br>University of Reims Champagne-Ardenne, 51100 Reims, France.
Validity and Reproducibility of the
Ergomo®Pro Power Meter Compared With
the SRM and Powertap Power Meters
Sébastien Duc, Vincent Villerius, William Bertucci,
and Frédéric Grappe
Purpose: The Ergomo®Pro (EP) is a power meter that measures power output
(PO) during outdoor and indoor cycling via 2 optoelectronic sensors located in
the bottom bracket axis. The aim of this study was to determine the validity and
the reproducibility of the EP compared with the SRM crank set and Powertap
hub (PT). Method: The validity of the EP was tested in the laboratory during
8 submaximal incremental tests (PO: 100 to 400 W), eight 30-min submaximal
constant-power tests (PO = 180 W), and 8 sprint tests (PO > 750 W) and in the
field during 8 training sessions (time: 181 ± 73 min; PO: ~140 to 150 W). The
reproducibility was assessed by calculating the coefficient of PO variation (CV)
during the submaximal incremental and constant tests. Results: The EP provided a
significantly higher PO than the SRM and PT during the submaximal incremental
test: The mean PO differences were +6.3% ± 2.5% and +11.1% ± 2.1%, respec-
tively. The difference was greater during field training sessions (+12.0% ± 5.7%
and +16.5% ± 5.9%) but lower during sprint tests (+1.6% ± 2.5% and +3.2% ±
2.7%). The reproducibility of the EP is lower than those of the SRM and PT (CV
= 4.1% ± 1.8%, 1.9% ± 0.4%, and 2.1% ± 0.8%, respectively). Conclusions: The
EP power meter appears less valid and reliable than the SRM and PT systems.
Key Words: cycling, mobile power meter, comparison, field, laboratory
Many sport scientists and coaches now use power output (PO) instead of
heart rate to specify training intensity in cycling. PO can be estimated by using
mathematical models or measured directly on the cyclists bicycle thanks to mobile
power meters.
1,2
Such devices (eg, SRM, Powertap [PT], Ergomo®Pro [EP], Polar
S710) enable the measurement of PO, pedaling cadence, and velocity during field
(training and competition) and laboratory conditions.
The first goal of a power meter is to provide a valid and reproducible PO. All
mobile power meters, except the EP, have been studied before for their validity
and reproducibility. The SRM crank set (Schoberer Rad Messtechnik, Welldorf, New Power Meter for Cycling
271
Germany) is viewed as the best device for measuring a cyclists PO because of its
measures high validity and reproducibility.
3,4
Through its negligible error in PO,
the SRM has been used as a reference system to validate other mobile ergometers
such as the Polar S710 and the PT
5,6
and stationary ergometers such as the Kin-
cycle ergometer and the Axiom Powertrain ergometer.
7,8
The PT is also viewed
as a valid and reliable power meter when compared with the SRM
6
or a dynamic
calibration rig.
3
The EP (SG Sensortechnik GmbH & Co, KG, Mörfeldn-Walldorf, Germany)
power-monitoring system consists of an EP sensor (an instrumented bottom bracket
axis), an EP computer (that displays and saves data), a speed sensor to measure
the cyclists velocity on the front wheel, and a sensor plug. This device uses 2
optoelectronic sensors located at the bottom bracket axis (Figure 1). The advantage
of this technology is that, unlike the SRM and PT sensors (strain gauges), the EP
sensors are not sensitive to temperature. The shifted phase position between the 2
sensors is assumed to be proportional to the force applied to the pedal (and thus the
torsion of the bottom bracket). The EP stores 72 data points per crank cycle. The
manufacturers of the EP claim an accuracy of ±1%. The EP sensor only measures
the PO developed by the left lower limb because of the location of the 2 sensors on
the left side of the bottom bracket axis. Thus, the PO displayed on the EP computer
is calculated by multiplying by 2 the PO measured by the sensors. This method
seems doubtful because some cyclists show an asymmetry in pedaling technique
between their 2 lower limbs, notably in the force applied to the pedals.
9,10
Figure 1 The optoelectronic sensor of the Ergomo
Pro power meter. 272 Duc et al
Compared with the SRM and PT, EP has some advantages. First, it can be
used on every kind of bike (road, off-road, track, and BMX). Second, when com-
pared with standard equipment (Shimano), the additional mass of replacing the
10 Speed Dura Ace component with the EP power meter (0.074 kg) is lower
than the additional mass of the PT (0.152 kg) and the SRM (0.280 kg). Third, the
EP measures altitude. This additional function allows the determination of the
change in altitude during a training session or the calculation of the mean grade of
a mountain pass. Moreover, the EP computer displays the road grade in real time
with a slight delay.
To the best of our knowledge, the validity and the reproducibility of the EP have
never been studied. Therefore, the aim of this study was to assess the validity and
reproducibility of the EP by comparing it with the SRM and PT power meters.
Methods
Subjects
A regional-level male competitive cyclist (age 27 years, height 1.80 m, body mass
65 kg, maximal aerobic power 360 W) volunteered as the subject for this study.
Before testing and after having received full explanation concerning the nature
and the purpose of the study, the subject gave written informed consent. Before
participating, he underwent several habituation sessions to familiarize himself with
the testing procedure and material.
Design
The validity and reproducibility of the EP compared with the SRM and PT power
meters were studied for indoor and outdoor conditions. The testing procedure has
been described in a previous study.
6
By means of 4 different test protocols, PO
EP
was
compared with PO
SRM
and PO
PT
across different levels of PO, pedaling cadences,
and pedaling posture to establish validity. Those 4 test protocols were repeated on
8 days to establish reproducibility.
Methodology
Indoor Tests.
Three tests were performed in the laboratory: a submaximal
incremental test, a submaximal constant-power test, and a sprint test. The subject
performed these 3 different tests on the same day and repeated them on 8 different
days during a 5-week period (thus, in all, 8
× 3 tests = 24 tests). The submaximal
tests were completed on a large motorized treadmill (S 1830, HEF Techmachine,
Andrézieux-Bouthéon, France) 1.8 m wide and 3.8 m long, and the sprint tests were
performed on a Cateye ergometer (CS-1000, Cateye, Osaka, Japan).
Submaximal Incremental Test.
The submaximal incremental test consists of
cycling on a treadmill with different slopes (2%, 4%, and 6%). For each slope,
2 treadmill velocities were used (15 and 25 km/h) with 3 different gear ratios
(39/15, 39/19, and 39/23). Nevertheless, the subject used a 39/21 gear ratio when New Power Meter for Cycling
273
he pedaled against the 6% slope at 25 km/h because of difficulties of maintaining
high pedaling cadence at high PO (>350 W). The combinations of these velocities
and gear ratios resulted in 6 different pedaling cadences (47, 60, 75, 80, 100, and
123 rpm) and POs (100, 165, 175, 230, 280, and 395 W) and thus allowed a study
of the effect of the pedaling cadence on the PO
EP
. All the trials were performed in
the seated position. To test the effect of pedaling posture on the PO
EP
, however, 1
additional trial was completed in the standing position with a 6% slope, a velocity
of 15 km/h, and a 39/19 gear ratio. Each trial lasted 1 minute, and the subject per-
formed the 19 different trials ([3 slopes
× 2 velocities × 3 gear ratios] + 1 standing
position) in a random order.
Submaximal Constant-Power Test.
In order to study the EP validity across time,
a 30-minute constant-power test was performed in seated position against a 2%
slope at 25 km/h and with a 39/16 gear ratio to achieve a moderate intensity (PO
= 170 W) and a pedaling cadence of 85 rpm.
Sprint Test.
The sprint test consisted of three 8-second sprints (all-out exercise)
in the seated position to determine maximal PO (PO
max
). To test the validity of
PO
max
measured by the EP, 3 different gear ratios were used (53/15, 53/17, and
53/21), which led to 3 different maximal pedaling cadences (104 ± 5, 114 ± 5, and
134 ± 10 rpm, respectively). Sprints were separated by 5-minute active-recovery
periods at low intensity (<150 W). The PO
max
was defined as the maximal PO value
obtained in each sprint. For these sprints the racing bicycle was mounted on a
Cateye ergometer, which provides a wind and magnetic resistance to simulate field
conditions. The front wheel of the bicycle was removed, and the bicycle fork was
attached to the ergometer by a quick-release skewer. The rear wheel of the bicycle
was fixed by the rear-wheel quick-release skewer in the ergometer stand. These 2
fixation points restrained the lateral motion of the bike and the rear wheel. During
each sprint the magnetic resistance was set at a simulated grade of 4% (displayed
on the small Cateye monitor).
Field Test.
The field tests consisted of 8 actual road-cycling training sessions on
hilly ground that included various field conditions (flat, uphill, and downhill sec-
tions) and different pedaling cadences and cycling postures. Time, distance, and
PO
SRM
(mean ± SD) of the 8 field-training sessions were 181 ± 73 minutes, 77 ±
31 km, and 140 ± 13 W, respectively.
Material
All testing sessions were performed by the same subject with the same road-racing
bicycle (mass = 9 kg), which was equipped with clipless pedals. The mass of the
system (subject + bicycle) contributes to the power required to ride on a treadmill
at a given speed and gradi