Major Question

What are the optimal biomechanics of the American Football Forward Pass? 

Before analysing the biomechanics of a skill it is important to provide the context in which the skill is typically used. The motion of the forward pass has often been linked biomechanically to the baseball pitch because they often utilise a whipping or slinging motion. This link has some basis but it is important to remember that by the very nature of the game of American Football there are a ‘unique set of performance constraints, objectives and conditions.’ (Heppe, 1992)Within a typical game situation not only does the initial and final alignment of defensive players constantly change but the ‘constraints and objectives of which target to throw to and how to get the ball there constantly change as well.’ (Heppe, 1992)  Lastly and most importantly it is vital to remember that the size and mass of the projectile and the time available in executing the skill make the two different skills a less effective comparison.

The closest comparison to ball used in American Football would be a rugby ball but again there are several key differences. Testing of Quarter Backs at the combine highlights that the higher level athletes clock in at around 60 mph, whilst the Guinness world record for a rugby passing throw is set at 48 mph.  One can argue that it is because of the very different movement patterns and skill executions that allow an athlete to be able to throw the two very similar in size and mass balls at vastly disproportional velocities.

Thus I argue and aim to highlight the unique biomechanical constraints necessary to achieve the goals of velocity and to some degree accuracy when executing the forward pass skill. 

The Answer

To begin it is necessary to define the sequence of movements necessary for an optimal execution of the forward pass skill. (For the description of the skill the below explanation is for a right handed athlete)

• The Athlete begins by positioning themselves side on, with their non-throwing arm facing their target.
• The athlete ensures that they have both their hands on the ball at or just below chest level. The right foot is planted and supports the weight of the body whilst the left foot is non-weight bearing ‘and underneath or slightly in front of the left hip depending on individual technique’. (Heppe, 1992)
• The throwing arm draws back behind the body with the elbow joint at a 90 degree angle.
• The leading leg strides forward and the hip abducts thus rotating the pelvis.
• The back foot pushes and the shoulders and trunk rotate towards the target.
• The rotation of the shoulders ‘initiates rapid elbow extension and the external rotation of the throwing arm reaches its maximal position’ this then needs to be followed immediately by an internal rotation of the throwing arm to bring the ball towards the target.
• The athlete continues the rotation of the body whilst sequentially extending the muscles from the shoulder joint all the way to the tips of the fingers towards the target. 
• The follow through allows full wrist flexion and rotation so that the hand finishes its range of motion with the palm facing the ground and fingers pointing at the target.  As a result rotational energy is applied to the ball which forces  the ball to spin, exhibiting the marker for successful skill execution the ‘spiral’ 

Starting Position of the Ball

Feedback cue:

• Whenever executing the pass skill ensure movement always begins in the optimal zone.

The aim of the athlete before beginning any throwing motion is to ensure both hands are on the ball within the optimal zone. Sports scientists label this zone as; ‘no higher than the clavicle, no closer to the body than the sternum, no further back than the armpit and no lower than the bottom of the chest. This is important because it helps to reduce the required displacement and time required to bring the ball from this ready position up into the first movement of the sequence. A good example of this effect is Tim Tebow’ s old throwing technique for which he was heavily criticized and he underwent training as a result and improved his technique.

Video Courtesy of : https://www.youtube.com/watch?v=5AsbCFNauAw

Some footballers bring the arm far out behind their body and Tebow is a great example of this, he brings his arm all the way out and stretches down as explained in the video above. Whilst  this creates a longer lever and thus greater moment of inertia since the arm bends when it comes back up for the throwing motion most of this inertia is wasted just bringing the arm back up from behind the body. (Blazevich, 2013, p. 72) The quarterback needs to bring the ball up quickly not only because of the time constraints placed on him via an opposition but it is also important to flow through the entire skill without losing any velocity and momentum to ensure maximum transfer to the projectile.

Force summation

Feedback Cue:

• Take note of how the athlete stands before throwing, the aim is to start side on and begin with rotating the body ending with the use of the arm.

Figure 3: The Principle of Force Summation, highlighting the importance of sequential timing. Image Courtesy of: http://www.coachr.org/biomechs5.jpg 

The weight change to the front foot allows power to be generated by the largest muscle group first, through the rotation of the body’s vertical axis. This is also known as angular velocity or put  simply ‘the rate of change in angle of the thrower.’ (Blazevich, 2013, p. 16)  To ensure optimal force summation the sequences of the movement need to be completed in order and correctly timed. The optimal technique as highlighted by Figure 3 involves the largest muscle groups first and then follows down the kinetic chain to the smaller muscle groups whilst the previous body part is still at its maximum speed of movement. (Landlinger et. al, 2010) The greater the force generated the more angular velocity is generated by the rotation of the hips. The projectile and the angular velocity have a linear relationship thus the faster the angular velocity of the body, the faster the projectile will be moving. The angular velocity is. It is quite obvious that the more quickly the thrower rotates (that is, the higher their angular velocity), the faster the ball will be moving and thus meeting the goal of maximum ball velocity. 

Push through back leg.

Feedback Cue:

• Focus really planting your back foot after the stride step to encourage the push from the back foot.

Figure 4: Highlights an athlete planting his foot in preperation for the push of the back leg. Adapted from: http://getbetterfaster.tv/wp-content/uploads/2014/10/RennerThrowSequence.png

To begin the rotation of the hips and to generate momentum forward a Quarterback pushes through their back foot towards the intended target (Figure 4).  As the athlete pushes down on the ground Newton’s law of opposite and equal reaction becomes apparent; the flexion of the foot against the ground, will result in the reaction force applied to the ground transferring to rotational energy as well as momentum being directed towards the target. (Blazevich, 2013, p. 46)The energy generated by this change in momentum and angular velocity is then transferred to the ball through the movement and allows greater transfer of energy to the ball increasing its velocity and likely its range. By not pushing off the ground through their back leg athletes have to compensate for the lesser inertia and velocity by over compensating with increased effort through the upper body. (Heppe, 1992) The majority of injuries for quarterbacks come from poor throwing mechanics, so by not utilising the rotation of the whole body to generate force athletes place extra stress on the muscles and joints of their upper boy. 

Center and Line of Gravity

Feedback cues:

• Where do you feet start and finish after the movement.

Figure 5: Highlights the movement outside the line and center of gravity required. Adapted from http://getbetterfaster.tv/wp-content/uploads/2014/10/RennerThrowSequence.png

Robert Heppe’s own research and literature review highlights the importance of centre and line of gravity within the American Football throw. Typically in a sporting context athletes aim to plant their feet within shoulder width to widen their base of support, doing this will place the athletes centre of gravity within the base of support and keep the centre of mass and centre of gravity aligned, increasing stability. (Blazevich, 2013, p. 65). This stable position also allows for the greatest range of movement patterns an athlete has a t their disposal, but the specific rotational action of the skill changes the relevance of these principles.  Heppe argues that up to 49 percent of the balls total velocity is generated through stride and rotation of the body at the start of the throwing motion (Heppe, 1992, p. 4) Through testing of right handed athletes Heppe states that an effective athlete actually places their stride foot slightly to the left of the midline, putting the centre and line of gravity slightly to the non-dominant side. He argues by putting the stride and the line of gravity towards one side this allows complete rotation towards the target thus maximum angular velocity as well as better accuracy because of the superior vision afforded by the front on position.

It is important to note that the step and lean needs to be minimal to avoid decreasing the effective ness of the skill. If the athlete is too bent over the angle of release will be effected and if the athlete is leaning too far to one side stability and accuracy may be effected. 

Projectile Tragectory

Feedback cues:

• How did the ball travel through the air before hitting the ground?

The trajectory of the ball is of vital importance to ensure maximum distance of a thrown football and to hit a target accurately from range. The trajectory is influenced by the projection speed, the projection angle and the relative height of projection (that is, the vertical distance between the landing and release points). The importance of the factor is highlighted by the fact that most star quarter backs are tall athletes; this is favourable biomechanically because it means the athlete already has a higher relative height of projection than a smaller athlete. An athlete cannot change their height so to ensure maximum velocity and distance the focus should be upon the projection speed of the athlete. The distance a projectile covers,’ is chiefly influenced by its projection speed’ (Blazevich, 2013, p. 24) therefore the faster the projection speeds, the further the object will go. The angle of release is the final component too high and the object will lose velocity sooner into the throw and begin its ‘arching’ trajectory sooner, too low and the ball will not take advantage of the arch back down to the ground for maximum distance.  The optimal angle of release for this skill has a large number of variables and because it is dependent on the physical characteristics of the athlete an optimal angle is not possible. But through video analysis a coach can see and analyse the trajectory of the ball to work out whether this angle needs to decrease or increase. Analysis of the flight path can also allow a coach to find other errors in the movement; i.e. ball drifting to the non- dominant side likely means over- rotation of the hips. 

Wrist follow through

Figure 6: Highlights the correct follow through movement. image courtesy of http://cdn.instructables.com/FD4/083I/G068PYIM/FD4083IG068PYIM.MEDIUM.jpg

Feedback cues

• How did the hand finish after releasing the ball?

Extension and Rotation of the wrist lead to rotational energy being transferred to the ball and thus forcing the ball to ‘spiral’ along it’s trajectory.  This is done purposefully to attempt to reduce the amount of drag the ball has moving through the air thus retaining as much velocity of the ball as possible. Drag occurs when molecules of a fluid (‘fluid’ refers to any moveable medium, including air) collide with an object and take energy away from it. (Blazevich, 2013, p. 137). The unique shape of the American Football in comparison to the spherical balls commonly used throughout different sports helps to allow this rotation. The elliptical shape of the ball leads to the ‘gyroscopic effect’ which increases the rotation of the ball which in turn helps to reduce effect of drag upon the ball. (Rae, 2003, p. 5)The relationship between this shape and the laminar flow is also important; in comparison a spiraling football is able to lower the amount of laminar flow that becomes turbulent. The leading edge of the ball (the side hitting the air first) is pointed, thus ‘the direction of the fluid hitting the object will be changed more slowly’ in comparison to a rounded tennis ball in which the ‘fluid hits the object abruptly.’ (Blazevich, 2013, p. 137)  

The relationship between laminar flow and projectile shape: Round ball in comparison to elliptical.
Images courtesy of : Blazevich, A. (2013). Sports Biomechanics: the Basics: Optimising Human Performance. London: Bloomsbury. Pages 11 and 12

Whip throwing arm

Feedback Cue

• Rotation of hips immediately followed by the careful 'whipping' motion of throwing arm.

The final movement of the arm ends in a ‘whipping’ or slinging motion this allows the summation of force generated by the larger muscle groups to transfer to the ‘distal segments of the arm much like the transfer that occurs in a fishing rod’ (Blazevich, 2013, p. 186) The careful sequential pattern of the over arm throw allows the ‘joints [to] increase their velocity first and the more distal segments increase their velocity later. Once the bigger muscle groups reach their maximum range of movement the proximal segments of the arm halt and the angular momentum must be transferred to the more distal segments or else the velocity will decrease or be lost. (Blazevich, 2013, p. 187) Therefore, if we rotate the proximal segments of the arm and then halt them, the momentum is transferred to these lighter segments and so their velocity must increase which allows for the greatest transfer of momentum to the projectile. 

How else can we use this information?

The biomechanical principles underlying the whole analysis of the Quarterback throw can be applied to a wide range of skills across a variety of sports and game scenarios. A lot of the throwing mechanics and the principles attributed can be applied across a range of different throwing sports. Any sport in which a ball is thrown shares the same laws that apply to the projectile being thrown and the effect upon the body whilst throwing it. The understandings of drag and how the ball can influence this has many applications in particular within the Australian Rules context. Because the sports share a similarly shaped ball they both take a similar understanding and relationship with fluid mechanics and drag. By understanding how a quarterback uses a spiral to minimise drag a footballer can create several different applications, whether that be purposefully creating increased drag on one side with a check side kick or trying to keep the ball from wobbling too much with a Torpedo punt. A similar situation could be used to improve rugby players throwing velocity and speed by also trying to increase the spiral effect thus minimising drag. 

References

Blazevich, A. (2013). Sports Biomechanics: the Basics: Optimising Human Performance. London: Bloomsbury.

Heppe, R. (1992). "The kinematic variables related to the efficiency of throwing : football. Master's Theses, 4(Paper 394).

Landlinger, J., Landlinger, S., Stoggl, T., & Wagner, H. (2010). Key factors and timing patterns in the tennis forehand of different skill levels. Journal Of Sports Science And Medicine, 9(17), 643-651.

W. J. Rae, “‘Flight dynamics of an American football in a forward pass”, Sports Engineering, 6, 149–164 (2003).

Images and Videos

http://getbetterfaster.tv/wp-content/uploads/2014/10/RennerThrowSequence.png

http://cdn.instructables.com/FD4/083I/G068PYIM/FD4083IG068PYIM.MEDIUM.jpg

https://www.youtube.com/watch?v=5AsbCFNauAw