I.N. Buchatskaya, Ph.D.
R.M. Gorodnichev, professor, Dr.Biol.
Velikie Luki State Academy of Physical Culture and Sport, Velikie Luki

Keywords: archers, amplitude and frequency of muscle electrical activity, «leading» muscles, fatigue.

Introduction. It is known that strenuous motor activity leads to a decrease in functional reserves of the body and the development of fatigue. While performing different types of muscle work fatigue is being developed involving central and peripheral mechanisms. The predominance of a particular mechanism depends on the structure of movement, duration and intensity of its execution. Fatigue is accompanied by temporal discoordination of physiological functions, deterioration of quantitative and qualitative characteristics of performance, increase of its physiological value [3]. Competitive activity of an archer is an alternation of prolonged static and dynamic, relatively monotonous work performed for several hours [5]. Hence, the purpose of the research was to study the electrical activity of the muscles of archers while performing archery exercise similar to competitive activity.

Materials and methods. Fourteen archers took part in the experiment. The subjects were second-class athletes to Master of Sports of International Grade. The age of the subjects was from 16 to 25 years old. All the subjects were informed in detail on the ongoing research and gave a written consent to participate in the experiment in accordance with the Declaration of Helsinki. The athletes performed 30 shots using a classic bow under conditions simulating competitive activity – 10 sets of 3 shots each, 3 minutes allowed for each set, from the 18 meter distance. During the shooting a video footage was done with a frequency of 500 Hz and electrical activity of muscles was recorded (EMG). Video analysis was done using «Qualisys» (Sweden, 2010) and then used for the analysis of shooting technique. Bioelectric potentials of the skeletal muscles were recorded using a 16-channel electromyograph «MegaWin МЕ 6000» (Finland, 2008), and the obtained data were processed using «MegaWin» software. Owing to electromyography, EMG of 12 examined muscles was recorded at a considerable distance from the recording computer, the data were transferred on-line by means of the Bluetooth wireless technology. The sampling frequency of the signal was 1,000 Hz. We developed a mechano-optical sensor that was used to record the moment of arrow release by an athlete as well as a preceding moment of clicker response. Impulses generated by the sensor were used to synchronize the release of the arrow with EMG and 3D video analysis data, so that the limits of the phases of the studied technique could be accurately fixed and the possibilities of analysis of these phases could be significantly enhanced.

 In the shooting exercise the first two (shots 1-6) and the last two (shots 25-30) shooting sets corresponding to the beginning and the end of the exercise were analyzed.

Statistical processing of the data was carried out in the software Statistiсa 10.0. (Statsoft Inc, USA, 2010) using Basic Statistics and the nonparametric Wilcoxon Matched Pairs Test.

Results and discussion. On the basis of the estimation of the footage of performed shooting exercises the researchers divided the shot structure into working phases, indicating the boundary moments of the beginning and end of each of them [1]. Owing to the synchronization of time parameters of shots with the registered electrical activity of the examined muscles a precise electromyographic analysis of the most technically important shot phases was carried out. The phases were as follows: a phase of taking the main ready position – “expansion”; a phase of an arrow moving from under the clicker – “inching” and a phase of “finishing the shot”. Figure 1 shows a typical recording of EMG of the muscles involved in shooting of one of the subjects with an indication of the analyzed phases. Although the phases of the pre-ready position and the relaxation are also technically important, their electromyographic characteristics varied significantly in different shots. It should be noted that the “inching” phase has the highest electrical activity of the studied muscles. This is probably due to the fact that in this phase an archer moves to static work aimed at both holding the bow stretched out and maintaining the balance of the “archer – weapon” system. 

Figure 1. A typical recording of EMG of the muscles involved in shooting of a subject named A.D., aged 17.

Notes:   1 – radial flexor muscle of the right hand; 2 – ulnar extensor muscle of the right hand; 3 – triceps brachii muscle of the right arm; 4 – back of the right deltoid muscle; 5 – radial flexor muscle of the left hand; 6 – ulnar extensor muscle of the left hand; 7 -  triceps brachii muscle of the left arm; 8 – front part of the left deltoid muscle; 9 – upper bundles of the right trapezius muscle; 10 – upper bundles of the left trapezius muscle; 11 – lower bundles of the right trapezius muscle; 12 – lower bundles of the left trapezius muscle.

Markers 1 and 2 – the “expansion” phase, 2 and 3 – the “inching” phase, 3 and 4 – the phase of “finishing the shot”.

Figure 2 shows the group average EMG amplitude of the examined muscles of the archers while implementing technically important phases. According to V.N. Komantsev, V.A. Zabolotnykh [2], a surface EMG of muscles of less than 300 uV indicates an insufficient voluntary effort of the muscle. With this in mind, we have identified the most active muscles involved in carrying out the shot as a whole as well as its individual phases.

We believe that they are subject to the main load in the course of the shot. Further on we will call them «leading» muscles.

Figure 2.  Group average EMG amplitude of the muscles of the archers while carrying out technically important shot phases.

Notes:   1 – radial flexor muscle of the right hand; 2 – ulnar extensor muscle of the right hand; 3 – triceps brachii muscle of the right arm; 4 – back of the right deltoid muscle; 5 – radial flexor muscle of the left hand; 6 – ulnar extensor muscle of the left hand; 7 -  triceps brachii muscle of the left arm; 8 – front part of the left deltoid muscle; 9 – upper bundles of the right trapezius muscle; 10 – upper bundles of the left trapezius muscle; 11 – lower bundles of the right trapezius muscle; 12 – lower bundles of the left trapezius muscle.

According to the data in Table 1, at the beginning of the shooting exercise (1-2 sets of shots), during the “expansion” phase, a significant effort is developed by the front part of the left deltoid muscle and the upper bundles of the left trapezius muscle. During the “inching” phase a significant effort is developed by the front part of the left deltoid muscle, back of the right deltoid muscle, upper and lower bundles of the left trapezius muscle and the lower bundles of the right trapezius muscle, the radial flexor muscle of the right hand. The phase of “finishing the shot” is implemented by means of efforts of the front part of the left deltoid muscle and the upper bundles of the right and left trapezius muscles. In general, throughout the shot, a significant muscle effort was developed by the front part of the left deltoid muscle, the upper bundles of the left trapezius muscle and the back of the right deltoid muscle. We should note that during the “inching” phase that is very important in technical terms its “leading” muscles were most active, that is, developed a significant effort compared to the other phases. In addition, this phase is implemented by means of a large number of muscles in comparison with the other phases.  

While analyzing the obtained EMG parameters of the studied muscles throughout the shooting exercise it was found that these muscles increased their activity in the final shooting sets (shots 25-30) as compared to the initial ones (shots 1-6), mainly due to an increase of the EMG amplitude. As seen from Table 1, during the “expansion” phase the EMG amplitude of its “leading” muscles significantly increased, namely that of the front part of the left deltoid muscle and the upper bundles of the left trapezius muscle, while the frequency of their bioelectric potentials was constant throughout the exercise    (84.66±1.48 Hz and 82.92±1.66 Hz, respectively). By increasing the amplitude of the action potentials the activity of such muscles as the ulnar extensor muscle of the right hand, the triceps brachii muscle of the right arm and the radial flexor muscle of the left hand was significantly increased. During the considered phase the activity of the muscles increased as compared to the start of the exercise, but due to a significant increase in the frequency of bioelectric potentials in the final sets. The muscles were as follows: the triceps brachii muscle of the left arm (from 99.82±3.24 to 106.89±3.91 Hz, p=0.02) and the upper bundles of the right trapezius muscle (from 81.32±1.52 to 92.86±1.73 Hz, p=0.05).  

Table 1. Group average amplitude of interference EMG of the archers’ muscles during a shooting exercise (n=14, M±SE, uV)

Note: *- significance of differences with reference to the beginning (Wilcoxon Matched Pairs Test).

A significant increase of the amplitude of bioelectric potentials of the “leading” muscles – the front part of the left deltoid muscle and the upper bundles of the left trapezius muscle – was observed during the “inching” phase too (Table 1). At the same time the frequency of their electrical activity was also constant throughout the exercise. The back of the right deltoid muscle tended to increase the EMG amplitude with fairly significant increase in the impulse frequency (p=0.02). The EMG amplitude of the other “leading” muscles during the considered phase, on the contrary, decreased. Thus, the amplitude of the radial flexor muscle of the right hand with a constant impulse frequency at the end of a shooting exercise significantly decreased compared with the beginning. A tendency to change the EMG amplitude in the direction of decreasing in the last sets of the exercise implementation was noted also in the lower bundles of the right and left trapezius muscle, while in case of the lower left bundles the increase of the impulse frequency was statistically significant (from 70.54±1.98 at the beginning to 75.55±1.27 Hz at the end, p=0.004).  

 P.G. Symanovich [4] notes that the “inching” phase is implemented mainly due to the work of the muscles of the right shoulder and the back. The effort of the “leading” muscles during the “expansion” phase should be redistributed on the lower back muscles during the “inching” phase. Analysis of the EMG of the studied muscles showed that during the “inching” phase the increase in the activity of the “leading” muscles in the last sets of the shooting exercise was accompanied by a significant increase in the activity of the “auxiliary” muscles, for example, that of the triceps brachii muscle of the right arm with a simultaneous tendency to decrease of the EMG amplitude of one of the “leading” muscles – the lower bundles of the trapezius muscle of the back (Table 1). This fact indicates growing fatigue during multiple execution of shots from one set to another, that leading to the loss of the proper distribution of muscular effort in the “inching” phase. In its turn, an increase in the electrical activity of the “leading” muscles during the last sets of the exercise compared to the first ones may be due to the fact that contraction properties of each motor unit (MU) of these muscles decrease due to fatigue, on the one hand, so additional motor neurons that were not previously involved in the shots implementation start to get involved in the work to maintain the muscle tension at the same level. On the other hand, it is possible that activation threshold of certain MUs changed under the influence of long-term multiple execution of shots.  

During the “finishing of the shot” phase the activity of its “leading” muscles significantly decreased in the final sets of the exercise. Besides, for some, a decrease was marked not only in the amplitude, but the frequency of EMG. Thus, the EMG amplitude of the anterior left deltoid muscle decreased, although the frequency remained constant. Activity of the upper bundles of the left trapezius muscle of the back and the upper bundles of the right trapezius muscle significantly decreased (in this case a significant increase of its EMG frequency was noted (p=0.02). The decrease in the activity of the muscles examined in this phase must be associated with a statistically significant reduction of the duration of the phase by the end of the shooting exercise implementation from 1.47±0.04 seconds to 1.39±0.04 seconds (p=0.02).

The analysis of the EMG parameters of the muscles involved in the general implementation of shot revealed that the amplitude of the electrical activity of the “leading” muscles in the final sets just tended to increase with a constant frequency of their EMG. Only in case of the triceps brachii muscle of the left arm this value significantly increased from 112.66±1.84 Hz to 118.6±2.42 Hz (p=0.003) with the amplitude of its activity being constant.

Conclusions:

1. For the first time the “leading” muscles involved in the implementation of the most technically important phases of a shot were defined.
2. Electrical activity of the “leading” muscles increases during the final sets of shots that can be associated with two central processes – increasing number of recruited motoneurons and their phase locking.
3. By the end of the shooting exercise in the “inching” phase the activity of the lower bundles of the trapezius muscle of the back decreases, while the EMG amplitude of the triceps brachii muscle of the right arm significantly increases, indicating that the redistribution of muscle efforts during the studied phase is technically incorrect, probably due to growing fatigue because of recurrent shots.

References

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5. Shilin, Yu.N. Sportivnaya strel’ba iz luka. Teoriya i metodika obucheniya. Uchebnoe posobie (Sports archery. Theory and methods of learning: study guide) / Yu.N. Shilin. – Moscow, 2011. – 280 P. (In Russian)

Corresponding author: irina-buchackaja@rambler.ru