A maximal isokinetic pedalling exercise for EMG normalization in cycling

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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.

Introduction

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 Dieën 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 Dieën et al., 2003). It is widely accepted that a procedure of EMG amplitude normalization is required in order to: (i) make a between- and within-subject comparison of activation level in working 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 different 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, Kollmitzer et al., 1999), it is strongly dependent on the specific joint angles used during the iMVC. In fact, an EMG signal 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 contraction 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 muscle is difficult (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 normalization procedure is essential. A recent paper (Rouffet and Hautier, 2007) has widely addressed this issue. The authors underlined that while executing a specific task, physiological modifications in the neural drive should be reflected in the EMG signal. Other authors pointed out that when dealing with sports movements the electromyogram should be the expression of the dynamic involvement of specific muscles (Clarys and Cabri, 1993). In cycling, EMG is often performed in order to assess the muscular intervention during the pedalling action. For the normalization purpose it is therefore pivotal to choose a meaningful reference contraction so that its activation is regulated by the same neuromuscular pattern as the pedalling action. This means that the task parameters of the reference contraction (e.g. movement amplitude, joint position, speed, etc.) should reproduce, as much as possible, the pedalling action (Latash, 1998).

Despite the above considerations, several studies examining cycling have improperly implemented EMG normalization 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 normalization 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 Hautier, 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. Takaishi 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 recruitment 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 chainrings were adopted, used the highest EMG value observed across all trials for normalization purposes. Since the experimental design entailed a variation of pedalling biomechanical conditions (e.g. instantaneous crank angular velocity), the normalization procedure chosen could not represent all the different tests performed.

Hug et al., 2004a, 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-maximal 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 contraction 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.

Section snippets

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 consent 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

Results

Fig. 3 shows PO (mean ± SD) reached during the 10 trials. The POB (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 reproducibility of the variable, with no observable learning or fatigue effect. The POB obtained ranged from 664.6 to 1013.9 W (data not shown).

EMG activity (mean ± SD) is shown for VL (Fig. 4A), BF (Fig. 4B), TA (Fig. 4C) and GL (Fig. 4D), registered during the 10 trials. For each

Discussion

The aim of this study was to propose a new method for EMG amplitude normalization in cycling and assess its reliability. Different investigations suggesting isometric or sub-maximal dynamic contractions for EMG normalization in cycling have been gone over, and the limitations were highlighted.

A MIP was introduced as a reference contraction for the EMG normalization procedure. The task required, a maximal 6-s pedalling action, and the submaximal cycling are comparable for at least three reasons:

Conclusion

The new protocol proposed for the purpose of EMG normalization in cycling, which consists of three 6-s maximal isokinentic pedalling sprints, has very good logical validity and within-subject reproducibility. Further, the test is also highly specific to the actions associated with cycling. The chosen cadence of the normalization protocol should be the same as the sub-maximal exercises.

Acknowledgements

The authors would like to thank APLab engineers, especially Nunzio Lanotte, who have designed the data acquisition system, for their technical assistance; Mark Watsford, Ph.D., lecturer in Exercise and Sports Science School of Leisure, Sport and Tourism University of Technology (Sydney) and Francesco Felici, M.D., professor of exercise physiology, Istituto Universitario di Scienze Motorie (Rome) for their assistance and suggestions in revising the paper.

Eneko Fernández Peña received his degree in Physical Education from the Basque Institute of Physical Education (SHEE/IVEF, Vitoria-Gasteiz, Spain) in July 2003, and his Ph.D. degree from the University of Urbino “Carlo Bo” (Italy) in March 2007. He is currently a post-doctoral fellow at the Institute of Health and Physical Exercise (Urbino, Italy), and his research interest focuses on biomechanics of cycling.

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  • Cited by (0)

    Eneko Fernández Peña received his degree in Physical Education from the Basque Institute of Physical Education (SHEE/IVEF, Vitoria-Gasteiz, Spain) in July 2003, and his Ph.D. degree from the University of Urbino “Carlo Bo” (Italy) in March 2007. He is currently a post-doctoral fellow at the Institute of Health and Physical Exercise (Urbino, Italy), and his research interest focuses on biomechanics of cycling.

    Francesco Lucertini received his diploma in Physical Education (1998) and his degree in Exercise Sciences (2001) from University of Urbino “Carlo Bo” (Italy). He received the Ph.D. degree in February 2006 from the Faculty of Health and Sport Sciences of the same University and he is currently a post-doctoral fellow at the Institute of Health and Physical Exercise (Urbino, Italy). His research interest focuses on performance assessment in both sport- and health-related topics.

    Massimiliano Ditroilo has a diploma in Physical Education (1992), a degree in Biological Sciences (1999) and a master degree in Methods of Training (2001). He is currently working within the Institute of Health and Physical Exercise at Urbino University (Italy). His research focuses on biomechanics and performance assessment of cycling, athletics, swimming and team sports.

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