Downloaded from bjsm.bmj.com on 8 December 2008
VO2 requirement at different displayed power outputs on five cycle ergometer models-A preliminary study Thibaut Guiraud, Luc Léger, Arthur Long, Nicolas Thebault, Jonathan Tremblay and Philippe Passelergue Br. J. Sports Med. published online 6 Jun 2008; doi:10.1136/bjsm.2007.044826
Updated information and services can be found at: http://bjsm.bmj.com/cgi/content/abstract/bjsm.2007.044826v1
These include:
Rapid responses
You can respond to this article at: http://bjsm.bmj.com/cgi/eletter-submit/bjsm.2007.044826v1
Email alerting service
Receive free email alerts when new articles cite this article - sign up in the box at the top right corner of the article
Notes
Online First contains unedited articles in manuscript form that have been peer reviewed and accepted for publication but have not yet appeared in the paper journal (edited, typeset versions may be posted when available prior to final publication). Online First articles are citable and establish publication priority; they are indexed by PubMed from initial publication. Citations to Online First articles must include the digital object identifier (DOIs) and date of initial publication.
To order reprints of this article go to: http://journals.bmj.com/cgi/reprintform
To subscribe to British Journal of Sports Medicine go to: http://journals.bmj.com/subscriptions/
Downloaded from bjsm.bmj.com on 8 December 2008
BJSM Online First, published on June 6, 2008 as 10.1136/bjsm.2007.044826
VO2 requirement at different displayed power outputs on five cycle ergometer models A preliminary study. Thibaut Guiraud1, Luc Léger1, Arthur Long1, Nicolas Thébault12, Jonathan Tremblay1, Philippe Passelergue2, 1
Département de kinésiologie, Université de Montréal, Montréal, Canada. 2 Département STAPS, Université de Pau et des Pays de l’Adour, Tarbes, France Address for correspondence: L. Léger Département de kinésiologie Université de Montréal CP 6128 Centre ville Montréal, QC Canada H3C 3J7 Phone: 1 514 343 7792 Fax: 1 514 343 2181 Email:
[email protected]
Running Title: Accuracy of Displayed Ergometer Power Key words: Assessment, cycle ergometer, power output, oxygen uptake Words number: 3297
Abstract Background and aims: The validity of five brands of cycle ergometers was evaluated by the comparison of the VO2 requirements at different displayed power.
Copyright Article author (or their employer) 2008. Produced by BMJ Publishing Group Ltd under licence.
Downloaded from bjsm.bmj.com on 8 December 2008
2 Methods and results: Five physically active males performed a continuous incremental exercise test on five ergometers (Ergomeca, Lifecycle, Monark, Polar S710 and Computrainer). The latter was also compared with a standard dynamometer in order to associate VO2 values with the real power. Every test started by 5-min warm up on the same cycle-ergometer (Ergomeca) at 100 W to make sure that the VO2 differences do not come from VO2 measurement error. Only last minute steady state VO2 values of each 2-min stage were used for the VO2/Watt curve. Large differences (5 to 10 ml kg-1 min-1) at the same displayed power indicate inaccuracy of displayed power output (PO). Using corrected power values from the dynamometer revealed that for the same VO2 the Computrainer underestimates PO by ~30 W between 100 and 300 W while the Lifecycle overestimate it by 3 to 53 W from 100 to 300 W. The Monark and Polar S710 underestimate PO by 15 W and the Ergomeca by ~5 W. Conclusion: Inaccuracies between -10 to 18% in displayed PO of various cycle ergometers, question their interchangeability.
2
Downloaded from bjsm.bmj.com on 8 December 2008
3 INTRODUCTION Ergometers were designed to provide accurate measurement of power output (PO) during standardized testing or training but the displayed power is often different than the true power. [1-3] The power output is a key determinant of endurance performance of elite athletes, and is currently used by physiologists for research in metabolism, exercise therapists in obese or rehabilitation training. PO is often used to estimate energy expenditure and, associated with direct VO2 measurement; it is used to determine mechanical efficiency. It is thus important to have accurate value of displayed PO. This is a key issue in cycling. Brands and models of cycle ergometers are numerous, including conventional ergometers and new power output measuring devices fixed on different parts of a regular bicycle. Theoretically, they should offer the same resistance while cycling at the same displayed PO. We must make sure that this true. According to Attaway et al. [4] the Velodyne ergometer yields the same power output as the Monark gold standard. Most recent studies however take the SRM as the criteria ergometer to either estimate the workload demands during a Grand Tour [5-6] or to validate a new instrument such as the Powertec®-System. [7] Jones and Passfield noted that SRM provided a valid and reliable measure of power output when compared with Monark ergometer. [8] Same results were found by Martin et al. (∆=2.36%). [9] A PO error less than 5% is acceptable. [10] Recently, the Monark 814e was still used to valid the performance and metabolic measures of Lode Excalibur Sport ergometer during a Wingate Anaerobic Test. [11] The PowerTap device PO is approximately 8% higher than the one displayed by the SRM crankset. [12] These results differ from the study of Bertucci et al. showing that PowerTap device is a valid and reliable mobile powermeter during submaximal intensities when compared with the scientific SRM model. [13] In recent study Duc et al. found that Ergomo®Pro is less valid and reliable when compared with the SRM and PowerTap systems. [14] The Polar S710 measuring device yields PO higher than the SRM system in both field and laboratory conditions. [2] Another study however reported a 23% underestimation of PO with random variation of 24% of the Polar S710 as compared to the SRM. [15] Most studies have employed continuous protocols, or brief periods of cycling, at different pedal cadences. Few studies were done with incremental protocol between 100300W. Thus, five different cycle ergometers (Ergomeca, Lifecycle, Monark, Polar S710 and Computrainer) were compared using a standardized multistage protocol. Assuming that individual VO2 is not different at the same PO of any cycle ergometer device, any difference in VO2 reflects inaccuracy in displayed PO. The main purpose of this study was to demonstrate inaccuracy of displayed PO of commercially available ergometers from VO2 requirement and to show the amplitude of VO2 variability at any displayed PO. A secondary goal (phase two of the project) was to measure real PO on the Computrainer in order to establish the true relationship between VO2 and real PO and to disclose systematic errors of each ergometer under study. MATERIAL AND METHODS Subjects and ergometers Five physically active males with no particular competitive cycling experience performed a continuous incremental exercise test on five ergometers. Subjects were 31.4 ± 9 year old (mean ± S.D.), 177.6 ± 3.3 cm tall, weighted 74.2 ± 4.0 kg and had a VO2max of 52.8±6.3 ml kg-1 min-1.
3
Downloaded from bjsm.bmj.com on 8 December 2008
4
The ergometers or PO measuring devices that were investigated in this study (Figure 1) were: 1. Ergomeca friction-loaded ergometer, GP400, La Bayette, France 2. Monark (824E) friction-loaded ergometer, Monark Exercise, Varberg, Sweden 3. Lifecycle 9500HR lifefitness, electromagnetic brake, Schiller Park, Illinois, USA 4. Computrainer Pro RC1 model 8001, Racermate inc, Seattle, WA, USA 5. Polar S710 with power sensor kit, Kempele, Finland Prior to each individual test, each ergometer was calibrated according to the manufacturer’s instructions.
Place Figure 1 around here Incremental test Exercise tests started with a 5 min of warm up on Ergomeca cycle at 100 W, followed with a 1-min pause to switch for one of the five ergometers in a random order to perform the incremental maximal test (Figure 2). That warm-up period on the same ergometer was adopted to make sure that our VO2 measuring device yields constant values for each individual from one test session to another one (± 1 ml kg-1 min-1) and to adjust values if necessary, which happens only twice over the 25 trials. That supports the reliability of our VO2 measuring device. Three to five days rest was given between each test in order to avoid interaction between tests. Total experimentation lasted around a month.
Place Figure 2 around here To make sure that a VO2 steady state is reached before the end of each stage, the PO increment must not be too large nor the stage duration too short. In a previous non published treadmill study, we found that increments should not be larger than 3.8 ml O2 kg-1 min-1 per 2min stage. On a treadmill that corresponds to a 1 km h-1 increment but on a cycle ergometer that yields different PO increments depending on subject's weight. We used ACSM (2000) equation to estimate average steady state VO2 cost of cycling (VO2 in ml kg-1 min-1) from power output (PO in W) and body weight (BW in kg) VO2 = (10.8 × PO / BW) + 7 Then we obtained rounded values for combination of RPM and Watt increments and depending on subject's weight, crank speeds were set to 75, 80, 85 or 90 RPM with increments between 23 and 27 W corresponding to energy cost increments between 3.4 and 3.8 ml O2 kg-1 min-1. Laboratory trials Cyclist position affects energetic expenditure. In order to keep the body position constant, we instructed subjects to adopt a top bar position. For feet, optimal position corresponded to the junction of the metatarsophalange articulation of big toe and pedal axis. [16] The saddle height was adjusted to 109% of the subject’s inseam leg length, as measured from the point of the pubic symphysis palpation to the floor. [17] Each subject used toe-clips and had to stay seat down until unable to follow the metronome cadence. At that point only, the subject could stand up on the bicycle in order to get VO2max values. VO2 was measured with the MOXUS Modular VO2 System II, AEI Technologies, Inc., Pittsburgh, PA. Calibration of the system (volume, O2 and CO2) was made before each test. Air temperature and humidity were constant, 22.5ºC and 37%, respectively. As said previously, the intra subject reliability of the system was assessed each test with the same first
4
Downloaded from bjsm.bmj.com on 8 December 2008
5 load being done on the same ergometer. Results had to be the same within ± 1 ml kg-1 min-1. Furthermore our laboratory regularly checks systematic errors ensuring that energy cost of same subjects are not only the same but within 5% of the literature values. [18] Complementary measurements. In a second phase of the study, the true PO on the Computrainer was also measured with a calibrating device similar to a Prony brake dynamometer. The crank-gear axle is powered by an electrical motor and the true power is the product of the angular speed (optical reader) by the torque (Figure 3). These measurements were done in VELUS laboratory. [19] Results of the displayed power on the Computrainer are then converted in true power units. Then these values become absolute criteria for PO and VO2/PO curves. At the time of the study, it was not possible or feasible (crank shaft incompatibility with the Monark and Ergomeca) to do these measures on each ergometer. We felt that the calibration of one ergometer was sufficient enough to establish a true VO2/PO curve to compare all other curves obtained with displayed PO and to indicate where the systematic errors were located. Of course, it is assumed that the VO2 requirement is the same at the same true PO on different ergometers, although there might be some differences due to different ergonomic designs for example.
Place Figure 3 around here Statistical approach. Regression analyses of VO2 as a function of displayed PO were run on each subject and for the five subjects together. Since both individual curves and average curves yield similar positions of ergometer curves, only average curves are presented. Nevertheless, individual curves were used to estimate VO2 at 100, 150, 200, 250 and 300 W of displayed PO in order to find out significant differences in VO2 at different PO and ergometer using a two way repeated design ANOVA and Fisher a posteriori tests. RESULTS Comparing the Computrainer displayed PO to the true PO measured by the electronic motor (Figure 4), disclosed errors between the two values. For example at 130 RPM, when pedalling at 50 W on the Computrainer, the true power was 170 W. At the same Computrainer output, the error was however less at low speed (-20 W at 70 RPM). But as the PO increases, the error is less dependent on angular speed. For example, at 350 W, the underestimation was more or less constant at 50 W. In other words, at low RPM, the underestimation increases as the PO increases, but decreases at high RPM.
Place Figure 4 around here Correcting Computrainer displayed PO values with the true ones make it possible to draw a standard curve of VO2 as a function of true PO along with displayed PO of each ergometer under study (Figure 5). While regression lines in Figure 5 were obtained with all the subjects combined, almost the same pattern was obtained on individual basis (not illustrated). Except in one subject, the Computrainer curve was always on top while the Lifecycle one was below for all the subjects. The position of the Monark and Ergomeca individual curves relative to each other was not quite clear on an individual basis but always between the Computrainer and Lifecycle curves as in Figure 5. Thus, individual curves confirm the relative position of group curves between ergometers. Since these individual curves were linear with high correlations (r>0.98) and small standard errors of the estimate (SEE~1 ml kg-1 min-1), it was
5
Downloaded from bjsm.bmj.com on 8 December 2008
6 possible to calculate VO2 values at displayed PO of 100, 150, 200, 250 and 300W and to compute a repeated design two way ANOVA to compare VO2 values between displayed power and ergometers. It was found that Lifecycle VO2 values were lower than all other values (p
