Researcher – Robert Munt L1065554

Dissertation Supervisor – Claire Harrison

May 2014

BSc (Hons) Sport & Exercise Science

The effect of plyometric training on hamstrings in novice footballers

The effect of plyometric training on hamstrings in

novice footballers

Robert Munt

BSc (Hons) Sport & Exercise Science, Sport and Exercise Section, School of Social Sciences & Law, University of Teesside

May 2014

Abstract

Hamstring injuries are an increasingly prevalent problem within all levels of football due to the

nature of the sport which can affect sports performance. The hypothesis of this experiment is that

progressive plyometric training will help improve flexibility in the hamstrings, in turn leading to

injury prevention due to putting the muscles through high intensity exercises to prepare them for

exertive force. Eight Participants who fitted the criteria (18 year old novice footballer) filled in

the necessary documents (PAR-Q, medical questionnaire, and consent form) and began a 10

week plyometric plan involving squats, squat jumps, and knee tuck jumps. The squat exercises

were incorporated to build power in the quadriceps which is the hamstrings antagonist pair to

keep muscular balance, whereas the knee tuck jumps eccentrically contract the hamstrings. Each

subject was tested on sit & reach to measure hamstring flexibility and vertical jump to measure

quadricep power (used as comparative data.) The results showed that as a group the overall sit &

reach results improved by 0.57cm and individual results ranging in progression from 0.13cm to

0.9cm and show that putting the hamstrings through a high intensity plyometric training plan will

increase flexibility, preparing the muscles for high exertion tasks. The results of this study

support the original hypothesis that progressive plyometric training helps improve flexibility in

the hamstrings, therefore reducing the risk of injury.

Acknowledgements

I would like to thank Claire Harrison for her continued support and advice throughout this study period. I would also like to thanks Christine Steel for her advice on the subject.

List of Figures

Figure 1 – Graph of average Sit & Reach results for the group

Figure 2 – Individual Sit & Reach pre-test and post-test results

Figure 3 – Individual Vertical Jump Height pre-test and post-test results

Figure 4 – SPSS data

ContentsAbstract............................................................................................................................................3

Acknowledgements.........................................................................................................................3

List of Tables & Figures..................................................................................................................4

Introduction.....................................................................................................................................1

Problem........................................................................................................................................1

Research Hypothesis....................................................................................................................1

Assumptions................................................................................................................................3

Delimitations...............................................................................................................................3

Risks............................................................................................................................................3

Literature review..............................................................................................................................4

Stretching.....................................................................................................................................4

Power & Plyometric training.......................................................................................................5

Hamstring muscles......................................................................................................................7

Antagonistic Pairs & Muscle Contractions.................................................................................7

Injury & Postural Problems.........................................................................................................9

Relation to Sport........................................................................................................................11

Pre-screening.............................................................................................................................11

Methodology..................................................................................................................................13

Background................................................................................................................................13

Participants................................................................................................................................13

Research Design........................................................................................................................13

Instruments................................................................................................................................13

Validity......................................................................................................................................13

Procedure of the Study..............................................................................................................14

Analysis.....................................................................................................................................15

Results...........................................................................................................................................17

Discussion......................................................................................................................................21

Limitations.................................................................................................................................25

Conclusion.................................................................................................................................27

References.....................................................................................................................................30

Introduction

Problem

Hamstring injuries are becoming increasingly common in football due to the high intensity of the

sport and can affect the level of sports performance. According to Woods et al., (2004) in order

to reduce both initial and reoccurring hamstring strains whilst playing football it is imperative to

prevent initial injury. He also states that hamstring strains are very common in sport, particularly

in those which involve both sprinting and jumping and therefore reducing the risk of hamstring

strains is likely to positively impact sport performance. In a study conducted by Woods et al.,

(2004) it was found that 12% of all professional footballing injuries reported over two seasons

were hamstring strains. Additionally it has been reported that despite many attempts at

prevention, via methods such as stretching and strengthening, athletes who partake in power

based events including sprinting and jumping have long been frustrated by hamstring injuries

(Stanton & Purdam, 1989).

Hoskins & Pollard (2005) stated that there is a lack of literature to determine the prevention,

management, and treatment of hamstring injuries. This test was created to collect results that

would lead to relevant factors involving prevention of hamstring injuries.

Research Hypothesis

The hypothesis of this experiment is that progressive plyometric training will help improve

flexibility in the hamstrings, in turn leading to injury prevention due to putting the muscles

through high intensity exercises to prepare them for exertive force. “Flexibility has long been an

important component of physical fitness and health” (Corbin et al, 1980) and is important during

football due to the amount of high impact motions the body endures throughout the course of a

game or training session such as running, tackling, jumping, and kicking. Due to the high impact

nature of the sport, the body must be prepared to partake in all these movements and plyometric

1

training has been the answer.

Plyometrics - defined as “exercises based around having muscles exert maximum force in as

short a time as possible, with the goal of increasing both speed and power” (Donald, 1998).

Plyometric training is focused on putting the body through highly intense exercises which will

prepare the specific part of the body being trained for other high exertion

activities/movements/exercises it may endure, hence leading to injury prevention. Building an

exercise plan for the hamstring muscle that involves eccentric contraction (involves lengthening

of a muscle due to another muscle concentrically contracting), the hamstring will endure short

bursts of lengthening which may lead to increased flexibility.

Eccentric contractions – Enoka in 1996 defined concentric contractions as “When the force

applied to a muscle exceeds the force produced by the muscle it will lengthen” and characterizes

it as high force production. Gabbe et al (2006) suggests that eccentric training is a potential

preventative strategy to hamstring injuries and in the same journal the findings suggest that a

simple eccentric exercise programme could reduce the occurrence of hamstring injuries

especially during pre- season. This study researched whether eccentric contractions of the

hamstrings are linked to flexibility. Theoretically, if the hamstring lengthens through short quick

plyometric exercises, the length would eventually stay at the increased length.

It is of paramount importance that whilst training one muscle, the antagonist pair (in this case the

quadricep) must also be trained to maintain muscular balance. In a journal by Heiser et al, (1984)

it is discussed how rehabilitation and maintenance of muscular balance can prevent hamstring

strains. Plyometric training was also applied to the quadriceps throughout the 10 week training

plan and monitored to show progression and used as comparative data. Hewett et al, (1996)

studied the effect of a jump plyometric programme on hamstrings and quadriceps in volleyball

players. The results showed that hamstrings increased in power by 44% and hamstrings to

quadricep muscle peak torque ratios increased by 26%. Results showing that plyometric training

2

increase flexibility in hamstrings could potentially link power and flexibility together creating

opportunities for more research to be conducted into the matter.

Assumptions

Assumptions of the study involved:

1. All participants will attend all testing times and will complete the exercise plan to the best of their

ability

2. The subjects will follow the testing protocols and engage in all exercises including warm up

3. Maximal effort will be given throughout the full study period

Delimitations

As with most studies there are numerous delimitations which can be identified before the

research is carried out. The primary delimitation in this study is the number of participants; only

eight people were involved. A second delimitation is that all eight of the participants were novice

male footballers aged 18.

Risks

There were no major risks to this study; however, it was possible that one of the subjects could

have become injured whilst carrying out the exercise plan. The subjects were informed of the

physical demands that the study provided. If an injury did occur throughout the test, the full

results may not have been collected although the information that an injury was caused would

have been noted.

3

Literature review

The purpose of this study was to study the effect of plyometric training on hamstring flexibility.

The usual way to improve flexibility would be to stretch as most literature indicates.

Stretching

It is thought that stretching is one method of reducing the risk of hamstring injuries by

increasing flexibility and therefore widening the range of movement. Flexibility is an important

component of fitness in sport and is understood as the capacity of a joint or muscle to move

through its full range of motion. “Measures of flexibility are performed to assess the ability of

skeletal muscle and tendon to lengthen. Flexibility can be both static and dynamic.” Gleim et al,

1997. Flexibility is also improved by stretching in other methods such as:

1. Passive – A form of static stretching which an external force moves the limb in to a new

position

2. Ballistic – A form of stretching (either static or dynamic) that involves a bouncing

motion

3. PNF (proprioceptive neuromuscular facilitation) - A form of stretching that involves a

partner helping to bend a limb into a certain position. This stretch is held for longer than

passive and ballistic.

Thacker, 2004 emphasises the importance of stretching before participating in any sport at any

level, whether it be competitive or merely recreational. Additionally he highlights that the

majority of athletes, coaches, physiotherapists and even textbooks recommend stretching before

participating in a sporting activity in order to reduce the risk of injury and to enhance

performance. Despite some journals which attempt to prove this theory incorrect, Shrier for

example, found five studies all of which suggested that there was no significant difference in

injury rates between stretchers and non-stretchers (Shrier, 1999) a stretching warm up was

4

included in the exercise plan given to participants to ensure that the participants muscles were

well prepared for physical activity.

It is well documented that stretching is the best way to improve flexibility and would lead to

reduced injury chances, supported by Shellock and Prentice (1985) who found that warming up

before physical activities appears to reduce the chance of musculoskeletal injuries. However,

some journals state that there is no link between stretching and injury reduction, Thacker et al.,

2004 found no significant association between injury reductions and stretching. Another journal

by Andersen (2005) studied the effect of stretching before and after exercise and discusses the

outcome that stretching protocols do not meaningfully reduce injury. However, Gelen et al.,

(2008) mentions that most literature dictates that stretching during a warm up increases the range

of motion and should also be performed in the cool down.

This study looked at an alternative to stretching by preparing the muscles through plyometric

training.

Power & Plyometric training

Based on the previous evidence on stretching with regards to hamstring flexibility and injury,

plyometric training was chosen to be monitored over a ten week period. Hamstring flexibility in

football is generally poor as the kicking motion of the leg involves shortening the muscle then

rapidly increasing without gentle progression so giving participants a progressive plyometric

training plan may in fact help the body get used to high intensity physical movements and reduce

the risk of injury whilst playing football. In a journal by Askling et al., (2003), a test was carried

out on elite soccer players to see whether training on eccentric overload on the hamstrings could

affect the occurrence and severity of hamstring injuries, they found that the training group had a

lower occurrence of injury in the hamstring than the control group, (3/15 and 10/15

respectively), additionally, the training group displayed significant improvements in both

5

strength and speed. As well as building the hamstring flexibility, eccentric overload could be

used for people who have deconditioned as a result of previous injury (Arnason, 2008).

Power movements, such as squats and lunges are the major contributing factor in hamstring

injuries causing strains and tears whilst performing high intensity movements. Pulled hamstrings

or hamstring strains incorporate everything from aches to partial tears and they are predominant

injuries in sportsmen who run very hard, particularly in sprinting where there is a lot of stop

starts. Often part of the tendinous origins of the hamstring, i.e the bicep femoris from the ischial

tuberosity, tears away as a result of violent muscular exertion. (Tortora & Derrickson, 2011)

Furthermore Williams, (1980) states that muscle damage can occur either due to external forces

such as contusion or instead as a result of forces which are generated in the tissue, commonly

known as an intrinsic injury.

Plyometric training can be incorporated in to the training sessions and will help the hamstring

gain strength in performing sharp movements without injury such as sprinting, tackling, lunging,

jumping, the extension stage of kicking the ball and in some cases the concentric contraction.

According to Howley, (2012) “The principle of progressive resistance refers to continually and

progressively placing demands on the body that are greater than that to which it is normally

accustomed”. Based on this premise as the hamstring muscle grows stronger it should ideally

adapt to the stress placed on it. Another method that would benefit power is resistance training

according to Glenn, (2000), Mastery of a wide range of skills that are linked to components of

fitness such as power, strength and speed can create success in a variety of different sports.

Glenn also discusses how challenging designing resistance training programs especially with

well-trained athletes can be, but will prove to be beneficial with regards to a variety of

performance skills. This suggests that resistance training is beneficial for novice performers and

could potentially be an area to observe. However the hypothesis is to prove that plyometric

6

training will be the most beneficial training method for power in hamstrings.” These are some of

the guidelines ACSM point out to be key issues in pre-screening.

Hamstring muscles

The hamstring muscles are split into three:

Bicep Femoris – This part of the hamstring has two heads of origin. The first (long head) joins

from the lower and inner part of tuberosity of the ischium by a tendon and from the lower part of

the sacrotuberous ligament. The second (short head) joins from the lateral lip of the linea aspera

between the vastus laterallis and adductor magnis, and from the lateral intermuscular septum.

Both of the heads perform knee flexion.

Semitendinosus – This part of the hamstring is used to flex the knee joint and also extend the

hip joint. It helps rotate the tibia medially on the femur when the knee is flexed and medially

rotate the femur when the hip is extended. It also stops the hips bending forwards.

Semimembranosus – This particular part of the hamstring helps flex the knee joint and also

extend the hip. It also helps rotate the knee medially and helps the tibia rotate medially on the

femur when the knee is flexed. The semimembranosus also rotates the femur when the hip is

extended.

Antagonistic Pairs & Muscle Contractions

There are 3 types of muscle contraction:

Concentric – This muscle contraction involves a particular muscle shortening with force

e.g. a bicep curl (the bicep will concentrically contract)

Eccentric – This contraction involves lengthening of a muscle due to another muscle

concentrically contracting e.g. a bicep curl, (as the bicep contracts concentrically, the

tricep will eccentrically contract.) The muscle works to decelerate at the end of a

movement.

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Isometric – This muscle contraction occurs when a muscle maintains at the same length

but generates force creating stability e.g. holding an object in the same place without

moving or dropping it

Hamstrings and Quadriceps are each other’s antagonist pair. When one muscle contracts, the

other muscle will relax. A muscle will contract concentrically and the other will eccentrically

contract, for example, when kicking a football, first the quadricep muscle will contract

eccentrically as the hamstring contracts concentrically, and as the kicking motion starts, the

hamstring will eccentrically contract lengthening and the quadricep will concentrically contract

moving the leg towards the ball.

Aagard et al., (2000) created a study to find out the amount of coactiviation between the

hamstring and quadricep muscles during maximal quadricep contraction in slow isokinetic knee

extension. The data showed that substantial antagonist coactiviation of the hamstring muscles

may be present during slow isokinetic knee extension. “In consequence substantial antagonist

flexor moments are generated. The antagonist hamstring moments potentially counteract the

anterior tibial shear and excessive internal tibial rotation induced by the contractile forces of the

quadriceps near full knee extension. In doing so the hamstring co-activation is suggested to assist

the mechanical and neurosensory functions of the anterior cruciate ligament (ACL).”

Another study by Li et al., (1999) measured the effect of hamstring and quadricep muscle

loading on the knee and ACL (anterior cruciate ligament) during a simulated isometric extension

motion of the knee and results found that hamstring co-contraction with quadriceps is effective

in reducing excessive forces in the ACL, reducing the chances of injury. This is further evidence

that building the quadricep by plyometric training as well as the hamstring will help maintain

physical fitness, balance and reduce the chances of injury. In a study conducted on Australian

football players in 2005 to discover risk factors relating to hamstring injuries, data showed that a

hamstring injury was the first injury of the season for 20 players (16%) and found that decreased

8

quadricep power and flexibility were significant predictors of the time to sustaining a hamstring

injury Gabbe et al., (2005). Draganich, 1989 also states how increasing the strength in

hamstrings is paramount during the rehabilitation or reconstruction of the ACL - which proves as

further evidence that the ACL is affected by the hamstrings therefore protecting the hamstrings

from injury is paramount.

Injury to the hamstrings can affect other areas as well as the localised area:

Injury & Postural Problems

In a paper conducted by Jonhagen et al., (1994), he states that uninjured sprinters tend to have a

significantly higher eccentric hamstring torque at all velocities than those sprinter who have

previously suffered with hamstring injuries. In his study he measured hamstring flexibility and

eccentric and concentric muscle torque and found that sprinters with a significantly tighter

hamstring were those who had experienced prior hamstring injuries in comparison to the

uninjured sprinters who had significantly higher eccentric hamstring torques at all angular

velocities. Using this information, a decision was made that this test would not focus on

participants with previous hamstring injuries, but would focus on those who have not.

It is believed that hamstring injuries are often caused during the swing phase of a gait cycle. In

the second half of the swing when running, the hamstring eccentrically contracts and absorbs

energy from the swing limb before foot contact, so hamstrings are stretched while subject to

load. Petersen et al., (2011) conducted a study to discover if eccentric exercise would reduce the

overall new, and recurrent hamstring injuries and the results stated that in both professional and

male soccer players additional hamstring exercise did result in fewer injuries. This evidence

suggests that exercises such as knee tuck jumps that work on bursts of eccentric contraction of

the hamstrings as well as bursts of quadricep concentric contraction would reduce the chances of

hamstring injury due to strengthening.

The hamstring does not only affect the leg, but can also cause harm to the lower bag and pelvis

9

due to restricting him flexion, particularly when the knee is extended. Stokes & Abery, (1980)

hypothesised that individuals that had tight hamstrings were likely to have abnormal pelvis

tilting in some postures (namely seated), and would also have greater reversal of the lumbar

lordosis. In a study from the same journal, it was concluded that hamstrings did affect that

lumbar spine curvature in sitting. The study involved healthy adults completing a toe-touch test

which measured a range of hip flexion. Within standing, the lumbar spine curvature was

measured and in sitting with knees flexed and partially extended respectively. The results

showed large individual variations in hip flexion ranges and people with the smallest range

showed the most pronounced change in spine curvature. The results concluded that “attention

should be given to this effect of the hamstrings on the lumbar spine when workplaces are

designed”.

Muckle in 1982 discusses some of the problems recurrent hamstring injuries can cause:

Reoccurring hamstring problems can cause abnormalities to the Lumbar spine, problems in the

knee, inflammatory of rheumatic disease, quadricep power and adhesion of the lateral popliteal

nerve. Petersen & Holmich, (2005), also discuss how one of the major problems with hamstring

injuries is reoccurrence of the injury. They also discuss how the reinjury rate for hamstrings is

between 12% and 31% and how that the principle of “Rest, Ice, Compression, and Elevation”

exists for muscle strains, no consensus exists for their rehabilitation. A potential cause of

hamstring injuries and also reoccurring hamstring injuries is consistent mistakes and unusual

movements according to Croisier, 2004, who also gives hypothetical views that injuries can also

be caused by inadequate warm ups, invalid structure, content of training, muscle tightness and/or

weakness and antagonist imbalance. It is important to consider a muscles antagonistic partner

when strengthening as imbalance can cause injury and postural problems, hence the training

programme created for this study.

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Relation to Sport

Flexibility and power are two essential components of fitness in many sports and in particular

football. The essential movements including jumping, running, tackling, and kicking involve

sharp burst movements of both concentric and eccentric contraction. However, this does not only

apply to football, but is also required in most other sports. A journal by Hoffman et al 1996

discusses the importance of flexibility, power, speed and agility on playing time within

basketball and the same principle would apply to other sports. Tsolakis et al., (2010) deliberates

the importance of power within the sport of fencing within such movements as a lunge, and squat

jumps (incorporated in the exercise plan). This article also mentions flexibility as being an

important factor during the same movements. Another study by Gleim & McHugh, (1997) talks

about the changes of flexibility specific training plans within different sports and in some cases,

different positions. Applying a specific flexibility training plan to an individual’s sport could be

essential in injury prevention.

Pre-screening

A pre-screening process took place before the programme began to ensure all participants were

safe. Each participant received an exercise help sheet which provided information on each

exercise as well as a medical questionnaire and PAR-Q. They also received a sheet containing

their exercise plan over a ten week period, a participation information sheet and a consent form.

Filling out these documents allowed the plan to go ahead legally and also adhered with ACSM

guidelines for pre-screening activity. Tharrett, & Peterson, (2012) state that there are many

benefits of having an active lifestyle and they are well documented. Adhering to well written

guidelines will ensure an individual’s safety and increase benefits. The same piece of work also

discussed how pre activity screening procedures should be completed which would protect the

program designer from any legal issues and would also allow the participant to quit the testing at

any time if there was a problem. This would allow the participant to engage fully in the activity

11

with no issues. This would also highlight any problems (physically) that an individual may have,

such as cardiovascular problems, metabolic, pulmonary and any disease symptoms and would

therefore act as help to the program designer to decide whether that particular individual is fit

enough to start the design.

12

Methodology

Background

The hypothesis of this study is that progressive plyometric training will help improve flexibility

in the hamstrings, in turn leading to injury prevention due to putting the muscles through high

intensity exercises to prepare them for exertive force. This test has been carried out as a pre/post-

test design.

Participants

In this study eight male participants who fitted the criteria of a being a novice footballer were

selected from a volunteer group of 12 who had applied. All of the participants in the study were

18 years old.

Research Design

The experimental design used was randomisation. 12 people applied for the available positions

and so their names were written on to pieces of paper, folded up and placed in to a hat. The

researcher then picked out 8 names completely at random to fill the 8 positions available. The

required documents were then collected by the researcher who kept them confidentially and

handed out more documents to the participants.

Instruments

Quadricep power and lower back/hamstring flexibility was measured using the Vertical Jump

Matt (Probotics Just Jump System – JJS to measure quadricep power) which was tested before

hand to ensure it was possible to use in the training plan. The Sit and Reach box (Power Systems

70200 Flex Tester) was used to measure hamstring flexibility on each individual one at a time.

Validity

In terms of the validity of the study one possible threat was selection bias as a result of the

participant sample being chosen for convenience. Selection bias can negatively affect the ability

13

to generalise the findings as the eight male participants which were chosen may not be

representative of male novice footballers as a whole. In addition to this events may have

occurred which were not part of the plyometrics training plan such as participants undertaking

extra training sessions or partaking in physical activity which may impact the results.

Procedure of the Study

The eight randomly chosen participants completed the necessary documentation including a

PAR-Q (Appendix 2) which included questions about previous injuries, lifestyle, and basic

personal information such as name, address and contact numbers. A participant information

sheet (Appendix 3) was also required. This sheet was to inform the participants of the purpose

and a background of the study. It also stated who was responsible for the study and any issues

that may arise and included contact details of the researcher. A consent form (Appendix 4) was

filled in which detailed the rights that the participants have and to release the researcher from all

legal issues, and finally a medical questionnaire (Appendix 5) to give all specific details of any

physical injuries or issues that could affect the study and again more basic lifestyle questions.

Any issues on these documents may have led to withdrawal of a participant as they may have not

met a reasonable standard of fitness/well-being to partake in the study. Participants were also

provided with a ten week training plan that specified the exercises that they would perform

throughout the plan such as squats, squat jumps and knee tuck jumps and the days they would

take place (Appendix 1) as well as an exercise help sheet (Appendix 6). A pre-test interview was

also conducted prior to the commencement of the training plan. A general conversation took

place between the researcher and participant asking questions to indicate any potential barriers or

problems that may have occurred between the time at which the documents were completed and

that current time.

Questions that were asked:

- “Are you currently fit and well (both physically and mentally) to partake in the experiment?”

14

- “Have you had any consultations with a doctor or hospital since the forms have been filled in?”

- “Has there been any change from the forms you filled in?”

- “Is there any issues you believe to be important that could harm the experiment, the researcher

and most importantly yourself?”

The participants were tested on day one of the exercise plan prior to the commencement of

training (pre-test) and on the final day of week 10 (post-test). At the start of each week on a

Monday, quadricep power and lower back/hamstring flexibility were measured using the

Vertical Jump Matt (Probotics Just Jump System – JJS to measure quadricep power) and the Sit

and Reach box (Power Systems 70200 Flex Tester to measure hamstring flexibility) on each

individual one at a time. The subjects were tested three times on both the sit and reach and

vertical jump tests. The three results were then averaged to provide the mean result for each

week. The vertical jump test was rounded to the nearest centimetre and was only used as

comparative data to show that the subjects were completing the plan and to show progression of

the antagonist muscle of the hamstring. The first testing time-point (the pre-test) occurred on the

first Monday of the 10 week training programme before the exercise plan had officially begun in

order to obtain knowledge of the flexibility and power of the participants to allow for

comparisons pre and post-test. This was repeated every Monday (before the participants carried

out the exercise that day) recorded, and confidentially stored. Progression was measured and

analysed from pre-test to weekly data and eventually the post-test which took place straight after

the exercise plan had finished on the last Sunday of the 10 week plan.

Analysis

Once the data had been collated the results of each time point were averaged and displayed as a

line graph in order to visualise if the group as a whole had improved in their flexibility.

Additionally a second bar graph was produced in order to show the progression of each

individual participant from their pre-test and post-test data. Statistical analysis was then carried

15

out on the collected data using SPSS. As the experimental design was repeated measures,

involving the same group of participants throughout the study, a dependent t-test was selected

(synonymously known as repeated measures t-test) to compare the results from the pre and post

test data in order to identify if there had been any significant difference in the participants

flexibility.

Another line graph was added to show progression of pre-test and post-test results of the

vertical jump test. This was not a factor to be statistically analysed, but was used as comparative

data to show that the participants had took part in the plan and trained the hamstrings antagonist

pair muscle.

16

Results

The primary purpose of this research was to identify if a 10 week plyometrics training plan

positively affected hamstring flexibility in novice footballers. Alongside this the vertical jump

test was carried out based on the theory that as muscles work in antagonistic pairs this result

would also increase therefore acting as comparative data.

The sit and reach test results were recorded for each individual participant across a testing period

of 10 weeks. The results for each testing point were then averaged and displayed in a graph. This

graph is able to show the progression of the group as a whole to see if the plyometrics training

has had any impact of the flexibility of the group.

Pre 1 2 3 4 5 6 7 8 9 10 Post9.00

9.50

10.00

10.50

11.00

11.50

12.00

12.50

Group Sit & Reach Results

Series1

Weekly Test Points

Aver

age

Reac

h (c

m)

Figure 1 - A graph displaying the average sit and reach results (cm) of the eight participants at each testing period to portray the progression of the group as a whole. The graph shows an increase from the pre-test to the post-test data, 11.45 to 12.02 cm respectively. A slight decline in reach is evident after week one with the average reach of the group dropping from 11.45 to 10.33 cm. Overall the results show that the plyometrics training has caused an increase in the sit and reach results of the test group.

17

The sit and reach test results were recorded for each individual participant across a testing period

of 12 weeks. The following graph shows the reach of each subject both before and after

undertaking the 10 week training programme.

1 2 3 4 5 6 7 80.00

5.00

10.00

15.00

20.00

25.00

Pre & Post Results For Each Subject

PrePost

Subjects

Reac

h Re

sults

(cm

)

Figure 2 – A graph portraying the pre and post sit and reach test results for each individual subject. All participants have shown an improvement in reach following the exercise plan. Subject 7 showed the greatest improvement increasing from 5.93cm to 6.83cm whereas Subject 1 showed the least improvement with a smaller increase of only 0.13cm.

Typically, as muscles work as antagonistic pairs, as the training plan benefits the hamstring the

quadricep should also show improvement therefore the vertical jump test was used in order to

assess if the power of each participant was increasing. The following graph shows the pre-test

and post-test group vertical jump results.

18

1 2 3 4 5 6 7 80

1020304050607080

Group Vertical Jump

Pre-testPost-test

Participants

Aver

age

Jum

p He

ight

(cm

)

Figure 3 - A graph displaying the average jump height (cm) of the eight participants at each testing period showing the progression of results from the group as a whole. The graph shows each participants pre-test score was lower than the post-test result leading to the outcome that plyometrics training has increased the vertical jump height of each participant.

SPSS was used to perform a dependent t-test of the pre and post test data to identify if there was

any significant difference between the sit and reach results collected in the pre-test and those

collected in the post-test. Despite seeing an increase in the average results of the group it is

necessary to statistically analyse the results at the 0.05 level of confidence (p < 0.05) meaning

that there will be a 95% likelihood that a change in results is not merely due to chance.

Figure 4. SPSS data showing the results of the repeated measures t-test. The high level of correlation .999 indicates that participants who had lower sit and reach test results at the pre-

19

test were still typically lower than others in the post-test whilst those with higher reaches maintained a high level in the post test – basically indicating that although the group as a whole improved the correlation between each participant’s results was high. The results also show that p = 0.001 which means that the increase in reach (cm) is statistically significant.

In terms of answering the research hypothesis, that plyometric training will improve flexibility in

the hamstring muscles of novice footballers, the statistical analysis carried out using SPSS

software found p = 0.001 which is actually much lower than the set let level of significance (p <

0.05) meaning that the alternative hypothesis can be accepted and the null hypothesis rejected.

As the p value was so low there is a much lower risk of type 1 errors.

20

Discussion

This study was created to test the hypothesis that plyometric training will improve flexibility in

the hamstring muscle of novice footballers and therefore reduce the chances of injury. Putting

the hamstrings through intense high exertion plyometric exercises in a progressive 10 week plan

should prepare the muscles for a wide range of movements and in relation to football. Training

the hamstring’s antagonist pair muscle (the quadricep) would help to reduce the risk of muscular

imbalance and therefore injury.

The results found show an increase in the reach (cm) of all eight participants supporting the

hypothesis that flexibility would improve. Figure 1 shows that the average reach of the entire

subject group increased from the pre-test to the post test results. A decline in average reach can

be seen at data collection week one, declining from 11.45cm to 10.33 cm; this could be due to

muscle tightening as a result of the inability to adjust to the high impact training programme,

another explanation is that some subjects did not follow the training plan correctly therefore

reducing the benefit and increasing the risk of injury. The graph then shows a steady incline from

week one to the post-test result indicating a steady improvement in flexibility on average.

Figure 2. shows the pre-test and post-test sit and reach result for each individual subject. All

subjects have achieved higher reach results in the post-test, an average improvement of 0.56 cm,

signifying that the plyometric training has in fact improved their flexibility. Subjects 7 and 1

have shown the most progression reaching 0.90 cm further in their post-test scores than in their

pre-test scores.

Week 2 shows a steady increase from 10.33 cm to 10.52 cm for the overall group indicating that

the exercise plan is potentially starting to work. The subjects may have suffered from muscle

soreness/tightness or were just poorly adjusted to the intensity of the plan in week 1, but this

issue looks to have been resolved as clear progression is shown. As the exercise plan has a small

static stretching warm up to be carried out each day involving a toe touch hamstring stretch and

21

quadricep stretch for 10 seconds each, this may have prepared the particular muscles for high

exertion exercises. Weijer et al., (2003) found that when there is a significant increase found in

the length of the hamstring it is possible to maintain this increased for up to 24 hours through the

use of static stretching, which shows that although the stretch could decrease the risk of injury by

preparing the muscle whilst taking part in the exercise plan, the lengthening benefit would not

last longer than 24 hours, therefore increases validity of the results of this study.

Week 3 to week 4 and on to week 5 all show increases respectively moving from 10.52cm to

10.56 cm and then to 10.77 cm. Once again this shows progression on a smaller scale of only

0.04 cm from week 3 to 4 but the progression between week 4 and 5 increases to 0.11 cm. The

reasoning behind this is likely to be that after 3 weeks of exercise, the hamstrings and starting to

loosen, strengthen, and become suppler. The longer that a muscle is trained, the more effective

they become at that particular exercise. Also, the gaps between week 2 and 3 and on to week 4,

shows the least progression, and in the case of week 2, regression. A possible explanation for this

as previously discussed (un-adjustment) could lead to muscle fatigue and a build-up of lactic

acid.

Lactic acid is an acid that builds up within muscles during strenuous exercise, in layman’s terms,

an acid that stops the muscle performing efficiently and effectively telling the body to stop due

to fatigue. A preventing method of lactic acid would be to increase oxygen intake. Aerobically fit

people will intake a higher level of oxygen (V02) than an individual who is not aerobically fit.

This leads to a key point of aerobic/cardiovascular endurance training could increase oxygen

intake, preventing lactic acid building up in the muscles and in turn, producing better results

Wasserman & Mcllroy, (1964) conducted a study in which from a standard exercise test a

measurement of respiratory gas exchange ratio was taken and then used to determine the onset of

anaerobic metabolism during exercise. This is a result of the cardiovascular system failing to

adequately supply the tissues with oxygen. Spengler et al., (1999), also states that whole body

22

endurance training can substantially reduce the lactate concentration of blood at a given exercise

intensity. However, Cairns, (2006) opposes the idea that lactic acid may have been the cause of

decline in reach as he concludes that it is not a fact that lactic acid impairs exercise or

performance and infact research has suggested that acidosis may have little effect, or may even

improve performance of isolated muscles. On the contrary, it is suggested in the same study that

acidosis can actually exacerbate fatigue during full body dynamic exercise.

Week 6 progression increases once again to 0.17 cm due to an increase of repetitions of squats

and knee tuck jumps. The intention behind increased repetitions was to incorporate a small

amount of muscular endurance within the plyometrics which would be a component of fitness

essential to a footballer’s performance, this is supported by Dudley & Fleck, (1987) who found

that training plans to increase endurance often include low resistance, high repetition exercise of

large muscle groups.. An example of this would be cycling, swimming, or running as footballers

would use.

Week 7 results showed another 0.22 cm progression to reach an overall average of 11.16, again

showing that the hamstrings are becoming suppler due to the exercise plan.

Results of 11.33cm, 11.48 cm, 11.68 cm, 11.82 cm followed and finally to a post-test results of

12.02. The group achieved an overall increase of 0.57 cm of average reach, furthermore

supporting the evidence that increasing the repetitions and time spent doing the specific exercises

will increase the flexibility of the hamstrings and therefore supporting the hypothesis that

plyometric training will increase hamstring flexibility and reduce the risk of injury.

The overall improvement could be a result of the eccentric contractions caused from the knee

tuck jumps. The repetitions and time increased throughout the 10 week plan causing the

hamstring to rapidly lengthen. LaStayo et al., (2003) discusses how high force eccentric

contractions are traditionally associated with muscle damage response helping the muscle

improve structurally and functionally. “Because the muscle’s force can be maximized when

23

contracting eccentrically, damage to the contractile and cytoskeletal components of the muscle

fibre itself, weakness, and a perception of soreness, often occur” (Le Stayo et al., 2003). To

reduce the risk of injury as much as possible in a footballer, a wider range of training types

should be adhered to. According to Costello, (1984) speed training, flexibility, and plyometrics

should all be incorporated to gain the most beneficial results in power and flexibility. Costello

believes that incorporating these techniques is the best way to keep his footballers in the best

physical form possible.

As well as improving flexibility, the training plan was also designed to increase power in the

quadriceps (the antagonistic pair of the hamstring) to keep the muscular balance at a high

standard. The results from Figure 3 show an increase in each subjects pre-test and post-test

results ranging from 3cm (the smallest increase) to 12cm (the largest increase). Quadricep power

is used in football in the majority of movements and higher levels of this can increase

performance levels. The quadriceps also control how the knee moves such as flexion and

extension along with the hamstrings meaning it is important to maintain muscular balance.

Without it, postural problems can occur. If the quadricep and hamstrings are not muscularly

balanced, problems can be created around the knee area and can cause injuries to important parts

of the body such as the anterior cruciate ligament (ACL). This is emphasised further by

Renstrom et al , (1986) who found that anterior cruciate ligament strain occurred significantly

more often during simultaneous hamstring and quadricep activity as opposed to during normal

passive motion from full extension to flexion (30°). A study by Hortobágyi et al, on people with

osteoarthritis of the knee discussed how the patients had increased levels of hamstring activity

throughout daily living. “Altered muscle activation at the knee may interfere with normal load

distribution and facilitate disease progression”. It is suggested that further studies should not

only focus on the quadriceps but also on the hamstrings and how they work together (Hortobágyi

et al, 2005.) Wisloff et al., (2004) states that elite soccer players should focus on maximal

24

strength training with emphasis on maximal mobilisation of concentric movements, which may

improve their sprinting and jumping performance which suggests that maximal strength training

may be an effective way of improving sprinting and jumping performance. These two

movements are key in football and could possibly link in to plyometric and flexibility training to

reduce chance of injury in football and also increase performance levels.

The results showed a positive correlation between plyometric exercises and hamstring flexibility,

but within football, hamstring injuries are often reoccurring. The participants in this study did

not show a previous history of hamstring injury, a way to improve this study would have been to

include some participants that did show previous injury history and observe their progression

over a longer period.

However, given the lack of previous evidence to suggest that plyometric training will increase

flexibility through preparation of the muscle for high intensity and high exertion exercises,

questions were raised over the clear improvements. A potential answer could be that the small

stretch warm up plan when carried out a number of times a week would in fact increase

flexibility, despite the paper written by Weiier et al stating that this would only affect hamstring

length for 24 hours. Although in that particular paper, it is found that the group who only did

static stretching showed increased hamstring length between pre-intervention and all post

intervention. From this evidence, it could be pointed out that static stretching could affect

hamstring length/flexibility depending on the time spent stretching.

Limitations

Despite producing significant results there are numerous limitations associated with the validity,

methodology and undertaking of this research. The first way in which this study could have been

improved is to increase the sample size of participants. By increasing the sample size the results

would have been more reliable and would have been more representative of the population

reducing the risk of outliers or misinterpretation of results. A second way to improve the study

25

would be to use a wider range of participants, i.e using a larger range of age groups, studying

both male and female participants and using a variation of physical abilities all of which allow

the results to be generalised to a larger population and would have given a better overview of the

tested hypothesis.

Another limitation of the study was the amount of time spent with the participants. Improved

communication between the researcher and participants throughout the training plan would have

allowed participants to discuss aspect of the training plan such as intensity and difficulties

performing certain exercises, additionally participants would have been able to better express

any concerns for their health and well-being. Talk tests are frequently used whilst measuring

exercise components to ensure that they are not having any problems e.g. injury and fatigue. In a

study conducted by Foster et al., ( 2008), correlative data was used to create a hypothesis that a

Talk Test (TT) and Ventilatory Threshold (VT) are related in a robust way, adequate for exercise

prescription. The study used healthy young adults who were required to participate in four

independent series of experiments which were either designed to decrease or increase VT

through use of blood donation and training, or to systematically vary the exercise intensity above

and below VT. The interventions which were put in place were based on the result of the talk

test. The study confirmed the robust relationship between VT and the TT during various

interventions and suggests that the TT is suitable for exercise prescription. Using this

information, it would be relevant to use a talk test in future testing for exercise to gauge the level

of intensity given to the subjects which leads to another issue, handing out a set 10 week exercise

plan may prove to be too intense or not intense enough for a particular individual, so adding a

talk test and flexibility of the exercise plan may improve the outcome e.g. decreasing the

exercise plan slightly when a participant indicates muscle fatigue.

In this study, the sit and reach box was used to measure hamstring flexibility due to ease of use,

availability and low associated costs, however results may be more reliable if more advanced

26

equipment was used. Measuring torque of the hamstring and using other factors on a “Kinetic

Communicator(R) (KIN-COM)” could have provided more in depth analysis of the hypothesis.

This piece of equipment was used in a study by Worrell et al., (1991) who used it to measure

isokinetic strength and flexibility of the hamstrings in athletes with previous injury history to the

specific area.

The length of the study could be another limiting factor as research has shown that it an optimal

training plan will exceed 10 weeks in order to see a significant improvement (Randell et al.,

2011). A longer training plan would have provided more results and therefore a more thorough

analysis could be carried out. Increasing the amount of time for the exercise plan would create a

lot more relevant results and eradicate outliers, however, this could create a limitation in itself as

a longer time period also increases the chances of potential issues for participants such as

sustaining injuries or growing bored of the training plan.

A final limitation is that as the plyometrics training is accompanied with a stretching plan there

is a chance that the increase in flexibility displayed by the improved sit and reach results may be

a result of regular stretching. One way to counteract this would be to use an additional group

who carry out all aspects of the training plan aside from the stretching to provide a comparison.

This would allow the researcher to identify if the improved flexibility was a result of the

plyometrics or the stretching at the start of each session. However removing the stretching

element greatly increases the risks of injury in the participants.

Conclusion

In this study, a 10 week progressive plyometric training programme has shown to be effective on

hamstring flexibility in novice footballers. The results show that as a group, the sit and reach

results have increased by a level of 0.57cm meaning that hamstring flexibility has increased,

therefore reducing the chances of injury. In the swing stage of a gait cycle in football, the

hamstring eccentrically contracts causing a lot of sudden burst movements within the muscle.

27

This also happens during kicking the ball, jumping, and tackling. Progression of 8 subjects was

shown to be substantial and none of them were injured throughout the testing. However, the

testing could have been improved by increasing sample size and participant variation. As there

was only 8 subjects and all of the same age, the results were only specific and valid on a small

scale. Also, none of the participants had a previous history of hamstring injuries. As a lot of

literature explains that hamstring injuries are common within footballers, it is recommended that

this test may be used on footballers who have stated previous hamstring injuries to see whether

this method could be used as rehabilitation, building and as injury reoccurrence prevention.

The stretching warm up before the exercise plan began could potentially have been part of the

reason that the results increased, but without this plan, participants would be more at risk of

injury meaning they would be less likely to apply “The warm-up is designed to increased

muscle/tendon suppleness, stimulate blood flow to the periphery, increase body temperature, and

enhance free, coordinated movement” (Smith, 1994). However, as previously stated by Weijer et

al., (2003), the type of stretching that is involved in the warm up (static) usually affects

hamstring length up to 24 hours.

For future research, the level of physical fitness in certain components should be measured in

more detail to determine how much exercise (repetitions, time, and type) an individual should

partake in. Although the subjects all increased in their pre-test and post-test results, some

participants may have been able to achieve higher results and may have felt they were

underworked during the 10 week exercise plan. It would be recommended that a test for each

component of fitness such as power, aerobic endurance, muscular endurance and strength should

be measured before the exercise plan is created.

The vertical jump height results show an increase from pre-test to post-test results indicating that

the subjects have completed the plan which shows a correlation between the vertical jump results

(power) and sit & reach results (flexibility).

28

Overall, we can conclude that the hypothesis is correct and that plyometric training does affect

hamstring flexibility and reduce chances of hamstring injuries, but the results may have been

slightly influenced due to factors such as a small stretching warm up, although previous research

has stated that the amount of stretching completed in the warm up would not be enough to affect

the results. It can be concluded that plyometric training could enhance the physical aspect of

sports performance in relation to flexibility and a suggestion to further benefit the individual

would be to incorporate plyometrics with a stretching plan. Given previous evidence and the

results concluded from this study, applying the two training methods together would increase

sports performance physically, and also give the best chance of injury prevention. The same

application may work with other muscles in the body, however further data is required.

29

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34

Appendices

1. 10 week exercise plan

2. PAR-Q

3. Participant Information Sheet

4. Consent Form

5. Medical Questionnaire

6. Exercise help sheet

7. Raw data sheet for testing

35