Research participants and study design
This feasibility study was reviewed and approved by the Ethics Review Board of the Canton of Bern in Switzerland (Kantonale Ethikkommission Bern, KEK. Ref. No. 2016-01502). Written informed consent was obtained from all participants.
Five healthy males (age 31.0 ±3.9), all non-smokers with no known cardiovascular, pulmonary or musculoskeletal problems participated (Table 2). The five participants had been recruited among participants in a previous study, where they performed incremental cardiopulmonary exercise tests on the same training device [18]. The two studies were separated by 7 months. Peak cardiopulmonary outcomes (\(\dot {V}\mathrm {O}_{2\text {peak}}\) and HRpeak), and sub-maximal cardiopulmonary thresholds (first and second ventilatory thresholds \(\dot {V}\mathrm {O}_{2\text {VT}1}\), \(\dot {V}\mathrm {O}_{2\text {VT}2}\), and heart rates at the first and second ventilatory thresholds HRVT1, HRVT2) obtained from the previous study were used for the individual training prescription and analysis (Table 2).
Each participant carried out 2 formal training sessions: (i) a constant-load test (CLT), where participants trained for 30 min at their personal PVT1 (work rate that corresponds to \(\dot {V}\mathrm {O}_{2\text {VT}1}\) obtained from the incremental test); (ii) a high-intensity interval training (HIIT) session, where participants performed 5 high intensity intervals at their personal PVT2 (work rate that corresponds to \(\dot {V}\mathrm {O}_{2\text {VT}2}\) obtained from the incremental test). Training sessions were separated by at least 48 h. Participants were required to avoid strenuous activity within the 24 h prior to each formal test session, to refrain from caffeine for 12 h before, and not to consume a large meal within 3 h prior to testing.
The CLT protocol (Fig. 1a) consisted of 5 stages: a 3-min recorded rest; 2 training phases of 15 min each separated by a 4-min rest period; and a 3-min recorded rest. During the training phases participants were instructed to follow a predefined target work rate PVT1 which corresponds to their individual \(\dot {V}\mathrm {O}_{2\text {VT}1}\).
The HIIT protocol (Fig. 1b) started with a 3-min recorded rest, then 5 training intervals of 2 min at a work rate PVT2 corresponding to the individual \(\dot {V}\mathrm {O}_{2\text {VT}2}\), interspersed by 4-min rest periods. The HIIT session was ended by a 3-min recorded rest. For both CLT and HIIT, participants performed 2 min of unrecorded warm-up at low intensity prior to the training session.
Equipment
A commercial dynamic leg press exercise device (Allegro, Dynamic Devices AG, Switzerland) was used in this study. The dynamic leg press is pneumatically-actuated and was augmented with force sensors in the foot plates, angle sensors at the rotation axis of the pedal, a work rate estimation algorithm, and a visual feedback (Fig. 2).
Respiratory variables and heart rate were monitored and recorded using a breath-by-breath cardiopulmonary monitoring system (Metamax 3B, Cortex Biophysik GmbH, Germany). Pressure, volume and gas concentration were calibrated prior to each test according the manufacturer’s instructions: pressure was calibrated using a certified atmospheric pressure device; volume using a 3 L syringe; and gas concentrations were calibrated using ambient air and a precision gas mixture (15% O2, 5% CO2). Heart rate was recorded using a chest belt (T34, Polar Electro Oy, Finland). Analysis of the cardiopulmonary data was done using the proprietary software associated with the breath-by-breath system (Metasoft, version 3.9.9 SR5).
Work rate estimation and targets
During the training sessions, the participant’s feet were fixed on footplates, and his work rate was estimated using force and velocity data (Fig. 2). The work rate estimation algorithm was implemented in Matlab/Simulink (The MathWorks, Inc., USA). The subject’s work rate (P) was estimated using force and velocity data provided by the force and angel sensors, and using the following equation:
$$ \begin{aligned} P &= P_{\mathrm{l}} + P_{\mathrm{r}} \\ &= F_{\mathrm{l}} R \dot{\theta}_{\mathrm{l}} + F_{\mathrm{r}} R \dot{\theta}_{\mathrm{r}} \end{aligned} $$
(1)
where Pl and Pr are the work rates of the left and right legs, Fl and Fr are the forces applied by the left and right legs, R the radius of the pedal, and \(\dot {\theta }_{\mathrm {l}}\) and \(\dot {\theta }_{\mathrm {r}}\) are the left and right pedal’s angular velocity [18]. Subjects were asked to adjust their volitional effort to maintain the target work rate.
Based on the personal sub-maximal outcomes obtained from the incremental tests performed in the previous study [18], CLT and HIIT protocols were set individually for each participant: for the CLT, the target work rate PVT1 is the work rate during the ramp phase of the incremental test at the time the first ventilatory threshold occurred. For the HIIT, the target work rate PVT2 is the work rate at the time of the second ventilatory threshold. Participants were required to adapt their volitional leg effort to keep the estimated work rate as close as possible to the target work rate, and to maintain a constant stepping cadence of 60 steps/min following the tones of a metronome.
Outcome measures and data analysis
For the CLT, outcome measures were steady-state levels of \(\dot {V}\mathrm {O}_{2}\) and HR: \(\dot {V}\mathrm {O}_{2}\) is the mean oxygen uptake obtained during the active phases with a 15-breath moving average. HR is the mean heart rate obtained during the active phases.
For the HIIT, outcome measures were the peak values obtained during the active intervals: \(\dot {V}\mathrm {O}_{2}\) is the average of the five peak oxygen uptake values obtained from each interval, using a 15-breath moving average. HR is the average of the five peak heart rate values obtained from each interval. For both protocols, the respiratory exchange ratio \(\text {RER} = \dot {V}\text {CO}_{2}/\dot {V}\mathrm {O}_{2}\) was recorded with a 15-breath moving average.
Descriptive analyses were performed on all variables. To assess the results obtained from the training sessions of each participant, comparison of training results and incremental test results were performed: for CLT, the mean values of steady-state levels were compared to the corresponding values of \(\dot {V}\text {O}_{2\text {VT}1}\) obtained from the incremental test; for HIIT, means of the 5 highest values were compared to the corresponding values of \(\dot {V}\mathrm {O}_{2\text {VT}2}\) obtained from the incremental test. All analyses and displays were performed using MetaSoft and Matlab.
Individual outcome measures were recorded for each participant and are presented in “Results” section (Figs. 3, 4, 5, 6, and 7). For the CLT protocol, the target values (\(\dot {V}\mathrm {O}_{2\text {VT}1}\) and HRVT1) are presented with a green line, and the mean steady state value with a dashed black line; for the HIIT protocol, the green line represents the target values (\(\dot {V}\mathrm {O}_{2\text {VT}2}\) and HRVT2) and the dashed black line represents the average of the 5 peak values obtained from the intervals (the blue dots).
Feasibility assessment
The criteria employed for feasibility assessment of cardiopulmonary training using a dynamic leg-press were as follows [29]: (i) implementation - was the approach technically feasible for cardiopulmonary training?; (ii) acceptability - were the training protocols tolerable?; and (iii) responsiveness - was the approach able to provoke substantial cardiopulmonary responses in relation to individual peak values?