EFFECTS OF MAXIMUM ISOMETRIC CONTRACTION ON EXPLOSIVE POWER OF LOWER LIMBS (JUMP PERFORMANCE)
Erol Kovačević1, Armen Klino1, Fuad Babajić2, Asim Bradić1
1Faculty of sport and physical education, Sarajevo University, Bosnia and herzegovina
2Faculty of physical education and sport, Tuzla University, Bosnia and herzegovina
Abstract
The aim of the research was to determine acute effects of maximal isometric contraction on explosive power of lower limbs (jump performance). Nine elite senior tennis players were taken as sample examinees for this research. Tennis players from this group had not had any severe injuries of trunk and lower limbs in the last six months. To assess explosive power, this research used variables for assessment of vertical and horizontal component of explosive power (high jump and long jump). Maximum isometric concentric contraction was used in a semisquat exercise as an operator for stimulation of postactivation potentiation. There is a statistically significant difference with control variable of a vertical jump between a preactivation and a postactivation attempt after time period of 60 and 90 seconds. With other control variable  horizontal jump  there is no statistically significant difference between a preactivation and a postactivation jump.
Key words: postactivation potentiation, contrast methods, tennis players
INTRODUCTION
Scientists constantly search for new methods in sport which would improve training process and enable a sportsman to achieve a higher level of functioning and top results. Primary factor of success in many fields of sport is explosive strength. According to Marković (2008), explosive strength is defined as an ability to produce maximum muscle force in shortest possible period. Explosive strength is one of limiting factors in most monostructural and complex sports. Therefore, we can conclude that it is very important to improve training technology which deals in improvement of explosive strength and exploiting of sportsmen's potential. New training technology should be oriented towards development of explosive strength and it should affect it as fastest and as effective as possible. It is known that explosive strength has a high coefficient of genetic determination, which underlines a need for a method or a set of methods which could contribute to application of existing potential and improve results in realization of speed – explosive activities. A contrast method and contrast training were established by Russian and Bulgarian coaches in the 60s in the previous century. In one training, small and large load are changing and large load precede small load (Ebben, 1998). Chu (1996) claims that strength increased through complex training load is up to three times more effective than application of traditional training methods. One of ways which could possibly contribute to improvement of training process is postactivation potentiation (PAP), which could be more integrated into the training process itself.
METHODS
Participants
In this research participants were nine elite senior tennis players. All of examines were timely informed about the nature of this research. Examinees who participated in the research were completely healthy and they had not had any serious injuries of trunk and lower limbs in the last six months. All of the examinees had previous experience in strength training, plyometric jump training and testing of vertical and horizontal component of explosive power. Chronological age of examinees was 22,5 ± 2,4 years on average, while average number of years of experience in sport was 13,1 ± 2,8 years, at the level of the group. Morphologic variables show that average body weight at the level of the group was 80,8 ± 6,6 kilos, while average body height for the complete group was 181,6 ± 5,2 centimeters.
Instruments
Morphologic variables:
 Body height  BH
 Body weight  BW
 Body mass index  BMI
 Age  AGE
 Tennis experience  TE
Note: selected variables from the field of morphologic status are taken to define certain morphologic characteristics of selected entities in a better way, as well as factors related to chronological age and (tennis experience) time spent in this sport activity (tennis) and give a precise and clear picture of characteristics of a selected sample.
Control variables:
Control variable  Squat jump
This is a vertical jump of concentric type and a test for an assessment of a vertical component of explosive power of lower limbs. We obtain jump height on a basis of a difference between reached height and height achieved after the squat jump.
VJI  vertical jump initially (conducted before maximal isometric contraction)
VJF1  first vertical jump final (conducted 30seconds after maximal isometric contraction)
VJF2  second vertical jump final (conducted 60 seconds after maximal isometric contraction)
VJF3  third vertical jump final (conducted 90 seconds after maximal isometric contraction)
Control variable – Standing long jump
A test for an assessment of a horizontal component of explosive power of lower limbs. One measures a length of jump from standing position to landing position on a mattress marked with centimeters.
SBJI  first standing board jump initial (conducted before maximal isometric contraction)
SBJF1  first standing board jump final (conducted 30seconds after maximal isometric contraction)
SBJF2  second standing board jump final (conducted 60 seconds after maximal isometric contraction)
SBJF3  third standing board jump final (conducted 90 seconds after maximal isometric contraction)
Testing procedure
A complete testing procedure happened in the period of three days (Table 1). In those three days the examinees realized testing according to a previously created protocol. Testing lasted for about 60 minutes daily. To examine influence one preformed maximum isometric contraction with a maximum semisquat as an activator exercise, which, in authors' opinion should cause effects of postactivation potentiation.
Table 1 Testing procedure and protocol
1st day 
Measuring anthropometric characteristics 
Aerobic running 
Dynamic stretching 
Testing vertical and horizontal component 
Extensity 
5 minutes 
10 minutes 
5 minutes 
5 minutes 
2nd day 
Aerobic running 
Dynamic stretching 
Maximum isometric preactivation 
Testing vertical 
Extensity 
10 minutes 
5 minutes 
6 seconds 
90 seconds 
3rd day 
Aerobic running 
Dynamic stretching 
Maximum isometric preactivation 
Testing horizontal 
Extensity 
10 minutes 
5 minutes 
6 seconds 
90 seconds 
Activation (maximum isometric contraction) in semisquat
An examinee stands on a specially designed platform with adjustable chain length, taking a position of a semisquat where lower and upper leg are positioned in the angle of 90°. This position, thank to adjustability of the length of the chain, is set individually for each of examinees. The examinee performs maximum static contraction against vertically set metal bar placed in the upper part of the back also performing maximum static contraction against immovable load, as illustrated in Picture 1. Maximum isometric contraction was performed for six seconds.
Picture 1 – Maximum isometric preactivation in semisquat
Data were processed by the statistical software package SPSS 12.0 for Windows. Central and dispersive parameters were calculated for the both measurements. Univariate changes were tested by a series of ttests for dependent samples.
RESULTS
Table 2 represents descriptive parameters (range, minimum, maximum, mean, standard deviation) of all tested variables, vertical and horizontal jumps before and after application of maximal isometric contraction. No significant variability can be observed with tested jumps.
Variables 
N 
Range 
Min 
Max 
Mean 
Std. Dev 
VJI 
9 
16,7 
36,0 
52,7 
48,578 
5,0413 
VJF1 
9 
14,0 
40,5 
54,5 
49,233 
4,8192 
VJF2 
9 
14,5 
41,5 
56,0 
50,756 
4,9838 
VJF3 
9 
17,3 
40,0 
57,3 
51,400 
5,0995 
SBJI 
9 
228,0 
206,0 
434,0 
278,311 
86,5841 
SBJF1 
9 
217,1 
215,9 
433,0 
277,789 
85,6236 
SBJF2 
9 
217,5 
214,5 
432,0 
278,978 
84,6340 
SBJF3 
9 
218,2 
216,8 
435,0 
282,500 
83,6193 
Table 3 shows results after a t–test for small dependent samples for a variable of vertical jump (semisquat jump) which clearly indicate related t values, static significance of differences among arithmetic mean of formed pairs of variables from initial (preactivation) measuring, and results of final (postactivation) jumps with a time interval of performance of 30, 60 and 90 seconds after application of maximum isometric contraction in a semisquat. On a basis of obtained data, it is possible to claim that expected differences at a statistically significant level took place on pairs of variables under number 2 and 3, which is visible from values of coefficients of statistical significance (pair2 0,04 and pair3 0,00).
Table 3 Results of ttest for variables of explosive power of vertical type
PAIRS 
VARIABLES 
Paired Differences 
t 
df 
Sig. 

95% Confidence Interval of the Difference 

Lower 
Upper 

Pair 1 
VJI VJF1 
2,7715 
1,4604 
,714 
8 
,495 
Pair 2 
VJI VJF2 
4,3069 
,0487 
2,359 
8 
,046 
Pair 3 
VJI VJF3 
4,4894 
1,1550 
3,904 
8 
,005 
Table 4 shows results of ttest for small depending samples of variables of a long jump which show related values, statistical significance of differences of arithmetic mean of the initial (preactivation) jump and final (postactivation) jumps with time intervals of 30,60 and 90 seconds after maximum isometric preactivation. An insight in the table enables us to conclude that there are not any statistically significant differences in any of the pairs of selected variables between jumps of preactivation and postactivation phase, in regard to values of coefficients of statistical significance of differences at the selected level of p=0,05. Simply saying, it is possible to be 95% sure and claim that there were not any statistical differences between selected jumps in the preactivation and postactivation phase.
Table 4 – Results of ttest for variables of explosive power of horizontal type
PAIRS 
VARIABLES 
Paired Differences 
t 
Df 
Sig. 

95% Confidence Interval of the Difference 

Lower 
Upper 

Pair 1 
SBJI SBJ1 
6,9871 
8,0316 
,160 
8 
,877 

Pair 2 
SBJI SBJ2 
4,4225 
3,0892 
,409 
8 
,693 

Pair 3 
SBJI SBJ3 
10,2428 
1,8650 
1,596 
8 
,149 
On a basis of obtained results one can conclude that maximum isometric contraction in the semisquat exercise as an activator exercise caused a positive growth of effects visible through achievement of better results in postactivation jump.
Graph 1 –Differences in pre and post activation for variables of standing high jump in percents
The graph shows differences in mean value of achieved height of jumps with and without maximum isometric preactivation shown in percentage. Analyzing data one can conclude that there is a significant difference of 2,17cm or 5% on the second, and 7% or 2,8 cm on the third jump. This confirms that there is the effect of postactivation potentiation.
Similar results are found in a research by Verhosanski from 1974, who also used a maximum isometric semisquat of 6 seconds as an activation exercise. There were statistically significant differences between preactivation and postactivation jumps. Namely, this study researched the difference between maximum vertical preactivation jump and the same postactivation jump after a break of 1 or 5 minutes. Obtained results show that postactivation jumps differ from preactivation ones for 3 % after a break of 1 minute and 5% after a 5 minute break.
In a study that examines effects of postactivation potentiation on maximum willing isometric contraction on a basis of electromiographic measuring Baudry and Duchateau (2007) confirm existence of positive postactivation effects and increased electro muscle activity after maximum preactivation. These observations suggest that PAP can be concerned as a mechanism that can influence our daily activities during contraction and can be used for improvement of muscle performance in explosive sports.
Various authors (Chui 2003, Duthie 2002, Hamada 2000) achieved various results in regard to existence and effects and size of effects of postactivation potentiation. Gregov et al. (2006) conclude that appearance of PAP is not questionable, but training conditions in which it arises are not fully known. Similar effects of postactivation potentiation but with different activation exercise that reflects in maximum dynamic regime of muscle contractions were achieved by: Young et al. (1998), Webber at al. (2008) and Babajić (2010).
Babajić's research (2010) which used maximum parallel back squat as an activating exercise showed similar result. It is noted that there were statistically significant differences between formed pairs of variables and jumps performed in preactivation phase and postactivation phase of squat jump variable after the third and the fourth jump in the time period after 60 seconds and 90 seconds. A reply to absence of a statistically significant difference on pair 1 between variables (VJI/VJF1) can be searched in fatigue, as one of acute effects which existence was defined in numerous similar researches and which result from previous maximum load and maximum isometric muscle contraction in the activation exercise.
Weber et al. (1998) also noted a positive effect of postactivation jump after heavy submaximal parallel squat in regard to preactivation jump. In this research there was a shortterm improvement of vertical jump so that one can conclude that heavy load can cause acute improvement of jump performance.
In a research of Young et al. (1998) they examined effects of various protocols of warming up, and they also underlined improvement of vertical jump for 2,8% after submaximum load in regard to preactivation jump. The conclusion is that warming up protocol which includes semisquat with submaximum load and explosive performance can be used for a shortterm improvement of vertical jump performance.
Results from the second control variable which concerned performance of long distance jump show that there were not any statically significant differences between jumps in preactivation and postactivation phase but in all of the three postactivation jumps there was a better average result in postactivation phase in relation to the equivalent pair from preactivation phase. An analysis of the results presented in table 2 shows that there is not any statistically significant difference in any of formed variable pairs. This is supported by values of coefficients related to statistically significant difference (pair1 0,87 / pair2 0,69 / pair3 0,14).
Graph 2 Quantitative differences in preactivation and postactivation for variable long jump
Obtained results underline the fact that an operator or maximum isometric preactivation in a semisquat did not cause any statistically significant differences, according to author's presumptions. Graph 2 clearly shows differences in arithmetic mean expressed in centimeters and in percentage. Structure of performance of maximum isometric preactivation which is realized in semisquat differs in regard to the control variable of long jump. In this case, an examinee performs maximum isometric towards vertical direction which is different from the control variable which is realized in horizontal direction. This is one of possible explanations why there were not any statistically significant differences with control variable long jump. We find similar results in other researches where a pre activation exercise is structurally different from a control variable. In a research where a parallel back squat was a pre activation exercise there was not any significant effect on realization of sprint, but there were statistically significant differences in realization of a high jump. The author quotes that a parallel back squat with more or less factors is more similar to the structure of moving that appears during performance (of a semisquat jump) as the second control variable where postactivation potentiation effect actually appeared (Babajić, 2010). If exercising on training is more concurrent with tasks of moving that should be realized within a frame of specific moving activity it is more likely that a positive transfer would happen (Sale, 1991). However, these are only presumptions and logical thinking resulting from obtained results which leave space for future researches.
CONCLUSION
Results obtained from this research indisputably confirm existing of post activation potentiation as a phenomenon that appears when an exercise of speedexplosive character is preceded by maximum load which is identical to performance of the main exercise by structure. However, this research confirms that effects of PAP differ in regard to structure of preactivation exercise and control variable. Also, necessary time between preactivation and appearance of PAP is still questionable. However, it is evident that effects appear in the interval from 60 to 90 seconds after preactivation. According to the obtained results of the research we can conclude that there is a statistically significant difference with individual control variables. With control variable vertical semisquat jump (vertical component of explosive power) there is a statistically significant difference in results after influence of an operator and maximum isometric preactivation.
Examining effects of PAP in order to emphasise performance of strength and speed, one has to resolve two dilemas. First, an intensive and extended conditioning activity can activate PAP mechanism more, but it also produces more fatigue. The second dilema is that when recuperation between a conditioning activity and commence of performance is longer, then decaying of PAP mechanisam is huger. These two dilemas can be resolved only through attempts and mistakes. Potentiation, fatugue recuperation and super compenzation are mechanisms that have to be taken into consideration in an analysis of body reaction to load. It is noticeable in this research that with a control variable vertical jump 30 seconds after preactivation there is no statistically significant difference, which most probably is a consequence of acute fatigue.
REFERENCES
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 Baudry, S., Duchateau, J. (2007). Effect on the rate of torque development of tetanic and voluntary isometric contractions. Journal of Applied Physiology, 102: 1394–1401.
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 Ebben, W.P., & Watts, P.B. (1998). A review of combined weight training and plyometric training modes: Complex training. Journal of Strength and Conditioning Research, 20: 1827.
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EFEKTI MAKSIMALNE IZOMETRIČKE KONTRAKCIJE DONJIH EKSTREMITETA NA EKSPLOZIVNU SNAGU TIPA SKOKA
Sažetak
Cilj ovog istraživanja predstavlja utvrđivanje veličine razlika neposrednih efekata nakon aplikacije maksimalne izometrijske kontrakcije paralelnog na manifestaciju eksplozivne snage tipa skočnosti. Devet elitnih tenisera seniorske kategorije činili su uzorak ispitanika u ovom istraživanju. Teniseri koji pripadaju ovoj grupi u prethodnom periodu od šest mjeseci nisu imali ozbiljnije povrede trupa i donjih ekstremiteta. Za procjenu vertikalne i horizontalne komponente eksplozivne snage korišteni su skok u vis iz polučučnja i skok u dalj iz mjesta. Kao operator za stimulisanje postaktivacijske potencijacije korištena je maksimalna izometrička koncentrična kontrakcija u vježbi polučučnja. Postoji statistički značajna razlika kod kontrolne varijable vertikalnog skoka iz polučučnja između predaktivacijskog i postaktivacijskog pokušaja i to nakon vermenske distance od 60 i 90 sekundi. Kod druge kontrolne varijable, odnosno, horizontalnog skoka ne postoji statistički značajna razlika između predaktivacijskog i postaktivacijskog skoka.
Ključne riječi: postaktivacijska potencijacija, kontrastna metoda, teniseri
Correspondence to:
Fuad Babajić, MSc
Faculty of physical education and sport,
Tuzla University
2. oktobra 1,
75000 Tuzla,
Phone: 00387 35 278 536
Email: fuad.babajic@untz.ba