S.A. Moiseyev
Velikie Luki State Academy of Physical Culture and Sport, Velikie Luki

Keywords: variability, muscular synergies, archery, motor coordination, movement control.

Introduction. The issue of variability and its role in functions of different biological systems is considered today among the most promising areas for studies in human physiology science. Among many different approaches to such studies we would give the top priority to the one that assumes variability rather as an element of a nonlinear dynamic system giving additional information on the system condition than a random spread in the sensorimotor system functioning process [3]. To put it in other words, it may be advantageous to consider variability as a positive feature that helps the system adjust to the varying conditions of movement sequence performed. A.A. Shalmanov offers a definition of variability as the modification of biological systems when the same system goes through multiple changes of its conditions (that implies the internal individual variability), or when conditions of multiple similar systems are compared (meaning either the group-specific or inter-group variability) [2].

Variability may be measured using the mathematical method of statistics as a key tool that applies a set of the relevant factors including the variation range, dispersion, mean-square deviation, variation ratio etc. These values, however, represent only the degrees of deviations of the measured figures from the arithmetical mean values and, therefore, may provide only some understanding of how close or different the elements of the system are. Furthermore, it is obvious that when precise sports movements are studied, individual variations in the performance techniques and the relevant variations of the values indicative of the motor elements may be very significant, particularly when athletes of very different skills are subject to the study. Therefore, it may be appropriate to give the top priority in the studies of such movements to analyze the internal individual variability focused on variations of performance criteria for one subject athlete who repeats the same standard movements many times.

The purpose of the research was to study the inter-group variability in every phase of precise movement taking the archery shooting technique as a study case.

Materials and methods. Subjects for our study were 8 elite archers (ranked Masters of Sport and International Class Masters of Sports). The subject archers shot 30 arrows on the 18-meter distance indoors in the left-hand stance. During every shot sequence, we recorded the bioelectrical activity of the key 12 skeletal muscles of the upper shoulder girdle using the 16-channel ME 6000 Electromyograph (Finland) with the 3D video captures of the archer’s movements using the Qualisys Video System (Sweden) being taken at the same time. Based on analysis of the process video captures, the archery shot sequence was divided into the following phases: preparatory stance with expansion (drawing) phase; loading phase till the arrow triggers the clicker; and the shot completion phase from the bowstring release till the very first follow-through (bow lowering) moment.

The variability ratios for the following parameters of the internal and external elements of the shot sequence calculated were as follows: EMG amplitudes; muscle activation cycles (meaning the actions when the EMG amplitude value surges to at least 100mkV and keeps at this level for at least 0.1 second); duration rate of the muscle electrical activity surge in every activation cycle; path length, speed and acceleration rates for different body segments as indicated by the selected sets of anthropometric points; joint angle variation rates (meaning the difference between the starting and final angles of the joint movement); and duration of every phase in the archery shot sequence. The variability ratios measured in the study were classified as low when they were under 30%; average when they ranged from 31% to 60%; and high when they exceeded 61%. In addition, mean arithmetic and mean square values were checked to exclude from the variability analyses those values that could be attributed to the measuring errors (random variation).

Results and discussion. The lowest variability ratios of the EMG amplitudes were detected in the expansion phase of the shot sequence when they varied within the range of 4% to 21%, with the highest ratios (of 21%) being achieved in the front bundles of the left-hand deltoid muscle. The lowest amplitude variability ratios (of at most 4% for a few tested subjects) were found in the radial flexor and ulnar extensor muscles of the left-hand wrist. In the loading phase, the EMG amplitude variability ratios were somewhat higher than in the expansion phase, falling within the range of 4-23%. In the shot completion phase, the variability ratios were almost as low, varying under 29%. Overall, the EMG amplitude variability ratios were classified as low in the above phases of the shot sequence.

Having analyzed the numbers of the muscle activation cycles and the electrical activity duration rates, it may be important to note the high dispersion rates of the EMG amplitude variability ratios. There were even registered the cases when some muscles of a few subjects were activated and relaxed for several times (up to 5 and even more times) in some phase of the shot sequence and never reached the background (‘threshold’) activity level in the process. The posterior head of the right-hand deltoid muscle in the expansion phase, for instance, was activated only in four tested subjects, with the measured variability ratios making up 17%, 48%, 49% and 18% and the electrical activity duration rates varying within the range of 22-99%. The lower segment of the right-side trapezius muscle was activated in six subjects out of eight, with the variability ratios estimated at 48%, 31%, 18%, 41%, 38% and 18%, respectively, and the activity duration rate ranging from 20% to 90%. On the whole, the skeletal muscle activity duration rates for different activation cycles and phases of the shot sequence varied within wide ranges, and the variability ratios reached average and high values.

Furthermore, the variability ratios for the anthropometric point pathes were low for most of the tested athletes in the expansion phase, with the maximal ranges of 6-33% being registered for the neck anthropometric point. In the other anthropometric points they varied within the range of 2% to 16%, with a few surges of the variability ratios up to 74%. The same parameter in the loading phase was found much more variable, with the variability ratios ranging within 24-63%, and with occasional surges up to 98% in a few cases.

Our study of the joint angle movement variability ratios showed that the loading phase and the shot completion phase were the times of the highest variability ratios compared to that of the shot beginning phase; and it was the expansion phase when the left-hand joint angle movement ratios reached their maximums. Duration rates of the movement phases varied in the following relatively wide ranges: 5% to 13% in the expansion phase; 19-49% in the loading phase; and 7-30% in the shot completion phase.

In the attempt to come closer to understanding of variability as a phenomenon, it might make sense to find out first of all what mechanisms shape up the external and internal patterns of the complex and precise movement sequences. According to the theory of N.A. Bernstein, every movement of human body is controlled by different tiers of the CNS (i.e. the movement control levels) having hierarchical structure [1]. Every such CNS tier operates within the certain acceptable variability range to control the certain aspects and components of the movement. For example, sensorimotor corrections on the spatial field control tier (the so called pyramid-striatum level) show higher degrees of variability that means that the system allows the same result being achieved by a wide variety of similar means. The archery aiming process, in particular, was classified by N.A. Bernstein with the processes controllable by the above CNS tier. Therefore, when we consider the loading phase – that normally includes the aiming movement sequence – it is only natural that we find the variability ratios being very high for both the external and internal movement structures in this stage. In addition, one of the key features of the spatial field control tier is its objectivity that means that it is closely linked to the actual conditions of the external environment by the data flow from mostly visual and sound perception and analyzing systems. In this case we refer to the visual control of the movement sequence in the aiming process and the archer’s response to the sound of the clicker.

One of the key factors of the movement structuring by any CNS tier is the acceptability of high variability for the less important aspects and components of the movement sequence and low variability for the top-priority ones within this tier. Therefore, in the archery shot sequence we should give preference to the muscular electric activity values (characteristic of the efforts in one or another phase of the movement sequence) associated with the low variability ratios; and treat as less important the internal structure elements of movements, including certain sequence controller ones, numbers of muscle activation/ deactivation cycles, and the electrical activity duration rates. We obtained very high variability ratios for these parameters for every skeletal muscle contributing to the movement process.

It should be mentioned in particular that in the expansion phase the variability ratios for the kinematic parameters of the radial and awl-shaped anthropometric points of the right and left arms and their angles were in most cases notably low. It may be due to the fact that variability decreases in proportion to increase of the speed and weight of the moving element along the movement path. In case of fast movements, every degree of freedom is bound by the reactive forces with the exception of the only one that controls the subject movement structure. In view of the fact that the external forces are beyond the reach of internal correction mechanisms, they are virtually uncontrolled, and therefore higher movement speed contributes to lower sensorimotor corrections. In the loading phase, the archery shot sequence is slowed down and the movement routes are shorter, and for this reason the variability ratios of the kinematic parameters turn to be notably higher than in the phases when the archer’s movements are faster.

The movement variability ratios are to some extent related to the motor skill formation and perfection process. This process is largely determined by the muscular synergies structured on the thalamo-pallidal level [3]. And it is this CNS tier that controls the highly harmonized body movements with involvement of many muscles naturally contributing to the sequence.

To detect the muscular synergies in the studied movement sequence we conducted a factor analysis (and its principal component method) using the Statistica 10 application software. The analysis demonstrated that the correlations of the subject skeletal muscles, with due consideration for the movement phases, can be grouped into the following 3 factors. Factor 1 covers the radial flexor muscle of the right wrist and the rear deltoid muscle of the right hand. These muscles are strongly interconnected in the expansion and loading phases. In addition, these muscles work together with the lower part of the right-side trapezius muscle in the shot completion phase and with the triceps muscle of the right shoulder in the expansion phase. Factor 2 refers to the radial flexor muscle of the left wrist, the triceps muscle of the right shoulder and the upper trapezius muscle on the right and left sides. These muscles actively cooperate in every phase of the subject movement sequence. And Factor 3 refers to the interrelations of the ulnar extensor muscle of the left wrist and the front bundles of the left-hand deltoid muscle. It should be noted that the muscles closely interrelated within one of the above Factors never work together beyond the scope of this Factor.

Our analysis of the EMG amplitude variability ratios for the detected muscular synergies found no differences with the other muscles that fall beyond the one or another synergy. Variability ratios of this process parameter were low in every phase of the shot sequence in most of the study subject cases. The numbers of the muscle activation cycles and the electrical activity duration rates for the synergized muscles showed lower variability ratios than the non-synergized muscles only in a few subject cases. Therefore, the factor analysis may be applied to detect the functionally interrelated muscular structures (synergies) contributing to the complex movement sequence performance process on the whole or its specific phases in particular; and the synergies are possibly controlled by the same control mechanism based in some specific CNS tier; however, it is not completely clear at this stage how they can be assessed based on the variability ratios of the bioelectrical activity parameters.

Conclusion. The study data on the variability ratios for the parameters of the external and internal structure of complex precise movements in the archery shot sequence make contribution to our understanding of the movement control mechanisms in the sport activities of this category. The study shows the potential for detecting the muscular synergies contributing to the movement sequence by means of factor analysis with its principal component method.

References

1. Bernstein, N.A. Fiziologiya dvizheniy i aktivnost' (Physiology of movements and activity) / Ed. by O.G. Gazenko; USSR Academy of Sciences. – Moscow: Nauka, 1990. – 494 P.
2. Shalmanov, A.A. Issledovanie variativnosti sportivnoy tekhniki (na primere tolkaniya yadra): avtoref. dis… kand. ped. nauk (Study of variation of sports technique (case study of shot put): abstract of Ph.D. thesis) / A.A. Shalmanov. – Moscow, 1977. – 24 P.
3. Latash, M.L. Variability of fast single-joint movements and equilibrium-point hypothesis // K.M. Newell, D.M. Corcos. – Human kinetics publishers, 1993. – P. 157–182.

Corresponding author: moiseyev.s.a.vl@vlgafc.ru