Key Biomechanical Principles
A projectile is
any object that can be launched, hurled or thrown, such as javelin (Hede, Russell & Weatherby, 2010). The path of a
projectile is called its trajectory. The trajectories of all projectiles are affected by the external forces
of gravity and of air resistance. Other factors affect the flight of a projectile
and ultimately the distance it will travel, the three main ones being the
speed, height and angle at which a projectile, in this case a javelin is
released (Hay, 1993). The speed of
release is by far the most important single factor in determining the distance
of the javelin throw (Hay, 1993). So how do we come to produce such speed with
reference to the kinetic chain?
With respect to the javelin throw, the transfer of momentum,
explains how large muscle forces generate momentum in the
proximal segments, those being the shoulder, which then transfers force
to the distal segments, being the fingers (Blazevich, 2012). Due to
conservation of momentum, the angular momentum as you slow down the proximal
segments of the arm is transferred to the distal segments of the arm and
finally in this sequence passed onto the javelin at the release
point. Angular momentum (H) is the product of moment of inertia (I) and
angular velocity (w). Therefore if moment of inertia decreases,
angular velocity must increase for angular momentum to stay
the same (Blazevich, 2012). As we give our arm angular momentum
during the throwing action an angular velocity is produced. As the
more distal segments of our arm, for example hands, are lighter compared
to the proximal segments of the arm, for example the shoulder, they have a
decreased moment of inertia, and therefore have a greater angular velocity for
the same angular momentum.
When performing a
throw-like movement, angular velocity increases towards the distal
segments of an arm. Angular momentum is conserved through both segments,
and moment of inertia decreases towards the distal segments. The transfer
of momentum involves accelerating proximal segments of the arm and then
stopping them to get a transfer of momentum along the arm that results in
a high velocity at the end point, being in the case of a javelin throw, the
fingers (Blazevich, 2012).
Next, we look at summation of force. Newton’s second
law of motion implies that the greater the force applied to the javelin, the
greater its acceleration. To obtain maximum force, it is necessary to combine
the forces applied by different body parts. This concept is known as the
summation of force (Hede, Russell &
Weatherby, 2010). The summation of force principle explains that the force produced during
the movement of one body segment (for example, the lower leg) will be added to
the force produced by the next body segment (the thigh), to the next (trunk,
chest and arms), and the next (wrist and fingers). Important for force
production is the sequence in which body parts are used. For best results,
movement begins with the larger, slower body parts and finishes with the
smaller, faster body parts. Javelin throwers are able to propel the projectile
further through the air by using the combined force of many parts of their
bodies, namely legs, trunk, shoulders, arm, wrist and fingers.
Kinetic chain:
The kinetic
chain can be defined as the complex co-ordination of individual
movements about several joints at the same time, with kinetic chain movements
classified as either a push-like movement or a throw-like movement (Blazevich,
2012). For the analysis of the javelin throw, the throw-like movement pattern which involves the movement of the
joints sequentially or one after the other, will be utilised. The delivery
stride within the throwing action is a good example of a throw-like pattern
with the most proximal segment within the whole action, the trunk, being
accelerated initially before a braking force is applied to the trunk region.
This braking force and Newton’s Third Law allows for the transfer of energy
sequentially from the most proximal point, the shoulder, to the most distal
point, the fingers (Blazevich, 2012). A factor that influences the quality of energy transfer to the javelin
is the coordinated motion of the upper limb starting from the
acceleration-deceleration of the sequences in the upper kinetic chain. These
sequential motions from the proximal to the distal segments are one of the
fundamental keys to performance in overarm throwing (Campos, Brizuela, &
Ramon, 1999). Figure 1 displays the kinetic chain
sequence that occurs from the javelin throw action.
Figure 1. The Kinetic Chain for javelin
The javelin throw utilises the biomechanical
principle of leverage. Third-class levers are levers which have the force
between the resistance and the fulcrum (Hede,
Russell & Weatherby, 2010). Our forearms act as third-class levers, with the
elbow as the fulcrum or point of rotation for the lever. Force is applied to
cause the lever to rotate around the fulcrum. Getting a lever moving can be
difficult as levers have inertia, so they have a reluctance to begin rotation.
To make it easier to move and swing through with speed, athletes will often
shorten the length of the levers in their body, such as by bending their arm in
the approach phase of the javelin throw. It is important that the lever (arm) is
straightened at the point of contact or release to ensure the maximum speed
and, therefore, enable force to be transferred to the javelin.
The Answer
(A Case Study comparison
of approach styles, before and after technique correction)
Austin is a
19 year-old male from country NSW, who is now in his second year of University
in Adelaide. In high school, Austin competed in the local inter-school
athletics Javelin event each year, without explicit javelin throwing instruction
and just for a bit of fun. In year 12, Austin was keen to become competitive in
the Javelin event and trained for it. Austin increased his knowledge through watching
multiple YouTube clips and with the assistance of his Physical Education
teacher who provided him with individualised feedback concerning his javelin
execution particularly with his approach and angle of release. Austin is not often
involved in Javelin events but is a keen football and cricket player.
Austin
agreed to take part in a biomechanical analysis of his javelin throw technique.
Austin warmed up, then had 3 throws using his preferred style, with the
distance of each measured. During the attempts and through observation of video
footage of Austin's throws, it was noted that Austin extended his arm straight
down during the approach run as opposed to the conventional method of an elbow bend
of approximately 90degrees. Austin otherwise displayed sound technique. Austin’s
initial natural, straight-arm approach technique is displayed in the photo
sequence below (figure 2).
Figure 2. Austin's natural, extended straight-arm approach (pre-biomechanical analysis)
Post biomechanical analysis and upon discussion with
Austin, he agreed to modify his approach with a flexed, bent arm for a consecutive
3 attempts. Again the distance of each throw was recorded. Austin’s flexed arm
run-up correction is displayed in the photo sequence below (figure 3).
Figure 3. Austin's flexed arm modified approach. (post biomechanical analysis and technique correction)
*Note
slides 6-8 on figures 2 & 3 show that Austin’s release and follow through
technique is almost identical. This is important as it shows it was just the
approach that has been modified, not his whole technique.
Figure
4. The results from Austin’s throw attempts, before and after correction:
Austin
thought the increase in his javelin throw distance may have been because he had
a few practices and was better warmed up, so a re-throw using his initial
technique was measured. The above results proved the increase in throw was due
to technique modification.
So how can
we understand this heightened performance in terms of biomechanics,
particularly with reference to the kinetic chain and approach technique?
The
approach run provides the body’s momentum and delivers a preliminary velocity
for the javelin, before muscular acceleration in the drive and throwing phases
(Valleala, 2009). During the approach run, the javelin is usually carried in an
over-the-shoulder position, elbow bent, with the hand moving back and forth
close to the ear and in unison with the athlete’s leg action (Hay, 1993). The palm of the hand must face upwards to help the
wrist, elbow and shoulder to relax, and therefore lead to an easy running
action, which gives momentum to the body. The javelin is lined up approximately
parallel to the ground (Stander, 2006). The
withdrawal of the javelin begins around 5 strides before the final throwing
position is reached. This is achieved through the direct pulling back of the
throwing hand into a position in which the arm is fully extended and the hand
is at approximately shoulder height (Hay, 1973). This is displayed by Austin in
the second round of attempts, post biomechanical analysis and technique correction,
and is in preparation for the throw-like movement. The
javelin throw is an overarm action, and begins with the shoulder
extending while the elbow and wrist are flexing during the wind-up
phase. The extension velocity of the hands and fingers in the latter part
of the throw increases significantly resulting in a high release
velocity (Blazevich, 2012).
We refer back to Newton’s Third Law, which relates to every action,
having an equal and opposite reaction. Newton’s Third Law can assist in analysing the javelin throw,
particularly in the run-up, which generates projectile speed by utilising the
ground reaction forces to decelerate the lower body causing the inertia of the
upper body to rapidly accelerate the shoulders, hips and throwing arm resulting
in a throw-like pattern where appropriate javelin release speed is applied.
Austin’s initial technique, as noted, differs from this, and consequently this
technical error inhibits his potential maximum throw distance.
Tying this back to
the principles of transfer of momentum, summation of force and the kinetic
chain, once approach phase is over, the momentum is passed from the feet to the
legs to the torso to the upper body and arms and finally through the wrist to
the fingers. Towards the final stage this momentum accelerates the
proximal segments of the throwing arm which then decelerates to allow
the transfer of momentum along the arm, resulting in a high velocity of the end
point. In addition, the tendons of wrist and fingers stretch and substantially
recoil also contributing to the speed of the throw (Butler-Bowdon, 2013).
A return is now made to the consideration of levers. The principle of
leverage means that when using a 3rd class lever to produce force,
it is often best to maximise the length of the lever being used and to throw
the object at the end of the lever. For example, in cricket, bowlers will
generate more speed on the ball if they use a straight arm; a straight arm
lengthens the lever. Given Austin’s past extensive involvement in cricket, this
may account for why it feels natural for him to ‘bowl’ rather than ‘throw’ a
javelin, as he associates faster speed with an extended arm.
View from
start to 0:30. This clip demonstrates a reasonable javelin approach.
Final thought
Practitioners
tend to rely on a perceived image of an idealised technique to determine
whether a change was required (Davids, Button & Bennett, 2008). Austin’s
javelin approach technique was corrected to suit this ‘text-book model’ of a
javelin throw. While this approach to technique correction is not always
beneficial, as it does not specifically take individual performer
characteristics and changing environments into account, the increase in
Austin’s throw distance proved otherwise in this particular case. Austin voiced
his concern that the change in his technique ‘does not feel right’, so we needed to determine if it was
worth perusing this particular technique modification. If an average of 2m can
be gained directly after instruction, one can assume after much practice with a
focus on his approach, that Austin will improve the distance of his javelin
throw if he takes this approach recommendation into account.
How else can we use and apply
this information?
The biomechanics
behind the javelin run-up and summation of forces is very similar to that of
the cricket bowling action. Athletes and coaches, within the discipline of
javelin are able to analyse and transfer the information that is provided in
the bowling run-up and apply the biomechanics to the skill to obtain the most
effective and efficient javelin throw. Through understanding the
biomechanical principles of the javelin throw, coaches, athletes and Physical
Education teachers can apply these transferable biomechanical principles to
other skills and sports and so analyse, develop and refine selected sport
skills. The tennis serve, baseball pitch and the volleyball spike, all demonstrate
variations of the throw-like kinetic chain movement pattern and are sport
skills to which this information can be applied.
References
Butler-Bowdon, C. (2013, April 23). What are the
biomechanical principles to maximise a javelin throw? [Blog]. Retrieved
from http://biomechanicsofajavelinthrow.blogspot.com.au/2013_04_01_archive.html
Blazevich, A. (2010). Sports biomechanics, the
basics: Optimising human performance. London: A&C Black Publishers.
Campos, J., Brizuela, G & Ramon, V. (1999).
Three-dimensional kinematic analysis of elite javelin throwers at the world
athletics championship “Sevilla’99”. International
Association of Athletics Federations: New Studies In Athletics. 19(21), 45.
Davids, K., Button, C., & Bennett, S. (2008). Dynamics
of skill acquisition: A constraints led approach. United States of
America: Human Kinetics.
Hay, J.C. (1993). The Biomechanics of Sports Techniques (4th edn). New Jersey: Prentice-Hall.
Hede, C., Russell, K., & Weatherby, R. (2010). Senior Physical Education for Queensland:
Applying Biomechanics to Sport. South
Melbourne, VIC: Oxford University Press..
Stander, R. (2006). Javelin Throw. Athletics Omnibus, South Africa. Retrieved from http://www.bolandathletics.com/5-13%20Javelin%20Throw.pdf
Valleala, R. (2012). Biomechanics
in Javelin Throwing. Retrieved from http://www.kihu.fi/tuotostiedostot/julkinen/2012_val_biomechani_sel72_42228.pdf
