Biomechanically what is the optimum way to perform a field hockey penalty stroke?

Introduction

Hockey is a sport popular among men and women around the world. It is one sports that can be player over a lifespan, in Australia players range from five to eighty-one years of age (Anonymous, 2014). Australia contains 143,000 registered hockey players in 880 clubs, it is also a popular sport chosen to teach in physical education classes in Australian schools. There are many skills involved in the sport such as hitting, flicking, pushing, dribbling and running (Anonymous, 2014).

The penalty stroke is a vital aspect of the game, proper execution will often result in a match winning goal, it is a one-on-one competition against the goalie. A penalty stroke is only awarded when an infringement occurs inside the circle to stop a goal from being conceded and to decide the winner of a tied match (Isport, 2014). It is a set shot taken from the ‘spot’, 6.7 meters directly in front of the goal and is highly technical; the ball needs to be delivered into a goal 3.6 meters by 2.1 meters whilst avoiding the goalie (Isport, 2014).

Figure 1. View of the goal from where the player will take the shot

To score a goal the penalty flick needs to perfectly executed with power and accuracy to ensure it goes in the goal and the goalie is unable to defend it (Hussain et al., 2010). For this to happen the biomechanics of the skill need to be executed effectively and efficiently. Biomechanics refers to the study of the human body as a machine, how forces act upon it to cause movement and how these forces can be used to help the body move with precision, resulting in optimum performance (Hede, Russell, & Weatherby, 2011). Knowledge of biomechanics can be very useful to physical educators, exercise scientists and sports professionals to improve their own or others sporting performance whist decreasing the chance of injury (Wuest & Fissette, 2012). This blog will discuss the important biomechanical principles relevant to performing the optimum penalty stroke in field hockey.

Video 1. Successful penalty stroke

Biomechanical principals of the penalty stroke.

When preparing to take a penalty shot it is important the attacker bends their knees. Doing so will produce a low centre of gravity. The centre of gravity is the point where the body’s weight is evenly distributed which changes through movement. Creating a low centre of gravity increases stability and therefore performance accuracy (Blazevich, 2012).

Does taking a step whilst taking a shot increase the speed of the ball?

The Coach’s Eye application on the iPad was used to record the same player executing two styles of penalty strokes. The first recording shows a step forward being taken as the shot is taken, the second without. Velocity of the ball was then calculated to see which movement produced the most force.

Calculations for velocity of the ball

Scaling Factor = 0.023

Frame to frame = 0.7m - Divided by scaling factor = 30.43m

Time between frames on an ipad = 0.033seconds

Velocity = Distance / time

30.43m/0.033s/100

Velocity = 9.22 m.s  

Scaling Factor = 0.023

Frame to frame = 0.4m - Divided by scaling factor = 17.39m

Time between frames on an ipad = 0.033seconds

Velocity = Distance / time
17.39m/0.033s/100
Velocity = 5.27 m.s  

The above results show that taking a step forward during the shot will cause the ball to travel at a higher velocity. This step forward widens the players base of support, ‘the area within an outline of all ground contact’ (Whiting & Rugg, 2012) which is effective in increasing stability. To maintain balance throughout the skill the centre of gravity needs to be maintained through static and dynamic aspects by keeping it above the base of support (Kwon, Park, Jefferson & Kim, 2013)

Figure 2 , This demonstrates the importance of having a wide base of support to maintain stability. In this picture the centre of gravity is the same for all stools. As stool (c) has a smaller base of support once movement occurs it falls over. Stools (a) and (b) are able to  return to its original position due to their wider base of support providing extra stability.

Force summation refers to a combination of forces produced by separate parts of the body (Anonymous, 2012). Stepping forward during skill execution is a way to maximise this, it is important to step in the direction of the force  to heighten force summation. To enhance the velocity of the ball it is effective to use the maximum number of muscles in the correct sequence. During the penalty stroke if the player starts with a step the movement begins in the legs, a large muscle group that can produce lots of energy. The most powerful flick will occur if that energy then travels through the hips, torso and then upper body (Wuest & Fissette, 2012).

All movement in sport require the synchronisation of many joints, this process is known as the kinetic (moving) chain. The kinetic chain has two categories, push –like and throw-like movements (Blazevich, 2012). The push-like movement is when a single movement occurs and joints are extended simultaneously. In this process torques  are produced at each joint collectively to produce and overall force. Torques refers to a force applied at a distance that causes rotation on an object, also known as moment of force (Blazevich, 2012). The push-like movement is produced when doing a bench press, squat lift and dart throw and is often used to lift heavy object and for straight line movements due to its accuracy (Blazevich, 2012). The field hockey penalty stroke would be a throw-like movement, this is when movement from each body part occurs sequentially starting proximal to the body and finishing distal, allowing time for forces to be summated (Blavevich, 2012).

According to Newton’s Third Law all actions have an equal and opposite reaction. A hockey flick does not allow any back swing and therefore needs to find another way to produce as much force as possible. Newton's second law states the ball needs to be acted on by a force comparative to it for it to accelerate, the more force applied the faster the acceleration (Adrien & Cooper, 1989). When taking a penalty stoke, as the player steps forward the pelvis rotates to the left as the hockey stick begins to move the ball. This is a way of producing great force through the ball; it is congruent with the kinetic chain and summation of forces. Hip rotation allows more momentum to be produced and transferred to the hockey ball. The principle of momentum states that the greater the momentum the body has, the greater the effect on the object. To be effective in momentum transfer, the body must be stable (Hede, Russell & Weatherby, 2011). The rotation of the body also relates to the angular velocity. The faster the angle of the player changes, in this case the hips moving from a side on position to facing the goalie, the faster the ball will move (Blazevich, 2012).

Levers are another biomechanical principle relevant to hockey; a hockey stick acts as a third class level. A lever can be used to produce speed, however in the case of the penalty stroke when the grip on the stick is shortened accuracy of the shot will improve. To do this the player must have there right hand further down the stick, this is demonstrated below (Hay, 1993). In the follow through phase of the skill it is important the momentum of the stick continues until it reaches waist height of the player, it should be pointed toward the target in order to guide the hockey ball in the desired direction (Pappas, 2012).
 

Figure 3. Image (B) shows the correct grip for the penalty stroke. Right hand should be further down the stick to increase control and accuracy.

The Answer

There are many different biomechanical principles impacting the field hockey throughout the skill phases. To execute the shot effectively and efficiently with power and accuracy, players and coaches need to consider these. The most effective way to deliver this skill is to have knees bent during preparation; this helps the player to be stable due to a low centre of gravity being formed. During the contact phase the player must step forward to initiate the movement. This does two things: it creates a large base of support to maintain balance through the movement and allows for force summation to occur. More energy is produced by the larger muscles groups, if the movement starts in the legs a large amount of energy will then be transferred through the hips, torso and upper body to the ball, building force to help the ball travel at a high velocity. It is important to ensure the player starts in a side on position to the goalie to allow hip rotation during the shot, this produces angular velocity which will generate force to increase the velocity of the ball. To increase accuracy the players right hand need to approximately 30cm down the hockey stick from the left hand to shorten the lever. The follow through should continue to help guide the ball in the required direction.

 

    
Figure 4. The penalty shot is best accomplish with knees bent (A) followed by a step forward (B) and finally hips rotating and stick follows the ball (C).

The penalty stroke in field hockey is often a match winning play so it is important to execute the skill efficiently and effectively. As it is a set shot with minimal variation it is a skill that can be practiced and perfected, knowing the biomechanical principles associated can help do this.  In 2012 only one research paper had been written in relation to the biomechanics of the field hockey penalty shot, coaches and players would benefit from further research.

How else we can use this information

The biomechanical principles discussed in this blog would be helpful to professional sports people and coaches as they can be applied to many different sports. For example, being aware that a wide base of support improves stability is useful in sports such as tennis, rugby and gymnastics, just to name a few.

          Figure 5. Base of support effective in tennis      Figure 6. Base of support used in Rugby

                                     Figure 7. The base of support can also be the hands.

Taking a step forward to commence the kinetic chain is another action effective in many sports, helping players to build forces. This is also demonstrated by baseball pitchers and batters, soccer players and Australian rules football players. Using the information on angular veloctity can enhance the game of tennis players, base ball players and golfers. This is determined by the amount of torque applied during the balls release. When a baseball player is a pitcher they need to train to produce a greater angular velocity, this will increase the velocity of the ball and increase the chance of the batter miss hitting it (Hay, 1993).

The biomechanics aspects of levels that are relevant to the hockey penalty stroke are also applicable to other sports with striking implements. In golf when a player is putting, if their hands are spread apart they will have more control over the club and ball.

Having the basic knowledge of biomechanical principles relevant to the sport someone is playing or coaching will allow them to practice and be aware of the best way to execute a skill to improve their overall performance.

 

References

Adrian, M., & Cooper, J. (1989). Biomechanics of human movement (2^nd ed.). Wisconsin: Brown & Benchmark Publishers.

Biomechanical analysis. (2012). Retrieved from the New Zealand Qualifications Authority Web site: http://www.nzqa.govt.nz/assets/qualifications-and-standards/qualifications/ncea/NCEA-subject-resources/PE/91499-B-2/91499-EXP-B-student1-001.pdf

Hockey Australia. (2014). Retreived from Hockey.com.au Web site: http://www.hockey.com.au/hockey-australia

Blazevich, A. (2012). Sports biomechanics the basics: optimising human performance. London: Bloomsbury

Gibbs, K. (2013). Stability [image]. Retrieved from http://www.schoolphysics.co.uk/age11-14/Mechanics/Statics/text/Stability_/index.html

Hede, C., Russell, K., & Weatherby, R. (2011). Senior physical education for Queensland. Melbourne: Oxford University Press.

Hussain, I., Mohammad, A., Mohd, A., Ban, A., Ahmad, A., & Ahmad, S. (2011). Penalty stroke in field hockey: A biomechanical study. International Journal of Sports Science and Engineering,5(1), 53-57.

Isport. (2014). How to take a penalty stroke in field hockey. Retrieved from http://fieldhockey.isport.com/fieldhockey-guides/how-to-take-a-penalty-stroke-in-field-hockey

Kwon, Y. J., Park, S. J., Jefferson, J., & Kim, K. (2013). The effect of open and closed kinetic chain exercised on dynamic balance ability of normal health adults. Journal of Physical Therapy Science, 25(6), 671-674.

Pappas, D. (2012, February). Hit me with your best shot. Push hockey magazine. 41-46. Retrieved from http://www.slideshare.net/depappas/push-hockey-magazine-hitting-article-part-1

Whiting, W., & Rugg, S. (2012). Five factors determine stability and mobility. Retrieved from http://www.humankinetics.com/excerpts/excerpts/five-factors-determine-stability-and-mobility

Wuest, D., & Fisette, J. (2012). Foundations of physical education, exercise science and sport (17^th ed.). New York: McGraw Hill.