CHALLENGING THE GOLDEN

STANDARD IN HAMSTRING INJURY

PREDICTION COMPARING THE VALIDITY OF ISOKINETIC AND FUNCTIONAL

STRENGTH ASSESSMENTS IN RECOGNIZING AND PREDICTING

HAMSTRING INJURY IN MALE SOCCER PLAYERS

Word count: 5,160

Esmoreit Groosman, Cedric Hubrechsen, Loewis Kinget Student numbers: 01403238, 01402443, 01407934

Supervisors: dr. Joke Schuermans, Prof. dr. Erik Witvrouw

A dissertation submitted to Ghent University in partial fulfilment of the requirements for the degree of

Master of Science in Rehabilitation Sciences and Physiotherapy: Rehabilitation Sciences and

Physiotherapy with Musculoskeletal Afflictions

Academic year: 2018 – 2019

Expression of Gratitude

After five good years, we finish our Master in Rehabilitation Sciences and

Physiotherapy with this master's thesis. Writing it was a difficult but fascinating task.

This would not have been possible without the help and support of a number of people,

we would like to thank them.

First of all, we would like to thank our supervisor, prof. dr. Erik Witvrouw, for providing

us with a topic that we were very interested in. We also want to thank our supervisor,

dr. Joke Schuermans, for her time and patience, for her help and feedback, for her

kindness and enthusiasm on the subject.

A big thank you to all the participants who participated in the study, for their time and

energy that they put in coming to Gent and performing the tests. Research is not

possible without them.

We also want to thank our friends for the necessary distraction, relaxation and support.

We want to thank our parents for letting us study, grow and invariably believe in us from

our first day at university.

Table of Contents

Expression of Gratitude.......................................................................................................... 3

Table of Contents................................................................................................................... 4

List of Tables and Figures ...................................................................................................... 5

List of Abbreviations ............................................................................................................... 6

1. Abstract .......................................................................................................................... 7

1.1 Abstract (ENG) ........................................................................................................ 7

1.2 Abstract (NL) ........................................................................................................... 8

2. Introduction ................................................................................................................... 10

3. Methods ........................................................................................................................ 12

3.1 Design ................................................................................................................... 12

3.2 Participants ............................................................................................................ 12

3.3 Procedure .............................................................................................................. 13

3.4 Follow-up of hamstrings strain ............................................................................... 14

3.5 Statistical analysis ................................................................................................. 14

4. Results .......................................................................................................................... 16

4.1 Incidence of hamstring injury ................................................................................. 16

4.2 Comparing injured and non-injured players ............................................................ 16

4.3 Correlation between IKD and RDL ......................................................................... 20

5. Discussion .................................................................................................................... 21

5.1 Detecting previous hamstring strain injury .............................................................. 21

5.2 Predicting future hamstring strain injury ................................................................. 21

5.3 Correlation between RDL and IKD ......................................................................... 22

5.4 The value of IKD .................................................................................................... 22

5.5 The value of RDL ................................................................................................... 23

5.6 Limitations ............................................................................................................. 23

5.7 Conclusion and implications for future research ..................................................... 24

6. References ................................................................................................................... 25

7. Lekenabstract ............................................................................................................... 28

8. Bewijs indiening ethisch comité..................................................................................... 29

9. Appendix ....................................................................................................................... 32

9.1 Appendix 1 ............................................................................................................. 32

9.2 Appendix 2 ............................................................................................................. 35

List of Tables and Figures

Table 1: Division of the participants: mean values of each group’s properties ...................... 12

Table 2: Mean values of isokinetic parameter relative peak torque (in N.m/kg.100) at different

angular velocities ................................................................................................................. 17

Table 3: Mean values of the isokinetic parameter average relative peak torque (in

N.m/kg.100) at different angular velocities ........................................................................... 18

Table 4: Mean values of the isokinetic parameter average power (in Watt) at different angular

velocities .............................................................................................................................. 18

Table 5: Ratios at different angular velocities and RDL values for each goup ...................... 19

Table 6: Significant mean differences between the hamstring injury history group and the

retrospective control group................................................................................................... 19

Table 7: Significant correlation coefficients between isokinetic hamstring strength measures

at different angular velocities and RDL values ..................................................................... 20

List of Abbreviations

HSI: Hamstring strain injuries

IKD: Isokinetic dynamometry and Isokinetic dynamometer

RDL: Romanian deadlift

DCR: Dynamic control ratio

PT: Peak torque

AvrPt: Average peak torque

AvrPower: Average Power

BW: Bodyweigth

Con Q: Concentric quadriceps

Con H: Concentric hamstrings

PT/BW : Relative peak torque

7

1. Abstract

1.1 Abstract (ENG)

Background: Hamstring strain injuries (HSI) are the number one most common injury

in professional male football players. To prevent these injuries from (re)occurring, it is

important to determine if there is a valid assessment tool available to predict future HSI

in players (at risk). To what extent hamstring strength assessment can be used as a

valid tool for hamstring injury risk evaluation and which strength testing modalities are

the most valid ones, has been a subject of discussion throughout the years.

Study design: A mixed cross-sectional and prospective cohort study.

Objectives: The purpose of this study was to investigate whether isokinetic

dynamometry (IKD) and/or a more functional strength test can detect previous

hamstring injury and identify players at risk. Furthermore, the correlation between the

isokinetic parameters and the outcome of the functional strength test were explored.

Methods: A total of 49 male football players were tested in the inter-season period of

the 2018-2019 football season. Knee flexion and extension torques were gathered

bilaterally at angular velocities of 60 (°/s) and 240 (°/s) concentrically for both

quadriceps and hamstrings followed by a protocol at velocities of 30 (°/s) and 120 (°/s)

concentrically and eccentrically for the hamstring muscles on a isokinetic

dynamometer. A maximal effort Romanian Deadlift (RDL) protocol was used as

functional strength test in which the number of repetitions to exertion was used as a

functional estimate of hamstring strength and endurance. Different groups of injured

players were compared to a control group, and injured legs were compared to non-

injured legs of the same group.

Results: Looking at eccentric isokinetic parameters of the hamstring muscles, there

were no significant differences found between any of the injured groups and the control

group. Multiple concentric isokinetic parameters, such as peak torque (PT) and power

at different velocities for the quadriceps and hamstrings, were significantly higher in the

group of football players with a hamstring injury history than in the control group.

Looking at the RDL as a parameter for injury detection or injury risk, no significant

association was found with injury history nor injury occurrence during follow-up. More

8

so, RDL performance showed no meaningful correlation with any of the isokinetic

parameters.

Conclusion: IKD and RDL testing did not reveal hamstring strength deficits in players

who have sustained a hamstring injury in the past seasons or during the follow-up

period. Thereby, strength testing alone is not enough to detect a previous HSI, or to

predict if a player is at risk for future injuries.

Key words: Isokinetic dynamometry, hamstring strain injury, Romanian Deadlift,

football.

1.2 Abstract (NL)

Achtergrond: Hamstringblessures zijn de meest voorkomende blessure bij

professionele mannelijke voetballers. Om te voorkomen dat deze blessures zich

(opnieuw) voordoen, is het belangrijk om te bepalen of er een geldig

beoordelingsinstrument beschikbaar is om toekomstige hamstringblessures te

voorspellen in spelers (met een verhoogd risico). In hoeverre de beoordeling van de

hamstringsterkte kan gebruikt worden als een valide instrument voor de evaluatie van

het risico op een hamstring letsel en welke testmethoden het meest valide zijn, is in de

loop der jaren onderwerp van discussie geweest.

Onderzoeksdesign: Een gemengd cross-sectionaal en prospectieve cohortstudie.

Doelstellingen: Het doel van deze studie was om te onderzoeken of isokinetische

dynamometrie (IKD) en/of een meer functionele krachttest eerdere hamstringblessures

kan detecteren en risicospelers kan identificeren. Verder werd de correlatie tussen de

isokinetische parameters en de uitkomst van de functionele krachttest onderzocht.

Methode: In totaal werden 49 mannelijke voetballers getest in het tussenseizoen van

het voetbalseizoen 2018-2019. Knieflexie- en extensie draaimomenten werden

bilateraal verzameld bij hoeksnelheden van 60 (°/s) en 240 (°/s) en 240 (°/s)

concentrisch voor zowel quadriceps als hamstrings, gevolgd door een protocol bij

snelheden van 30 (°/s) en 120 (°/s) concentrisch en excentrisch voor de

hamstringspieren op een isokinetische dynamometer. Een Romanian Deadlift (RDL)

protocol fungeerde als functionele krachttest, waarbij het aantal herhalingen tot

uitputting werd gebruikt als functionele inschatting van de kracht en het

uithoudingsvermogen van de hamstrings. Verschillende groepen van geblesseerde

9

spelers werden vergeleken met een controlegroep, en gekwetste benen werden

vergeleken met niet-gekwetste benen van dezelfde groep.

Resultaten: Als we kijken naar de excentrische isokinetische parameters van de

hamstringspieren, zijn er geen significante verschillen gevonden tussen de

geblesseerde groepen en de controlegroep. Meerdere concentrische isokinetische

parameters, zoals piek draaimoment en vermogen bij verschillende snelheden voor de

quadriceps en hamstrings, waren significant hoger bij de groep voetballers met een

geschiedenis van hamstringblessures dan bij de controlegroep. Kijkend naar de RDL

als een parameter voor letseldetectie of letselrisico, werd er geen significant verband

gevonden met een eventuele geschiedenis van een letsel of het optreden van letsel

tijdens de follow-up. Sterker nog, de RDL-prestaties toonden geen betekenisvolle

correlatie met een van de isokinetische parameters.

Conclusie: IKD- en RDL-testen hebben geen tekort aan kracht in de hamstrings aan

het licht gebracht bij spelers die een hamstringblessure hebben opgelopen, in de

afgelopen seizoenen of tijdens de opvolgingsperiode. Het testen van de sterkte alleen

is dus niet voldoende om een eerder hamstringletsel te herkennen of om te voorspellen

of een speler in de toekomst risico loopt op een blessure.

Trefwoorden: Isokinetische dynamometrie, hamstringblessure, Romanian Deadlift,

voetbal

10

2. Introduction

Football is a popular sport, played by a substantial portion of men and women

worldwide, regardless of age and level. Besides the obvious health benefits related to

sports participation, it is also associated with a decent injury risk.1 Hamstring strain

injuries (HSI) are the number one most common injury professional male football

players cope with.2 Not only has HSI a high incidence rate, but it also is associated with

significant recurrence rates.3 Therefore a valid assessment tool to prevent injuries and

detect players at risk is necessary, if this is possible taking into account the

multifactorial origin of hamstring injuries.4

The biceps femoris is the most commonly (53%) strained muscle of the hamstring

complex.3 The hamstring muscles must control the torque towards knee extension and

hip flexion eccentrically during running activities. Due to the maximized load on the

hamstrings in the elongated position, most of the strains occur during the late swing

phase in running.5

In the current literature many predisposal risk factors have been appointed. Age,

previous injury, ethnicity, strength imbalances, insufficient flexibility, fatigue,

inadequate warm-up, are all factors that have been proposed relating to HSI risk.6,7 Lee

et al. have proved that the following three factors were linked to increased risk of acute

hamstring strains: significant lower isokinetic hamstring strength, a Hecc/Qcon ratio

lower than 0.55 and a previous injury.8 The Dynamic Control Ratio (DCR), also known

as a functional or mixed ratio (Hecc/Qcon), is suggested to identify strength imbalances

by Wright et al.9

In this regard it is of essential importance to measure the eccentric strength accurately.

A now commonly used way to measure eccentric strength is by using an isokinetic

dynamometer (IKD), but this strength might not compare to the strength needed for

running and might therefore not give valid information for injury prediction. When we

measure strength on the IKD, we ask the person to hold a position against a greater

force. This is not the same movement as the late swing phase in running.16 Therefore

the IKD, which is considered the golden standard for strength measurement, can be

called into question.

The inconsistencies about HSI predictors found in the previous part of this dissertation

suggest that IKD might not be the best assessment tool for predicting HSI. The present

literature uses many assessment tools but none of them seem to be completely valid.

This might imply a ‘missing link’ in HSI risk identification, which hampers the predictive

11

value of the baseline screening protocols used to evaluate the athlete’s future injury

susceptibility nowadays. As multiple researchers have questioned the methodological

quality of eccentric strength testing using IKD, this missing link could potentially consist

of valid eccentric hamstring strength objectivation and as such, an actual reliable

measure of hamstring muscle function. Maximal eccentric hamstring strength potential

is generally evaluated using IKD, but it is unknown whether this muscle activation

actually accords to the same neuromuscular activation patterns needed during high

speed running activities (during which hamstring injuries mostly occur). More so, the

majority of hamstring strength testing is executed in a seated position, which is a non-

functional position and does not allow functional hamstring muscle activation as is the

case in high speed running.

In this study we want to investigate the value of isokinetic strength measurements in

HSI risk identification, by comparing eccentric hamstring performance investigated by

means of isokinetic strength assessment with eccentric hamstring performance during

an RDL task. The latter is a functional movement similar to the movement happening

on the field. Besides there is a maximized hamstrings activity relative to other functional

exercises.10 The authors wish to investigate to what extent isokinetic eccentric strength

correlates with functional eccentric performance objectified by a standardized RDL test

and to verify whether these measurements can predict if an athlete is at risk of future

injury. In other words, can we predict and detect injuries by only looking at a single

strength measurement? And if yes, is the functional strength measurement an

appropriate assessment tool?

We hypothesize that the functional strength measurement will present the best

association with hamstring injury history and occurrence, whereas isokinetic eccentric

results will be less sensitive for hamstring injury risk prediction. In addition we also

expect to find no correlation between the RDL and IKD values.

12

3. Methods

3.1 Design

A retrospective cross-sectional and prospective cohort study design was used in this

study.

3.2 Participants

As it is hard to convince a complete football team to participate in a study like this one,

the authors recruited individual football players by advertising on social media and by

word-of-mouth communication in the authors’ social circles. Ultimately, forty-nine male

football players competing at recreational level were willing to participate. They were

divided into different groups, based on the presence of HSI and the time of the injury.

Table 1: Division of the participants: mean values of each group’s properties

HSI last season

(SD)

HSI history

(SD)

HSI during

follow-up (SD)

Control group

(retrospective)

(SD)

Control group

(prospective)

(SD)

n 7 15 6 19 15

Age in years 24.0 (5.6) 22.3 (2.0) 21.2 (1.9) 21.3 (2.1) 21.4 (2.0)

Height in cm 181.0 (7.3) 180.0 (7.7) 179.7 (5.0) 179.5 (6.2) 179.5 (5.8)

Weight in kg 74.9 (5.5) 74.2 (7.2) 74.8 (9.0) 71.0 (8.4) 71.1 (8.1)

BMI in kg/m² 22.9 (2.5) 22.9 (1.8) 23.1 (2.4) 22.0 (1.8) 22.0 (1.7)

Experience in

years

16.6 (7.3) 14.7 (3.9) 15.0 (1.7) 14.4 (3.7) 15.1 (2.2)

Weekly sport

time in hours

5.1 (0.99) 5.2 (0.88) 5.6 (0.58) 5.4 (1.1) 5.2 (0.96)

Thirty-eight subjects (78%) reported that they were right-leg dominant with respect to

kicking and eleven (22%) were left-leg dominant. Participants completed an injury

history questionnaire (Appendix 1) regarding their general injury history and HSI in the

past. Footballers were eligible if they were over 18 years of age and competing on a

recreational level within the Belgian Football competition series. Participants were

excluded if they reported suffering from any injury or physical complaint at the time of

testing, preventing them to perform maximally during training and game play.

13

3.3 Procedure

All players were tested during the interseason period of the 2017-2018 and the 2018-

2019 football season (June, July and August 2018). The subjects were asked not to

train vigorously or participate in gameplay 48 hours prior to testing, to avoid fatigue

related bias. All participants were evaluated under the same conditions in a research

laboratory at the Ghent University Hospital (UZ Gent, Ghent, Belgium) and were

informed about the purpose of the study. They read and signed an informed consent

prior to testing. The study was approved by the Medical Ethics Committee of the

University of Ghent. Each participant underwent a standardized testing procedure,

consisting of a 10-minute warm up on a stationary bike, followed by a maximal

isokinetic strength evaluation and a standardized functional RDL testing procedure.

Isokinetic data collection was performed using a Biodex Isokinetic dynamometer

(BIODEX System 4, Biodex medical systems, NY, US). The subjects were positioned

sitting upright and secured using torso, thigh and shin stabilization straps. The axis of

rotation of the IKD was aligned with the axis of rotation of the subject’s knee. The shin

stabilization strap at the end of the lever arm was positioned 2 cm proximal to the lateral

malleolus. Subjects were asked to place their hands on the handles at either side of

the dynamometer. Concentric and eccentric knee flexion and extension torques were

systematically recorded within a standardized knee range of motion going from +/- 0°

up until 100° of knee flexion. Extension and flexion torques were gathered bilaterally,

the dominant leg being evaluated first. A comprehensive isokinetic testing protocol was

used, consisting of both concentric and eccentric measurements at different angular

velocities. Unilateral maximal voluntary torque for the hamstrings and quadriceps were

measured in the following order; 60 degrees/second (°/s) concentrically, 240°/s

concentrically for both quadriceps and hamstrings. This was followed by a protocol only

for the hamstrings at 30°/s and 120°/s concentrically and eccentrically. Between the

different velocities, subjects had 45 seconds rest. During the test a standardized

protocol with verbal instructions was used. An adequate familiarization with the IKD

was given by a few submaximal test repetitions at each velocity. In case of pain the

subjects could always terminate the test by pushing the release button.

After the isokinetic testing, a short cooling down of 5 minutes on the stationary bicycle

was foreseen, in order to allow the legs to recover dynamically before submitting the

participants to subsequent RDL testing. The reason why they had to perform a RDL is

because of the maximized hamstring activity in the RDL relative to leg curl and the

good morning exercise.10 It also corresponds with the movement happening on the

field. The RDL testing protocol was initiated with a standardized explanation on how to

14

perform an RDL, providing the athletes with both verbal and visual instructions. They

were instructed to lower the bar just underneath the patella with extended arms, which

accords with circa 80° of hip flexion, keeping both knees flexed approximately 20°

throughout the entire RDL procedure. They were asked to do this while maintaining a

neutral position of the spine. Participants were instructed to perform these deadlifts at

a standardized pace, using the guidance of a metronome. Each deadlift needed to be

performed within a 3-second time window, in which the eccentric downward phase had

to be covered within the first 2 seconds, whereas the concentric upward phase needed

to be performed in only 1 second. In this way, angular velocity was estimated to be

approximately 40°/s for the eccentric phase, and 80°/s for the concentric phase, which

is in line with the isokinetic testing speed (resp. 30°/s eccentrically and 60°/s

concentrically). RDL testing performance was objectified by counting how many times

the deadlift was executed correctly (constant speed, no compensations).

For the data analysis, the value corresponding to the following calculation was used:

Value = number of correctly executed repetitions, multiplied by the weight lifted (olympic

barbell + weights), divided by body weight.

3.4 Follow-up of hamstrings strain

After strength testing, participants were monitored prospectively on injury occurrence

throughout the first part of the 2018-2019 football season (from September until

December 2018). This was done by sending out questionnaires (Appendix 2) on a

weekly basis, via which they were asked to report whether they were experiencing any

pain or discomfort in the lower limbs as a result of football exposure (training or

gameplay). In case of a major injury concerning the hamstrings more information was

asked. They were asked to report how long the injury lasted, how long they were absent

from the game and training and how the injury was treated.

3.5 Statistical analysis

Parameters obtained by isokinetic testing were Peak Torque (PT), average Peak

Torque (avrPT) and average Power (avrPower). For the hamstring muscles these

parameters were measured both concentrically at 30°/s, 60°/s, 120°/s and 240°/s, and

eccentrically at 30°/s and 120°/s. Quadriceps muscles were only measured

concentrically at 60°/s and 240°/s. The bodyweight (BW) reported in the questionnaire

was used to calculate relative PT and avrPT (PT/BW and avrPT/BW). Ratios (H:Q and

DCR) were calculated with the measured parameters.

15

Participants were divided into groups depending on the type and time of injury. The first

group consisted of players who had sustained a hamstring injury in the season just

before testing (2017-2018) but were recovered and fit to play at the time of testing. A

second group was made up out of players with a history of hamstring injuries, before

the season of 2017-2018. If players had a history of hamstring injuries and injured

themselves again in the previous season, they were put in the first group. This choice

was made because a more recent injury was assumed to have more impact on the

strength testing than an older one. Players who got injured on their hamstring muscles

during our follow-up period (from September to December 2018) where classified in a

third group.

Because we had to start analysing the results before the end of the follow-up period,

there are two control groups. The first control group consists out of all the players who

did not sustain an injury that could influence the hamstring function. These injuries

include a direct hamstring injury, injuries at the knee or hip joint, or low-back pain. This

first control group was used to analyse the data of the players injured in previous

seasons. After the follow-up period a second control group was created. This was

necessary because some players of the first control group injured themselves during

the follow-up period. Their injuries included hamstring related injuries, but also other

injuries that caused an absence longer than one month. A player might be susceptible

for a hamstring injury, but not get injured because of little exposure time.

The Independent-Samples T Test was used to compare the means of the injured legs

of the injured groups and the control groups. The Paired-Samples T Test was used to

compare injured and non-injured legs in the different injured groups and dominant and

non-dominant leg in the control groups. Correlation between the RDL values and the

isokinetic parameters for the concentric and eccentric hamstring strength was

investigated using the Pearson test.

16

4. Results

4.1 Incidence of hamstring injury

Seven of the forty-nine players (17.5%) had recently, being last season prior to testing,

injured their hamstring muscle. One of those seven players injured his dominant leg,

while three injured their non-dominant leg. Another three players sustained an HSI on

both of their legs. When asked if they had injured their hamstring muscles before, three

players answered yes. The injuries occurred at the same leg.

Looking at history of HSI, i.e. injuries that occurred more than one season ago, fifteen

players (31.6%) indicated to have injured their hamstring muscles in the past. Nine of

those injuries occurred at the dominant leg, three at the non-dominant leg, and another

three at both legs.

During the follow-up period six players (12.5%) sustained an HSI. Four injuries were

on the dominant leg, two on the non-dominant leg. Two of the players who got injured

during the follow-up period had already sustained an HSI in the past. One player

sustained it during the previous season, the other one the season before that.

Combining these figures shows that 46.9% of the study population has suffered from a

hamstring injury at least once in their career.

4.2 Comparing injured and non-injured players

Mean values obtained by IKD are shown in tables 2, 3 and 4. Table 5 shows the

calculated ratios and RDL values for the different groups. All groups of injured players

were compared to a control group. Players with a recent hamstring injury and players

with a history of hamstring injury were compared to a control group who had never

suffered from an HSI. This control group is named ‘control group (retrospective)’

because it is used to detect a previous injury.

The isokinetic parameters of players with a recent HSI, sustained during the 2017-2018

season, showed no significant difference (p>0.05) to this control group. Furthermore,

neither the functional test, nor the RDL, nor any of the ratios show a significant

difference (p>0.05) when these groups are compared.

However, differences are found when the group of players with a history of hamstring

injuries is compared with that same control group. Concentric quadriceps (con Q) and

17

hamstring (con H) relative peak torque (PT/BW), measured at 60°/s, is significantly

higher in the injured group (p=0.003 and p=0.043, respectively).

For the same angular velocity, the average PT/BW is also higher in the injured group

when quadriceps (p=0.027) and hamstrings (p=0.016) are measured concentrically. At

the same angular velocity of 60°/s, the concentric avrPower of the quadriceps and

hamstring muscles of the hamstring injury history group is significantly higher (Q:

p=0.033, H: p=0.008) than the values measured in the retrospective control group.

AvrPT/BW of the hamstring muscles concentrically measured at 240°/s is also higher

in the injured group (p=0.021). Moreover, the avrPower of Q and H measured at this

speed shows higher values for the players with a history of hamstring injury (p=0.018,

p=0.001). Comparing the hamstring injury history group with the retrospective control

group gives no significant differences in RDL values or any of the ratios (p>0.05).

At last, a group containing the players that got a hamstring injury during the follow-up

period was compared to a control group with players who never have had an HSI

(control prospective). This comparison showed no differences in isokinetic parameters

for Q or H, concentric or eccentric, at any measured angular velocity (p>0.05). Ratios

and RDL values are not significantly different either between both groups (p>0.05).

Overall, none of the eccentric isokinetic hamstring parameters were significantly

different between any of the injured groups and control groups. The same goes for the

ratios and RDL values (p>0.05).

Table 2: Mean values of isokinetic parameter relative peak torque (in N.m/kg.100) at different angular velocities

Hamstring

injury last

season (SD)

Hamstring

injury history

(SD)

Hamstring

injury during

follow-up (SD)

Control group

(retrospective)

(SD)

Control group

(prospective) (SD)

Q con 60

H con 60

248.7 (62.9)

121.8 (31.2)

296.7 (26.6)*

150.5 (24.6)*

265.2 (42.9)

130.1 (39.6)

259.7 (37.6)

131.3 (27.5)

263.7 (39.1)

139.5 (27.7)

Q con 240

H con 240

148.9 (31.0)

93.8 (30.5)

165.0 (16.8)

104.8 (14.0)

155.8 (28.2)

88.1 (27.9)

158.2 (23.4)

95.0 (18.7)

154.6 (22.5)

98.3 (17.3)

H con 30

H ecc 30

172.4 (47.2)

193.4 (50.0)

191.1 (47.9)

223.0 (50.0)

202.6 (38.3)

226.7 (51.7)

190.9 (63.2)

221.8 (53.4)

191.1 (56.6)

224.9 (51.0)

H con 120

H ecc 120

199.7 (62.8)

202.3 (63.7)

207.2 (47.0)

226.0 (44.3)

194.4 (50.9)

211.7 (48.5)

199.3 (49.3)

214.6 (50.7)

202.5 (42.3)

216.0 (45.3)

18

Q: quadriceps muscle, H: hamstring muscle, con: concentric, ecc: eccentric, SD: standard

deviation

Significant values are indicated with *.

Table 3: Mean values of the isokinetic parameter average relative peak torque (in N.m/kg.100) at different angular velocities

Hamstring injury

last season (SD)

Hamstring injury

history (SD)

Hamstring injury

during follow-up

(SD)

Control group

(retrospective)

(SD)

Control group

(prospective)

(SD)

Q con 60

H con 60

215.3 (52.4)

109.4 (28.0)

267.5 (34.1)*

141.3 (23.7)*

228.2 (44.3)

110.5 (26.3)

235.1 (44.9)

120.3 (24.3)

238.9 (44.4)

128.2 (24.7)

Q con 240

H con 240

138.6 (36.1)

84.3 (28.3)

150.2 (14.1)

95.7 (12.2)*

137.2 (26.8)

77.2 (27.4)

139.0 (18.3)

83.8 (15.8)

135.4 (18.2)

86.9 (14.9)

H con 30

H ecc 30

153.3 (44.7)

167.7 (49.4)

173.2 (46.9)

199.1 (52.8)

176.0 (43.3)

193.6 (64.8)

164.9 (63.2)

190.8 (41.0)

164.5 (54.3)

197.7 (49.7)

H con 120

H ecc 120

176.1 (60.8)

163.8 (57.9)

178.0 (41.4)

198.3 (46.2)

178.9 (54.7)

186.8 (59.4)

166.8 (46.6)

180.5 (47.3)

169.2 (41.2)

182.7 (43.2)

Q: quadriceps muscle, H: hamstring muscle, con: concentric, ecc: eccentric, SD: standard

deviation

Significant values are indicated with *.

Table 4: Mean values of the isokinetic parameter average power (in Watt) at different angular velocities

Hamstring injury

last season (SD)

Hamstring injury

history (SD)

Hamstring injury

during follow-up

(SD)

Control group

(retrospective)

(SD)

Control group

(prospective)

(SD)

Q con 60

H con 60

110.1 (28.6)

63.7 (22.2)

138.4 (23.0)*

84.7 (19.0)*

125.5 (21.0)

66.3 (22.0)

118.1 (28.6)

67.4 (16.8)

121.2 (30.6)

73.8 (19.2)

Q con 240

H con 240

254.1 (77.8)

134.7 (52.1)

275.7 (30.8)*

167.6 (28.2)*

250.8 (72.4)

135.2 (66.1)

241.3 (45.8)

130.9 (28.8)

135.4 (18.2)

140.0 (34.4)

H con 30

H ecc 30

31.3 (10.4)

47.4 (13.4)

37.2 (11.8)

57.5 (15.5)

34.7 (11.6)

59.5 (21.3)

29.4 (11.8)

50.8 (13.3)

30.7 (12.7)

54.0 (16.1)

H con 120

H ecc 120

74.3 (34.7)

151.1 (62.2)

81.2 (24.8)

191.9 (56.0)

79.5 (32.0)

175.9 (61.3)

70.3 (24.2)

163.3 (49.7)

71.7 (26.0)

171.3 (54.5)

Q: quadriceps muscle, H: hamstring muscle, con: concentric, ecc: eccentric, SD: standard

deviation

Significant values are indicated with *.

19

Table 5: Ratios at different angular velocities and RDL values for each goup

Hamstring

injury last

season (SD)

Hamstring

injury history

(SD)

Hamstring

injury during

follow-up (SD)

Control group

(retrospective)

(SD)

Control group

(prospective)

(SD)

H:Q 60

H:Q 240

0.50 (0.098)

0.63 (0.15)

0.51 (0.070)

0.64 (0.10)

0.49 (0.13)

0.56 (0.15)

0.51 (0.078)

0.61 (0.12)

0.53 (0.070)

0.64 (0.12)

DCR 30/60

DCR 30/240

DCR 120/60

DCR 120/240

0.78 (0.098)

1.31 (0.28)

0.81 (0.15)

1.36 (0.37)

0.75 (0.13)

1.36 (0.31)

0.76 (0.12)

1.38 (0.29)

0.86 (0.20)

1.47 (0.32)

0.81 (0.19)

1.36 (0.23

0.86 (0.18)

1.41 (0.31)

0.83 (0.17)

1.37 (0.30)

0.86 (0.19)

1.47 (0.34)

0.82 (0.16)

1.42 (0.32)

RDL 20.6 (5.28) 17.4 (8.80) 22.1 (11.2) 17.4 (9.52) 16.2 (7.21)

H:Q stands for the conventional concentric hamstring:concentric quadriceps ratio

DCR: Dynamic Control Ratio, eccentric hamstring:concentric quadriceps

RDL: Romanian Deadlift

Significant values are indicated with *.

Table 6: Significant mean differences between the hamstring injury history group and the retrospective

control group

mean difference (SD) significance = p

PT/BW Q con 60 H con 60

36.94 (11.48) 19.12 (9.08)

0.003 0.043

avrPT/BW Q con 60 H con 60 H con 240

32.42 (13.99) 21.04 (8.31) 11.96 (4.95)

0.027 0.016 0.021

avrPower Q con 60 H con 60 Q con 240 H con 240

20.24 (9.08) 17.32 (6.15) 34.49 (13.81) 36.77 (9.86)

0.033 0.008 0.018 0.001

Q: quadriceps, H: hamstring, con: concentric, PT: peak torque, BW: bodyweight, avrPT: average

peak torque, avrPower: average power

20

4.3 Correlation between IKD and RDL

Concentric and eccentric hamstring strength parameters, obtained by IKD, were

compared to the RDL values. Because the RDL is a bilateral exercise, isokinetic

measures from both dominant and non-dominant leg were used in the comparison.

The Pearson correlation test showed significant (p<0.05) coefficients between these

two strength measurements, but all were below 0.37. This is shown in table 7.

Table 7: Significant correlation coefficients between isokinetic hamstring strength measures at different angular velocities and RDL values

Pearson correlation coefficient

significance = p

PT/BW con 30 ndom con 120 ndom ecc 30 ndom ecc 120 dom ecc 120 ndom

0.312 0.308 0.282 0.348 0.359

0.029 0.032 0.049 0.014 0.011

avrPT/BW con 30 ndom con 120 ndom ecc 30 ndom ecc 120 dom ecc 120 ndom

0.307 0.368 0.296 0.330 0.307

0.032 0.009 0.039 0.021 0.032

avrPower ecc 120 dom

0.334

0.019

PT: peak torque, BW: bodyweight, avr: average, con: concentric, ecc: eccentric, dom: dominant leg, ndom:

non-dominant leg

21

5. Discussion

This study wanted to investigate the value of strength testing in HSI risk evaluation in a sample

of male football players, by using both IKD at different speeds and a more functional RDL task.

The first hypothesis was that a functional strength measurement would be more valuable in

detecting previous hamstring injury and predicting injury than IKD. This could not be confirmed

by the empirical study.

Secondly, it was hypothesized that the values of the RDL test would not correlate with the

isokinetic hamstring strength measures. The study supports this second hypothesis.

5.1 Detecting previous hamstring strain injury

Significant results were achieved only in the analysis of the players with a history of hamstring

injury (>1y). With this group, the concentric quadriceps and hamstring strength parameters

obtained by IKD were higher than those of the control group. This was also established in the

study of Bennell et al.11, suggesting that hamstring strengthening throughout rehabilitation

could be indeed effective. It should however be noted that only the concentric parameters were

higher, not the eccentric ones. This could suggest that rehabilitation programs should focus

more on eccentric rather than concentric strength training for lowering the risk of future HSI.12

However, the higher concentric parameters could not be found with the players who got injured

last season, which could mean that the higher parameters were rather a coincidence. This

should be further explored.

All of these findings show that it is not possible to accurately identify players with a previous

HSI out of a group, based solely on strength testing.

5.2 Predicting future hamstring strain injury

Higher concentric quadriceps strength could lead to an increased risk of HSI, if the hamstring

muscles are not capable to counteract the movement of the lower limb created by concentric

quadriceps activation. To evaluate this, the DCR was used. In the DCR we compare the

eccentric hamstring strength to the concentric quadriceps strength. This is based on the idea

that the hamstring muscles are contracting eccentrically to control the late swing phase during

running.13

22

Yet, our results showed no significantly higher concentric quadriceps strength, nor a lower

DCR for the group of players who sustained an HSI during the follow-up period. This leads to

believe that concentric quadriceps strength is not a useful parameter to consider when trying

to predict HSI. This is in line with the findings of Bakken et al. who have found a difference in

concentric quadriceps strength of injured and non-injured groups, but the difference is too small

to clinically distinguish both groups.14

Our results did not show lower eccentric hamstring strength in players who sustained an HSI

during the observation period. A study by van Dyck et al. identified lower eccentric hamstring

strength, adjusted for BW, as a risk factor for HSI, but only with a weak association.15 They

however note that their findings have little clinical value.

Research by Van Hooren et al. on the other hand, suggests that the hamstring muscles are

only contracting isometrically while other, elastic structures are lengthened passively.16 This

implies that the conditions under which the hamstring muscles must function during running

activities does not correlate with the conditions under which they are tested during strength

measurements. Therefore, this leads to inconsistencies in the literature.13,16 These

inconsistencies were pointed out in the previous part of this dissertation.

5.3 Correlation between RDL and IKD

Results showed no correlation between the concentric or eccentric isokinetic hamstring

strength parameters and RDL values: the coefficients for the significant parameters were lower

than 0.37. This is in line with what was expected and hence confirms the hypothesis that the

IKD and the RDL exercise both ask a different type of activity from the hamstring muscles.

5.4 The value of IKD

IKD might not be valuable in the prediction and evaluation of risk concerning HSI, but this does

not mean this strength test is to be abandoned. The test is an objective way to establish

strength deficits, and thereby identify players who might need specific training. Strength

training has actually shown to reduce the incidence of lower limb injury.17 Another valuable

advantage of IKD is that it can measure the effect of a training programme.

On the other hand, there are several reasons to question the value of IKD as a predictor. First,

measuring muscle strength alone is not enough to establish an injury risk profile because of

the multifactorial cause of injury. A player can be predisposed for injury because of an internal

risk factor, such as muscle strength, and become susceptible for injury because of an external

risk factor, bad weather for example. Still, to actually sustain an injury an inciting event has to

23

take place, such as an increase in gameplay.18 All of these factors should be taken into account

when establishing a risk profile.

Another reason why IKD did not qualify as valid predictor for HSI in this study, might be

explained by the fact that IKD was used only to measure the peak torque and not the strength

at a certain angle. However, hamstring injuries are most likely to occur in 20°/30° of knee

flexion.19 Further research should investigate this.

5.5 The value of RDL

If the function of the hamstring muscles during running, controlling the motion of the leg

eccentrically or isometrically, differs from what is tested during IKD, a different strength test

might be needed to evaluate the strength deficiencies associated with HSI.12,16 The RDL

seemed a promising alternative.

While IKD is an open kinetic chain exercise performed in a seated position, the RDL is a closed

kinetic chain exercise that incorporates core strength and shows a high hamstring EMG

activation.10,20 In this study, the RDL test was also used to evaluate the strength endurance of

the hamstring muscles. HSI are more common at the end of the game when fatigue sets in,

implying that strength endurance could be an important parameter in the injury mechanism.21

However, the RDL test did not allow us to distinguish injured players from healthy subjects, as

the differences were not significant.

5.6 Limitations

Although being the first to evaluate the value of a functional strength testing protocol using

isokinetic strength testing, the golden standard reference, in function of HSI history and

occurrence in a sample of male football players, this study is not without limitations.

The first limitation is the participant recruitment. Because of the selection process, that was

not completely random, it is plausible that players with a previous injury and thus higher risk

for reinjury showed more interest to participate in the study. This possible bias would not have

been present if we would have been able to test complete football teams. Moreover, the

limitations due to the selection process could also explain the high HSI incidence rate of 49.6%

in our study. However, the participants’ average age was rather low, given that most of them

were friends of acquaintances of the three authors. The proven risk factor of higher age

therefore was not present within the group of participants.22,23

24

Secondly, it is possible that the RDL performance value was influenced by some of the

participants’ unfamiliarity with the exercise. The RDL has a specific movement pattern and

must be performed correctly to activate the hamstring muscles and thus be a valid test.

Another consideration here is the time lag between testing and injury. Several intervening

factors may have altered a player’s strength profile during the three-month follow-up. This

would have been a less important factor if the study had tested complete teams, of which the

players get more or less the same training schedule.

A last important limitation concerns the fact that this study did not control for football exposure

during the follow-up period. The incidence of injuries is strongly related to the number of games

played, as a player is seven times more likely to be injured during a game than during a training

session.24 As a result, the players who have played more games have been exposed to a

greater risk of getting injured. The number of games played was not considered in the follow-

up.

5.7 Conclusion and implications for future research

In this study we established that it is not possible to detect players with a previous HSI or

players that will sustain an HSI by solely testing their muscle strength.

Although we could not confirm that an RDL test is able to detect or predict HSI, we are

convinced that a functional strength test is more suitable for measuring hamstring strength

than IKD. Further research should investigate which functional strength test resembles the

hamstring function during high speed running the most. This functional strength test should

then be implemented in a larger test battery, looking at internal and external risk factors, as

well as possible exposure to inciting events. Only when all of this is taken into account, we

might be able to establish an athlete’s injury risk profile.

25

6. References

1) Krustrup, P., Nielsen, J. J., Krustrup, B. R., Christensen, J. F., Pedersen, H., Randers, M.

B., ... & Bangsbo, J. (2009). Recreational soccer is an effective health-promoting activity for

untrained men. British journal of sports medicine, 43(11), 825-831.

2) Ekstrand, Jan, Martin Hägglund, and Markus Waldén. "Epidemiology of muscle injuries in

professional football (soccer)." The American journal of sports medicine 39.6 (2011): 1226-

1232.

3) Woods, C., Hawkins, R., Hulse, M., & Hodson, A. (2002). The Football Association Medical

Research Programme: an audit of injuries in professional football—analysis of preseason

injuries. British journal of sports medicine, 36(6), 436-441.

4) Mackey, C., O'Sullivan, K., O'Connor, A., & Clifford, A. (2011). Altered hamstring strength

profile in Gaelic footballers with a previous hamstring injury. Isokinetics and Exercise Science,

19(1), 47-54.

5) Stanton, P., & Purdam, C. (1989). Hamstring injuries in sprinting—the role of eccentric

exercise. Journal of Orthopaedic & Sports Physical Therapy, 10(9), 343-349.

6) Opar, D., Williams, M., & Shield, A. (2012). Hamstring strain injuries: Factors that lead to

injury and re-injury [accepted manuscript].

7) Small, K., McNaughton, L., Greig, M., & Lovell, R. (2010). The effects of multidirectional

soccer-specific fatigue on markers of hamstring injury risk. Journal of Science and Medicine in

Sport, 13(1), 120-125.

8) Lee, J. W., Mok, K. M., Chan, H. C., Yung, P. S., & Chan, K. M. (2018). Eccentric hamstring

strength deficit and poor hamstring-to-quadriceps ratio are risk factors for hamstring strain

injury in football: A prospective study of 146 professional players. Journal of science and

medicine in sport, 21(8), 789-793.

9) Wright, J., Ball, N., & Wood, L. (2009). Fatigue, H/Q ratios and muscle coactivation in

recreational football players. Isokinetics and exercise science, 17(3), 161-167.

10) McAllister, M. J., Hammond, K. G., Schilling, B. K., Ferreria, L. C., Reed, J. P., & Weiss, L.

W. (2014). Muscle activation during various hamstring exercises. The Journal of Strength &

Conditioning Research, 28(6), 1573-1580.

26

11) Bennell, K., Wajswelner, H., Lew, P., Schall-Riaucour, A., Leslie, S., Plant, D., & Cirone,

J. (1998). Isokinetic strength testing does not predict hamstring injury in Australian Rules

footballers. British journal of sports medicine, 32(4), 309-314.

12) Askling, C., Karlsson, J., & Thorstensson, A. (2003). Hamstring injury occurrence in elite

soccer players after preseason strength training with eccentric overload. Scandinavian journal

of medicine & science in sports, 13(4), 244-250.

13) Chumanov, E. S., Heiderscheit, B. C., & Thelen, D. G. (2011). Hamstring musculotendon

dynamics during stance and swing phases of high speed running. Medicine and science in

sports and exercise, 43(3), 525.

14) Bakken, A., Targett, S., Bere, T., Eirale, C., Farooq, A., Mosler, A. B., ... & Bahr, R. (2018).

Muscle strength is a poor screening test for predicting lower extremity injuries in professional

male soccer players: A 2-year prospective cohort study. The American journal of sports

medicine, 46(6), 1481-1491.

15) van Dyk, N., Bahr, R., Whiteley, R., Tol, J. L., Kumar, B. D., Hamilton, B., ... & Witvrouw,

E. (2016). Hamstring and quadriceps isokinetic strength deficits are weak risk factors for

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1789-1795.

16) Van Hooren, B., & Bosch, F. (2017). Is there really an eccentric action of the hamstrings

during the swing phase of high-speed running? part I: A critical review of the literature. Journal

of sports sciences, 35(23), 2313-2321.

17) Thorborg, K., Krommes, K. K., Esteve, E., Clausen, M. B., Bartels, E. M., & Rathleff, M. S.

(2017). Effect of specific exercise-based football injury prevention programmes on the overall

injury rate in football: a systematic review and meta-analysis of the FIFA 11 and 11+

programmes. Br J Sports Med, 51(7), 562-571.

18) Meeuwisse, W. H. (1994). Assessing causation in sport injury: a multifactorial model.

19) Brockett, C. L., Morgan, D. L., & Proske, U. W. E. (2004). Predicting hamstring strain injury

in elite athletes. Medicine & Science in Sports & Exercise, 36(3), 379-387.

20) Cresswell, A. G., & Thorstensson, A. (1994). Changes in intra-abdominal pressure, trunk

muscle activation and force during isokinetic lifting and lowering. European journal of applied

physiology and occupational physiology, 68(4), 315-321.

27

21) Ekstrand, J., Hägglund, M., & Waldén, M. (2011). Injury incidence and injury patterns in

professional football: the UEFA injury study. British journal of sports medicine, 45(7), 553-558.

22) Opar, D., Williams, M., & Shield, A. (2012). Hamstring strain injuries: Factors that lead to

injury and re-injury.

23) Freckleton, G., & Pizzari, T. (2013). Risk factors for hamstring muscle strain injury in sport:

a systematic review and meta-analysis. Br J Sports Med, 47(6), 351-358.

24) Roe, M., Murphy, J. C., Gissane, C., & Blake, C. (2018). Hamstring injuries in elite Gaelic

football: an 8-year investigation to identify injury rates, time-loss patterns and players at

increased risk. Br J Sports Med, 52(15), 982-988.

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28

7. Lekenabstract

Hamstringletsels zijn de meest voorkomende letsels in het voetbal, goed voor 12-15%

van alle voetballetsels. Naast de hoge incidentie heeft deze blessure ook een hoge re-

injury rate en krijgen spelers dus vaak opnieuw te maken met dezelfde blessure.

Daarom is het van groot belang om spelers die risico lopen op de blessure te kunnen

identificeren en om toekomstige blessures te kunnen voorspellen. In onze studie gaan

we dan ook na of dat mogelijk is. Dat doen we aan de hand van twee krachtmetingen.

Een eerste meting is een niet-functionele meting via een isokinetische dynamometer.

Een tweede is een functionele krachtmeting, in dit geval een Romanian deadlift. We

gaan in het prospectieve deel van deze studie na of het via de krachtmetingen mogelijk

is om blessures bij spelers te voorspellen. In het retrospectieve deel kijken we dan weer

of we aan de krachtmetingen kunnen zien welke spelers een verleden van

hamstringletsels hebben. Daarnaast zullen we ook onderzoeken of deze twee

metingen dezelfde resultaten geven.

De hypothese van deze studie was dat de functionele krachtmeting superieur zou zijn

aan de niet-functionele, omdat de functionele meer lijkt op de bewegingen die de

voetballers op het veld uitvoeren. Die hypothese werd door het onderzoek niet

bevestigd. We kunnen besluiten dat het niet mogelijk is om blessures te herkennen of

te voorspellen op basis van een enkele factor. In de toekomst zullen er meer complexe,

functionele tests gebruikt moeten worden om te proberen blessures te voorspellen.

29

8. Bewijs indiening ethisch comité

30

31

32

Ma Di Woe Do Vrij Za Zo

9. Appendix

9.1 Appendix 1

Beste kandidaat, gelieve een aantal minuten de tijd te nemen om onderstaande vragenlijst in

te vullen. Bij vragen of onduidelijkheden kan u zich steeds wenden tot de onderzoekers.

1. Demografie Naam/Code:…………….………………………………………………………………..………………… Geboortedatum: …………………………………………………………………………….…………….. Geslacht: ……………………………………………………………………………………….………….. Nationaliteit: ……………………………………………………………………………………….………. Beroep: ……………………………………………………………………………………………….……. Hobby’s: ………………………………………………………………………………………………….… Woonplaats: ………………………………………………………………………………………………..

2. Antropometrie Gewicht: ………kg Lengte: ………..cm Voorkeursbeen: Links / Rechts (omcirkel wat past)

3. Sport en Training Algemeen Gelieve hieronder per dag het aantal uur dat u aan sport doet in te vullen. Competitief Sport: …………………………………………………………………………………….…………………. Competitieniveau: ………………………………………………………………………………..……….. Ervaring: ………jaren Training: ………u/week Wedstrijd: .…….u/week Type trainingen die de club organiseert: (omcirkel wat past) Kracht / Lenigheid / Cardio (uithouding / interval) / Sportspecifiek

33

Andere: …………………………………………………………………………………………………….. Wordt er op training aandacht besteed aan lenigheid en kracht? Ja/Nee Zo ja, hoeveel? ……u/week

Aanvullend/recreatief Sport(en): 1) ………………………… ……u/week 2) ………………………… ……u/week 3) ………………………… ……u/week

4. Blessures Algemeen Heeft u op dit moment ergens klachten, pijn of een blessure? Ja / Nee Zo ja, waar? ……………………………………………………………………………………………….. Bent u hiervoor in behandeling bij een arts, kinesitherapeut, of andere? Ja / Nee Zo ja, welke behandeling? ……………………………………………………………………………...…………..…………………… Volgt u een aangepaste training? Ja / Nee Zo ja, welke aangepaste training? ………………………………………………………………………………………………………………. Verleden: 1) Klacht, pijn, blessure ( + datum): ….………….…….…………………………………. Behandeling, aangepaste training: ……………………………………………………. Tijd afwezig van training: ………………………………….……………………………. Tijd afwezig van wedstrijd: …………………………………………………………....... 2) Klacht, pijn, blessure ( + datum): ….………….…….…………………………………. Behandeling, aangepaste training: ……………………………………………………. Tijd afwezig van training: ………………………………….……………………………. Tijd afwezig van wedstrijd: …………………………………………………………....... 3) Klacht, pijn, blessure ( + datum): ….………….…….…………………………………. Behandeling, aangepaste training: ……………………………………………………. Tijd afwezig van training: ………………………………….…………………………….

34

Tijd afwezig van wedstrijd: …………………………………………………………....... Specifiek Heeft u al eens een hamstringblessure opgelopen? Ja/Nee Indien Ja: Type blessure: overbelasting / verrekking / scheur (omcirkel wat past) Locatie: links / rechts Omstandigheden: begin / einde van training / match lopen / trappen / springen / andere: …………………………………..

5. Opmerkingen ……………………………………………………………………………………………………….……………………………………………………………………………………………………………….……………………………………………………………………………………………………………….……………………………………………………………………………………………………………………….

35

9.2 Appendix 2