Abstract
The event of a ‘tackle’ in a game of rugby union is the main cause of injury for players and yet, is something that is rarely the same on the training ground as it is on the pitch. There are a variety of types and techniques that players use to bring the opponent to the ground. However, I am going to investigate whether or not there is an ‘optimal’ type or technique of tackle to use. What I mean by ‘optimal’ is in terms of bringing the opponent down efficiently as well as a technique with a reduced risk of injury in the body areas that are involved in the contact. Initially, to narrow my search I am going to find the most commonly used types of rugby tackle in the modern game. Then I will perform them, analysing the forces involved on specific joints and body areas in each of the types of tackle. This will highlight the benefits and flaws with each type and from all the data we can deduce an ‘optimal’ tackle.
Introduction
Although personally I have only played it from since the age of 14, the sport of rugby union has truly become a passionate hobby of mine. I adore watching and analysing international and local fixtures as well as playing on a weekly basis. During my experience I have noticed that the one aspect of the game that is never the same on the training pitch compared to a live game is the tackling area. So the main reason for this project is to explore the tackling area in as much detail as possible to not only aid my own personal game but also perhaps others. Since I am also studying Physics and Maths A-Level, I intend to look into the underlying forces and biomechanics that come into play during a tackle and then apply statistical tests to the data.
As my question is quite broad, I will have three main stages of the experimental investigation. My aim with the first stage is to find a few techniques that are most commonly used and are the most effective in bringing a player to ground. I will also use video analysis where I will record the outcome of each specific type of tackle from multiple games. The second part of my experiment involves performing these types of tackles and analysing the physical forces behind the collisions and applying a statistical test to them. Stage three will be applying statistical tests to the data which help find the tackle with the least statistical chance of injury for the tackler. My hypothesis is that the ‘smother’ tackle is the optimal tackle, in contrast to the general view that a tackle with shoulder contact is the most effective. I believe that this is because a smother technique minimises contact to the head as well as having the priority to secure the ball from the ball carrier.
However, before all this I will need to provide background information and dive into the sport itself, looking at aspects such as the history, laws and general information that has made the game what it is today. All this will provide us with a basic understanding of the sport. In addition, it must be noted that my investigation will be drastically affected by the application of laws surrounding the tackling area as it is those laws which lay the boundaries of what an ‘optimal’ tackle can be.
Literature Review
Background
History
Rugby is a worldwide sport and as all sports, it has a history. Rugby originated from football, a game that has been played in various forms throughout history. In its various forms, ‘football’ was played from the times of the ancient Romans to the Medieval Era. Surprisingly throughout large periods of history many deemed the game too violent to be legal and so in England it was not until 1424 where the game of ‘futebol’ was finally legalised. Football then was variably different to the football we know today as it included areas of the game where one can throw and catch the ball, however one could not run with the ball in hand. Rugby as a game is believed to have been created by a student called William Webb Ellis in 1823 at his English school in a town called Rugby. During a game of football he simply picked up the ball and started to run with it. Only later in 1845 was this version of football fully legalised and named ‘rugby’ by three ex-Rugby School boys who were lawyers at the time. (Jones & Jackson, 2001[1]). If you want to see the difference between rugby and football just watch the Carabao Cup Final between Chelsea and Liverpool.
The sport began to be somewhat of a success as local clubs were being founded and many schools in the UK began to adopt the game of rugby. The culture spread further and now the sport is played globally in many forms including Rugby Union, Rugby League, Rugby 7s and Touch Rugby which are all governed by the Rugby Football Union (RFU). I will focus on the Rugby Union because this is the original version of rugby. The others are a progression of rugby union and have slightly different rules and regulations. The game also had a considerable influence in later sports such as American Football and Australian Rules Football. Internationally, now over one hundred countries have a national rugby union team, ranging from the All Blacks of New Zealand to the Red Roses of England to the Pumas of Argentina.
Famously, the prestigious Rugby World Cup trophy is called the Webb Ellis Cup (named after William). This is the prize for winning the most challenging rugby tournament between the best international rugby teams on the planet. In 1987 the first tournament for the Rugby World Cup was held in New Zealand. This marked the start of the professional era for rugby, where the best rugby players were beginning to get paid. Subsequently the image of the ‘rugby player’ began to change (Wikipedia, 2016[2] and Origins of Rugby, 2007[3])
General Information About Rugby Union
Professional rugby union involves two teams of fifteen players as well as eight substitutes. They are split up into two types of players: eight forwards and seven backs (the substitutes can be a variety). The players are numbered 1-15, 1-8 being the forwards, 9-15 being the backs. The forwards, commonly known as “The Pack”, are the players who take part in the ‘lineout’ and ‘scrum’ set pieces as well as specialising in securing the ball in rucks. The lineout and the scrum are both means of restarting play. The lineout is when the ball has gone out of play and the ball is thrown back into two parallel lines of players who can challenge for possession. The scrum is where after a foul the ball is thrown into two groups of interlocked forwards. This means that in the past they have usually been the bulkier, stronger, taller and consequently the slower players on the field. However recently, the highest level tier one international rugby teams (e.g. The New Zealand All Blacks) have been employing more mobile, technique-driven forwards for quicker play which is dominating the international scene and also evolving the image of ‘The Forward’. The backs are the players who tend to specialise in passing and kicking the ball. They are usually the faster players on the field who break through the defensive line and tend to score the most tries.
There are two halves in a game of rugby, each half lasting 40 minutes. However, unlike most major team sports the time can go on for as long as the ball is still in play after the 40 minutes pass. There is no extra time added on for injuries. Simply the play stops for a maximum of one minute for the injury to be treated and then play resumes. The points system in rugby is unique as at each end of the pitch there is an area called the in-goal area, designated behind the try-line. There is a set of posts on each try-line where the ball can be kicked through (similar to American Football). If a player, with control and downward pressure, places the ball on the try-line or in the in-goal area then they score a valuable five point ‘try’. This allows the attacking team a ‘conversion kick’ that allows the five points to be turned into seven points if a player from the attacking team kicks the ball through the posts. This has to be done from any position on the same horizontal line where the try was scored so players try to place the ball down as near to the posts as possible.
During the game, if a team commits a foul, they concede a penalty that can be turned into three points if a player from the opposing team kicks the ball through the posts from the spot where the foul was committed. Anytime during open play any team can attempt a ‘drop goal’. This is where a player drops the ball on the ground and then kicks it through the posts; they will be awarded three points. This area of the game is most famously recognised by the “golden boot” of Johnny Wilkinson in the Rugby World Cup Final of 2003 where a last minute drop goal in extra time (which changed the score from 17-17 to 20-17) gave the game to England over Australia. However, the main area of the game that I will be focusing on will be the tackling and contact area. (Wikipedia, 2016 and Mathew Brown[4] and World Rugby, 2016[5])
The Breakdown
As a player goes into contact, the player will get tackled and taken to ground. The attacking team will tend to have players close by in order to secure the ball. This then creates what is called a ‘Ruck’ and is often called the area of ‘The Breakdown’. A Ruck is defined by the RFU as:
‘A phase of play where one or more players from each team, who are on their feet, in physical contact, are close around the ball on the ground… Players are rucking when they are using their feet to try win or keep possession of the ball, without being guilty of foul play’ – World Rugby
This is also the point where the tackled player must let go of the ball (or pass it away) and players (usually forwards) attempt to secure possession of it, while supporting their own body weight, standing over it and holding off opposition players who try to get hold of it. No one can use their hands to secure the ball so they attempt to push each other off it and therefore players are only allowed to enter the ruck from the backmost position. Entering a ruck from the sides is deemed illegal as it creates an unfair advantage when ‘rucking over’ for the ball. Top tier teams want to recycle the ball from rucks as quickly as possible in order to get an opportunity to attack while the opposition’s defensive line is disorganised or even offside. This is important because both the attacker and defender who enter a tackle want to end up in a beneficial position for the ruck. (Wikipedia, 2016 and World Rugby, 2016)
Laws and Rules
The original concept of rugby was simple: grab the ball, either run into each other or pass it backwards. However there are many rules and laws that have been developed throughout the years in order to make the game flow smoothly for spectators and ensure that players are as safe as possible. Safety has been a more prioritised area since the beginning of the professional era. This is because players began to get bigger, faster, more specialised and consequently the collisions in the high tier games have become more forceful. Injuries are therefore much more frequent and severe. The main principles of the game, according to World Rugby, is said to be discipline, integrity, solidarity, respect and passion. In my opinion, as physically brutal as the game of rugby is, the discipline, sportsmanship and respect displayed by the players is like no other sport. (Morgan MSc MSST & Herrington PhD MCSP, 2013).
There are many laws and rules in the modern game. However, nowadays, ‘foul play’ is a key area focused on very carefully by officials due to the safety of the players. Offenses can range from obstruction to collapsing the scrum to playing the ball on the ground in the ruck. Examples of offenses around the tackling area that I will be looking into are the ‘high tackle’ and the ‘tip tackle’. Importantly though, Laws 15 and 16 are about the tackle and ruck respectively. Specifically Law 15 that is a key area that I will look into as this will define what actually a ‘tackle’ is. This of course will drastically influence what would make the optimal tackle. (World Rugby, 2016)
Tackling
The Basics
Before we can start looking at the various types of tackle, we need to understand what ‘a tackle’ actually is. The definition of a tackle by the RFU is:
‘A tackle occurs when the ball carrier is held by one or more opponents and is brought to ground…Opposition who held the ball carrier and do not go to ground are not tacklers’ – World Rugby
The laws surrounding tackling are very clear and there are many potential fouls that can be committed if the tackle is not performed well. In a tackle scene, there are two players: the tackler and the ball carrier. The tackler’s primary task is to stop the attacking player as quickly as possible. For the majority of situations on the pitch, the tackler will try to bring the ball carrier to ground (when the ball carrier has one or both knees on the floor). After the ball carrier has gone to ground, the next few moments of play are crucial for the tackler. He must release and get away from both the player and the ball immediately. If the tackler does not do these things he can concede a penalty and a potential three points against his team. The positioning of the tackler after a tackle is also crucial because if he finds his body obstructing the ball from the attacking team (even if he’s stuck there!) he will concede a penalty from slowing down the attacking play (another potential three points). (World Rugby, 2016)
During a tackle, the ball carrier also has his obligations. As soon as the ball carrier is tackled he must immediately release the ball by putting it on the ground or push it in any direction (preferably towards his own team). The sanction again being a penalty conceded against his team. Unfortunately for a ball carrier who is tackled, if an opposition player (which could be the tackler) is on his feet (supporting his own body weight) and competing for the ball by trying to rip it out of the ruck, the ball carrier may be deemed to be ‘holding onto’ the ball which is also illegal (even if the ball carrier is not actually holding onto the ball). This form of trying to get the ball from a ruck for either a penalty or a ‘turnover’ (change in possession between the teams) is called ‘jackaling’. This means for the tackler, being able to present the ball carrier in a way for either himself or a teammate to jackal is also important. (World Rugby, 2016) (Morgan MSc MSST & Herrington PhD MCSP, 2013). A former All-Black captain called Richie McCaw stated: “I try and have the mind-set of that the tackle isn’t finished until I\’m back on my feet” (McCaw)[6]
Types
For my investigation I am defining six main types of tackle:
- Shoulder Active – The tackler’s shoulder is the first point of contact then there is a leg drive and forward momentum for the tackler.
- Shoulder Passive – The tackler’s shoulder is the first point of contact however there is no forward momentum.
- Smother – The tackler wraps his arms around the ball carrier trapping the ball with any part of their bodies.
- Jersey – The tackler grabs the ball carrier’s jersey to try to bring the ball carrier down.
- Ankle Tap – The tackler taps the ball carrier’s foot or leg with force enough to trip him up, usually done when the ball carrier has already passed the defender and is running with some pace.
- Arms Short to Long – The tackler has no shoulder contact but contact with only his arms which extend while they bring the player down.
Figure 1[7] SHOULDER ACTIVE
Figure 2[8] SHOULDER PASSIVE
Figure 3[9] SMOTHER
Figure 4[10] JERSEY
Figure 6[13] ARM SHORT-TO-LONG
Injuries
General Information
As we all know with every high performance sport, injuries come with it. However as a comparison, rugby injuries are on average three times more frequent in a season than in football. Injuries occur at all ages yet most are experienced in the developing bodies of 10-18 year olds. Tackling is the main cause of injury in the game of rugby and therefore the knowledge of how to tackle is vitally important for young players. The tackles in a game of rugby are often the most forceful impacts any player will face during the game. Not only is it the event with the most forceful impacts but it is also very difficult to emulate it during training. I will be focusing on the potential injuries inflicted; specifically, those on the tackler. (Power[14] 2016) (Burger[15] & den Hollander, 2017).
Causes
There are a variety of potential causes of injury on and off the pitch while playing rugby. Tackle-related injuries are usually associated with dynamic and high-speed events: events that occur frequently in a typical game of rugby. (Seminati, Cazzola, Preatoni, & Trewartha, 2015)
Using data from the 2011 Rugby World Cup census there was an average of 3.6 injuries per game. A total of 56% occurred in the second half, highlighting that fatigue obviously has an impact. There were many causes including the scrum and rucks; however, tackling represented the main cause of injuries by a considerable margin, being the cause of a massive 44.4% of the total injuries:
Figure 7 – Cause of injury to players participating in the Rugby World Cup 2015[16]
Furthermore, according to data from the Rugby World Cup of 2015, a grand total of 52.3% of injuries were caused by either being tackled or performing a tackle. Between the two World Cups there have been major changes in key laws to make the game safer for players. For example, the engagement at the scrum was changed in 2012 where the two front rows pre-interlock before the engagement (i.e. pushing into each other). Notwithstanding this, the injury toll has still increased in the four-year period. Another 20.5% of injuries were caused by collisions so the contact and tackle area are a huge cause of injury in the modern game. Fatigue also continues to be a problem with 33% of the total injuries taking place in the final twenty minutes of the game. Injuries also occurred in training, as in any sport – ranging from the warm-ups to conditioning. (Colin Fuller, 2017)
Looking into academy and schoolboy level rugby, a study was done by the University of Bath in cohesion with the RFU, England Rugby and the RFU Injured Players’ Foundation that looked into the injuries of 12 premiership academies as well as 7 prestigious rugby schools. The data collected highlighted that schoolboy and academy level had around half the injuries per 1000 player hours when compared to international and premiership rugby teams (School = 35/1000, Academy = 44/1000, Premiership = 87/1000, International = 84/1000). Nevertheless, the severity of injury (in terms of days off playing) for academy and schoolboy level rugby players, at an average of 30 days off, is almost double that for International and Premiership players, at 18 days. In short, the injuries are fewer but more severe at lower tier rugby and with younger players. (Seminati, Cazzola, Preatoni, & Trewartha, 2015) (Palmer-Green, Trewartha, & Stokes, 2009)[17]
TYPES
Rugby injuries span the entire human body; however, some regions are more frequently-injured than others. From 639 international rugby players who took part in the 2015 Rugby World Cup, a table was constructed conveying the main types of injuries. Here are the main body areas affected:
| Percentage of Injuries / % | |||
| INJURY LOCATION | BACKS | FORWARDS | ALL PLAYERS |
| Head/Face | 22.2 | 21.7 | 22.0 |
| Shoulder/Clavicle | 6.7 | 4.8 | 5.8 |
| Trunk | 13.3 | 6.0 | 9.8 |
| Thigh | 13.2 | 22.9 | 16.8 |
| Knee | 17.8 | 14.5 | 16.2 |
| Lower Leg/Foot | 15.6 | 18.0 | 16.8 |
(Colin Fuller, 2017)
From the data collected a total of 49.8% of injuries were below the hip, which is the area that players are internationally taught to aim for when tackling. A further 22.0% of injuries were to the head (including concussion). The head is obviously an area of common contact when tackling. This evidence further depicts the number of injuries by tackles executed badly.
This next data set depicts the broad type of injuries from the 2015 World Cup:
| Percentage of Injuries / % | |||
| INJURY TYPE | BACKS | FORWARDS | ALL PLAYERS |
| Bone | 5.6 | 9.6 | 7.5 |
| Concussion | 16.7 | 12.0 | 13.9 |
| Joint/Ligaments | 34.4 | 31.3 | 32.9 |
| Muscle/Tendon | 38.9 | 42.2 | 40.5 |
(Colin Fuller, 2017)
Here, we can see that around 40% are muscle injuries. These tend to be strains and tears that occur at every high level sport. What particularly stands out to me is the 33% of ligament/joint injuries, the vast majority of which occur in the knee. The knee is a big contact area in the event of a tackle. This also agrees with another major study that concluded that tacklers were most at risk of injury when making low level tackles where their heads are vulnerable. It also concluded that overall, the tackle was the most common cause of injury and the head was the most common site of injury. (Kenneth & Will, 2008)
Data Collection Methods
There will be three main stages to my investigation. First, video analysis will be used from various sources to narrow the search from the six types of tackle to two or three, by recording what happens after each tackle and thereafter assessing their effectiveness. Secondly, there will be a practical element. This will include filming different types of tackle in a controlled environment and then using a computer program to obtain data such as velocity and momentum. In the final stage of my investigation a statistical test will be applied to my momentum data that will determine the validity of the data collected.
STAGE 1: EFFECTIVENESS AFTER TACKLE
The initial primary data will be collected from a total of six rugby games, two rugby union and four rugby 7s.
- The videos will be paused at every tackle. I will decide which type of tackle each individual tackler has done and subsequently record the outcome of that tackle, be it a miss-tackle or the ball being offloaded.
- Two tables will be constructed. One will show the type of tackle; the other will show the result of that specific type of tackle such as offload or missed.
- From this data we will not only find out which tackles are the most frequently used on average but also how effective each technique is.
STAGE 2: INITIAL METHOD INJURY RISK
Primary data will be obtained by analysing my friends tackling each other with a high speed camera (iPhone 6) and recording the velocity. The masses of each participant will be recorded to the nearest kilogram using an electric scale. Only the most successful (effective) tackles from my effectiveness data will be analysed further (Shoulder Active, Shoulder Passive and Smother).
- Initially the attacker will be stationary with the tackler coming in with his shoulder active, testing that type. Repeated 5 times.
- Next will be the tackler stationary with the attacker running in, demonstrating the shoulder passive tackle. Repeated 5 times.
- Like the last step, the tackler will be stationary but this time will perform a smother tackle. Repeated 5 times.
- Using a program called Tracker, a point of mass will be selected on the shoulder of the tackler and the program will graph out the velocity over the relevant time interval.
- From that data we can calculate momentum over a certain time before and after the tackle by the equation: p = mv (momentum = mass x velocity).
RE-WORKING STAGE 2:
Unfortunately, I was unable to obtain valid data from my initial tackling videos. This is because the light difference between the shirts of my participants and the background was indistinguishable by the tracker program (i.e. it was just too dark) and so the point of mass could not follow a set pixel.
Rectifying this issue, I have made a second data collection plan for my stage 2.
- Rugby footage of some of the games of my college’s 1st XV rugby team will be obtained. Quality of play is not so much of an issue as the team were national champions the previous year.
- Going through all the footage, 5 second clips of 5 clean tackles in each type and technique will be collected.
- These will be submitted into Tracker. Since the games occurred during the middle of the day, the light difference will not be an issue.
- The velocity of the tackler one second before and after the collision will be recorded. The mass of each tackler will be obtained to the nearest kilogram via inquiry.
- Using the same equation (p = mv), momentums before and after the contact will be calculated.
- This will give us the impulse[18].
Essentially, the only difference is that rather than using videos (where I practically performed the tackles), I have used primary video analysis done for real game situations. The same computer program is being used.
STAGE 3: APPLYING A REGRESSION ANALYSIS TEST
A Regression Analysis will be applied to the momentum data collected from the re-worked stage two of my investigation.
- For the fourteen samples the difference in velocities will be calculated (Velocity before – Velocity after).
- A regression analysis (on excel) will be applied on these fourteen data points and their corresponding impulses (Regression Analysis: Excel > Data Analysis > Regression).
- This will give me the correlation of my data, a value for the F-Test, T-Test and a P-value which will help decide the validity of the data that will be collected.
- The P-Value will give the probability that the samples collected occurred by chance. The F and T Tests will depict the validity of the data collected as well as their correlation.
Results and analysis
For the initial stage of my investigation, cup games 1 and 2 this year between New Zealand and Australia and the specific outcome of each tackle attempted by each individual player was recorded. I have highlighted the key pieces of data that, in my opinion, are the most useful to be taken from this research. The process was then repeated with four Rugby 7s games from the various tournaments around the world this year. I have subsequently added a secondary source to analyse and compare. Here is all the data:
STAGE 1:
PRIMARY DATA – RUGBY UNION
Figure 8 – Data of the type and effectiveness of each tackle technique from two international Rugby Union Fixtures [19]
PRIMARY DATA – RUGBY 7s
Figure 9 – Data of the type and effectiveness of each tackle technique from two international Rugby Sevens fixtures[20]
From the primary rugby union data (Figure 8), 250 out of the total 440 (56.8%) successful tackles were made with the shoulder. This highlights that the use of the shoulder is not only the most frequently used technique but also a reliable and effective way to tackle. Furthermore, of the total of 314 tackles using the shoulder, only 64 (20.4%) were either missed or had the ball offloaded. Surprisingly, the most effective way to tackle from this sample was the ‘smother’; only 9 of the total 146 (6.1%) smother tackles attempted were unsuccessful. The ‘jersey’ and ‘ankle tap’ tackles weren’t used frequently enough and so the data is negligible to determine their effectiveness. They are clearly not frequently used. The ‘arm short to long’ tackle from this was shown to be the least reliable way to tackle. Out of the total 117 tackles attempted, only 43 (36.8%) were successful. The rugby 7s data (Figure 9) supports the rugby union data in that the use of the shoulder and smother were the most successful types of tackle and also the most frequently used. Once again, I can discard the ‘jJersey’ and ‘ankle tap’ tackles for very infrequent use and the ‘arm short-long’ tackle for being the least effective.
SECONDARY DATA
Figure 10 – Secondary data of the frequency of certain tackle types taken from a large sample (McCrory & Mcintosh, 2010).
This secondary data does not depict the outcome of each tackle; rather, it highlights the most frequently used tackles from a much bigger sample, that covers a much broader range of levels of Rugby Union. I highlighted the main points of data that I believe are worth looking at, which include the frequency of the shoulder active and smother techniques being used. I would also like to point out the large quantity of ‘obscured’ tackles recorded.
REWORKED STAGE 2:
PRIMARY DATA
For the reworked stage two of my investigation, tackles of all three techniques were recorded and analysed. The footage was obtained from games during my college 1st XV team’s most recent two seasons. The velocities of the tackler were obtained via Tracker from their centre of mass one second before and after each clear tackle. The mass of each tackler was recorded to the nearest kilogram and from this, the momentums and impulses for the tackler could be calculated. Below depicts an instance of me using this program to obtain the velocities one second before and after impact.
The figures above depict how I used the program Tracker. There was around 2 minutes of continuous tackle footage. For each tackle, I selected ‘Create > Add Centre of Mass’ and clicked on the cluster of pixels that made up the tackler. Helpfully, the difference in colour of kit was not an issue in any tackles. When I clicked ‘Search’, the program would follow this tackle (the red dots) and provide the displacement and velocities in a tabulated form. Here, I picked the velocities one-second prior and one second after each collision.
There were many obscured tackles that could not be analysed due to kits being the same colour at times, as well as other players obstructing the tackle. This limited the sample size. Below are raw data points that were collected:
PRIMARY DATA
Figure 11[21]
Figure 12
Figure 13
There were a total of five samples obtained from the footage of both the shoulder active and shoulder passive techniques, however, only four in the case of the smother tackle. The momentums before and after were collected by multiplying the mass of the tackler by the velocity before and after, respectively. From the difference in momentum, the impulses were gathered.
Initially, looking at the shoulder passive data (Figure 11), the initial velocities of the tackler were slightly lower on average than the velocities of the others. This is because generally because a large momentum to stop the ball carrier and then drive him back is generally required for the execution of a shoulder active tackle. This, however, did not decrease the overall impulses, as the velocities after were negative. The very first sample, with an impulse of 180.5 kg m/s, was quite low. However, it is worth noting that these tackles were taken from a real game situation, where the variety in situations faced is a variable that needs to be considered.
In terms of the shoulder active data samples (Figure 12), the data collected seems to correlate well. There was one slightly high impulse value of 524.4 kg m/s, which was a result of a very quick winger lining up early and tackling his opposite number in a thunderous hit. This further highlights the importance of variance in situations. In Figure 13, the final impulses collected for the smother tackles were surprisingly the ones that correlated the most to each other. However, there were only four samples, as these tackles tended to be the most obscure and were definitely the hardest to obtain data from.
Figure 14[22]
Figure 15
Positively, of my average impulses there are no outliers. An outlier is essentially a point that is so far from the mean value that it can be discarded from the sample. Interestingly (from Figure 15 and 16) the shoulder active tackles tended to, on average, have the most force in the time involved (345 kg m/s). This is what I expected as this is the style of tackle in which the tackler requires the most velocity and therefore momentum to perform. Out of the other two techniques, the smother tackle on average had the lesser impulse of 273 kg m/s compared to 282 kg m/s. I feel that the difference in 10 kg m/s is negligible as it is such a tiny difference. Nevertheless these data points do signify that the shoulder active tackle tends to involve the most force on the tackler than either the shoulder passive or smother tackle.
Figure 16
SECONDARY DATA
There are a few secondary data sources that I will compare in the discussion section. Here are secondary data samples with stripped data and my analysis of them. The first source states that the aim of this article was to relate physical components (mass, velocity and momentum) to tackle dominance (where the tackler ended up with the forward momentum), rather than an ‘optimal’ tackle, which needs to be more carefully considered.
Figure 17 (Hendricks, Karpul, & Lambert, 2014[23])
The only raw data from this article that is of interest to this investigation is the average initial momentums of tacklers performing front-on tackles pre-impact. The average initial momentums here are in a range of 471 kg m/s (± 212) to 620 kg m/s (± 268) across three different levels of rugby. In the diagram, ‘Super 14’ is the most elite and ‘Under 19’ is the most amateur. Their total sample count of 30 tackles is a good amount of trials; their calculated momentums seem valid once cross-examined with other sources and my own raw data. (Hendricks, Karpul, & Lambert, 2014)
The University of Bath provided this next secondary source. Similarly to my initial methodology, they had a video analysis and a practical element. The practical element consisted of a 50kg punch bag that was held three metres away from a stationary tackler standing on force platforms to simulate a ‘typical tackling scenario’. Four pressure sensors were used to collect pressure data and derive force data. Two sensors were attached to the bag and one on each shoulder of the tackler. Here are their findings:
Figure 18 (Seminati, Cazzola, Preatoni, & Trewartha, 2015)
This graph here depicts that the maximum force experienced by the tackler’s right shoulder is 1.78 ± 0.35 kN. This force was over a time period of 0.2 seconds. If we then multiply these together, the resultant impulse experienced on the tackler’s shoulder is 356 kg m/s. This is very similar to the average impulse calculated earlier (345 kg m/s).
STAGE 3: STATISTICAL TESTING
The total of fourteen samples with the changes in velocity and their corresponding impulses are shown here:
Figure 19
These changes in velocities were obtained by the equation: Velocity before – Velocity after from figures 11-13. These have been lined up with their corresponding impulse values in the tables above (Figure 19). A graphical representation is shown in Figure 20.
Figure 20
Figure 21
Figure 22
With regards to my momentum data in the statistical tests (Figure 21), we can see it passed both the F-Test and T-Test, as both values are above the critical values (Figure 22). Both tests help to determine the correlation and validity of the data collected. The P-Value of 3.64e-7 is good, meaning that there is an extremely low chance that my data occurred by chance.
Discussion
STAGE 1
Here is the main secondary data sample that I will be comparing my data with:
My data samples are similar to the secondary data in terms of most commonly used tackles (Shoulder Active and Smother). It also correlates with the arm short-long tackle being the next most used tackle. However, as shown with my data, the effectiveness of said tackle technique was quite poor. Where it disagrees with my data is in lack of recorded shoulder passive tackles, which I believe is a difference in definition, or perhaps a lack of quality of film supported by the many more ‘obscured’ tackles recorded. This secondary data source also does not depict the outcome of each tackle, whereas I considered the outcomes using the definitions stated earlier. (McCrory & Mcintosh, 2010)
From my initial data collected in the primary stage of the investigation, it could be said that my sample size of 799 tackles is too small a sample size to depict which tackles in Rugby Union are effective. I disagree as they have been taken from six individual games with a variety of players and teams. Another issue one may find with the data collected is that I have only explored ‘Rugby Union’ and ‘Rugby 7s’ many other types of rugby exist. The other forms of rugby, however, are outside my initial question. ‘Rugby 7s’ is not technically classed as ‘Rugby Union’, but the styles of tackle that need to be performed are very similar to the tackles made in Rugby Union due to the formation of rucks. (Wayne Smith, 2012[24].) (Rugby Sevens: The game explained, 2016)
I therefore found that the shoulder active, shoulder passive and smother tackles are the most effective techniques of tackle to successfully bring the ball carrier to the ground.
STAGE 2
The initial momentums from Figure 17 are consistent with my average momentums before the tackle for the shoulder active and smother tackles – 509.1 kg m/s and 468 kg m/s, respectively. However, the average momentum before impact for the shoulder passive tackles is quite a bit lower at 273 kg m/s. Even though this lies within the valid correlation of the secondary data (471 ± 212 kg m/s) this could be explained by the lower level of rugby. The game was played at a slightly slower pace, the tackles were more random, and the tackler was moving with less velocity. Furthermore, their very high average impulse value of 620 kg m/s could be explained by having a heavier average weight in that specific league. Their total sample size of thirty does, however, outnumber my sample size of fourteen; the fact that the vast majority of my samples are similar to their raw data is a pleasing sign. (Hendricks, Karpul, & Lambert, 2014)
The purpose of the University of Bath study was to describe the biomechanical loads experienced by tacklers’ shoulders that may lead to a cause of cervical spinal injury. However in order to devise this experiment they used video analysis which concluded that there is no single reason that is the sole cause of cervical spinal injury. This is in tandem with some of the variety in my data and further highlights the variety and random nature of the tackle situation in rugby. The main similarity to my data is the depiction of the much higher forces involved in shoulder active tackles in comparison to the other two techniques. (Seminati, Cazzola, Preatoni, & Trewartha, 2015)
My primary momentum data could initially be interpreted as unreliable as the sample size is only limited to fourteen. However, I believe that the variety and random selection of tackles I analysed, as well as the fact that the tackles were done in a real game situation, credits my samples. However, the elephant in the room is that injury can occur whenever and however, not only as a result of the force of impact. Yet, it is also true that generally, the less force applied to a contact event, the lower the chance of risk of injury to the tackler.
In summary, my stage one research depicts that the use of the shoulder in a tackle and the smother tackle are the most effective techniques to use. From stage two, we can deduce that the force or impulse over a given time can be reduced by the technique of tackle used. From this data produced, we can conclude that technique with the least risk of injury is the Smother tackle. Therefore, across both stages, the ‘optimal’ tackle technique, in tandem with my hypothesis, is the smother tackle. (Jones & Jackson, 2001)
Conclusion
Before beginning my investigation, there were two main aims I wished to achieve. The first was to add knowledge about the area of the Rugby Union ‘tackle’ for not only others but also myself. The next, and main aim, was to find out if there was an ‘optimal’ tackle and if so, what technique it was. I believe that these aims have been met and as a result, it was possible to develop some conclusions.
One of the first conclusions that we can deduce is that the event of a tackle is the most common cause of injury in the game of rugby union. This has not only been shown by my findings but is also supported by extensive research that has already been done (Kenneth & Will, 2008) (Colin Fuller, 2017).
During the initial stage of my investigation, I analysed videos and recorded the outcome of 799 samples consisting of six different types of tackle to determine effectiveness. I concluded that the three most effective styles of tackle were, in popular view, the shoulder active and shoulder passive techniques but also (surprisingly) the smother technique. These methods had the highest percentages of successful IRB tackles (as defined by the RFU) (World Rugby, 2016) to tackle attempts out of all the techniques.
For the stage two of my project, I once again used video analysis, however this time using a program called Tracker to obtain velocity data pre and post impact. I compiled 14 samples for the three most effective techniques of tackle. These changes in velocities were used, along with the masses of the tacklers, to calculate the momentum and thus impulses of each type of tackle. Here, the smother technique had on average the lowest impulse (force in a given time) per tackle and thus was concluded to be the technique with the least chance of injury. I ran these data through statistical tests of validity and ‘goodness of fit’, which the sample passed.
In conclusion, from the data obtained in this investigation, the ‘smother’ technique is the ‘optimal’ tackle in Rugby Union. On the other hand, I do also believe that, in order to obtain a more reliable answer, there would need to be many more samples of accurate testing into the injury area of this investigation. This can be achieved using more advanced technology and methodology.
Evaluation
Personally, I found that the background research of the subject was by far the most straightforward part of this investigation. This is mainly because rugby is a huge interest of mine and I had already known much of the sport. In other words, I was only looking for reliable sources that agree with me. I believe that I completed my first aim, which was to learn more about the contact area in Rugby Union and develop my own (and hopefully other people’s) rugby game and knowledge. After the Literature Review section, I notably changed the project from a dissertation to an investigation EPQ. However, this did not change my main aim in finding an ‘optimal’ tackle; I just felt that an investigation was better suited to answer this type of question and topic area. My hypothesis of the ‘smother’ technique of tackle being the ‘optimal’ technique was initially a shot in the dark because everyone with basic Rugby knowledge knows that a good shoulder contact tackle has been used effectively ever since the game was created. Yet, after the initial data collected in stage one, it was shown that the ‘smother’ technique was the more successful in both Rugby Union and 7s, which came as a pleasant shock. This was further supported when my stage two momentum data came through with the smother technique having the least impulse involved.
There were a variety of challenges I faced and had to overcome during this process. Firstly, in regards to another one of my aims which was to learn how to conduct a professional investigation. Learning how to compile and write a professional investigation has been a tough journey – the research of sources and citations in particular was a new experience for me when writing any document. The data collection was even more strenuous, notably the shock of realising a couple months’ worth of filming of primary video footage could not get me any valid data. I initially performed and filmed the different types of tackle with my friends and wanted to use that to obtain velocities and further on impulses; I had great fun filming those videos. Unfortunately, when I put the videos into Tracker, the program could not distinguish between the shoulders of the tacklers and the background and no valid velocities could be obtained. This was mainly because I was filming the videos during November, when it gets very dark by about 4 PM, but I also feel that the program felt quite out-of-date. This definitely made me learn the need for a trial run with videos taken with dark backgrounds, as I could have foreseen this error.
The one aim I feel that I did not wholly complete was to find a tackle with a statistically smaller chance of injury. I did conclude that the Smother tackle was the technique with a less chance of injury; however, that only considered the very broad forces with many methodology limitations. The limitations were in my Stage 2 methodology, as there I was attempting to find which style of tackle had the least impulse involved. This is, of course, very broad because every tackle is unique. I first had to obtain the velocities from the centre of mass of the tackler rather than a specific contact area because the video footage was quite poor in quality. Furthermore, I did not take into account the unresolved forces involved; for example, if the shoulder was moving up or down, which would change the forces involved in certain areas. Injury wise, I have not even considered the angle of tackle or area of impact, which would have a big effect on the chance of injury for both the tackler and the ball carrier.
If I were to do this project again, there are many improvements and alterations that I would employ, mainly because I am really keen to answer this question, as the tackler area in Rugby Union is an area that has not had enough research done on. In order to investigate the injury aspect of this topic further, there needs to be considerably more data points. I believe one would need GPS systems fitted to individual players for time periods as long as an entire rugby season, which would allow the measurements of their velocities, momentums and other pieces of data, for a long duration of time. Force sensors attached to the shoulders of a tackler, while standing on pressure plates in a controlled environment, would also help. This would aid calculate the stresses and strains of the certain joints involved, such as the shoulder, during certain types of tackle. All these methods would add to the information that can be taught in future to hopefully minimise the chance of injury for rugby players of all ages and skill level.
More generally, what I have learned from doing this project is how to professionally construct an investigation document. If I were to do one in future, I will be able to research, cite and analyse reliable sources that allow me to have an interesting discussion. I definitely now know the importance of having a strong plan of methodology, foreseeing any potential problems and coming up with solutions to them. I believe the combination of video analysis as well as a practical element is also important to get truly involved with the ideas in question.
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About the Author
Sebastian Peace is currently a student at Bath University studying Integrated Electrical and Mechanical Engineering. Sebastian Peace wrote this article for his A-Level EPQ, having a keen interest in both Rugby and Physics.