What are the basic biomechanical principles to consistently kick an AFL football drop punt successfully?

Through out this analysis I intend to discuss the biomechanical benefits of performing an AFL drop punt and also the physical factors that contribute to successfully kicking a drop punt. For the use in this blog, I will define a successful drop punt as having power, accuracy and being able to consistently reproduce the skill. I will discuss why the drop punt is widely accepted as the preferred  type of kick and also discuss the vital parts of the skill, which I have broken down into 6 key phases:

  • Run up
  • Planting of the support leg
  • Ball drop
  • Leg swing
  • Impact
  • Follow through

Why is a drop punt widely accepted as the preferred AFL kick?

Firstly we must discuss the key factors of an AFL style drop punt. The ball must be dropped from the players hands onto the foot. The most important factor of a successful drop punt is the way the ball spins off the boot and through the air. An ideal drop punt has the ball positioned in an upright manner and spinning backwards along its a vertical axis (AFL 2013).

The spin is a vital part to performing this skill and is stressed as being important when teaching the drop punt to developing players, as shown in the below video at 0:47.

Thinking in a biomechanical sense, there must be a reasonable justification for why back spin is seen as a valuable trait to a successful drop punt. The Magnus Effect greatly explains how different types of spin effect the trajectory of a ball in flight in different ways.

As the ball is moving forward through the air, if it is spinning, the air is flowing around the ball- from the front to the back. The ball is spinning in the same direction as the ball at the top but in the opposite direction at the bottom. This causes friction between the ball and the air, causing air to be dragged around the top of the ball towards the back. At the bottom of the ball, the air is faced with a ball spinning towards it, which causes the air to move past the ball much slower when compared with the air above the ball.

With the air above the ball being pushed down as it passes the ball and the air below remaining relatively neutral, we can observe the net force acting upon the air surrounding the ball is in a downwards direction. Newton’s third law, ‘for every action, there is an equal and opposite reaction’, we can conclude that the downwards force on the air is creating an equal upward force acting upon the ball.

We can safely assume a similar effect would occur with an oblong shaped football. The fact that the ball is spinning under itself we could infer that the turbulence and therefore effect of the spin is increased. Although nearly all the creditable research is developed using a spherical ball (Briggs 1959 & Bray 2003), we can assume (due to its shape) the football would cause more turbulence through the air and should exhibit a greater effect.

The main reasons why a drop punt is widely accepted as the ideal form of kicking:

  • Magnus Effect shows back spin increases the time the ball is in flight, making it easier for team mates to judge.
  • The end-over-end style of the kick makes it easier for team mates to mark. The horizontal perimeters of the ball change minimally which makes it easier for players to judge where to position their hands to trap the ball on either side for best control.
  • If the ball were to hit the ground, the bounce is much more uniform and easy to read than a ball with uncontrolled spin.

How does the run-up affect kicking?

A run up serves two main biomechanical purposes, the first is to increase stability and the second is to increase the force transfer into the ball. To explain stability I’ll use an example of riding a bicycle; it is much easier to stay balanced while riding a bike if it is moving rather than stationary. This phenomenon is caused by centrifugal and gyroscopic force (Jones 1970). This extra stability allows more control in the ball drop and leg swing which will help increase accuracy and consistent repetition in the kick.

photo

The final step of the run up (non kicking foot forward) is important as it allows the kicking leg to be left at a greater angle behind the body and for the hips rotate on a horizontal plane away from the kicking leg.  While the hips are rotated away from the kicking leg, the shoulders remain relatively perpendicular to the target, allowing for recoil of the trunk. These factors allow for a greater force and time of force to be applied to the kicking leg prior to impact with the ball, I will go into more detail of this in the  leg swing section.

Why is the planting of the support leg important?

The plant foot is meant to brace the player by absorbing force generated by the speed of the run up. This bracing of the support foot allows the trunk and hips to rotate toward the kicking side and increase force generated in the kicking leg.

photo (1)

Plantar Flexion (extension of the ankle joint) on the support foot during kicking pushes the body away from the ground, which adds vertical lift to the kicking foot, marginally increasing the force transferred into the football on impact.

How does the ball drop influence the success of a drop punt?

The ball drop is an essential factor in successfully performing a drop punt. there are many small differences in types of ball drops but the key factor is to control the flight of the ball after it leaves the hand and before it strikes the boot.

The above video shows the extended guiding time of the ball in the hand prior to release. The longer the contact time in the ball drop, the more control the player will have. An important thing to remember in the ball drop is trunk position, while leaning forward and dropping the ball closer to the boot may allow for more control and, in turn, accuracy but it inhibits the full body movements involved in generating leg speed. having a forward leaning trunk position will produce a less powerful and more accurate kick and having a backwards leaning trunk will cause the ball to be dropped from higher which will decrease the control in the ball drop. It is important to consider external factors such as wind and rain, which will increase the variability of the ball drop.

Although the guiding of the ball drop begins around the torso, it is recommended to release the ball around hip height. This allows ideal trunk position and enough time and distance for the ball to be guided while in the hand. This may mean the ideal height of ball drop is lower in such conditions.

This video shows the ball guide and drop being directly over the kicking leg, this is ideal as it encourages ball movement along the sagittal plane which is the plane intended for the ball to travel. Dropping the ball from outside of the hip will impart sideways movement on the ball or the arc of the kicking leg which will decrease the likelihood of the kick travelling straight.

What are the contributing factors to achieving a powerful and controlled leg swing?

The key goal of the leg swing is to increase the speed, and therefor force, of the foot during collision with the ball to exert more kinetic energy into the ball. It is important to note that in this kind of action, leg length (or length of lever) and leg mass are two very important factors to the amount of energy that can be transferred to the ball. Since these two factors are almost completely out of the control of the person performing the kick I will not go into detail.

The act of kicking a ball can be compared to cracking a whip (Philadelphia Hawks Australian Rules Football Club 2013), where there is a sequence of events which all contribute to the end of the whip, or the foot, ends the sequence at high speeds.

As discussed prior, the momentum built in the run-up adds kinetic energy to the leg swing and the football. The final step leaves the leg behind the body, this is important because it increases the duration that force can be applied to the leg swing to help generate foot speed. The pelvis also rotates to leave the hip of the kicking leg behind while the shoulders remain square. Now that the leg is ready to begin swinging, the core recoils and brings the pelvis back to square and beyond, this propels the hip and kicking leg forward.

As we can see in the above video, as the thigh begins to swing through, the knee bends back, leaving the lower leg bent and ready to swing. In the last 3 frames, the upper leg is finished swinging and does not change angle from the hip. The sequence is ready for its final phase, knee extension. The knee extends, and combined with all the energy built up swings the foot through at high speeds to collide with the ball. It is also relevant to see that through the 8 frames, the body itself moves significantly forward, increasing foot speed.

How does the ball contact with the foot effect kicking performance?

The key factor in this step is to ensure the toes are pointed and muscles in the leg are tight. The benefit of having the toes pointed is to ensure the platform on the foot for contact is larger and flat. Also, if the foot is soft and gives way under the impact of the ball, some of the energy of the collision will be lost and ball speed will be reduced. It is therefore vital to keep the foot tightly extended and make contact the ball on the instep rather than near the toes to reduce the absorption of energy in the foot.

photo (1)

It is important to note that at this point that the mass, or weight, of the foot at this point is important. A 2kg foot impacting the ball will travel further than a 1.5kg foot, assuming both feet are travelling at the same speed. It is thought that stiffening of the joints at point of impact can increase the effective mass of the foot by incorporating some of the weight of the leg into the transfer of force.

How does the follow through affect kicking performance?

Although the follow through does not directly impact the contact of the ball, it is important as the form of the body can be used to show some flaws in kicking action.

photo (2)

As seen in the photo above, the follow through of the leg should be straight along the sagittal plane of the hip joint. This shows that the leg is swinging through on a straight arc. If the follow through of the leg were not straight, it would show the arc of the leg is not parallel to the intended direction of ball delivery.

The Answer?

The action of kicking a drop punt is a whole body, complex skill. I have detailed many small factors that contribute to a successful drop punt and how they each impact the outcome. In this section, I will list each of the key phases of a kick in a simple format.

  • Steady run-up
  • Longer final step
  • Hip rotation (kicking side back with shoulders square)
  • Ball guide one hand
  • Guide to hip before release
  • Drop over kicking leg
  • Thigh swings forward before knee extension (may be difficult to distinguish between)
  • Impact on boot laces
  • Impact with toes pointed
  • Impact with the ball standing upright
  • Ball spin backwards
  • Leg swings through in a straight direction after the contact

How else can we use this information?

I intend this information to be used by players and coaches alike, to allow a greater understanding and appreciation of the many key phases that go into a successful drop punt. This information can be used to justify why an ‘incorrect’ action is important to achieving a beneficial outcome. I believe this blog could also be used to find what phase of the kicking action needs improvement to fix a discovered error. For example, if a coach had discovered a player was having difficulty getting the ball to spin properly through the air. They could read this blog and ascertain that the ball drop and foot posture are paramount to correct ball spin and work on improving in those areas.

As with all technical adjustments, the coach should have experience in both teaching skills and an intimate knowledge of the drop punt. Video and photos are recommended to analyse flaws as the give the kicker a greater understanding into where they need to improve.

References

AFL (2013) AusKick Coaching Manual, accessed 16 April 2013,
<http://mm.afl.com.au/portals/0/afl_docs/development/coaching/junior_manual/AFL_Junior_Coaching_Manual_5.pdf>

Briggs, Lyman J. “Effect of spin and speed on the lateral deflection (curve) of a baseball; and the Magnus effect for smooth spheres.” American Journal of Physics 27 (1959): 589.

Bray, K., & KERWIN, D. (2003). Modelling the flight of a soccer ball in a direct free kick. Journal of sports sciences21(2), 75-85.

Jones, D. E. (1970). The stability of the bicycle. Physics today23(4), 34-40.

Philadelphia Hawks Australian Rules Football Club (2013) Biomechanics of Kicking, accessed 22 April 2013, <http://phillyhawks.com/z_NOT%20WEB%20SITE%20STUFF/pdfs/newkick.pdf>

Richardson, G. (2013). Filmed and edited on Apple iPhone5, using Coaches Eye, ver. 3.0.2. TechSmith Corporation.

What is Magnus Force? 2011, YouTube Video, accessed 21 March 2011, <http://www.youtube.com/watch?v=23f1jvGUWJs>

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