Plyometrics is a specialized high-intensity training technique that enables an athlete's muscles to deliver as much strength as possible in the shortest period for power development to take place (9,13,50). Based on this, it is apparent why plyometric training is regarded as a useful training tool for athletes who participate in dynamic explosive types of sports (52). Research also seems to indicate that team sports such as soccer, baseball, basketball, and volleyball (8,35,53,55) will benefit from plyometric training. Despite the power requirements of rugby union, no studies to date have made an attempt to determine the possible benefits of a combined rugby conditioning and plyometric training program on the physical and motor performance components of rugby union players. It is also unclear whether plyometric training will benefit rugby players' anthropometry in general.
Rugby players need a higher degree of power in the execution of tackles, in acceleration from a static position and during rucking and mauling when scrumming and forceful play take place (17). Line out jumping, breaking through tackles, and fast and effective changes in running direction (agility) when attacking will also require from players to develop their muscle power output optimally (31).
Despite reports that plyometric training does have a significant positive effect on the physical and motor performance components such as agility, speed, the explosive and anaerobic power output of team sport players, few researchers have investigated the effects of a combined sport-specific and plyometric training program on the different components of team sport players. Dodd and Alvar (14) reported greater increases in the vertical jump performance of baseball players when combining heavy resistance and plyometric training programs, over 12 weeks, than when doing either only heavy resistance training or plyometric training alone. Marques et al. (35) found that combining regular volleyball training with plyometric training led to increased performances in maximal strength and in medicine ball throw and countermovement jump tests. Similar findings were reported by Martel et al. (36) who found that combining plyometric training with a traditional volleyball training program led to significantly bigger increases in vertical jump height compared with volleyball training alone. In contrast to the previously mentioned studies, Ronnestad et al. (53) implemented a 7-week plyometric program in addition to regular soccer and resistance training and found no additional benefits by including plyometric training.
Despite the fact that body size and composition (anthropometry) play an important role in the performances of rugby players and determine playing position (5,16), no studies could be traced in which the anthropometric changes of team sport players because of a plyometric training program have been reported. Despite this observation, studies by Paavolainen et al. (43,44) and Luebbers et al. (30) found that the body mass values of runners, cross-country skiers, and physically active college students, respectively, increased significantly during a 9-week period of plyometric training. A 1.3% change in lean body mass after 9 weeks of plyometric training among competitive male cyclists (2) seems to suggest that the positive change in body mass because of plyometric training may possibly be attributed to an increase in lean body mass. This conclusion was also supported by Witzke and Snow (66) who presented evidence that a plyometric program of 9 weeks among adolescent girls brought about a significant increase in peak bone mass, which may possibly influence an athlete's lean body mass positively. An average increase of 7.3% in type II muscle fiber cross-sectional area, which may also benefit a lean body mass–related component, namely, muscle mass, after 8 weeks of plyometric training, might also explain the increase in lean body mass (49). The results of research by Ronnestad et al. (53), Mikkola et al. (40), Paton and Hopkins (45), and Spurrs et al. (56) on soccer players, endurance athletes, cyclists, and distance runners did not coincide with the results of the last-mentioned studies. They all concluded that a plyometric program between 5 and 8 weeks did not appear to offer any significant benefit in terms of body mass increases. Similarly, Luebbers et al. (30) and Potteiger et al. (49) failed to demonstrate that a plyometric training program between 4 and 8 weeks among physically active males would lead to any significant changes in body fat percentage.
According to literature, both bone mass and muscle fiber cross-sectional area can be improved by making use of load-bearing activities such as jumping (37), running, and climbing (29). A bone and muscle mass–related body composition characteristic, namely, somatotype and especially the mesomorphic component, will probably also be influenced positively because of an increase in bone and muscle mass (7). In this regard, plyometrics will probably benefit these anthropometric variables because of the load-bearing activities it contains. Also, because of the direct relationships between these anthropometric variables and power, which is a requirement for rugby (17,25), certain plyometric-related anthropometric changes would benefit rugby performance in the long run.
Despite the availability of ample literature that demonstrates the positive effect of plyometric training on performance (10,13), it is still not clear whether combined sport-specific conditioning and plyometric training programs should be implemented and whether the benefits could be extended to team sports such as rugby union. Literature also does not seem to give a clear indication of the period over which a plyometric program must be followed in order to gain significant benefits. The plyometric program durations vary between 4 weeks (6) and 12 weeks (19) with one study that showed improvements after just 3 weeks of training (39).
The purpose of the present study therefore was to investigate the effects of a 4-week combined rugby conditioning and plyometric training program on selected physical, motor performance and anthropometric components of university-level rugby players compared to the effects of a rugby conditioning program alone. This study was the first to explore the effects of a combined rugby conditioning and plyometric training program on rugby players' physical, motor performance and anthropometric components. The results from this study may possibly provide coaches and other sport professionals with information and guidelines that will enable them to plan and set up more effective combined sport-specific conditioning programs.
Experimental Approach to the Problem
The specific hypothesis under scrutiny was that a rugby conditioning program, combined with plyometric training, will lead to significantly bigger changes in selected speed, agility, anaerobic power output values, body size, lean body, muscle, fat, and skeletal mass and somatotype among university-level rugby players than a rugby conditioning program alone. Therefore, a pre- to posttest randomized design was used for the study, and subjects were randomly assigned to either a control or experimental group. The experimental group completed 4 weeks of plyometric program in addition to their normal rugby training, whereas the control group continued with their normal rugby conditioning program. Both groups were subjected to the same testing protocol before and after the 4-week conditioning program.
Forty rugby players (18.94 ± 0.40 years) from the first and second U/19 rugby teams of the North-West University (Potchefstroom Campus, South Africa) were randomly selected to participate in the study. Approval for the research was granted by the Ethics Committee of the North-West University (number: 06M02). The competitive rugby playing experience of these players varied between 10 and 12 years with an average of 11.26 years. The study design, purpose, and possible risks were explained to the subjects, and written informed consent was obtained from the subjects before the investigation. Subjects also completed a general information questionnaire regarding their exercising habits, injury incidence, and competing level and were randomly assigned to either a control (n = 20; age = 18.94 ± 0.38 years; competitive experience in rugby union = 11.25 ± 1.00 years) or experimental group (n = 20; age = 18.94 ± 0.42 years; competitive experience in rugby union = 11.26 ± 0.99 years). Positionally, each group consisted of 7 backs (numbered 9–15), whereas 12 forwards (numbered 1–8) made up the experimental group and 9 forwards the control group.
Subjects volunteered to participate in the study and were healthy and free of any injuries during the time of testing and participation in the rugby conditioning training programs. Each subject was instructed to sleep at least 8 hours during the evening and morning before different testing sessions. They also had to abstain from ingesting any drugs or participating in strenuous physical activity that may influence the physical or physiologic responses of the body for at least 48 hours before the scheduled tests. Subjects had to maintain the same diet during the weeks of testing. The subjects arrived at the testing sessions in a rested and fully hydrated state. Only 16 subjects in the control group and 19 subjects in the experimental group executed all the tests, which meant that 5 subjects were excluded from the study. One subject in each of the groups also did not complete the agility T-test but was still included in the study because of the fact that they completed all the other tests.
All subjects were participating in the same rugby conditioning program before, during, and after the testing period. At the time of the study, players were following a preseason program, which was conducted by the same coach and sport scientist to ensure consistency in coaching techniques and programming. The program consisted of field sessions once a day and resistance training sessions 3 times a week that each lasted for more or less 2 hours per training session. The field sessions included skill activities, offensive and defensive drills, and conditioning intervals. Resistance training sessions consisted of more or less 12–16 medium- to high-intensity resistance exercises (70–85% of the 1RM) that were focused on the attainment of muscle hypertrophy and strength. The experimental group also had to participate in plyometric training sessions 3 times a week, for a 4-week period, over and above their normal rugby training. Subjects completed 2 sets of 10 repetitions with a 30-second rest period between sets. These guidelines were followed throughout the 4-week training period. A qualified sport scientist, who was in charge of the U/19 rugby teams' conditioning, supervised all sessions. The plyometric exercises executed are presented in Table 1. All control group subjects were requested to refrain from any plyometric training. Subjects were required to attend at least 92% of all training sessions and not join other type of fitness training programs to be included in the study. The 4-week training period was deemed to be sufficient because of the suggestion made by Luger and Pook (31) that the necessary period for preseason rugby players' training is 4–6 weeks. Furthermore, combined sport conditioning and plyometric training or combined resistance and plyometric training programs of 4 weeks seem to be sufficient in causing significant improvements in speed (over 20, 40, and 60 yd), standing broad jump, T-agility performance and vertical jump height and power, respectively, among groups of trained baseball (14) and volleyball players (39).
The players underwent 4 days of testing: 2 pretest and 2 posttest days, respectively. A week before the official testing week, each player was familiarized with the testing procedures and plyometric training programs. On the first pretest day, subjects completed a questionnaire, together with the informed consent form after which the anthropometric measurements were taken and this was followed by the execution of an intensive, dynamic, rugby-specific warm-up for more or less 15 minutes. Finally, a test battery that consisted of the 3-kg medicine ball put, vertical jump, acceleration, and speed and the Wingate anaerobic tests (WAnT) was performed. After a period of 48 hours, the next testing session followed on the exact same time of day so as to minimize the effects of circadian variations in different test results. Again, a warm-up was performed before the completion of the agility T-test. All experimental group subjects were then subjected to 4 weeks of plyometric training, which was performed in conjunction with their normal rugby training program. The control group only continued with their normal rugby conditioning program for the 4-week period. After the 4-week period, the players were again tested at the exact same time of day (posttest day) and same day of the week as the pretest day to minimize the effect of circadian variations in the test results.
Firstly, each subject was landmarked by one of the certified anthropometrists, after which they were directed stations where the different anthropometric measurements were taken. Body fatness was determined by means of a Harpenden skinfold caliper (Holtain Limited, Crosswell, Crymych, Pembrokeshire, United Kingdom) with a constant pressure of 10 g/mm2, to measure subcutaneous adipose tissue, and was calculated through the sum of the following skinfolds: triceps, subscapular, abdominal, supraspinal, front thigh, and calf skinfolds as per the formulas of Withers et al. (65). All measurements were taken at the right side of the body and recorded to the nearest 0.2 mm. Muscle and skeletal mass was calculated according to the formulas of Lee et al. (28) and Drinkwater and Mazza (15). Body stature was recorded to the nearest 1 cm by means of a stadiometer (Harpenden Portable Stadio-meter; Holtain Limited, United Kingdom), and body mass was recorded to the nearest 0.1 kg with a portable electronic scale (BFW 300 Platform Scale; Adam Equipment Co. Ltd., Milton Keynes, United Kingdom). Ankle, femur, humerus, and wrist breadths were measured by making use of a small sliding caliper (Holtain Bicondylar Calipers; Holtain Limited, United Kingdom) and recorded to the nearest 0.1 cm. Girth measurements were taken with a flexible steel tape (Lufkin W606PM; Cooper Industries, Sparks, MD, USA) and were also recorded to the nearest 0.1 cm. Girth measurements included the relaxed and contracted upper arm, forearm, thigh, and calf girths. All measurements were taken by International Society for the Advancement of Kinanthropometry Level 2–accredited anthropometrists.
Arm, midthigh, and calf girth were corrected for the different skinfolds at these sites, by using the following formula: corrected girth = girth − (π × skinfold thickness). The average technical error of measurement (46) for all the anthropometric measurements was 6.36%. The average test-retest reliability coefficient of the pretest day's anthropometric measurements was calculated to be 0.88 compared with the average value of 0.95 for the posttest day.
Explosive Power Tests
Upper-body explosive power was measured by means of the seated 3 kg Medicine Ball Put Test (3 kg MBPT) according to the method of Ball (1). The seated medicine ball put test is regarded as an objective (r = 0.99) (20), valid (r = 0.77–0.90) (21,26), and reliable test (r = 0.77–0.99) (20) to assess the muscular power of the arms and shoulder girdle (1). Subjects were instructed to sit up straight with the upper back area against a wall and the legs extended straight to the front. Subjects were not allowed to move the upper back from the wall during the put action with a view to eliminate the use of momentum. Subjects were instructed to place the palms of their hands on the sides of the ball in a manner as to prevent cocking of the wrists. When ready, the subjects drew the ball back against the chest and forcefully pushed it forward and upward. The arc of the ball was controlled by a ring that was positioned 2 m in front of the subject at a height that controlled the angle of release to be approximately 45°. Subjects were given 2 practice trials, followed by 3 maximal efforts with a rest period of 30 seconds between each effort. The best distance of the 3 maximal efforts was recorded to the nearest centimeter. The test-retest reliability for the 3 measurements of the pretest day was found to be 0.63 and for the posttest day 0.53.
Lower body explosive power was measured by means of the vertical jump test (VJT) according to the method of Ellis et al. (18). The VJT is regarded as an objective (r = 0.90) and valid test (r = 0.93) to determine the peak anaerobic power output of subjects (54). Subjects were instructed to stand against a wall to which a measuring stick was attached, with the dominant arm's shoulder and the dominant leg's foot against the wall. By keeping the heels on the floor, the subjects were requested to reach upward as high as possible. An arm swing and countermovement was allowed after which the players had to jump as high as possible and touch the measuring stick at the highest possible point. This distance was then recorded as the highest jumping distance. The difference between the reaching and jumping distance was then calculated and recorded to the nearest 1 cm. The subjects performed a minimum of 2 trials with a 30-second rest period between each trial. The better of the 2 trials were recorded for the purpose of data analysis. The test-retest reliability for the 2 measurements of the pretest day was found to be 0.84 and for the posttest day 0.95. Power values were derived from the formula of Foster et al. (20): power (W) = 21.67 × body mass (kg) × vertical displacement (m)0.5.
Acceleration and Speed
The acceleration and running speed of the players were determined by means of a 5-, 10-, and 20-m maximal sprinting effort. The sprint over a specified distance is seen as an objective, reliable, and valid test to determine the acceleration and speed of subjects (22). Ellis et al. (18) reported that players rarely run further than 20 m in a straight line during a game, and this is the reason for a 20 m sprint test. Intermediate beam electronic timing gates (Brower Timing Systems, Draper, UT, USA) were set at 0-, 5-, 10-, and 20-m intervals on a section of the rugby field. The subjects were instructed to start when ready from a standing position with the front foot on the starting line, so as to eliminate the possible influence of reaction time. Subjects were also instructed to wear their rugby boots during testing. The subjects were requested to sprint as fast as possible through the finishing line, making sure not to slow down before the finishing line. Split times (at 5 and 10 m) and final time (20 m) for 3 trials, with a 2-minute rest period between each, were recorded to the nearest 0.01 seconds. The best times for 5, 10, and 20 m were used in the final analysis. The average test-retest reliability coefficient of the pretest day's speed measurements was calculated to be 0.80 compared with the average value of 0.47 for the posttest day.
Players' agility was evaluated by using the ATT according to the method of VanHeest et al. (62). The T-test was also performed on the rugby field, and subjects were again instructed to perform the test in their rugby boots. The subjects were instructed to sprint from a standing starting position to a cone 9 m away, followed by a side shuffle left to a cone 4.5 m away. After touching the cone, the subjects side shuffled to the cone 9 m away and then side shuffled back to the middle cone. The test was concluded by back pedaling to the starting line. The test score was recorded as the best time of 2 trials, to the nearest 0.01 second. A 2-minute rest period was allowed between each trial. Subjects were disqualified if they failed to touch the base of any cone, crossed the one foot in front of the other, or failed to face forward for the entire test. The test-retest reliability for the 2 measurements of the pretest day was found to be 0.89 and for the posttest day 0.80.
Wingate Anaerobic Test
The WAnT was implemented to evaluate the anaerobic power and capacity of the players. The WAnT is considered an objective (r = 0.84–0.88) and valid (r = 0.94–0.98) test to determine the anaerobic power and capacity of subjects (24). The test was conducted as per the method described by Inbar et al. (24). The WAnT consisted of a 30-second period during which the subjects were instructed to pedal maximally on a Monark 834 bicycle ergometer (Monark Exercise AB, Varberg, Sweden), at a resistance of 0.1 g/kg body mass for the duration of the period. The players prepared for the test with a 5-minute standardized submaximal warm-up. The test began with a pedal frequency of about 60 revolutions per minute and a low braking force to facilitate the control of pedal cadence. When the players were able to maintain a constant pedal cadence, a countdown started and the full braking force was applied to signal the start of the test. The feet were stabilized to the pedals with stirrups. The players were instructed to sprint maximally from the start of the test and were requested not to pace themselves through the testing period. The peak power, relative peak power, average power, relative average power, total work, relative total work, and fatigue rate of each subject were then calculated from the test. The average test-retest reliability coefficient of the pretest day's measurements was calculated to be 0.66 compared with the average value of 0.93 for the posttest day.
The Statistical Consultation Services of the North-West University determined the statistical methods and procedures for the analyses of the research data. The Statistical Data Processing Package (57), which is available on the North-West University Web site, was used to process the data. The descriptive statistics (averages, SDs, and minimum and maximum values) of each test variable and anthropometric measurement were first calculated. This was followed by the calculation of technical error of measurement for all the anthropometric measurements according to the method of Pederson and Gore (46). Next, the Cronbach's alpha coefficient of reliability was calculated for each measurement that was taken on the separate test days. Dependent t-tests were done to reveal the significant changes between the pre- and posttest results, and independent t-tests were then done to determine the significance of pre- and posttest changes between the control and experimental groups. In all analyses, the level of significance was set at p ≤ 0.05. Effect sizes (ESs) were calculated for pre- and posttest results in each group and for differences between the experimental and control groups to determine practical significance for all the values, which showed statistical significance. Effect sizes (expressed as Cohen's d value) can be interpreted as follows: an ES of more or less 0.8 is large, an ES of more or less 0.5 is moderate, and an ES of more or less 0.2 is small (59). Last, spaghetti graphs were compiled for each of the test variables and anthropometric measurements that revealed significant changes from pre- to posttesting to identify responders and nonresponders.
Results of the descriptive statistics for the pre- and posttest and group result differences (dependent and independent t-test results) for the experimental and control groups with regard to body fat–related measurements are presented in Table 2. No statistical or practical significance was observed for pre- to posttest changes in any of the body fat–related measurements.
Results of the descriptive statistics with regard to the girth and breadth measurements are presented in Table 3. No statistical or practical significance was observed in any of the girth measurement changes of the test subjects. However, statistically significant increases were seen for femur breadth among the control group subjects and for wrist breadth among the experimental group subjects. The pre- to posttest changes did, however, not obtain high practically significant values.
Figures 1 and 2 display the spaghetti graphs of the pre- and posttest values for femur breadth of the control group and for wrist breadth of the experimental group. From the figures, it is clear that 10 control group subjects responded positively to the rugby conditioning program with regard to femur breath, whereas 13 experimental group subjects showed positive responses with regard to wrist breadth after completion of the combined rugby conditioning and plyometric training program.
Results of the descriptive statistics for the pre- and posttest and group result differences (dependent and independent t-test results) of the experimental and control groups for body stature, body mass, muscle and fat percentage, and somatotype are presented in Table 4. Body stature showed a significant increase (p ≤ 0.05) from pre- to posttesting for both the control and experimental groups, whereas skeletal mass showed a significant increase (p ≤ 0.05) for only the control group. No statistically or practically significant changes were observed in any of the somatotype-related values for the different groups or between groups. Again, none of the last-mentioned measurements showed high practically significant changes.
Figures 3 and 4 display the spaghetti graphs of the pre- and posttest values for body stature of the control and experimental groups, respectively. Ten of the control and 10 of the experimental group subjects showed increases in body stature height from pre- to posttesting. The spaghetti graphs for skeletal mass changes of the control group from pre- to posttesting are displayed in Figure 5. Ten of the control subjects responded positively with regard to their skeletal mass values.
Explosive Power, Speed, and Agility Measurements
As Table 5 indicates, the experimental group experienced statistically significant decreases in speed over 20 m and ATT times during the training period. Cohen's effects size revealed a small (ES ∼ 0.2) and medium practical significance (ES ∼ 0.5) for the named measurements. The independent t-test results of the last-mentioned variables also showed statistically and medium and large practically significant values, respectively, when the control group was compared with the experimental group. The control group obtained a statistically (p ≤ 0.01) and practically significant (ES ≥ 0.8) lower medicine ball put test result from pre- to posttesting.
Figures 6–8 present the spaghetti graphs of the pre- and posttest values for the medicine ball put test of the control group and 20-m speed and ATT times of the experimental group, respectively. Eleven of the control group subjects responded negatively with regard to the rugby conditioning program when their medicine ball put test values were analyzed. Thirteen subjects of the experimental group decreased their speed and 15 subjects their ATT times from pre- to posttesting.
Wingate Anaerobic Test Measurements
Table 6 lists the WAnT results. A significant training effect (p ≤ 0.05) was seen in the experimental group for peak power, average power, relative peak power, relative average power, total work, relative total work, and average power over 5, 10, 15, 20, and 25 seconds. None of the variables, which displayed statistically significant changes, obtained large practically significant values. The change in average power at 20 seconds was significantly better for the experimental group than for the control group. Again, only a medium ES value was obtained when this change was analyzed.
Figures 9–16 present the spaghetti graphs of the pre- and posttest values for the different WAnT results of the experimental group, which displayed significant changes. The number of subjects who responded positively to the combined rugby conditioning and plyometric training program, with regard to the named WAnT variables, is as follows: 15 subjects in relative peak power and average power over 15 seconds, 17 subjects in relative average power and average power over 20 seconds, 16 subjects in relative total work and average power over 10 seconds, and 14 subjects in average power over 5 seconds and average power over 25 seconds.
The study succeeded in showing that a combined rugby conditioning and plyometric training program of 4 weeks led to significantly bigger changes in certain speed, agility, and anaerobic power output values among university-level rugby players than a rugby conditioning program alone. Furthermore, the results of the present study indicated that both types of conditioning programs had a significantly positive effect on the wrist breadth measurements of the players. Finally, the study results revealed that femur breadth and skeletal mass was significantly increased by participation in a 4-week long rugby conditioning program. None of the anthropometric components did, however, display a significantly bigger change because of the combined rugby conditioning and plyometric training program, compared to a rugby conditioning program alone.
No other studies have been conducted to assess the effects of a combined rugby conditioning and plyometric training program on selected physical, motor performance and anthropometric components, which made it difficult to directly compare the results of this study with similar studies. However, several studies have investigated the effects of combined sport-specific and plyometric programs on a wide range of variables. In this regard, a study by Wilkerson et al. (63) showed no significant improvements in ATT times after completion of a 6-week combined plyometric and basketball conditioning program by female basketball players. In the present study, the combined plyometric and rugby conditioning program resulted in a statistically and practically significant decrease in average ATT times. The group that participated in the combined program also displayed significant pre- to posttest ATT time changes compared with the group that only executed the rugby conditioning program alone. The combined program had a similar effect on the 20 m speed times when the pre- to posttest changes of the control and experimental groups were compared. Dissimilarly, a 7-week combined plyometric and soccer conditioning program did not lead to significantly lower 40 m sprint times than those of a soccer conditioning program alone in a group of professional soccer players (53).
Although this study design may not explain the reasons underlying the improvements in agility and speed because of the combined plyometric training program in this study, several authors have purposed the following: plyometric-related programs may promote changes within the neuromuscular system that enhances neuromuscular efficiency. In this regard, research evidence suggests that more motor units are stimulated and activated or the neural firing frequency is enhanced because of plyometric training (38). The activation of more motor units would enable the muscle to generate more power compared with what was previously possible. Furthermore, Swanik et al. (58) concluded that the sensitivity of the muscle spindle system may increase because of a plyometric training program and that this adaptation may lead to enhanced joint proprioception of the participants. Plyometric training also seems to enhance kinesthesia, which, together with an enhanced joint proprioception, may increase functional stability (58). Moreover, Kubo et al. (27) demonstrated that the jump performance gains after plyometric training can be attributed to changes in the mechanical properties of the muscle-tendon complex. Notably, the authors observed that plyometric training significantly increased the maximal Achilles tendon elongation and the amount of stored elastic energy together with an increase in the stretch-shortening cycle jumping performance. It can be postulated that a more compliant muscle-tendon unit would improve stretch-shortening cycle jumping performance by allowing the muscle fibers to operate at a more optimal length over the first part of the shortening phase (34).
Another possible neuromuscular adaptation that plyometric training appears to induce is the reduction in the time required for voluntary muscle activation, which may facilitate faster changes in movement direction and an accompanied decrease in the ATT time (63). This finding was also supported by Hutchinson et al. (23), who presented evidence that a leap training program led to significant improvements (p < 0.002) in floor reaction time among rhythmic gymnasts. According to Hutchinson et al. (23), it is also possible that a cognitive learned effect, rather than a purely motor strengthening effort, is the reason for an increase in the selected physical and motor performance components because of the plyometric training program.
No studies could be found that investigated the effects of a combined sport-specific and plyometric training program on the WAnT results of team players. Most of the WAnT-derived variables displayed significant positive changes because of the combined plyometric and rugby conditioning program, except for fatigue ratio and the average power output at 30 seconds. The finding of Pincivero et al. (47) that those subjects who exert a lower peak power output at the start of the WAnT will develop lower levels of fatigue may possibly serve as an explanation for the lack of significance in the last-mentioned variables. The experimental group displayed significantly higher relative and absolute peak power output values during posttesting than pretesting, which possibly had a detrimental effect on the fatigue ratios and average power output values at the end of the WAnT. Despite the favorable results the dependent t-test delivered with regard to the effects of a 4-week combined program, the independent t-test did not show the same kind of results. The experimental group only achieved significantly better pre- and posttest differences than those of the control group in average power output at 20 seconds. Considering these results, it is possible that the plyometric program had a more pronounced effect on the muscle power endurance than on the peak power output values of the rugby players. The prescribed rest periods of 30 seconds between sets of plyometric exercises may have resulted in an increase in muscle power endurance instead of muscle peak power because of the fact that the rest periods were too short to allow for the resynthesis of high-energy phosphates (11). The anaerobic alactic energy system is usually depleted after 5–10 seconds of high-intensity activities and needs at least 3–5 minutes for the total resynthesis of the relevant energy sources (10,11). The high-energy phosphates are the major contributors to energy for high-intensity plyometric exercises (11). Consequently, the short rest periods will result in insufficient high-energy phosphates for the following plyometric exercises and a reliance on the anaerobic lactic system. Naturally, players will therefore decrease their plyometric exercise intensities and focus more on the completion of the prescribed number of repetitions than on the quality of the exercises.
Somewhat unexpected results of this study were that the explosive power tests (3 kg MBPT and VJT) showed no significant changes because of participation in the combined rugby and plyometric conditioning program. Again, these results may be related to the short rest periods between the different sets of the plyometric program. The 3 kg MBPT result is, however, consistent with those of Lyttle et al. (32) and Mangine et al. (33), who also did not obtain significant increases in upper-body power output values after the completion of an upper-body plyometric program. In this regard, Bieze (4) states that only elite athletes are able to execute upper-body plyometric exercises in such a way that the amortization phase is kept short. Energy that is stored during the eccentric phase of the plyometric exercise will dissipate as heat and will not be used to increase the force of the concentric phase if the amortization phase lasts too long (9,48). What the statement of Bieze (4) therefore suggests is that the young inexperienced university rugby players in this study would not have been able to train the upper body successfully because of their inability to perform the upper-body plyometric exercises correctly.
It is interesting to note that the control group that followed the rugby conditioning program experienced a statistically and practically significant decrease in 3 kg MBPT distance. The results show that the rugby conditioning program alone was detrimental for the upper-body power development of players. As previously mentioned, the rugby conditioning program primarily consisted of field and resistance training sessions. The primary aim of the field sessions was to increase the players' fitness and to improve their rugby-specific skills. Resistance training focused on general conditioning, muscle hypertrophy, and strength. What this indicates is that rugby conditioning programs should include exercises and programs that are specifically aimed at improving explosive power. The fact that the experimental group, which also performed plyometric exercises in their program, maintained their 3 kg MBPT values from pre- to posttesting further accentuates the last-mentioned fact.
The nonsignificant VJT results after completion of a combined sport-specific conditioning and plyometric training program are similar to those of Marques et al. (35), Martel et al. (36), Mihalik et al. (39), Paavolainen et al. (43), Rahimi and Behpur (51), and Timmons (60) but in contrast to the findings of Bauer et al. (3), Chimera et al. (9), Mangine et al. (33), Moore et al. (41), Ronnestad et al. (53), and Turner et al. (61). The nonsignificant results with regard to the VJT pre- to posttest change was unexpected and can possibly be attributed to the following reasons: Outliers among the rugby players who completed the combined program could have “pulled” the t-test results skew because of the rather small sample size in this study. For example, 3 players of the experimental group achieved negative results (−5 cm) when the pre- and posttest VJT heights were compared. These players also achieved negative results with regard to the VJT power output values with values that ranged between −91.47 and −17.40 W. A further analysis revealed that these players lost 2.3 kg in body weight on average during the intervention period, which had a detrimental effect on their calculated power output values.
The significant increase in body stature after completion of the training programs is most likely because of the growth in body stature among the young group of male rugby players. The average age of the players in this study was 18.94 ± 0.40 years, and according to Wilmore et al. (64), some boys do not reach their mature stature until their early '20s. This would suggest that body stature was not influenced by the training programs but rather by the growth factor. Additionally, a statistically significant (p ≤ 0.05) increase in average wrist breadth was detected for the experimental group. Upper-body plyometric exercises such as the single clap push-up, medicine ball grab, and power drops and the upper-body resistance exercises executed in the rugby conditioning program may have facilitated bone growth in the load-bearing site. The control group also experienced a significant increase in femur breadth and skeletal mass despite the fact that they did not participate in the plyometric program. Again, the increase in femur breadth and skeletal mass can probably be attributed to the load-bearing resistance and on-field rugby-specific training exercises that the players performed. These results are similar to those of Nelson and Bouxsein (42) who found a significant increase in breadth measurements of the area that was subjected to a load-bearing activity among females who participated in racquet sports. However, a study of Dean et al. (12) showed that athletes who achieve lower pretraining values will normally experience the largest gains in terms of the variables measured. This may account for the significant changes experienced by the control group who displayed lower pretraining values compared with the experimental group in the majority of the variables.
The above-mentioned results would suggest that only the minority of anthropometric components were significantly affected by either the combined rugby conditioning and plyometric training program or the rugby conditioning program alone that the players followed during the 4-week period. The hypothesis that a 4-week combined rugby conditioning and plyometric training program will have a significantly bigger effect on selected speed, agility, anaerobic power output values, body size, lean body, muscle, fat, and skeletal mass and somatotype of subjects, compared with a rugby conditioning program alone, is therefore only partly accepted. Several researchers have made similar observations for a variety of sport events. For example, a study on cross-country skiers by Mikkola et al. (40) failed to show any training-induced hypertrophic adaptations as determined by means of the calf and thigh circumferences when a part of the 8-week training period was replaced with plyometric training. The same researchers also did not observe any significant changes in body weight or fat percentage because of the change in the training program. Similarly, runners and cyclists did not show significant changes after a 5-week and 4- to 5-week combined running and plyometric and a combined cycling and plyometric program, respectively (45,56). The findings of the present study are, however, not consistent with those of Luebbers et al. (30) and Bastiaans et al. (2) who found an increase in body mass and lean body mass after completion of a 4- and 9-week sport-specific and plyometric training program, respectively.
The nonsignificant results with regard to the anthropometric components of this study can also possibly be attributed to the high individual variability in the different pre- and postmeasurements, which might have influenced the t-test results. For instance, the individual skeletal mass pre- to posttest differences for the experimental group varied between −0.493 (minimum) and 1.656 (maximum) with an SD of 0.587; the values of muscle mass differences varied between −0.406 (minimum) and 0.582 (maximum) with an SD of 0.301 for the control group and between −0.461 (minimum) and 1.490 (maximum) with an SD of 0.766 for the experimental group. The values for fat percentage differences varied between −2.388 (minimum) and 4.227 (maximum) with an SD of 1.818 for the control group and between −4.126 (minimum) and 6.618 (maximum) with an SD of 2.409 for the experimental group. The variability of all these values could have influenced the t-test results negatively.
Another factor that could explain the lack of significance in, especially, the anthropometric results could be the fitness levels of the rugby players who participated in the study. Players in this study had already been subjected to a general rugby conditioning program for 6 months before this intervention. It might therefore be expected that their fitness levels were already high and their anthropometric profile already developed because of participation in the rugby conditioning program. In this regard, Paton and Hopkins (45) failed to prove any significant increases in the performance of competitive cyclists after the inclusion of plyometric training into their existing cycling training program. Notably, the authors attributed the outcome of their research to the fact that the cyclists were already in the competitive cycle of their training period and had already attained a high fitness level. Athletes who have already attained a certain fitness level and anthropometric profile will probably not be so sensitive and reactive to conditioning programs when compared with untrained subjects. The conclusion of the study of Turner et al. (61) , namely, that the significant improvement in running economy due to a 6-week plyometric training period was because of the inexperience and the untrained state of the subjects, as confirmed by the last-mentioned statement.
In view of the fact that several researchers adjusted the number of sets and repetitions of the plyometric program on a weekly basis in their studies (8,32,61), this may also be something to consider. A continuous adjustment in the last-mentioned exercise variables would probably give rise to more muscle overload and more pronounced changes in the different speed, agility, power, and anthropometric measurements.
To conclude, the research in this study seems to suggest that a 4-week combined rugby conditioning and plyometric training program may only lead to neural adaptations and not to morphologic changes. Hence, it is conceivable that a longer training period would have been more beneficial in a study in which changes in the anthropometric makeup of players was the aim. However, there is no agreement between different researchers concerning this aspect (2,30,40,56).
This study was the first to report on the effects of a combined rugby conditioning and plyometric training program on the physical, motor performance and anthropometric components of university-level rugby players. The study results revealed that if the goal of training is to significantly improve the speed, agility, and power of young rugby players, then a 4-week combined program of sport-specific conditioning and plyometric training can be implemented. Although not significant in altering the overall anthropometric profile of the rugby players, the significant results in some of the measurements do indicate that certain anthropometric components might be positively influenced by a combined sport-specific and plyometric training program. However, the results of the present study and those of Marques et al. (35), Martel et al. (36), Mihalik et al. (39), Paavolainen et al. (43,44), Witzke and Snow (66), Rahimi and Behpur (51), and Luebbers et al. (30) indicate that a combined sport-specific conditioning and plyometric training program of 4 weeks may not be as effective in increasing all power-related and anthropometric components as a longer combined training program. Therefore, in conclusion, practitioners can apply plyometric training in sport-specific programs in an attempt to improve rugby performance by increasing speed, agility, and power in rugby players. Future studies on rugby union should probably rather focus on the possible influence of a combined rugby conditioning and plyometric training on the performance outcome of players. Coaches, trainers, and sport scientists of rugby union teams can implement plyometric training in their regular training programs, and we would suggest a minimum training period longer than 4 weeks.
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