Identification of heart rate dynamics during moderade-to-vigorous treadmill exercise

BACKGROUND: Heart rate can be used to prescribe exercise intensity for development and maintenance of cardiorespiratory fitness. The aim of this study was to identify the dynamics of heart rate response during moderate-to-vigorous treadmill exercise and to explore parameter dependencies with respect to time, intensity level and step-change direction. The focus was on simple approximate models for subsequent design of heart rate control systems. METHODS: 24 healthy, able-bodied male subjects each did two separate, 35-min tests on a treadmill, one at moderate and one at vigorous intensity. Each test had four individual upward and downward steps (1–4). Heart rate responses were modelled as first-order transfer functions with steady-state gain k and time constant [Formula: see text] . Models were estimated both for the overall testing periods and for individual step responses within each test. RESULTS: There were no significant differences in the overall mean values of k [24.3 vs. 24.1 bpm/(m/s), [Formula: see text] ] and [Formula: see text] (55.7 vs. 59.5 s, [Formula: see text] ) between the two intensity levels. The overall nominal gain for both conditions was [Formula: see text] , 21.9–26.6 bpm/(m/s) (mean [Formula: see text] standard deviation, 95 % confidence interval), and the overall nominal time constant was [Formula: see text] , 50.9–64.3 s. Analysis of models estimated from the individual steps revealed a significant difference in steady-state gain k for upward and downward steps [30.2 vs. 23.6 bpm/(m/s), [Formula: see text] ], but no difference in time constant [Formula: see text] between these two directions (57.5 vs. 54.4 s, [Formula: see text] ). For gain k, there was no significant main effect of intensity ([Formula: see text] ) or intensity–time ([Formula: see text] ) interactions, but there was a significant main effect of time ([Formula: see text] ). Pairwise comparison with respect to time showed a significant difference between the upward steps at times 1 and 3 [33.0 vs. 27.3 bpm/(m/s), [Formula: see text] ], but no significant difference between the downward steps at times 2 and 4 [24.4 vs. 22.8 bpm/(m/s), [Formula: see text] ]. For time constant [Formula: see text] , there were no significant main effects of intensity ([Formula: see text] ) or time ([Formula: see text] ), or intensity–time interactions ([Formula: see text] ). CONCLUSIONS: The tight CI-bounds obtained, and the observed parameter dependencies, suggest that the overall nominal model with [Formula: see text] and [Formula: see text] might serve as the basis for design of a linear time-invariant (LTI) feedback system for real-time control of heart rate. Future work should focus on this hypothesis and on direct comparison of LTI and nonlinear/time-varying control approaches

Test-retest reliability of short- and long-term heart rate variability in individuals with spinal cord injury

Study design: Cross-sectional. Objectives: To investigate test-retest reliability of heart rate variability (HRV) metrics in SCI without restriction of activity over long (24-h) and shorter durations (5-min, 10-min, 1-h, 3-h and 6-h). Settings: University hospital in Khon Kaen, Thailand. Methods: Forty-five participants (11 with tetraplegia and 34 with paraplegia) underwent two 24-h recordings of RR-intervals to derive time and frequency HRV metrics. Relative reliability was assessed by intraclass correlation coefficient (ICC) and absolute reliability by coefficient of variation (CV) and Bland–Altman limits of agreement (LoA). Results: For 5- and 10-min durations, eight of eleven HRV metrics had moderate to excellent reliability (ICC 0.40–0.76); the remaining three were poor (ICC < 0.4). HRV values from 1-h and 3-h durations showed moderate to excellent reliability (ICC of 0.46–0.81), except for 1-h reliability of ULF and TP (ICC of 0.06 and 0.30, respectively). Relative reliability was excellent (ICC of 0.77–0.92) for 6-h and 24-h durations in all HRV metrics. Absolute reliability improved as recording duration increased (lower CVs and narrower LoAs). Participants with high AD risk (SCI level at or above T6) showed lower test-retest reliability of HF and LF values than participants with low AD risk. Conclusion: Relative reliability of HRV was excellent for 6-h and 24-h. The best absolute reliability values were for 24-h duration. Time-domain outcomes were more reliable than frequency domain outcomes. Participants with high risk of AD, particularly those with tetraplegia, showed lower reliability, especially for HF and LF

Comparison of peak cardiopulmonary performance parameters on a robotics-assisted tilt table, a cycle and a treadmill

Robotics-assisted tilt table (RATT) technology provides body support, cyclical stepping movement and physiological loading. This technology can potentially be used to facilitate the estimation of peak cardiopulmonary performance parameters in patients who have neurological or other problems that may preclude testing on a treadmill or cycle ergometer. The aim of the study was to compare the magnitude of peak cardiopulmonary performance parameters including peak oxygen uptake (VO2peak) and peak heart rate (HRpeak) obtained from a robotics-assisted tilt table (RATT), a cycle ergometer and a treadmill. The strength of correlations between the three devices, test-retest reliability and repeatability were also assessed. Eighteen healthy subjects performed six maximal exercise tests, with two tests on each of the three exercise modalities. Data from the second tests were used for the comparative and correlation analyses. For nine subjects, test-retest reliability and repeatability of VO2peak and HRpeak were assessed. Absolute VO2peak from the RATT, the cycle ergometer and the treadmill was (mean (SD)) 2.2 (0.56), 2.8 (0.80) and 3.2 (0.87) L/min, respectively (p < 0.001). HRpeak from the RATT, the cycle ergometer and the treadmill was 168 (9.5), 179 (7.9) and 184 (6.9) beats/min, respectively (p < 0.001). VO2peak and HRpeak from the RATT vs the cycle ergometer and the RATT vs the treadmill showed strong correlations. Test-retest reliability and repeatability were high for VO2peak and HRpeak for all devices. The results demonstrate that the RATT is a valid and reliable device for exercise testing. There is potential for the RATT to be used in severely impaired subjects who cannot use the standard modalities

Comparison of peak cardiopulmonary performance parameters on a robotics-assisted tilt table, a cycle and a treadmill

Robotics-assisted tilt table (RATT) technology provides body support, cyclical stepping movement and physiological loading. This technology can potentially be used to facilitate the estimation of peak cardiopulmonary performance parameters in patients who have neurological or other problems that may preclude testing on a treadmill or cycle ergometer. The aim of the study was to compare the magnitude of peak cardiopulmonary performance parameters including peak oxygen uptake (VO2peak) and peak heart rate (HRpeak) obtained from a robotics-assisted tilt table (RATT), a cycle ergometer and a treadmill. The strength of correlations between the three devices, test-retest reliability and repeatability were also assessed. Eighteen healthy subjects performed six maximal exercise tests, with two tests on each of the three exercise modalities. Data from the second tests were used for the comparative and correlation analyses. For nine subjects, test-retest reliability and repeatability of VO2peak and HRpeak were assessed. Absolute VO2peak from the RATT, the cycle ergometer and the treadmill was (mean (SD)) 2.2 (0.56), 2.8 (0.80) and 3.2 (0.87) L/min, respectively (p < 0.001). HRpeak from the RATT, the cycle ergometer and the treadmill was 168 (9.5), 179 (7.9) and 184 (6.9) beats/min, respectively (p < 0.001). VO2peak and HRpeak from the RATT vs the cycle ergometer and the RATT vs the treadmill showed strong correlations. Test-retest reliability and repeatability were high for VO2peak and HRpeak for all devices. The results demonstrate that the RATT is a valid and reliable device for exercise testing. There is potential for the RATT to be used in severely impaired subjects who cannot use the standard modalities

A new method for self-paced peak performance testing on a treadmill

Purpose: Self‐paced maximal testing methods may be able to exploit central mediation of function‐limiting fatigue and therefore have potential to generate more valid estimates of peak oxygen uptake. The aim of this study was to investigate the feasibility of a new method for self‐paced peak performance testing on treadmills and to compare peak and submaximal performance outcomes with those obtained using a non‐self‐paced (‘computer‐paced’) method employing predetermined speed and slope profiles. Methods: The proposed self‐paced method is based upon automatic subject positioning using feedback control together with an exercise intensity which is driven by a predetermined, individualized work‐rate ramp. Results: Peak oxygen uptake was not significantly different for the computer‐paced (CP) versus self‐paced (SP) protocols: 4·38 ± 0·48 versus 4·34 ± 0·46 ml min−1, P = 0·42. Likewise, there were no significant differences in the other peak and submaximal cardiopulmonary parameters, viz. peak heart rate, peak respiratory exchange ratio and the first and second ventilatory thresholds. Ramp duration for CP was longer than for SP: 494·5 ± 71·1 versus 371·3 ± 86·0 s, P = 0·00072. Concomitantly, the peak rate of work done against gravity was higher for CP: 264·8 ± 40·8 versus 203·8 ± 53·4 W, P = 0·0021. Conclusions: The self‐paced approach was found to be feasible for estimation of the principal performance outcomes: the method was technically implementable, it was acceptable to the subjects and it showed good responsiveness. Further investigation of the self‐paced method, with adjustment of the target ramp‐phase duration or modification of the work‐rate calculation equations, is warranted

Test-retest reliability and four-week changes in cardiopulmonary fitness in stroke patients: evaluation using a robotics-assisted tilt table

BACKGROUND: Exercise testing devices for evaluating cardiopulmonary fitness in patients with severe disability after stroke are lacking, but we have adapted a robotics-assisted tilt table (RATT) for cardiopulmonary exercise testing (CPET). Using the RATT in a sample of patients after stroke, this study aimed to investigate test-retest reliability and repeatability of CPET and to prospectively investigate changes in cardiopulmonary outcomes over a period of four weeks. METHODS: Stroke patients with all degrees of disability underwent 3 separate CPET sessions: 2 tests at baseline (TB1 and TB2) and 1 test at follow up (TF). TB1 and TB2 were at least 24 h apart. TB2 and TF were 4 weeks apart. A RATT equipped with force sensors in the thigh cuffs, a work rate estimation algorithm and a real-time visual feedback system was used to guide the patients' exercise work rate during CPET. Test-retest reliability and repeatability of CPET variables were analysed using paired t-tests, the intraclass correlation coefficient (ICC), the coefficient of variation (CoV), and Bland and Altman limits of agreement. Changes in cardiopulmonary fitness during four weeks were analysed using paired t-tests. RESULTS: Seventeen sub-acute and chronic stroke patients (age 62.7 ± 10.4 years [mean ± SD]; 8 females) completed the test sessions. The median time post stroke was 350 days. There were 4 severely disabled, 1 moderately disabled and 12 mildly disabled patients. For test-retest, there were no statistically significant differences between TB1 and TB2 for most CPET variables. Peak oxygen uptake, peak heart rate, peak work rate and oxygen uptake at the ventilatory anaerobic threshold (VAT) and respiratory compensation point (RCP) showed good to excellent test-retest reliability (ICC 0.65-0.94). For all CPET variables, CoV was 4.1-14.5 %. The mean difference was close to zero in most of the CPET variables. There were no significant changes in most cardiopulmonary performance parameters during the 4-week period (TB2 vs TF). CONCLUSIONS: These findings provide the first evidence of test-retest reliability and repeatability of the principal CPET variables using the novel RATT system and testing methodology, and high success rates in identification of VAT and RCP: good to excellent test-retest reliability and repeatability were found for all submaximal and maximal CPET variables. Reliability and repeatability of the main CPET parameters in stroke patients on the RATT were comparable to previous findings in stroke patients using standard exercise testing devices. The RATT has potential to be used as an alternative exercise testing device in patients who have limitations for use of standard exercise testing devices

Feasibility of cardiopulmonary exercise testing using a robotics-assisted tilt tabele in dependent-ambulatory stroke patients

BACKGROUND: We evaluated the feasibility of an augmented robotics-assisted tilt table (RATT) for incremental cardiopulmonary exercise testing (CPET) and exercise training in dependent-ambulatory stroke patients. METHODS: Stroke patients (Functional Ambulation Category ≤ 3) underwent familiarization, an incremental exercise test (IET) and a constant load test (CLT) on separate days. A RATT equipped with force sensors in the thigh cuffs, a work rate estimation algorithm and real-time visual feedback to guide the exercise work rate was used. Feasibility assessment considered technical feasibility, patient tolerability, and cardiopulmonary responsiveness. RESULTS: Eight patients (4 female) aged 58.3 ± 9.2 years (mean ± SD) were recruited and all completed the study. For IETs, peak oxygen uptake (V'O(2peak)), peak heart rate (HR(peak)) and peak work rate (WR(peak)) were 11.9 ± 4.0 ml/kg/min (45 % of predicted V'O(2max)), 117 ± 32 beats/min (72 % of predicted HR(max)) and 22.5 ± 13.0 W, respectively. Peak ratings of perceived exertion (RPE) were on the range "hard" to "very hard". All 8 patients reached their limit of functional capacity in terms of either their cardiopulmonary or neuromuscular performance. A ventilatory threshold (VT) was identified in 7 patients and a respiratory compensation point (RCP) in 6 patients: mean V'O(2) at VT and RCP was 8.9 and 10.7 ml/kg/min, respectively, which represent 75 % (VT) and 85 % (RCP) of mean V'O(2peak). Incremental CPET provided sufficient information to satisfy the responsiveness criteria and identification of key outcomes in all 8 patients. For CLTs, mean steady-state V'O(2) was 6.9 ml/kg/min (49 % of V'O(2) reserve), mean HR was 90 beats/min (56 % of HR(max)), RPEs were > 2, and all patients maintained the active work rate for 10 min: these values meet recommended intensity levels for bouts of training. CONCLUSIONS: The augmented RATT is deemed feasible for incremental cardiopulmonary exercise testing and exercise training in dependent-ambulatory stroke patients: the approach was found to be technically implementable, acceptable to the patients, and it showed substantial cardiopulmonary responsiveness. This work has clinical implications for patients with severe disability who otherwise are not able to be tested