Institution of Health Sciences
IV005A
Sports Science MA, Master Degree Project 30 credits,HT18-VT19.
Effects of combined isometric followed by plyometric exercise on vertical jump performance of soccer
players.
Roussos Angelos (anro1801) Thesis Supervisor: Helen Hanstock Examiner: Erica Schagatay
Date: 04/06/2019 Words: 3225 words
Östersund
Mid Sweden University
Masters Programme in Sports Science – Performance Optimisation with a Focus on Elite Sport
ABSTRACT
Background:
Currently, different combined protocols with lower bodyreinforcement followed by plyometric exercises provide positive results in different explosive variables such as jump capability, velocity, agility, and so on. Nevertheless, the effect of a combined protocol comprised of an isometric back squat followed by drop jump has never been examined.
Objective:
The aim of this study is to investigate and compare the effects of an isometric back squat followed by drop jump on drop jump height and reactive strength index between male soccer players.Methods:
Overall, the study included seventeen participants, nine in theexperimental group and ten in the control group. Data was collected with the use of a Kistler force plate. Two groups were used to compare the participants before and after the six weeks protocol to reveal if there was any positive difference. The combination protocol was focused on whether isometric contraction followed by plyometric exercise could provoke significant improvements in drop jump height and RSI.
Control group was performed 10 mobility exercises during the experimental group protocol.
Results:
Both groups improved drop jump height (p<0.05) and there was a trend towards the improvement in the experimental group being greater than in the control group (p=0.061). Reaction on the ground was reduced for both groups (p<0.001) but with no difference between the two groups (p=0.721). Reactive strength index was significantly improved for the experimental group (p<0.001) with a trend towards a significant interaction between groups (p=0.069).Conclusions:
Isometric back squat followed by drop jump produce significant improvements in the above variables. Future research with better organization and different parameters could be interesting in order to investigate whether different kinds of combination protocols (strength and jumping) could provide a positive interaction with respect to vertical jump ability and ground contact time.Key words:
Back squat, ground contact time, drop jump, performance optimisation, quantitative protocol, reactive strength indexTable of Contents
ABSTRACT………
1. Introduction/Background ………...…………1
2. Methods ………...……….4
2.1 Procedures……...………4
2.2 Participants…...4
2.3Familiarisation……….………...……….5
2.4 Training protocol………..………...5
2.5 Pre/Post measurements ……….………7
2.6 Statistic analysis………...……...7
3. Results………...….…8
4. Discussion ……….……10
4.1 Limitations of the study………..…………..………11
4.2 Implications for future research……….………..12
4.3 Conclusion………..13
4.4 Acknowledgements………..13
References ……….…………14
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1. Introduction / Background
Soccer is one of the most popular sports in the world. This is the main reason for having a lot of different scientific approaches from different perspectives regarding soccer and performance (11, 14, 18). Physical fitness is receiving global attention and is closely associated with sports performance. The higher an athlete’s level of condition, the better their capability to achieve top level performance (28).
Plyometric exercise is a way of training to enhance the performance in sports requiring skills such as velocity, strength, and jump ability. Plyometric training increases vertical jump ability, which allows faster ground reaction time and can improve performance in explosive drills (7, 15, 17). Moreover, plyometric training as an individual method of training has been shown to increase vertical jump height, maximum body power, reactive strength index and change of direction capability while it was found to decrease 10, 20 and 40 meter sprint time and agility test time.
(27, 35). Following a two-week plyometric protocol, Maćkała&Fostiak, (23) reported a significant increase in countermovement jump, squat jump, standing long and triple jump. After the short-term plyometric regime, a decrease was seen in contact time and an increase in stride frequency (23)
A combination of plyometric jumps with lower body strength exercise can develop explosive strength, evidenced by improvements in lower body strength, vertical jump height, reactive strength index, change of direction, 10, 20, 40 meters time and agility test (1,10,12,20). Moreover, combination training protocols have produced greater performance improvements in vertical jump ability (countermovement jump, hurdle jumps) and sprint ability (50m dash and shuttle run test) compared with a simple plyometric jump regime (19, 32, 33). For example, a lower body reinforcement back squat followed by plyometric jumps and sprints increased the performance in countermovement jump, drop jump, one repetition maximum and drop jump reactivity index (13).
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Rather than training effects, there are also acute effects which demonstrate positive results regarding performance such as lower body reinforcement, vertical jump height, and speed (16, 30, 34). Postactivation potentiation is the acute effect method of increasing muscular performance especially in activities involving speed and power (16, 30, 34). After a postactivation potentiation protocol (back squat followed by countermovement jump) there was a significant improvement in jumping performance in youth soccer players (39). Similarly, a 5.51 cm increment in vertical jump performance was accomplished after a postactivation potentiation protocol with a heavy back squat at 87% of 1 repetition maximum followed by a vertical jump in hockey players (5)
A combined intervention with weight training and plyometric jumps was performed in intercollegiate male and female athletes (soccer, basketball and volleyball). After a 6 weeks training regime there was no improvement in vertical jump performance (8). Similarly, after 6 weeks plyometric jump program there was a very small increase in the vertical jump and a slight decrease in 5, 10 and 20 meters sprint time respectively (19). However, some postactivation potentiation interventions have not demonstrated any effects on performance (jump height, jump power and sprint) after back squat and isometric maximal voluntary contraction (30, 36, 38).
When it comes to isometric contraction there are few findings in training regimes. Comparing isometric, concentric and eccentric contraction, isometric contraction achieved higher percentage activation (95%) compared to 90% and 88%
for concentric and eccentric contractions respectively (3). Isometric squat produced positive effects on squat jump height (21). Postactivation potentiation protocols with isometric back squat + countermovement jump after a 140° angle was more efficient compared with 90° angle (40). Similarly, outcomes between resistance trained and untrained subjects have shown positive effects only for the trained group (6).
Many studies show relationship between the vertical jump and back squat with specified soccer performance variables such as speed and jump ability. A report indicates that there is a correlation between 5 meters sprint performance with countermovement jump in a smith machine (24). Moreover, there was a positive relationship between drop jump and 20 meters split time (32). Similarly, several studies revealed that there is a relationship between vertical jump height and back
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squat (4, 22, 26) with sprint time test and agility time test performed by elite soccer players.
To perform isometric back squats at a specific angle of 140° in complex training can also improve subsequent countermovement jump height (40). Depth jump training has also been shown to improve countermovement, squat and drop jump performance (15). Nonetheless, a combination training using isometric back squats instead of using eccentric or concentric back squat with a slow descent rhythm followed by drop jumps has never been examined. Important issue was the higher muscle fiber activation after and isometric contraction. This could possibly be used as evidence if there would be an improvement in the specific protocol. Therefore, the purpose of this investigation is to investigate the efficacy of a 6-week combination training programme consisting of a back isometric squat followed by drop jumps to improve vertical jump and reactive strength index. It would be interesting for extra knowledge and interpretation to examine if the combination of a back isometric squat followed by drop jumps will improve adaptation to plyometric training and induce positive results in post intervention vertical jump performance.
The aim of this study is to compare the effects of a combination regime on drop jump height (DJ) and reactive strength index (RSI) between male soccer players. RSI is evaluated by dividing jump height by the ground contact time (30, 31). This paper focused on answering two main research questions:
1. Does a combination regime of back squat followed by drop jump improve vertical jump performance?
2. Does a combination regime of back squat followed by drop jump improve RSI?
The main hypothesis is that there will be an improvement in the experimental group but not the in the control group in drop jump height and RSI.
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2. Methods 2.1 Procedures
The following training protocol was utilized for 6 weeks, twice-weekly, interspersed by 48 hours. The standard duration of the protocol was approximately 15 to 20 minutes with a 10 minute warm up including running and stretching with 5 minutes stretching recovery after the main regime.
2.2 Participants
Nineteen male soccer players from the same club were originally recruited to participate in this study. They were assigned to the training group or an active control group. The players were engaged in standardized characteristic measurements (e.g.
age, height, body weight) before the 6 weeks protocol commencement (training group, mean±Standard deviation, n=9; age=21.2±2.7 years; Body weight=77.4±3.4 kg; height=178.7±4.7cm), (control group, mean±Standard deviation, n=10;
age=24.3±2.9 years; Body weight=85.9±6.9 kg; height= 182.6±4.2 cm). Prior to the study, the participants were informed about the possible risks and benefits of the study and that their participation in the study was voluntary. Before the investigation, the athletes were required to sign an informed and medical history consent form.
According to inclusion criteria, it was a requirement for athletes to be healthy without injuries at least 3 months, having a 3 year experience in soccer and at least 6 months experience in weight and plyometric training. Athletes having an injury, high blood pressure background or did not fulfill the 3 out of 6 weeks protocol were excluded from the research. Therefore, the final sample included 17 participants (training group, mean±Standard deviation, n=8; age=20.7±2.3 years; Body weight=77.1±3.3 kg; height=179±4.5 cm), (control group, mean±Standard deviation, n=9; age=24±2.7 years; Body weight=86±6.9 kg; height=183±4.2 cm).
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2.3 Familiarisation
Before the commencement of the intervention, participants were familiarised with the experimental procedures. Only the experimental group completed 2 familiarisation sessions interspersed by 48 hours after the first pre measurements.
Before each familiarisation, training, or testing session, participants performed a general warm up consisting of 5 minutes running or cycling with low intensity with 5 minutes light active stretching exercises. Additionally, a specific warm up routine consisting of exercises that mimicked the testing / training content (isometric back squats without kilos) was performed after a 2 minutes rest interval.
2.4 Training protocol
The semi back squat isometric exercise was considered to be used with ~90%
of 1 maximum repetition load for each athlete individually. The precise execution contained a controlled descend (eccentric phase) until ~145° angle squat (semi back squat), with three seconds isometric contraction at this angle. Finally, an ascending movement back to the starting position was achieved within a total of three consecutive attempts. After the back squat, three drop jumps with 8 seconds rest in between each jump was accomplished from a 40 cm drop box. Three sets were performed with three minutes rest interval between sets. It also important to highlight that half of the players lost 5 out of 12 in total training with result lower reliability with respect to the outcomes.
Both groups do pre tests ( 3maximum drop jumps in the
laboratory)
Experimental group ( 12 sessions over 6 weeks) - Combination
Protocol
Experimental group do post test ( 3 maximum drop jumps in the
laboratory)
Control group (12 sessions over 6 weeks)
- Mobility Exercises
Control group do post test ( 3 maximum drop
jumps in the laboratory)
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Table 1. Six weeks combination protocol for the experimental group.
Week (biweekly)
Body part
Exercises Intensity Reps(total) Sets(total) Rest between
sets
1-6 Lower
body
Isometric back squats-drop
jumps
High 18 6 2-3 min
For the control group, the mobility session was conducted by different exercises with a purpose to improve the mobility of the football players. The participants from the control group performed ten mobility exercises. For each exercise, the duration was three sets of six repetitions or seconds (depending on the nature of the exercise) with one minute rest between sets.
Diagram 1. Control group training through the six weeks intervention.
1)Rotate upper body (sit on the exercise mat with
straight legs) 2) Lunge position-hips forward-full range rotation
of both arms
3) Supine position-raise each leg straight up- and down 4) Deep squat position-rotate
upper body
5) Prone position- raise straight each leg
6) Swift in lunge position with straight posture 7) Push up position- bouncing
each leg forward alternately 8) Downward facing dog 9) Chair pose-on toes-and
down 10) Pigeon pose
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Week: 1-6, Intensity: Low, Reps (total): 180, Sets (total): 30, Rest between sets: 1 minute 1 2 3 4 5 6
7 8 9 10
2.5 Pre/Post measurements
The testing session was accomplished in 1 week before and after the training protocol and consisted of a drop jump (drop box 40 cm height). The drop jump test was included by 3 maximum effort trials with 45 seconds rest interval between each.
The best trial was used for analysis to estimate the drop jump height and RSI. The same test was applied in the pretest (1 week before) and posttest (1 week after) of the 6 weeks training protocol. Kistler force plates were used to measure these variables in the Gothenburg University laboratory. All procedures were supervised by the same experienced researcher.
2.6 Statistic analysis
The statistical analysis was performed using the SPSS V24. Two Way Mixed ANOVA was used to compare the drop jump height and RSI between the experimental and the control group for the pre and post test.
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3. Results
There was an improvement in drop jump height for the experimental group (from 38cm to 43 cm, +13.1%, p<0.05) after the six weeks combined protocol with an improvement of +2.8% for the control group (p=0.326). There was a trend towards a significant interaction between groups (F (1, 15) =4.1, p=0.061; d=1.36; Figure 1).
On the other hand, both the experimental and control group were reduced the time of reaction on the ground. Although the reaction on the ground (contact time) does not differ between the two groups with a reduction of 16.7% and 15.4% for the experimental and control group respectively (F (1, 15) =0.13, p=0.721; d=0.45; Figure 2).
For the third variable, there was an improvement in RSI for the experimental group (from 1.6 ms-1 to 2.2 ms-1, +35.6%, p<0.001) after the six weeks protocol with an improvement of +20.7% for the control group (p<0.001). Although the RSI does not differ between the two groups significantly (F (1, 15) =3.8, p=0.069; d=1.12;
Figure 3).
Calculations of Cohen’s d (small=0.2, medium=0.5, large=0.8) indicates the difference between the two groups for jump height, reaction on the ground and RSI.
Figure 1: Drop jump height (Exp + Control Group)
*Exp=Experimental group, Con=Control group
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Figure 2: Reaction on the ground (Exp + Control Group)
Figure 3: RSI (Exp + Control Group)
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4. Discussion
To our knowledge, this is the first study to examine the effects of a combined protocol with an isometric back squat followed by drop jump in football players. To better understand the challenges faced, this research employed a quantitative approach. It was interesting to distinguish positive differences between pre and post test for the training group. Drop jump height was improved compared with the control group. For the RSI, there was an improvement for both groups. A better improvement was distinguished though for the training group. When it comes to reaction on the ground, there was reduction on contact time equal for both groups. Being an innovative study, it was noteworthy that the combination of isometric contraction with plyometric exercise induced positive results on experimental group.
Accordingly, the questions that were aimed to be answered were to provide evidence if the combination of a back squat followed by drop jump would cause any positive changes on drop jump height and RSI. Therefore, initial research questions of the study regarding jump height and RSI are partially confirmed. There was a significant improvement in both variables in the experimental group. Also, a trend towards a significant interaction between the two groups compared to previous study in which the improvement was smaller than the present study by 4% (13).
Comparing previous study with the present study there is only a 3%
difference versus the current results when it comes to drop jump height improvement after a 6 weeks protocol (27). Moreover, it is known regarding previous studies that plyometric training could possible enhance explosiveness (2, 29, 37). This was also distinguished in the present study with explosiveness improvement for the training group throughout the 6 weeks protocol. There were evidence regarding plyometric training and ground reaction time improvement (15, 17), which is not distinguished in the present study when it comes to interaction between groups. However, ground reaction time showed an improvement (~ 16%) but for both groups. We do not have evidence why this occurred. Perhaps learning - observing effect of the technique through the 6 weeks protocol from the control group. In the present study, RSI increased by 36% for the experimental group and 20.7% for the control group.
According to RSI previous study (7, 13) indicates that RSI increased by 22.2% and 18% respectively. Additionally, regarding specific findings which include
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combination regimes with plyometric jumps and lower body strength, there are indications of development on lower body strength (10, 20). After the current 6 weeks protocol, we notice a lower body power increase. Players started lifting in the isometric back squat with a mean of 77 kg± 15 in the first week and at the end of the week number 6 they managed to lift with a mean of 130kg±20 (70% increment).
Taking under consideration the study by Babault et al. (3) who indicates that isometric contraction can activate muscle fiber more, compared with concentric and eccentric, we can base that isometric contraction in our intervention had an important role of reinforcement with result the drop jump height and reactive strength index improvement.
A very important factor was the physiology when it comes to isometric contraction and plyometric training. Regarding previous study, isometric contraction could provoke hyper stimulation of the neuromuscular system. Moreover, there is a potentiality of and increment when it comes to neuromuscular response. There was an indication that is more beneficial to perform functional isometric in explosive sports such as football to enhance performance (6). Additionally, plyometric training program induced mean thigh cross sectional area increase, with result jump performance improvement (9).
4.1 Limitations of the study
One limitation could probably be the small number of football players who took part in the study. This means that there is a possibility that the sample may not be adequate to have reliable results. Other investigations (1, 12) showed a significant improvement in jump height with a larger number of participants (48 and 41 respectively). Another limitation could be the 6 weeks duration of the protocol and the participants’ compliance. On the other hand different investigations (10), (12) demonstrate protocols of 8 and 12 weeks duration respectively with a significant jump capability improvement. Furthermore, a very important factor for the current study was that the participants did not comply 100% on the protocol. Half of the players lost 5 out of 12 in total training with result lower reliability with respect to the outcomes.
Regardless of the above limitations (small subject’s number, the duration of the whole
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protocol, not 100% compliance from the participants) there were some positive results to interpret and understand, and maybe a potential for significant results in future investigations of the effect of isometric + plyometric training. Another limitation could probably be the drop box height. At the present study, all the football players performed the drop jump from a drop box of 40cm height. Regarding of previous finding (25) the optimum drop jump height depends on the subject’s neuromuscular capacity to produce maximal muscle strength. The precise investigation demonstrate that stronger individuals showed optimal results regarding the power output, on the higher drop box heights (0.62 cm) compared with weaker individuals (0.32cm).
Regarding the present study, participant’s maximal muscle strength was not at high level which means that 40 cm drop box height was probably less optimum height for the football players. Maybe a lower drop box height (~30cm) would be more efficient for the specific subject for more significant improvements in drop jump height and RSI.
4.2 Implications for future research
In the current study, implications can be made for future research. This study aims to identify the effect of a combined protocol with an isometric back squat followed by drop jump in non elite football players of Sweden. The present study is valuable for future research, as it focuses on an innovative protocol that has never been examined. A good interpretation could probably be a comparison between two different combination protocols. For instance, a combined protocol included a strength type of training (eccentric, concentric, isometric) followed by an explosive kind of exercise (plyometric, sprint, reaction exercises and so on) compared with another combined protocol. According to reaction on the ground, which was not part of the research questions for the present study, better adaptation is encouraged to be made in future interventions in order to notice a difference between the experimental and the control group. Generally, future research with better organization (participant’s compliance, experimental planning), and different parameters (different performance variables to be examined) could be interesting to be made in order to
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investigate if a different kind of protocol could also provide positive interaction when it comes to vertical jump capability, contact time and so on.
4.3 Conclusion
The difference between the two groups was distinguished for drop jump height with a trend towards significant interaction between the two groups. At this part it is important to highlight that the improvements that was found through this protocol are very promising for the future. Isometric contraction (as a simple or combined) can be used more in the training programs in order to distinguish performance optimisation in all kind of sports. The reason could be the higher stimulation of muscle fiber, the neuromuscular response and the performance enhancement. Maybe that could probably be a reason of improvements for the current study. It was interesting to see a hyper stimulation weightlifting exercise combined with a plyometric training that induces also improvements on explosive variables.
4.4 Acknowledgements
I would like to thank the Professor Stefan Grau and Mikael Gustafsson for their vital help when it comes to my measurements in Gothenburg University. I am also very grateful to my supervisor Helen Hanstock helping me with all the issues when it comes to my master thesis.
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