Postgraduate student K.V. Sergeeva¹
Dr. Biol., Professor R.V. Tambovtseva^1
1Russian State University of Physical Education, Sport, Youth and Tourism (SCOLIPE), Moscow

Keywords: electromyography, EMG, fast muscle fibers, spectral analysis, medium frequency.

Background. Mode of muscular work is the nature of changes in such parameters as length and stress during exercise. The pulling power of the muscles during work of overcoming character (concentric mode) is greater than the moment of external force. If the moment of the applied muscular force is less than the moment of counterforce, the muscle is extended by the action of loading in the ceding or eccentric mode. Due to these features, eccentric contraction requires lower muscle activity at a similar concentric torque. That is why muscle activity, as evaluated by surface electromyography (sEMG), is generally lower during the ceding phase of movement [3], but the muscle can produce significantly greater force than the muscle that contracts isometrically or concentrically under other conditions. In terms of active muscle strain, a single sarcomere can exert a force that exceeds the pure isometric force at an optimal length of 20-50% on average due to the structural protein titin, which role in the muscle contraction goes far beyond a purely passive structural one. Titin promotes "active" production of force, working as a molecular spring, which increases its rigidity by binding calcium and reducing the free length by attaching the proximal titin region to actin myophiliants [5]. Given the potentially higher force capacity of the muscles in the ceding mode, the eccentric phase loading should be greater than that in the concentric phase, thus equalizing the neuromuscular activity of both types of contraction.

Objective of the study was to conduct a comparative analysis of the electrical activity of the quadriceps muscle of 9 trained males performing sitting knee extension in the eccentric and concentric contraction modes with increasing intensity in order to indirectly assess possible differences in the recruitment of motor units.

Methods and structure of the study. The experiment was carried out in the Sports and Sports Medicine Research Laboratory at the premises of the Russian State University of Physical Education, Sport, Youth and Tourism (SCOLIPE). During the experiment, we measured the bioelectric activity of three heads of the quadriceps muscle: m. rectus femoris, m. vastus lateralis and m. vastus medialis in 9 representatives of speed-strength sports. The subjects performed a maximum voluntary concentric contraction (100% MVC) of the lower leg extensors, followed by contractions of submaximal intensity of 80%, 50%, and 25%. This sequence was performed separately first in the concentric and then in the eccentric contraction modes in a randomized order. The eccentric maximum corresponded to 140% of the concentric maximum. This percentage was chosen according to the experimental data in the difference between the maximal eccentric and concentric dynamic force [2]. The work tempo was set by the metronome and was one movement per 2 sec. The test movement was performed on command with fixation of the initial and final posture, which was reflected in the EMG pattern, which was then used to determine the time limits of the bioelectrical activity of the muscles under study. The biopotentials of the skeletal muscles were recorded using a 16-channel ME6000 Biomonitor System electromyograph (Mega Electronics Ltd, Finland). The EMG signal was recorded using Ag/AgCl electrodes (50 mm in diameter). The signal sampling frequency was 2000 Hz. The electrodes were fixed above the myogaster (in the motor zone projection) with an interelectrode distance of 20 mm. The oscillographic signals were analyzed using the following characteristics of electromyograms: moving root mean square (RMS) and mean power frequency (MPF), which was determined using the fast Fourier transform (FFT) algorithm with an array of 4096 signal points with the Hann window function.

Results and discussion. The representation of the time-domain EMG amplitude expressed as the temporal evolution of the root mean square RMS, is an indicator of the mean signal power and shows the number and speed of discharges of the active motor unit action potential [1]. As expected, the RMS rate increased with the increase in the loading rate in each contraction mode, thus indicating an increasing number of the muscle fibers involved. At the same time, the eccentric contractions showed a smaller amplitude in the whole range of efforts, which was unexpected to us, as sampled for the study were the trained subjects (Fig. 1). In addition, in our previous similar study of the biceps brachii (unpublished data), the maximal eccentric contractions were accompanied by the significantly larger RMS, which was probably primarily due to the involvement of additional high-threshold motor units due to the preponderance of external forces rather than an increase in the speed of discharge of action potential, since the RMS rate reflects changes in recruitment rather than the discharge speed [4].

Fig. 1. Dynamics of RMS rates of quadriceps muscle of thigh ( electromyogram) with increasing intensity from 25% to 100% MVC.

The values are represented as an average ± σ. The asterisk indicates the statistically significant (р<0.01) differences between the eccentric and concentric modes.

We assume that the identified discrepancy between the two studies may be due to the functional properties of the tested quadriceps muscle of the thigh, which is characterized by the predominance of the slow muscle fibers, as compared to the biceps brachii. Moreover, the differences in the amplitude may be due to other circumstances, such as the use of insufficient load for the muscle group, biomechanical features of the exercise, lack of familiarization sessions for the subjects.

However, the test of the hypothesis at p=0.01 shows conclusively higher mean frequencies of the eccentric EMG signal than in the concentric mode, which is totally in line with the idea of greater activity of the fast-contracting motor units, despite the lower amplitude (Fig. 2, 3).

Fig. 2. Dynamics of MPF rates of quadriceps muscle of thigh (electromyogram) with increasing intensity from 25% to 100% MVC.

The values are represented as an average ± σ. The asterisk indicates the statistically significant (р<0.01) differences between the eccentric and concentric modes.

The mean frequency increased in parallel with the increase in the voluntary effort, thus indicating gradual involvement of the fast fibers, and reached an 80%-100% MVC plateau. This dynamics of MPF at maximum and near-maximum levels of force production corresponds to the results of our previous studies [4] and suggests that most motor units are already activated and do not reflect any physiological phenomena.

Fig. 3. Dynamics of MPF rates of quadriceps muscle of thigh (electromyogram) with increasing intensity from 25% to 100% MVC.

The values are represented as an average ± σ × t (0.95, 9). The asterisk indicates the statistically significant (р<0.01) differences between the eccentric and concentric modes.

Conclusion. Based on the results obtained and theoretical substantiation, we can conclude that the higher average EMG frequency in the eccentric contraction mode is consistent with the theory that rapidly contracting motor units are recruited selectively during the eccentric contractions. Thus, the high level of reflex excitability of high-threshold afferents, improvement of their contractility, ability to achieve a higher torque (i.e., mechanical load) when performing supramaximal loads in the eccentric phase of movement can give a competitive advantage for the development of strength indicators and hypertrophy of muscle tissues which is most relevant in speed-strength sports.

References

1. Filligoi G.C., Felici F. Detection of hidden rhythms in surface EMG signals with a nonlinear time-series tool. Medical Engineering & Physics.1999. Vol.21, no.6-7. pp. 439–448.
2. Maximal eccentric and concentric strength discrepancies between young men and women for dynamic resistance exercise. J. Strength Cond. Res. 2007. Vol. 21, no.1. pp. 34–40.
3. Pasquet B., Carpentier A., Duchateau J. Specific modulation of motor unit discharge for a similar change in fascicle length during shortening and lengthening contractions in humans. Journal of Physiology. 2006. Vol.577, no.2. pp. 753–765.
4. Christie А. Relationships between surface EMG variables and motor unit firing rates. Eur. J. Appl. Physiol. 2009. Vol.107, no.2. pp. 177–185.
5. Herzog W.G. et al. Residual force enhancement following eccentric contractions: a new mechanism involving titin. Physiology. 2016. Vol.31, no.4. pp. 300–312.

Corresponding author: sergeeva_xenia@mail.ru

Abstract

Objective of the study was to conduct a comparative analysis of the electrical activity of the quadriceps muscle of 9 trained males performing sitting knee extension in the eccentric and concentric contraction modes with increasing intensity in order to indirectly assess possible differences in the recruitment of motor units.

Methods and structure of the study. During the experiment, we measured the bioelectric activity of three heads of the quadriceps muscle: m. rectus femoris, m. vastus lateralis and m. vastus medialis in 9 representatives of speed-strength sports performing knee extension in the simulator.

Results and conclusions. The analysis of the surface electromyogram showed that the average frequency (MPF) within the range of the studied intensities was higher in the eccentric contraction mode, which indicates a higher level of activity of rapidly contracting motor units as opposed to the concentric mode.

Based on the results obtained and theoretical substantiation, we can conclude that the higher average EMG frequency in the eccentric contraction mode is consistent with the theory that rapidly contracting motor units are recruited selectively during the eccentric contractions. Thus, the high level of reflex excitability of high-threshold afferents, improvement of their contractility, ability to achieve a higher torque (i.e., mechanical load) when performing supramaximal loads in the eccentric phase of movement can give a competitive advantage for the development of strength indicators and hypertrophy of muscle tissues which is most relevant in speed-strength sports.

The data obtained can be used when selecting the strength training methods.