Jakub Grzegorz Adamczyk, Dr.
Jozef Pilsudski University of Physical Education in Warsaw
Anna Slupik, Master
Dariusz Boguszewski, Dr.
Dariusz Bialoszewski, Dr.Hab.
Andrzej Ochal, Master
Warsaw Medical University, Warsaw, Poland

Key words: power, force, progressive resistance, plyometric exercises, electromyography, thermography.

Introduction

Warm-up consists of proceeding of the proper effort with the muscular work of short duration, which with its intensity and character is similar to the following activity. The basic aim of warming up is the endeavour to competitor’s achievement of the optimum-psychophysical state before a training, a game or a contest. It serves also as the preparation of those exercising for the utilization of their maximum effort possibilities of the body [31].

Potential advantages refer to many physiological aspects of the physical exertion ability, among other things acceleration of the process of metabolism and efficiency of the nervous system [31], increase of speed of nerve impulses transmission [5], enlargement of contraction speed of muscles, risk diminution of the contusion of muscles, tendons and ligaments [31].

The selection of the suitable warming up must take into account the individually adapted intensity, content and form (active-passive), and the estimation of efficiency must be based on physiological or biomechanical indicators. It refers not only to the control of effects of warming up, but also the manner of its execution. Favouring the general and special warming up we must choose exercises from an angle of muscles and joints, which play the main role in the given discipline. Therefore, also in its course one ought to apply special strengthening and stretching exercises [32].

The role of warming up is universally recognized and it should be treated as the foot-bridge between the everyday activity and the specialistic effort within the framework of training or contest. It seems that too large pressure is put onto body temperature rise, which occurs during so called continuous warm-up) , while the basic aim should be neuromuscular adaptation to the foreseen type of work. Therefore, over the recent years other forms of warming up aid or exercising its active forms have become popular. We can list among other things plyometric exercises or progressive resistance with the utilization of elastic bands [28].

Plyometric exercises consist of stretching of the muscle and following it immediate explosive cramp, known as the SSC - stretch-shorten cycle. These exercises are a method of training which shapes the ability of muscles to produce the power at high speed (power output) in dynamic movements. With reference to formations of the maximum power or speed there exist an ample evidence of their large efficiency [18].

Athletes customarily use elastic bands (EB) as a training method used in the dynamic warming up for the enlargement of the power or improvement of conative abilities [2]. A theory of the efficiency of this form of exercises is based on the foundation that with their utilization one notices changes in the mechanics of exacting exercises of force and power. On the example of the knee bending as opposed to the stable weight, the use of variable resistance causes decreasing of the weight in lower position, when muscles are in mechanical disadvantage because of less cross-bridge activation. In turn, in the final part of the exercise it creates additional resistance when lower limbs are in more mechanically advantageous position to produce greater force. This mechanical advantage arises from the fact that the length of the quadriceps is shortened, allowing more opportunity for cross-bridge contractile activity [29].

In the most of disciplines a key-indication of efficiency is developed power.  This ability together with force is essential also in everyday life, permitting to limit the risk of falls, especially in advanced years [23]. One of its forms is the ability of high lifting of the barycentre of the body. Various tests, with the utilization of the maximum perpendicular jump (eg. CMJ – counter movement jump, VJ – vertical jump, DSJ – deep squad jump), are possible to be quickly executed almost in every conditions and simultaneously they credibly inform about the generated power.

The measurement of force moment in the isometric condition is one of most often practiced measurements. It rates this parameter only in the chosen articular angle, but it is safe (also for people after movement system traumata). As a result it is sometimes the most often used parameter for testing healthy persons, sportsmen within a period of preparatory and patients in the early period after injury, operations or reconstructions within the organ of the movement, when the inclusion of isokinetics is impossible [16, 20].

From the point of view of the physiology of effort, the moment of force is the essential parameter determining the physical exertion ability of muscles. The value of the moment of force is relative to many factors among others: degree of stretching of muscle fibres, the cross-section of a muscle, quantity and the quality of motor units forming the given muscle, and also frequency of their stimulation, and consequently to the occurrence of muscle fatigue. The estimation of the moment of force makes the comparison between the limbs, the calculation of the relation of the power with relation to the body mass or readiness of the competitor for undertaking defined effort possible [7, 13, 16].

Other test rating the state of muscle is the surface electromyography, sEMG. It is a non-invasive test evaluating the bioelectric activity of muscles by assembling and comparing the electric potential from the skin with two electrodes stuck on the pad of the same muscle (the electrodes are arranged in the line of muscle fibres). It permits examination of chosen muscle, and even chosen piece of its functional part (heads or parts). The sEMG testing evaluates, among other things, the frequency of stimulation of motor units, forming the given muscle (measurement of signal frequency) on which the nervous system has its influence [8]. In case of readiness of the nervous system for work, this frequency is high (especially in maximum efforts of short duration, such as the isometric tension of the muscle) and drops in case of pronouncement of fatigue (there appears enlarged postsynaptic potential of the neuromuscular synapse). The frequency of stimulations influences also generated muscular power [24].

Besides an often evaluated parameter of EMG testing is the value of the electric potential. It depends on the degree of the stimulation of the muscle and separately various factors, such as thickness and density of the adipose tissue, the genus and the number of motor units forming the given muscle, and also factors occurring during the cramp: angular speed in a joint and displacement of a muscular pad during the cramp. That is why functional sEMG testing must be a measurement taken in the isometric conditions or constant angular speed (isokinetics), to analyse received data quantitatively. Due to singular differences in generated sEMG signal, data should not be compared interindividually. Above two parameters of the sEMG signal – frequency and height of the electric potential objectively rate the state of the stimulation and the fatigue of a muscle; therefore they also evaluate the state of muscle preparation for the physical effort and the influence of this effort on the muscle [13, 24].

The purpose of the research was to make an attempt of qualification of efficiency of plyometric exercises and those with the use of progressive resistance put-upon in the power, force and bio potential of muscles warming up. Additionally one made an attempt answering the question whether there is any dependence between above-mentioned parameters and the superficial temperature of the quadriceps of a thigh.

Material & Methods

Participants

In research partook 20 non-training women aged from 21 to 25 years (tab. 1) who at random one allotted to two groups.

Tab. 1.  Biometrical data of the examined groups

The first (PLYO) group performed plyometric exercises in the warming up, while the second (EB) exercised with elastic bands using the progressive resistance. The examined were characterized with correct proportions of height and body mass (BMI in the norm). Within the range of the biometrical characterization one did not show any essential differences between classes.

Warm-Up protocol

During the 5-minute-lasting warming ups the examined performed defined exercise set (tab. 2), targeting the rise of physical effort ability.

Tab. 2. The schedule of preparatory exercises

Counter Movement measurement

The measurement of the power one made with the counter movement jump (CMJ) and the time of contact with the basis during the rebound with both feet. The women performed the test of the counter movement jump (CMJ) on the platform twice. To do the test one used Kistler′s platform (Switzerland) with the sampling rate of 1000 Hz with the BioWare 3.24 software. For further analyses one used the height of the maximum jump in every test.

Maximal torque measurement

The estimation of moments of powers one run on the dynamometric Sumer UPR-02 arm-chair with the software the Moment II.  This measurement was taken in isometric conditions at the knee of a dominant limb bent to 75º and hip joints bent to 90º (the sitting position). The limb not under examination was freely let hang. The point of resistance fastenings was every time individually adapted to the examined and was situated on the tibia just above its upper ankle bone joint. By means of girdles with Velcro one stabilized hips and the chest, and the arms of the examined were crossed. The measurement lasted 20 seconds, and the person examined had a task to reach the highest moment of force and to hold it until the command "stop". For the analysis one used the maximum moment of force in the count on the kg masses of the examined body [7, 13].

Electromyography measurement

Measurement of the surface electromyography one took with the utilization of the telemetric system Trigno 16ch (Delsys Inc., U.S.A.) and EMG software Works 4.0 Acquisition. The electrodes were stuck after the preparation of the skin (disinfection, peeling of the dead epidermis) in compliance with the SENIAM guidelines [www.seniam.org] on muscles: vastus lateralis, rectus femoris and medial. The distance between the electrodes amounted 1 cm, and their length 1 cm (divided into two parts) and the width 1 mm [11]. The Electrodes of the Trigno system are made of silver, and the preamplifier is found directly over them (strengthening of the signal 10 times), what eliminates possible interferences of the signal due to external factors. The frequency of assembling data was 4000 Hz per channel. The time of data assembling was 30 seconds, while in middle 20 seconds they were assembled simultaneously with the measurement of the moment of force in isometric conditions. The research was done by means of the official record assembling raw data which then one subjected to the analysis in the electromyogram Works 4.0 Analysis program. One analysed only the segment of data, gathered during the isometric cramp with the exclusion of the rise time and the essential fall of the force (15 seconds). The analysis of the average frequency (ν) was run on the raw data, meanwhile the calculation of the average myotonus (x) demanded the use of RMS (Root Mean Square) filter of window length of 0,125 seconds [11].

Thermography measurement

Temperature scanning were done on a quadriceps thigh muscle (musculus quadriceps femoris) between the place of diffraction in the hip joint and with the head of knee-cap. Extreme areas of marked fields of the measurement became appointed markers so, that every time the analysis referred to the same surface [1]. For the analysis one used the average and the maximum temperature from the field of measurement. The place of the measurement was bare (the lack of clothing) and devoid of hair which could disturb the measurement result.

Thermometries were performed with the camera with the valid certificate of making samples. Additionally before the beginning of the examination one made the geometrical and thermal calibration of the camera and one run tests on the repeatability and the stability of shown results in compliance with the Glamorgan guidelines [3].

The measurement one made with the MobIR M3camera, equipped with a micro biometric detector assuming the shape of detectors matrix (FPA) of the measurements: 160 detectors horizontally by 120 detectors vertically.  The temperature resolution of the camera differentiates the temperature up to 0,12ºC. The distance between the camera and the object photographed one marked as 0,7 m what helped the diminution of attached movement artifacts connected with the examined movement during the respiration. The average temperature of the room was 23,4ºC (±1,1), while the moisture 48,9% (±0,5). Before every thermometry one qualified the moisture with the means of the hygrometer.

Before the beginning of measurement the examined were prepared for the thermometry with 15-minute-long adaptation to conditions prevalent in the room, wherein one made the test. The adaptation took place with the exposure of the place of measurement which was pictured with a thermo vision camera. The purpose of the adaptation was the achievement of the state of thermal equilibrium in prevalent room conditions so that obtained thermograms would be authoritative and potential changes of the thermal image resulted from undertaken exercises. One was also making sure, that the direction of the camera observation would be perpendicular to the examined surface – in order to avoid the false temperature reduction.

Thermograms were worked out with the use of the IR Analyser program. From the images one eliminated errors of distortion disfiguring the geometry as well as one removed taxonomical measuring deviation of the temperature appearing on the edges of the images.

Statistical analysis

The statistical analysis was performed with the use of the Statistica 10.0 software. One used basic statistics and not parametric tests – the test of the order of pairs by Wilcoxon and the Spearman’s indicators of correlation of ranks. As the essential one acknowledged the results of the coefficient p<0,05.

Results

Exercises undertaken by the EB group led to the essential temperature reduction of average (p=0,007) and maximum (p=0,009) of quadriceps (fig. 1). The higher altitude of the jump in spite of the improvement of the class of about 1,53 cm, did not prove to be significant (p=0,139). In this class one did not obtain either the essential improvement within the generated range moment of force (p=0,047). One noted down instead the height of the bioelectric activity of the straight thigh muscle during the isometric cramp (p=0,047), whereat the activity of remaining measured heads of the quadriceps (vastus lateralis and medial vastus) did not surrender to the essential change (respectively p=0,086 and p=0,139). One did not observe either characteristic difference in the frequency of stimulations of the examined muscles (tab. 3).

In the class doing plyometric exercises one did not show essential changes of the average (p=0,343) and maximum (p=0,678) temperature of working quadricepses (fig. 2). The PLYO group noted instead some essential improvement in the height of the jumps (1,52 cm ; p=0,028) and maximum moment of force. The frequency of stimulations of the straight thigh muscle increased and was evaluated in the EMG signal (p=0,033). The frequency in the vastus lateralis muscle rose imperceptibly (p=0,386), and in medial vastus muscle dropped imperceptibly (p=0,263). The average bioelectric activity of the examined muscles did not undergo any change (tab. 3).

The correlation analysis performed within the whole examined class did not show the existence of dependence of no matter which examined parameters from the superficial temperature of the body. Similarly one did not show the influence of the frequency of stimulations on other evaluated values.

The mutual essential dependence was discovered. It was dependence of the bioelectric activity of three measured heads of the quadriceps – of straight, lateral vastus and medial vastus among each other. Those were the correlations on a considerable level (medial vastus muscle and rectus femoris muscle R_s=0,67) or high level (vastus lateralis and rectus femoris R_s=0,82 , lateral and medial vastus muscle R_s=0,75). One showed also average and considerable dependence (p<0.05) between the obtained maximum moment of force and the activity of vastus lateralis muscle (R_s=0,44), medial vastus muscle (R_s=0,64) and straight muscle of a thigh (R_s=0,55) also one noted the fundamentally negative correlation between the height of the jump and the activity of the lateral head (R_s=-0,379) and medial head of the quadriceps (R_s=-0,329).

Tab. 3. Results of EMG measurement and the maximum moment of powers obtained in EB and PLYO classes before and after the warm up

The undertaken research allowed the qualification of the influence of two types of exercises on the physical exertion ability connected with the strength and the power. To the analysis one subjected only the height of the jump as the target indicator within the range of the power, because, as one proved, there is no need for the dependence between the top power reaction of the basis and the height of a jump. In this situation, the key to sporting achievements is not the magnitude of the power itself but the ability of the body lifting [19].

On the height of obtained results, and also the magnitude of the improvement in the following test, apart from well-chosen method of the warming up, other factors can have the influence too, such as, for instance, the time of a day. Therefore, in order to maximize achievements, as the time of the examination one chose afternoon hours in which, as one proved, possibilities within the range the explosive power and the bioelectric potential are higher than in the morning [21].

In the light of the research the temperature seems to be negatively correlated with the time of the stimulation of the muscle potential and positively with the potential and the speed of the conduction. Earlier one proved also the relation between the body temperature and the ability of power producing [10]. The fact that at those examined the body temperature dropped after starting of exercises may be the result of the active character of the warming up during which muscles being all the time on the run, were cooled by circumjacent air. Besides, one ought to note that the correctly run warming up raises the efficiency of thermoregulatory mechanisms, so in spite the effort, we need not observe the temperature rise [26]. The most probable explanation seems to be however the fact that the loss of the warmth by the working muscle took place mostly as the result of transpiration and further steaming of the sweat from the skin surface. The more dynamic form of exercises we use, the higher fall of temperature we can await [9]. The utilization of thermo vision can inform us about the efficiency of the warming up, though paradoxically its effect will be rather the superficial temperature reduction of working muscles [1].

The analysis of results of each exercise classes showed first of all the lack of the improvement of key for physical effort abilities parameters – the maximum moment of force and the height of a jump in the class in which one had the warming up with elastic bands. We consequently did not confirm the Wallace’s and co. [29] nor Aguilar’s and co. [2] findings, who obtained the essential rise of peak power and peak force with the utilization of exercises with the elastic resistance. In this class, the bioelectric activity of the straight thigh muscle increased too. Striking is the fact that the enlargement of the bioelectric activity of the muscle did not translate into the height of its power and force. It seems that the rise of the bioelectric activity of only one of heads of the quadriceps cannot influence fundamentally the rise of the moment of force of this muscle, and for the improvement of the result in the isometric test the rise of bioelectric activity of also lateral and medial muscles is required. Additionally, the positive effect appearing with the rise of the number of stimulated motor units, could be levelled by other effect of the warming up which worked inversely. It seems probable that applied elastic resistance was too small for the obtainment of significant effects.  Because suggested by Baker and co. [4] resistance of the height 47-63% 1-RM for the obtainment of highest MPO values in the perpendicular jump could not be realized. In turn the effect of the small resistance and the large speed of movement will be rather the enlargement of the speed of movement and the not the examined power in the perpendicular jump. In the EB group one did not obtain the change in the frequency of stimulations of the examined muscles too, what on one hand is a positive signal that the warming up was not painful for these groups of muscles, but then again lack of the rise of frequency of stimulations can testify about its inefficacy in the activation of the neuromuscular system to the work [14, 22].

The group in which one used the warming up with the utilization of plyometric exercises obtained indeed better results of the examined functional parameters – of the power and force. One reached then the desirable aim of the warming up. Simultaneously one did not show the enlargement of the bioelectric activity of none of the examined muscles, what suggests that the rise of the power followed as result of other mechanism than the rise of the number of stimulated motor units. It can be due to sex and proportion of muscle fibres at the examined non-training persons or from the fact that EMG signals one assembled during the measurement of the power in isometric conditions and not the dynamic effort which CMJ was [27].  Besides, the improvement within the range of perpendicular jump can be in the greater degree connected with the proper intermuscular co-ordination and not inside-muscular. What is more, speaking about the CMJ improvement, apart from the essential neuromuscular processes, the equally essential part was played by elastic structures of the muscle, eg. tendons which as a result of mechanical stretching gather „the elastic energy”  which freeing in concentric phase allows gaining greater power during the jump [17, 30]. The positive effect of the plyometric warming up is doubtless the increase of the frequency of the EMG signal in the straight muscle of a thigh. The plyometric exercises (down jumps) engage not only the knee joint, but also muscles of the hip joint and ankle bone, so their essential influence on the straight muscle of a thigh, which is biarticular, seems to be well-founded [6].

The correlation between the bioelectric activities of each heads of the quadriceps shown in the examination, finds its own confirmation in other research [15]. The existence of dependence between the bioelectric activity of the quadriceps and the force generated by this muscle does not seem to be well-founded for individual differences in the EMG signal which were described above.  This dependence one did not show either in other research [12] and it can be a coincidence.

Conclusions

The plyometric exercises applied in the warming up allowed the obtainment of desired effect of the improvement of the power and force and the frequency of stimulations while the exercises with the utilization of the elastic resistance did not fulfill their own assignment. Therefore one ought to acknowledge plyometric exercises as more efficient in the preparation for generating of the maximum power and force of lower limbs. The fact that one did not find the dependence between the temperature and exertion parameters can be the result of chosen procedure of research. One confirmed however that a result of the exercises was superficial temperature reduction. This can be the indirect indicator informing about the physical exertion readiness.  The electromyographic evaluation helps with the analysis of change mechanisms of other examined parameters. However, it should be extended in the future with the measurement during CMJ and the analysis of power spectre. Obtained results encourage to further research of optimum-forms of the preparation for the effort. 

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Correspondence to:

Jakub Adamczyk, Ph.D.

Assistant professor

Theory of Sport Department

Józef Piłsudski University of Physical Education in Warsaw

Marymoncka St. 34

00-968 Warsaw, Poland

tel. +48 22 834-41-54

E-mail: jakub.adamczyk@awf.edu.pl