Biomechanical Analysis

 

Variables:

Independent variable:

- Type of approach (Straight, curvilinear)

- Radius of curvature

Dependent Variable:

Horizontal displacement – total horizontal vector of displacement from initial point of contact to first point of ground contact

Vertical displacement – Peak vector of vertical displacement following initial contact

Controlled variables:

- Football used

- Dominant kicking leg

- Controlled surface

- Controlled approach distance

- Controlled kicking target (goals)

- Individual participant

Equipment

- Match standard Australian rules Football

- Cones to define approach path

- Ipads with Kinovea tracking system

- High speed video camera

- Trundle wheel

Method: 

1. Mark a fixed kicking point roughly 40-50m from the goal

2. Set three different approach paths:

- One with zero deviation from the mark

- One with a 2m radius of curvature from the mark

- One with a 4m radius of curvature from the mark

3. Players are to complete ten kicks from each point, randomised in order as to avoid fatigue or skill repetition interacting with data

4. For each kick, utilise a trundle wheel to measure horizontal displacement at first point of ground contact. 

5. Use Kinovea tracking system to determine the peak vertical displacement of the ball

For the sake of consistency, it is worth noting there will be no manned mark, as a curvilinear approach would allow for a point of first contact closer to the goal than a straight approach should there be a manned mark. 

According to the hypothesis, a curvilinear run up will increase angular velocity and momentum of the hip and trunk rotation. This will improve the efficiency of the kinetic chain, thus increasing ball velocity and finally, greater displacement. Altered body positioning at contact will influence the launch angle, the angle of parabolic motion, and consequently the vertical displacement. 

Limitations:

Individual skill level will provide an uncontrolled variable that questions the validity of comparing data. Furthermore, environmental variability may also impact the results, as wind and rain will be uncontrollable variables during the enactment of the method. While the learning effect will be negated by randomization, it cannot be entirely controlled, and thus individuals may see a positive trend of displacement between their first kick and their final kick. Finally, Individuals may find difficulty in controlling the consistency of their radius of curvature and may see variations in this regard. 

Potential improvements:

An increased sample size of elite level footballers would minimise the impact of skill deficiency on the testing and make data more comparable on a larger scale than just within the individual. Run ups could be standardised with marked painted lanes instead of cones, to ensure no deviations between approaches. Finally, speed gates or wearable motion sensors could determine individual variances in velocity or acceleration, opening the discussion of this variable’s implications on results

Results

Results recorded were horizontal displacement, initial velocity and angle of the kicker considering the ground as perpendicular. The angle of the kicker was only recorded on one kick per sample due to lack of resources. The mean data points of each participant are observable below:

Figure 2: Results table displaying the mean data points of three participants

Observing the data in figure two, the independent variable changed each test is the radius of curvature. Following this, typical behaviour displayed by the kicker is to decrease the angle of their leg in relation to the ground, rotating their body to lean on a sharper kicking angle to retain kicking accuracy. Resultantly, increases in horizontal displacement and initial velocity are typically observed on each kick. Outliers within this trend may have been influenced by a multitude of factors to be discussed further. 

Utilising the horizontal displacement and radius of curvature, a visual representation of the relationship between this data has been constructed in the figure below:

Figure 3: A column graph representing the relationship between the mean horizontal displacement of each participant in relative to the radius of curvature

As is observable in figure 3, each participant demonstrated a positive correlation between increase in radius of curvature and mean horizontal displacement of their kick. Relating these results back to the initial hypothesis, the positive linear relationship between horizontal displacement and radius of curvature suggests the hypothesis is correct.

To calculate vertical displacement from these values, we must first find the launch angle of the ball. It is worth noting that these calculations have been done without factoring air resistance, something to observe in the following discussion. This process begins by utilising the range equation, rearranged to find the launch angle, where R refers to range, and u refers to initial velocity.

This equation will produce two different angles, each the opposing integer resulting in a sum of 90. The more plausible angle for the kick has been chosen for each point of data. From here, each kick’s vertical velocity can be found using this formula

Finally, to find the peak horizontal displacement, all relevant data can be run through this formula, where s refers to vertical displacement.

Following the initial summary, the following means can be produced for each player. These values are in degrees in reference to the ground.

Figure 4: Table of data displaying the mean launch angle of each participant’s kicks between each radius of curvature

Two external variables that have not been accounted for is the air resistance on the ball, and the vertical height difference between the first point of contact with the foot, and the point of contact with the ground. Finally, the below tables have been constructed using the mean values of each participant to examine horizontal displacement, and all relating factors

Figure 5: Table demonstrating the data obtained from the operations above

Following this, a graph was constructed showing the correlation between the radius of curvature and vertical displacement.

Figure 6: A chart examining the relationship between vertical displacement and the radius of curvature

Examining the data sets above in relation to the hypothesis, no conclusion can be made regarding the relationship between the radius of curvature and horizontal displacement. Examining the dataset from participant 1, with an increase in the radius of curvature, the angle to the ground is decreased, as well as the launch angle of the kick. Subsequently, the initial velocity of the kick increases drastically with a curved approach. The parabolic profile of a kick with a 5m radius-curved approach is much flatter, and faster than its counterpart. This relationship is not present within the other two profiles, with participant two showing the inverse profile of this and participant 3 showing no trend. 

To summarise, three participants were made to take three kicks with varying degrees of curvature dependent on the radius. The results suggest that there is a direct correlation between an individual’s curved approach and horizontal displacement of their kick. The initial velocity of the ball also consequently increased as a result of implementing a curved run up. No relationship can be observed between curvature of an approach and the vertical displacement of a kick however, and discovering a relationship would require further testing, greater resources, and idealistic conditions.

 

 Analysis

Individual differences between participants

As seen in the results, participant one appeared to have the biggest increase in performance from the increased curvature run up. Participant 1 consistently demonstrated increased velocity and horizontal displacement as the run up opened up. Participants 2 and 3 showed weaker relationships in all areas. The effectiveness of a curvilinear run up may depend on many factors like technical proficiency and kicking experience. Participant 1 evidently played at a high level in the junior ranks compared to the other 2 participants whose experience was at a more beginner level. In addition, The success of the run up can also be i9nfluenced by hip mobility and trunk rotational capacity that can vary between the kickers. This information suggest that curved run ups in AFL should utilise as an individualised approach rather then applied to every single player.

Limitations

Many factors may have contributed to the results being inaccurate. Firstly the sample size of 3 does not provide a lot of credibility to conclude whether or not the curved run up is affected. The results proved that performance varied based on kicker attributes, so it is therefore recommended to increase the sample size to get more definitive data. In addition, the was increased wind conditions during the testing which may have influenced the ball flight. This may have caused the distance measurement to be skewed. Another limitation in the testing was the inconsistency in kicking technique. By altering the angle of the run up, the participants may have unintentionally changed technical features such as hand placement and ball drop that may have influenced the distance and ball flight after making contact with the boot.

 

Horizontal displacement

The hypothesis suggested that a curvilinear run up may enhance kicking performance when maximum distance is required. This was supported by the results in the method. Figure 3 showed a positive relationship between radius of curvature and horizontal displacement in all three of the participants. The radius of curvature was increased for every kick, which saw the distance of the ball increase. When kicking off a curved run up, the participants generated a greater transfer of force throughout the entire kicking action. In AFL, players should used a curved run up when they are trying to kick for distance in situations such as clearing the ball out of a kick out.

 

Summation of forces

The increase in horizontal displacement is explained by the principle of summation of forces. During the kicking action, force is transferred through the kinetic chain that starts at the ground. The force then travels through the pelvis, trunk, thigh, shin and then foot. The participants showed very little hip rotation in the straight run up. In contrast, the curved run up saw the kickers hips and trunk rotate significantly more which helped transfer more force through the body. This then allowed a greater force to be applied to the ball on impact. As seen in the results, the ball travelled at a higher horizontal distance during the curvilinear testing.

Acceleration and velocity

The data in figure 2 shows that the initial ball velocity increased as the radius of the curvilinear run up was greater. This is closely linked to newtons second law that suggests that increased force applied increases the acceleration (Pourciau, 2006). The ball left the foot at a faster rate on the curved run ups, which can be explained by the leg moving faster in the kicking motion. This then resulted in the ball receiving a larger impulse at impact which ultimately increased the initial velocity. The data reflects this where curvilinear run ups where closely linked with increases in ball velocity and horizontal displacement.

 

Momentum transfer

Momentum is determined by the product of mass and velocity(Schilling, Falvo, & Chiu, 2008).  The same size football was used throughout testing, so the mass remained the same. Increases of momentum were dependent on increases in velocity. The faster ball speeds seen in the curvilinear run ups meant that the kicker had more momentum transfer from them to the ball. As a result, the momentum helped the ball travel longer and further before hitting the ground which lead to a greater horizontal displacement measurement.

 

Vertical displacement

The results did not show a clear relationship between the radius of curvature and vertical displacement. Figure 6 shows a high level of variation between participants and does not display a consistent trend across all testing conditions. Participant 1 showed changes in launch angle and vertical displacement when the curved run up was widened, however participant 2 and 3 did not show the same pattern. This result does jot support part of the hypothesis. Lauch angle can commonly affect the vertical displacement more than kicking speed. Whilst the curvilinear run up increases the speed of the kick, it does not always change the launch angle consistently which can lead to varying results in vertical displacement across the participants.

 

Overall, the findings partially supported the hypothesis. The data showed that increased curvilinear run ups generally linked to increases in initial ball velocity and horizontal distance. The results are supported by key biomechanical principles including summation of forces, acceleration and momentum transfer. The vertical displacement aspect of the hypothesis was not supported. The main benefit of using a curved run up in AFL is to increase kicking distance.