THE INFLUENCE OF THE COMPLEX TRAINING METHOD ON MAXIMAL ISOMETRIC FORCE PRODUCTION OF JUNIOR BASKETBALL PLAYERS INFLUENCIA DEL MÉTODO COMPLEJO DE ENTRENAMIENTO EN LA MANIFESTACIÓN DE FUERZA ISOMÉTRICA MÁXIMA DE LOS BALONCESTISTAS JUVENILES

Correspondence with the author: Aleksandar Kukrić, e-mail: aleksandar.kukric@ffvs.unibl.org ABSTRACT During the period of ten weeks, a study has been conducted on the effects of a complex training method on the maximal isometric muscle force, its peaking time, and the rate of force development in the semi-squat test. The participants of the study were twenty junior basketball players (average age 16.4+/-0.7; mean body height 186.2cm+/-9.2; mean body weight 75,4+/-7.5kg; mean body fat percentage 12,83%+/-1.15). The participants were divided into experimental (n=10) and control group (n=10). The experimental group, besides technical-tactical training sessions, had additional complex training, while the control group had technical-tactical training sessions only. At the final testing, the results of maximal isometric muscle force and the explosive power index, have significantly improved in the experimental group, while the control group did not make significant progress. There were no significant changes in the maximal isometric muscle force’s peak time at the final testing. The study findings indicate that application of the complex training method has positive effects on the development of maximal isometric force, and the rate of force development.


INTRODUCTION
Complex training belongs to the group of reactive training methods, whose main role is to develop maximal muscle force through overcoming large and small loads in high speed movements (Siff, & Verkhoshansky, 1999). Reactive training method is one of the methods with explosive dynamic efforts, alongside complex training method, and the plyometric method. The complex training has been founded by Russian scientists, and its mechanism of operation draws more attention during the 80s. National Strength and Conditioning Association -NSCA, in 1986 organized a visit of 40 coaches from the USA and Canada to the Institute of Sports in Moscow, where they for the first time saw a new training method that rest on the overcoming of large and small loads within a single set (Xenofondos et al., 2010). The method was based on the execution of several exercise sets with large loads and low movement speed, followed by a series of exercises with a relatively small load and high movement speed. The exercises ought to be performed in a biomechanically similar way, and to be anatomically congruent, hence to activate the same muscle groups in both exercises of one complex (i.e. squat and vertical jump). Apart from the complex method, other known methods include contrast and traditional method. Contrast training means alternating large and small loads within one set, while traditional training connotes alternating small and large loads within one set (Duthie, Young, & Aitken, 2002).
At the heart of the complex training method is the physiological mechanism of post-activation potentiation (PAP). Robins (2005) defines PAP as a physiological phenomenon, which due to muscle activation, enables improvement of the following muscle activation. Muscle excitability resulting from acute physiological adaptation will lead to an improvement in the exertion of muscle force in subsequent muscle activation. The force gradient, which represents the rate of increase of force in a unit of time, will increase particularly. Studies have shown that after coping with large external loads, in the next period, for a few seconds, up to several minutes, significant effects of PAP can be induced, especially in activities such as jumping, sprinting, throwing (Robbins, 2005;Jensen, & Ebben, 2003). Two physiological mechanisms are thought to be responsible for the existence of PAP. The first one is based on the phosphorylation of myosin regulatory light chains, which makes actin and myosin more sensitive to calcium, which is released from the sarcoplasmic reticulum during explosive muscle contraction (Robbins, 2005; Weber et al., 2008; Hodgson, Docherty, & Zehr, 2008). The second mechanism is based on the existence of increased synaptic excitation within the spinal cord, leading to increased generation of muscle force (Wilson et al., 2013). In programming a complex training method, consideration should be given to reconciling two variables: the magnitude of the external load and the length of the break between the two exercises in the complex. The study has mainly examined the effect of training levels, types of muscle fibers, gender, varying intensity and volume of preload, as well as types of preload (dynamic or isometric), on different output parameters of the athlete's motor skills (jump, sprint, throw, force and power parameters) ( Some studies have shown that trained athletes are more responsive to PAP than recreational athletes (Gourgoulis et al., 2003;Chiu, et al., 2003). The effects of PAP are more evident in fast muscle fibers compared to slow muscle fibers (Seitz, de Villarreal, & Haff, 2014). Concerning gender, Jensen et al. (1999), concluded that there was no difference in the effects of PAP between men and women. Wilson et al. (2013), in a meta-analysis of PAP effects, concluded that the greatest effect is achieved by applying an external load in the range of 60-84% of 1RM and that the break between the two exercises in the complex is 7-10 minutes. Besides, it was concluded that there were no statistically significant differences between the application of dynamic and isometric preloads. More research recommends a break of 3 to 4 minutes between the two exercises in the complex because then the effects of PAP are most pronounced ( The effects of PAP have generally been tested under conditions of dynamic movements (jump, sprint, throw). The purpose of this study is to investigate the effect of PAP on the isometric muscle force of leg extensions. Since these are young athletes, changes in isometric muscle force are expected at the end of the

Variables
The variables are divided into two groups. Independent variables related to the morphological characteristics of the participants: body weight (BW), body height (BH) and body fat percentage (BFP). The dependent variables represented the maximum isometric muscle force in the semi-squat test (SST), the time of maximum isometric muscle force (MIF) and the rate of force development (RFD). The maximum isometric muscle force is expressed in newton (N), the time to reach the maximum isometric muscle force is expressed in seconds (s), while the RFD is expressed in N/s.

Procedure
The plan and program of the experiment were presented to the participants in detail. At the first gathering, the morphological characteristics of the participants were measured, after which the maximum isometric muscle effort was measured with the semi-squat test. Since the muscle can develop maximum muscle force only at the appropriate joint angle, called the optimum joint angle, testing was performed at a 90° angle at the knee joint. At this angle, due to the longest force arm, the total torque in the knee joint is the greatest. One study found that there was a high correlation (r = 0.77) between an isometric semi-squat at an angle of 90° in the knee joint and one repetitive maximum in a semi-squat exercise performed in the dynamic muscle mode. Performing an isometric semi-squat at a knee joint angle of 90° and 120° will provide a strong effect on one repetitive maximum in the semi-squat exercise (Bazyler, Beckham, & Sato, 2015).
The testing was performed on a semi-squat fitness machine. The testing protocol implied that the participants take the standard position of the body for performing the semi-squat, with a goniometer determining the angle of 90° in the knee joint. This fulfills the biomechanical prerequisites for proper semi-squat. The testing of isometric effort took at least 3 seconds from the beginning of the development of muscle force. The participants were required to perform the contraction as fast as they could. The next day, following the recommendations of Brzycki (1993), participants performed the semi-squat test with one repetitive maximum (1RM). experiment. There are studies that have examined the effects of plyometric training, as a reactive training method, on isometric muscle force. Behrens et al. (2016), tested the effects of plyometric training on isometric, concentric and eccentric muscle contraction. The results showed that regardless of the mode of muscle work, plyometric training has positive effects on the exertion of muscle force. Another study confirmed that plyometric exercises, consisting of maximal vertical jumps and deep jumps from different heights, increase the maximal isometric force of the extensor muscle in the knee joint (Clutch et al., 2013).
On the other hand, it should be noted that there are also research that have not proven the effectiveness of PAP ( & Sale, 2000).
The aim of this study is to investigate the impact of a complex training method on maximal isometric muscle force, its peak time, and rate of force development. Using the special training program, the effects of dynamic training on the force-velocity curve under isometric stress conditions were tested.

METHOD Participants
Twenty junior basketball players, with at least five years of playing experience, participated in the study (average age 16.4+/-0.7; mean body height 186.2cm+/-9.2; mean body weight 75,4+/-7.5kg; mean body fat percentage 12,83%+/-1.15). Before the commencement of the experimental program, participants were divided into experimental and control group. There were 10 participants in each group. Based on the Zvalues, the groups were homogenized, which was one of the important methodological conditions for starting the experiment with parallel groups. The participants are of normal health status, free of injuries and orthopedic limitations that could affect the results of the study and are fully aware of the goals and objectives of the experiment and have voluntarily accepted to participate in the research.
Based on all of the measurements, the participants were divided into two homogeneous groups. The experiment was organized over 10 weeks, in which the participants of the experimental group, in addition to regular basketball training, had twice a week sessions with a complex training model. The training program consisted of 3 to 5 exercises aimed at the development of the lower extremities. The exercise complex consisted of the back semi-squat exercise, with a high external load (80% of 1RM) in 4 sets, with 4 to 6 reps, with a break between sets of 3 minutes. This would be followed by a 2-minute break and after that a set of vertical jump exercises in 4 sets, with 10 reps each, with a 3-minute break between sets. In addition to this set of exercises, other exercises were used that are anatomically and biomechanically very similar to the example above (e.g. semi-squat on one leg and jump with one leg; front squat and long jump...). Young et al. (1998) recommended a very similar training program, with a break between two exercises in the complex for 2 minutes. The control group, except for regular basketball training, had no additional work. After 10 weeks, the final measurement of the variables was done in the same way as the initial measurement.
All measurements were performed under laboratory conditions. Body height was measured with a Seca (Germany) stadiometer, while body weight and body fat percentage were measured by a specialized in stru ment for body composition measurement Tanita bc-418ma (Japan). Using the Globus Ergo Tesys System 1000, Real power (Italy) dynamometer, the maximum isometric muscle force and the time at which it was reached in the semi-squat test were measured with dynamometric method.

Statistical data processing
Using the appropriate operational statistical program (SPSS), the arithmetic mean and standard deviation were calculated for all variables. Post hoc analysis (Tukey's HSD criterion) was used to determine whether the groups differ from each other in the initial measurement. By applying the t-test for dependent samples, the difference of the results at the initial and final measurements for each group was tested. The significance level was set at p = 0.05. Table 1. shows the mean and standard deviations of the morphological characteristics of junior basketball players. The values of body height, body weight and body fat percentage are approximately equal in the experimental and control groups at the initial and final measurements.  Table 2. shows the mean values and standard deviations of the maximum isometric muscle force, its peak time, and the rate of force development in the semi-squat test at the initial and final measurement.  Table 3. shows the results of the analysis of variance at the initial measurement. There were no statistically significant differences observed in the tested variables between the experimental and control groups at the initial measurement. This satisfied one of the basic methodological requirements of research with parallel groups, where is necessary that at the beginning of the experiment, there is no statistically significant difference in the tested variables between the groups.  Table 4. shows the results of the t-test for dependent samples. The significance of differences between the means of the experimental and control group vari-ables at the initial and final measurements was tested. Measurement results indicate that there is no statistically significant difference in the experimental group in the SST and RFD variables, whereas no statistically significant differences were observed in the MIF variable. No significant differences were observed in the control group in any of the tested variables.

DISCUSSION
The values of the measured morphological characteristics indicate that the participants are of the appropriate ratio of body height and body mass, with a relatively small proportion of fat tissue in the body composition (Table 1.). There were no significant changes in the participants' morphological attributes observed on the final measurement. The whole exercise program made no significant effects on body weight and the body fat percentage. Concerning body height, no significant changes were expected during the short period of the experimental program. Considering the impact of a complex training method, based on the performance of explosive and rapid movements, it is quite clear why there was no activation of metabolic processes, which would significantly affect the percentage of fat tissue and body weight.
The measured values of maximal isometric muscle force, its peak time, and the rate of force development at the initial measurement indicate that the participants have approximately similar abilities in producing isometric effort in the semi-squat test ( Table 2). The results of the analysis of variance at the initial measurement indicate the absence of significant differences between the sample groups, which allowed the commencement of the experimental program ( Table 3).
The results of the final measurement of isometric effort in the semi-squat test indicate that, after a tenweek exercise program, certain changes occurred in the production of maximal isometric muscle force (Table 4, Figures 1, 2 and 3). On the final measurement, statistically significant differences were found in the SST and RFD variables in the experimental group. Monitoring the parameter changes in the semi-squat test revealed an increase in the maxi-mum isometric force by 14. 22%, as well as improving the rate of force development by 17.41% over initial measurement. Although no significant changes in the peak time of reaching maximal isometric muscle force occurred, the improvement in the rate of force development was the result of a significant increase in maximal isometric muscle force. No significant changes in the tested variables were registered in the control group.
The absence of significant changes in the control group variables was fully expected since the participants did not undergo the training program for the development of force and strength; therefore no significant changes could be expected on the forcevelocity curve. As no significant changes occurred in the control group, any resulting improvements in the experimental group, with a high level of probability, could be attributed to the implementation of the experimental ten-week exercise program.
Significantly higher values of maximal isometric muscle force in the semi-squat test indicate that dynamic exercises have a positive effect on isometric effort. Although a change in the rate of muscular contraction was also expected, since the exercise program involved the execution of explosive movements in eccentric-concentric contraction mode, this change did not occur this time. Some studies have confirmed that resistance training can affect the speed of muscle force development in isometric conditions. One of the studies concluded that resistance training can increase the frequency of nerve impulse discharges and that this increase may cause a greater rate of muscle force development (Sale, 2003). As one of the possible reasons why there was no change in the time of manifestation of maximal isometric muscle force, the relatively short break between potentiation and explosive exercise can be considered.
As it is known, the effects of performing highspeed explosive exercises are greatest under conditions of complete muscle recovery. It may be possible that the two-minute break was not sufficient for a full recovery, especially for the recovery of the neural component of muscle activity, which largely depends on the speed of exertion of muscle force. Seitz et al. (2014) concluded in their study that the most pronounced effects of PAP were observed in trained athletes after a 6-minute break between exercises in the complex, while in less-trained athletes breaks were as long as 9 minutes. As only significant changes in isometric force were observed, it can be concluded that the effects of complex training are primarily directed to muscle adaptation and that the changes probably occurred as a result of alterations in physiological muscle cross-section.
There are two components that determine the exertion of maximum muscle force. The neural component refers to the activity of the central nervous system (CNS), which in many ways affects muscle activity. From the aspect of CNS influence, a maximal muscular force can be developed by recruiting as many motor units as possible, optimal frequency of nerve impulse discharge in order to create tetanus contraction and simultaneous action of motor units in a short period during maximal voluntary contraction (Sale, 2003). Muscle component refers to the influence of muscle structure and architecture on the maximum muscle force. Maximum muscle force may increase at the expense of an increase in physiological muscle cross-section ( The greatest changes in muscular architecture result from hypertrophy rather than hyperplasia. Häkkukinen, Komi, & Alén (1985) examined the influence of explosive strength training on the isometric force of the leg extensor muscle over 24 weeks. They applied electromyographic measurement and analyzed the force-velocity curve and found a statistically significant improvement in the maximal isometric force of the leg extensor muscles, as well as an improvement in the rate of isometric muscle force generation. As one of the causes of these improvements, the authors cite an increase in muscle physiological cross-section, especially in the conditions of performing plyometric exercises in combination with additional external loading. Similar results were found in a 12-week study where muscle changes were followed by biopsy in the cross-section of m. bicep brachii. An increase was found in the cross-section of fast twitch fibers by 17% and slow twitch fibers by 10% (McCall et al., 1996).

CONCLUSION
In recent decades, the effects of complex training, which is a reliable method in the preparation of athletes, have been increasingly explored and many coaches recommend it as an effective method in training process. The special significance of this research is reflected in the examination of the effects of complex training on the parameters of force and time at isometric muscle effort. The results of the study confirmed the effectiveness of a complex training method to exert maximum isometric muscle force. Statistically significant improvements in results were obtained in the group that trained with the complex method of training compared to the control group. The application of the experimental program did not lead to significant changes in the time of maximum isometric force. It is evident that under the influence of the experimental program only adaptations of muscle tissue to force and strength training occurred, which led to significant improvements in the production of maximal muscle force in conditions of isometric effort.