe (5530 series, Hans Rudolph, Inc., MO, USA). Sensor adjustments (ambient air calibration) were performed after bringing the MetaMax outside and before the first cycling session in each person. Speed and elevation were measured from GPS-coordinates using a GPS Kit for MetaMax® and cycling time was measured on-site by the test leader.
Total measurement period lasted between 120 and 150 min, where the participants cycled on two “cycling-to-work” routes, using a conventional bicycle (Cannondale 50/50 with an internal Shimano Nexus 8 speed hub, wheel size 26 `and bike weight of 12 kg (2005 model), Wilton, Connecticut, USA) and an E-bike cycling on maximal electrical power (RIXE with a 17 Ah/612 Wh Impulse battery, wheel size 28 `and bike weight of 27 kg (2014 model), Cloppenburg, Germany). The E-bike had a motor capacity of 250 W and a maximal speed of 25 km/h, with an active engine.
The two routes (a flat route of 8.1 km, and a hillier, but shorter, route of 7.1 km) started at the same place of departure, reached the same destination, and ended at the place of departure (simulating commuting, back and forth, from residence to a place of work). Altitude at both departure and destination was 18 m above sea level. The hilly route reached a maximal altitude of 83 m above sea level, including one hill that was climbed twice (to and from destination giving a total height difference of 130 m). Conversely, the flat route had no steep hills, and reached a maximal altitude of 35 m above sea level (total height difference 34 m). The route profiles are shown in Fig. 1.
Each participant performed four trips, the order of which was randomised; these were 1) E-bike hilly, 2) E-bike flat, 3) Bike hilly and 4) Bike flat. There was a two minutes’ break between the experimental conditions. The participants were told to cycle at the intensity they would choose when commuting to work.
Laboratory tests
Before the field tests, RMR was obtained by indirect calorimetry with a canopy hood (Oxycon Pro, Jaeger BeNeLux Bv, Breda, Netherlands) according to international guidelines [27], using a standardised protocol. Prior to using, the system was turned on for at least 30 min, and then calibrated prior to every test. First calibrating of the gas analysers by using a reference gas (15% O2, 6% CO2), and then verifying the calibration against ambient air. Secondly, an automatic volume calibration was performed according to manufacturer’s recommendations. Participants were instructed to abstain from coffee, alcohol, smoking, exercise and not eat food, 12 h before measurements. The participants were placed under the canopy hood in a relaxed, supine position for 30 min and instructed to stay awake. Measurements were repeated on all participants until a 15-min steady state and the lowest \( \overset{.}{\mathrm{V}}{\mathrm{O}}_2 \) (ml/kg/min) value, average of one minute, defined RMR.
After the field tests, the participants had a 15 min’ break before they performed a test of cardiorespiratory fitness, measured as \( \overset{.}{\mathrm{V}}{\mathrm{O}}_2 \max \). The protocol was performed using an electronically-braked cycle ergometer (Monark 839 Ergomedic, Varberg, Sweden), which was calibrated electronically before each test and mechanically after being moved. MetaMax 3B-R2 was calibrated before each test. During \( \overset{.}{\mathrm{V}}{\mathrm{O}}_2 \max \) tests, the ambient air temperature was 22 – 23 °C, relative humidity 50 – 60%, and barometric pressure 99.3 – 101.2 kPa. The initial workload was 50 W and was increased by 25 W every 2nd minutes until exhaustion. The pedalling rate (between 60 and 80 rpm) was instructed, and height of the saddle was adjusted individually. Minute ventilation (\( \overset{.}{\mathrm{V}}\mathrm{E} \)), respiratory exchange ratio (RER) and \( \overset{.}{\mathrm{V}}{\mathrm{O}}_2 \) were recorded every minute by using the same portable gas analyser as in the field tests. Data was analysed with Metasoft Studio v.4.9 (Cortex Biophysik, Leipzig, Germany). The main criterion for having reached maximal effort was a subjective assessment by the test leader. The second criteria were RER above 1.00 and reporting perceived exertion (RPE) above 17 using the Borg-RPE-Scale [28].
Data processing
Data from start to end of cycling the sessions were imported into Microsoft Excel®, computed at one-minute intervals, and synchronised for further analysis. Percentage of \( \overset{.}{\mathrm{V}}{\mathrm{O}}_2 \) max was calculated by dividing \( \overset{.}{\mathrm{V}}{\mathrm{O}}_2 \) (ml/kg/min) from the field tests by maximal \( \overset{.}{\mathrm{V}}{\mathrm{O}}_2 \) (ml/kg/min) from the laboratory test. Measured METs and estimated METs were calculated by dividing participants RMR (\( \overset{.}{\mathrm{V}}{\mathrm{O}}_2 \) ml/kg/min) and the standard 1-MET value (3.5 ml O2/kg/min), respectively, by oxygen uptake during the field tests. Time spent in moderate and vigorous intensity physical activity was based on measured METs, which was categorised as moderate if 3 – 5.9 METs or vigorous if ≥6 METs [29]. Data are presented as median and interquartile range (IQR).