A Maximal Isokinetic Pedalling Exercise for EMG.pdf

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Journal of Electromyography and Kinesiology xxx (2008) xxx–xxx

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A maximal isokinetic pedalling exercise for EMG

normalization in cycling

Eneko Fern´ndez-Pe˜a a,b , Francesco Lucertini a , Massimiliano Ditroilo a,b, *

a Istituto di Ricerca sull’Attivit` Motoria, Universit` degli Studi di Urbino ‘‘Carlo Bo”, Via I Maggetti, 26/2, 61029 Urbino, Italy

b Scuola Regionale dello Sport – Coni, Comitato Regionale Marchigiano, Ancona, Italy

Received 26 April 2007; received in revised form 27 November 2007; accepted 27 November 2007

Abstract

An isometric maximal voluntary contraction (iMVC) is mostly used for the purpose of EMG normalization, a procedure described in

the scientiﬁc literature in order to compare muscle activity among diﬀerent muscles and subjects. However, the use of iMVC has certain

limitations. The aims of the present study were therefore to propose a new method for the purpose of EMG amplitude normalization in

cycling and assess its reliability. Twenty-three cyclists performed 10 trials of a maximal isokinetic protocol (MIP) on a cycle ergometer,

then another four sub-maximal trials, whilst the EMG activity of four lower limbs muscles was registered. During the 10 trials power

output (CV = 2.19) and EMG activity (CV between 4.46 and 8.70) were quite steady. Furthermore, their maximal values were reached

within the 4th trial. In sub-maximal protocol EMG activity exhibited an increase as a function of exercise intensity.

MIP entails a maximal dynamic contraction of the muscles involved in the pedalling action and the normalization session is

performed under the same biomechanical conditions as the following test session. Thus, it is highly cycling-speciﬁc.

MIP has good logical validity and within-subject reproducibility. Three trials are enough for the purpose of EMG normalization in

cycling.

Keywords: Surface electromyography; Maximal dynamic exercise; Sub-maximal dynamic exercise; Reproducibility; SRM ergometer

1. Introduction

malization is required in order to: (i) make a between-

and within-subject comparison of activation level in work-

ing muscles ( Bolgla and Uhl, 2007; Lehman and McGill,

1999; Mirka, 1991 ), (ii) facilitate comparison between

two diﬀerent muscles, or right and left side muscles of the

same subject ( Lehman and McGill, 1999 ), (iii) allow for

comparisons between diﬀerent joint angles, namely diﬀer-

ent speciﬁc positions throughout the range of motion of

a joint ( Mirka, 1991 ), (iv) compare results with similar data

from other studies ( Soderberg and Knutson, 2000 ).

Most published studies have used an isometric maximal

voluntary contraction (iMVC) for the purpose of EMG

normalization ( Arokoski et al., 1999; Lobbezoo et al.,

1993; Smith et al., 2004 ). Although this method has been

demonstrated to be reliable ( Dankaerts et al., 2004; Kollm-

itzer et al., 1999 ), it is strongly dependent on the speciﬁc

joint angles used during the iMVC. In fact, an EMG signal

Surface electromyography (EMG) is a non-invasive

method used to obtain information on muscle activity.

Absolute EMG amplitude level is of interest, for instance,

in clinical studies, since patients usually can not perform

maximum contractions ( van Die¨n et al., 2003 ), or to make

diﬀerences in EMG activity between a pain and a non-pain

group come to light ( Danoﬀ, 1986 ). However, absolute

EMG values depend on many factors unrelated to the level

of muscle activation (e.g. van Die¨n et al., 2003 ). It is

widely accepted that a procedure of EMG amplitude nor-

* Corresponding author. Address: Istituto di Ricerca sull’Attivit`

Motoria, Universit` degli Studi di Urbino ‘‘Carlo Bo”, Via I Maggetti,

26/2, 61029 Urbino, Italy. Tel.: +39 0722 303413; fax: +39 0722 303401.

E-mail address: m.ditroilo@uniurb.it (M. Ditroilo).

doi:10.1016/j.jelekin.2007.11.013

Please cite this article in press as: Fern´ndez-Pe˜a E et al., A maximal isokinetic pedalling exercise for EMG ..., J Electromyogr Ki-

nesiol (2008), doi:10.1016/j.jelekin.2007.11.013

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collected during an iMVC performed at a reference joint

angle should be used only for normalization of muscle

activity recorded at the same speciﬁc joint angle, otherwise

a considerable error can occur ( Enoka and Fuglevand,

1993; Mirka, 1991 ). A second potential limitation is the

assumption that subjects can actually perform an eﬀort

involving maximal force generation, especially if they are

not trained and well motivated.

The use of normalization to sub-maximal isometric con-

traction is present in studies conducted with the patient

population and when assessing low level of muscle activity

( Dankaerts et al., 2004; Hunt et al., 2003 ). This method

was found to be even more reliable, compared to iMVC,

in between-days repeated measures, although the correct

determination of relative sub-maximal loads for every mus-

cle is dicult ( Dankaerts et al., 2004 ). Moreover, the EMG

associated with a dynamic activity has also been proposed

as reference value (e.g. Prilutsky et al., 1998 ).

The problem of a correct selection of an EMG normal-

ization procedure is essential. A recent paper ( Rouﬀet and

Hautier, 2007 ) has widely addressed this issue. The authors

underlined that while executing a speciﬁc task, physiologi-

cal modiﬁcations in the neural drive should be reﬂected in

the EMG signal. Other authors pointed out that when deal-

ing with sports movements the electromyogram should be

the expression of the dynamic involvement of speciﬁc mus-

cles ( Clarys and Cabri, 1993 ). In cycling, EMG is often per-

formed in order to assess the muscular intervention during

the pedalling action. For the normalization purpose it is

therefore pivotal to choose a meaningful reference contrac-

tion so that its activation is regulated by the same neuro-

muscular pattern as the pedalling action. This means that

the task parameters of the reference contraction (e.g. move-

ment amplitude, joint position, speed, etc.) should repro-

duce, as much as possible, the pedalling action ( Latash,

1998 ).

Despite the above considerations, several studies exam-

ining cycling have improperly implemented EMG normal-

ization using an iMVC as a reference contraction, and then

expressing the dynamic EMG activity as a percentage of it

( Ericson, 1986; Ericson et al., 1985; Hautier et al., 2000;

Marsh and Martin, 1995; Neptune et al., 1997 ). In 2002,

Hunter et al. published a paper comparing four normaliza-

tion protocols: three of them involved an iMVC, the fourth

a dynamic pedalling action against a constant load, which

was repeatedly increased until the subject could no longer

complete a full revolution of the pedal. The authors found

that the iMVC test performed on an isometric leg extension

dynamometer yielded the highest iEMG amplitude values

and concluded suggesting that, for this reason, the use of

iMVC as a normalization procedure for dynamic cycling

activity would be better. This assumption, however, has

been recently questioned since the reference EMG signals

collected during iMVC can hardly represent the maximal

neural drive obtained during cycling ( Rouﬀet and Hautier,

2007 ). Furthermore, other authors compared the EMG

amplitude signal during iMVC and maximal dynamic

cycling contractions ( Hautier et al., 2000; Rouﬀet and Hau-

tier, 2007 ) and found that the electrical activity of some of

the analysed muscles were not signiﬁcantly diﬀerent

between the two methods, or even higher when the

dynamic contraction was used.

More recently, alternative dynamic methods for the

EMG normalization in cycling have been proposed. Takai-

shi et al. (1998) set the integrated EMG corresponding to

the lowest cadence (45 rpm) as reference value, while Hug

et al. (2004b) normalized the vastus lateralis EMG activity

with a 40 W intensity exercise. However, it could be argued

that due to the low intensity chosen, the muscular recruit-

ment pattern could be quite diﬀerent from a pedalling

action at higher intensity; furthermore, the vastus lateralis

activity at 40 W intensity is probably not diﬀerent from

baseline.

Neptune and Herzog (2000) , assessing the adaptation of

muscle coordination when traditional and elliptical chain-

rings were adopted, used the highest EMG value observed

across all trials for normalization purposes. Since the

experimental design entailed a variation of pedalling bio-

mechanical conditions (e.g. instantaneous crank angular

velocity), the normalization procedure chosen could not

represent all the diﬀerent tests performed.

Hug et al. (2004a) and Laplaud et al. (2006) normalized

the EMG of a graded pedalling exercise as a percentage of

the highest intensity step. Interestingly, Taylor and Bronks

(1995) showed that the reference EMG amplitude value (a

maximal ‘‘unfatigued” EMG value obtained by rapidly

increasing the resistance until the subject could no longer

maintain the ﬁxed cadence) was about twice than the one

reached during the last step of the graded exercise. It could

be maintained therefore that when the EMG reference

value is the latter, a normalization to a sub-maximal

dynamic contraction is performed and the limit of this

procedure, as previously reported, is the determination of

equivalent sub-maximal eﬀorts for diﬀerent muscles

( Dankaerts et al., 2004; Marras and Davis, 2001 )and

subjects.

Several methods have been proposed for the purpose of

EMG amplitude normalization in cycling but, based on the

above evidence, the best reference contraction to use is still

controversial. Methods grounded on iMVC or sub-maxi-

mal dynamic contractions have evidenced limitations.

Accordingly, the main aim of this paper was to present a

maximal isokinetic protocol (MIP) as a new method for

the purpose of EMG normalization in cycling. Brieﬂy,

this protocol should produce a maximal dynamic contrac-

tion of the muscles involved in the pedalling action.

Furthermore, the normalization session is performed under

the same biomechanical conditions as the following test

session, thus making the protocol highly speciﬁc. It is

therefore hypothesized that the cyclists do not need to

learn the required task as it is inherent in their pedalling

patterns.

The second aim of this investigation was to detect the

intra-individual variability of the method proposed.

Please cite this article in press as: Fern´ndez-Pe˜a E et al., A maximal isokinetic pedalling exercise for EMG ..., J Electromyogr Ki-

nesiol (2008), doi:10.1016/j.jelekin.2007.11.013

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2. Methods

exercises, while the subject was pedalling at 50 W, the load was

increased and as soon as a steady pedalling cadence of 80 rpm was

reached, data were collected for 20 s. Trials were separated by a

2 min active rest period. The 80% intensity was too demanding to

be maintained for at least 20 s, hence it was not included in the

SMP.

2.1. Subjects

Twenty-three recreational and competitive healthy male

cyclists (age 29.3 ± 9.0 yr, height 177.5 ± 7.4 cm, weight

71.6 ± 9.8 kg) volunteered and gave their written informed con-

sent to participate in the study, which was previously approved by

the Human Ethics Committee of the University of Urbino (Italy).

All the cyclists used to train about 10 h per week with a quite

homogeneous training programme. Competitive cyclists, unlike

recreational ones, competed during weekends at Masters level.

They all had covered an average of 9000 km during the last sea-

son. None of them had previous experience in riding an isokinetic

cycle ergometer, and they were asked to refrain from exhausting

exercise 24 h before testing.

2.3. Recording of EMG and angular crank position

Following the recommendations of the SENIAM project

( Freriks et al., 1999 ), EMG of four muscles of the right leg was

recorded during the MIP and the SMP. The selected muscles were

vastus lateralis (VL), biceps femoris (BF), tibialis anterior (TA)

and gastrocnemius lateralis (GL). Skin was shaved, slightly

abraded with sandpaper and cleaned with alcohol. Ag/AgCl

bipolar electrodes (Blue Sensor N-00-S, Ambu Medicotest A/S,

Ølstykke, Denmark) were placed over the muscle belly of selected

muscles at an interelectrode distance of 20 mm. To avoid artefacts

from lower limb movements, the wires connecting electrodes were

well secured with tape.

Signal was ampliﬁed at a gain of 600. Common mode rejection

rate and input impedance were respectively 95 dB and 10 GX.

Raw electromyographic data were band-pass ﬁltered using a

fourth order Butterworth ﬁlter, with cut-oﬀ frequencies of 10 and

350 Hz. Fig. 1 depicts an individual example of the raw EMG

signals as a function of time, related to the four analysed muscles.

In order to measure the instantaneous angular position of the

crank, a rotational encoder (EL40B, Eltra, Sarego (VI), Italy)

with a resolution of 2000 pulses per turn was coupled to the left

crank of the ergometer by a chain drive. Since the gear ratio

between the gear wheel of the left crank and the sprocket of the

encoder was 53/15, the total resolution of the system was 7066.7

pulses per pedal cycle ( Picture 1 ).

The EMG and angular position of the crank signals were

synchronized, sampled at 1000 Hz and stored on a PC using a

16 bit A/D converter data acquisition system (APLabDAQ,

APLab, Rome, Italy).

2.2. Exercise protocol

An SRM ergometer (Schoberer Rad Meßtechnik SRM

GmbH, J¨ lich, Germany) was used for all tests, mounted with the

two ﬂywheels and in the ninth gear. The SRM crankset, equipped

with strain gauges, directly measured the torque produced by the

force applied to the pedals perpendicularly to the crank length.

The ergometer was customized with subject’s own bicycle’s mea-

sures and clipless pedals. A display was available close to the

handlebars of the ergometer to let the cyclists check their pedal-

ling cadence.

2.2.1. Warm up

Cyclists performed a 10 min warm up at a recommended

cadence of 80 rpm. The power constantly increased from 75 to

200 W during the ﬁrst 6 min (25 W min 1 ), the intensity was then

set to 125 W for the next 2 min and increased to 200 and 250 W

for the last 2 min. Depending on the performance level of the

subjects, the warm up intensity could be increased by no more

than 25 W per step.

2.2.2. Maximal isokinetic protocol (MIP)

MIP was performed in the isokinetic mode of the ergometer, at

a ﬁxed pedalling frequency of 80 rpm. This mode allows the

subject to pedal without resistance up to the ﬁxed cadence, while

resistance is automatically and proportionally increased when the

subject tries to overcome it. Prior to the maximal eﬀort, cyclists

pedalled at 80 rpm and low intensity (50–100 W) and at the signal

they started to pedal as forcefully as possible for 6 s, while a

vigorous verbal encouragement was given. They were instructed

to remain seated and to hold the hands on the low part of the

handlebars during the trial. Every cyclist completed a total of ten

6 s all-out sprints. A full recovery was ensured by a 3 min rest

period between sprints, in which they were allowed to drink water

and pedal at a low intensity.

2.2.3. Sub-maximal protocol (SMP)

After the MIP, cyclists rested for 10 min, pedalling at 50 W at

a freely chosen cadence. They were then asked to perform four

sub-maximal exercises at 0%, 20%, 40% and 60% of the maximum

power output obtained during the MIP. In order to perform the

0% exercise, the brake was turned oﬀ. It is however useful to know

that, due to the friction of the moving parts, the workload was

actually about 30–35 W. Concerning the other sub-maximal

Fig. 1. Raw EMG signals from a single, representative trial recorded

during the maximal isokinetic protocol. The window corresponding to a

pedal cycle is also shown. VL = vastus lateralis; BF = biceps femoris;

TA = tibalis anterior; GL = gastrocnemius lateralis.

Please cite this article in press as: Fern´ndez-Pe˜a E et al., A maximal isokinetic pedalling exercise for EMG ..., J Electromyogr Ki-

nesiol (2008), doi:10.1016/j.jelekin.2007.11.013

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Picture 1. A rotational encoder was coupled to the left crank of the SRM ergometer in order to measure the instantaneous angular position of the crank

and synchronize it with the EMG activity.

2.4. Data processing

of the right crank from top dead center (TDC at 0) to the next

TDC. The mean RMS was calculated averaging the RMS values

of the eight pedal cycles completed in every MIP trial, and aver-

aging the complete pedal cycles of the last 10 s (about 13 pedal

cycles) in every SMP trial.

For MIP assessment, the highest EMG activity achieved for

each muscle was set to 100% and the other trials were calculated

as a percentage of the highest. In contrast, for SMP assessment,

the EMG activity of all muscles corresponding to PO B was set to

100% and the values obtained during the submaximal exercises

(0%, 20%, 40% and 60% of PO B ) were expressed as a percentage

Torque applied to the crankset during the MIP was recorded

at 200 Hz for power output calculation purposes. Average power

output of each maximal trial (PO) was calculated as the product

of the average torque over the 6 s (in Nm) and the actual average

cadence (in rad/s). For each subject, the best PO (PO B ) was set to

represent 100% and the other trials were calculated as a per-

centage of PO B .

Raw EMG data were processed by root mean square (RMS)

determination for each complete cycle, deﬁned as a full revolution

Fig. 2. Example of individual muscle activity, as a function of crank angle, obtained at ﬁve diﬀerent intensities for the vastus lateralis (A), biceps femoris

(B), tibialis anterior (C) and gastrocnemius lateralis (D).

Please cite this article in press as: Fern´ndez-Pe˜a E et al., A maximal isokinetic pedalling exercise for EMG ..., J Electromyogr Ki-

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of the former. An example of individual EMG patterns obtained

for the four analysed muscles is represented in Fig. 2 . The raw

data were root mean squared with a moving window length of

100 ms.

assistance of a reliability spreadsheet ( Hopkins, 2007 ). The CV is

1 Þ , where SD is the standard deviation

of the change scores of natural log of the measure. The ICC is

deﬁned as (V v)/V, where V is the between-subject variance

averaged over the two trials analysed, and v is the square of the

standard error of measurement.

2.5. Statistical analysis

In order to evaluate the within-subject reproducibility of PO

and EMG for the four muscles analysed, the variables were

checked for normality and homoscedasticity and were log-trans-

formed when these assumptions were violated. Thereafter the

intra-subject standard error of measurement (SEM), the coe-

cient of variation (CV) and the intra-class correlation coecient

(ICC) were calculated as proposed by Hopkins (2000) with the

3. Results

Fig. 3 shows PO (mean ± SD) reached during the 10 tri-

als. The PO B (100%) was achieved during the 4th trial. The

PO values were, however, very close to each other, ranging

from 98.0 to 100.0, thus indicating a quite high reproduc-

ibility of the variable, with no observable learning or fati-

gue eﬀect. The PO B obtained ranged from 664.6 to

1013.9 W (data not shown).

EMG activity (mean ± SD) is shown for VL ( Fig. 4 A),

BF ( Fig. 4 B), TA ( Fig. 4 C) and GL ( Fig. 4 D), registered

during the 10 trials. For each of the muscles included in

the analysis, the 100% activity was achieved within the

3rd trial, although BF ( Fig. 4 B) and GL ( Fig. 4 D) tend

to decrease thereafter.

Reliability measures from consecutive pairs of trials are

summarized in Table 1 . SEM and CV are presented as a

mean value, whilst for ICC maximal and minimal values

are shown. PO has the lowest CV (2.19), indicating very

good consistency between repeated measures. Among the

EMG activities, VL and TA show, respectively, the lowest

(CV = 4.46) and the highest variability (CV = 8.70). ICCs

in all the variables analysed, ranging from 0.922 to 0.994,

are considerably high.

Fig. 3. Power output (mean ± SD) reached during the 10 trials of the

maximal isokinetic protocol. The best power output was set equal to 100

and the other trials’ were calculated as a percentage of the best one.

Fig. 4. EMG activity (mean ± SD) for vastus lateralis (A), biceps femoris (B), tibialis anterior (C) and gastrocnemius lateralis (D) registered during the 10

trials of the maximal isokinetic protocol. The highest EMG activity was set equal to 100 and the other trials’ were calculated as a percentage of the best

one.

Please cite this article in press as: Fern´ndez-Pe˜a E et al., A maximal isokinetic pedalling exercise for EMG ..., J Electromyogr Ki-

nesiol (2008), doi:10.1016/j.jelekin.2007.11.013

deﬁned as 100 ð e SD = 2

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