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 body

reinforcement 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 the

experimental 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 index

Table 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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