FROM: IAAF "Introduction to Coaching Theory"
Expert coaches are able to analyze the techniques involved in athletics and
modify them to make desired improvements with a particular athlete. The novice
coach often has difficulty deciding which technique to use and what
modifications to make. The simplest and most often used approach to overcome
this difficulty is to copy the event techniques used by current champions. The
problem that arises is top athletes frequently have different techniques and
additionally, coaches and athletes copy bad, as well as good, aspects of each
Every athlete has individual strengths and weaknesses. The technique of the champion is frequently built on training and practice over many years and is developed to suit his particular strengths and weaknesses. This highly developed technique is usually not suitable for a developing coach or athlete. How can coaches improve their ability to select the best techniques and identify the causes of faults they observe? To answer this question an understanding of what produces movement and an ability to analyze movement is essential for the modern coach.
Forces produce movement and a force is simply a pull or a push. We cannot see force, but are aware of it because of the effects it produces. For example, a high jumper applies force to the ground. We do not see the force but we observe the results, the athlete leaving the ground. Biomechanics is the science concerned with understanding the internal and external forces acting on a human body and the effects produced by these forces. Internal forces are those forces created inside the athlete's body by the action of muscles pulling on bones. External forces are those acting outside the body such as gravity and friction.
In this unit we will look at the basic language and principles of biomechanics to help your analysis of movement. These principles applied in practice, combined with the development of a good "coaching eye", will make you a more effective coach.
Linear and Rotational Motion
Linear motion is movement along a straight line and rotational motion is movement about an axis of rotation. In athletics, movement is usually a combination of linear and rotational motion and is called general motion. A sprinter's body, for example, has linear motion but the movement is caused by the rotational motion of the legs. Both forms of motion take place to produce the general motion of running. A discus thrower uses rotational motion to build up speed before releasing the discus. He also moves with linear motion from the back to the front of the throwing circle. This is another example of general motion.
Velocity and Acceleration
Speed tells us how fast a thing is moving. This thing may be the human body or a throwing implement. Velocity tells how fast a thing is moving and in which direction. A sprinter may cover 100 metres in 10 seconds. His horizontal velocity is determined by dividing the distance covered by the time taken. In this example 100 metres divided by 10 seconds gives a velocity of 10 metres per second.
Athletics has standard distances, so we can compare times to see which athlete has greater velocity. From experience, we know that an athlete who runs 100 metres in 10 seconds is faster, or has a higher velocity, than an athlete who takes 12 seconds. An athlete who runs 1500 metres in 3:40 has a higher velocity than an athlete who runs 4:00.
When you race any distance your velocity changes. At the starting line you are not moving and have zero velocity. After the gun has fired you gain velocity or accelerate. Acceleration tells us how fast the velocity of something is changing. Running acceleration may be to a maximum velocity, as in the 100 metres or to a velocity which is optimal for the event.
An athlete who slows down, loses velocity and is said to be decelerating. If we look at the speed-time graph for a sprinter we see an initial phase of acceleration. This is followed by maximal velocity sprinting and finally a phase of deceleration as the athlete fatigues.
Momentum is the quantity of motion a body has and is a product of weight and
velocity. In the human body there can be a transfer of momentum from one body
part to another. In the long jump, for example, the "blocking" of the free leg
when the thigh is parallel to the ground transfers momentum as additional force
to the take-off leg.
Angular momentum is the quantity of angular or rotational motion a body has and is the product of the moment of inertia and rotational velocity. When a body is rotating the moment of inertia is proportional to its size. If the arms are bent in sprinting, for example, their moment of inertia is less than if they are straight. A rotating body has a given quantity of motion or momentum and any reduction in the moment of inertia will cause acceleration to an increased rotational velocity. In sprinting this principle affects arm action and leg recovery. Any increase in the moment of inertia has the opposite effect of reducing rotational velocity. This increase of moment of inertia is used in the different flight techniques of the long jump to slow down forward rotation.
There can also be a transfer of angular momentum from one body part to another. This is applied in the throws when, for a right handed thrower, "blocking" the left side of the body
immediately before delivery transfers angular momentum to accelerate the right, throwing side.
Implications for the Coach
There are two practical principles that apply specifically to running, jumping and throwing where the athlete is concerned with creating optimal force and speed:
● Use all the joints that can be used
● Use every joint in order
Use All the Joints That Can be Used
The forces from each joint must be combined to produce the maximum effect. This is best done when all joints that can be used are used. This will help to get the most speed or acceleration out of a movement.
In the shot put, for example, the knee, hip, shoulder, elbow, wrist and finger joints should all be used to exert the greatest force on the shot. Beginners frequently miss out early joint movements such as the knee or hip action, or fail to complete a movement fully by not using the wrist or fingers.
Use Every Joint in Order
When several joints are used in a skill, their sequence and timing are important. This principle tells us when the joints should be used. Movement should begin with the big muscle groups and move out through the progressively smaller muscles, from big to small. This pattern produces optimal forces and flowing, continuous movement.
The continuous, flowing movement produces a summation of forces, forces adding together. The force generated by one part of the body is built on by the force of subsequent joints. In the well timed shot put, the hip action commences just as the leg extension decelerates. The shoulder action commences as the hip rotation decelerates and so on.
The release velocity of an implement depends on the speed of the last part of the body at release. The correct sequence and timing allow the athlete to attain maximal release velocity.
LAWS OF MOTION
Understanding the relationship between force and motion owes much to the work of
an English scientist, Sir Isaac Newton. He is best remembered for his three laws
Newton's First Law of Motion
It is important to know the definition for each of the three laws of motion and more important, know how to apply the laws in practical situations. Newton's first law of motion states:
"All bodies continue in a state of rest or uniform motion in a straight line unless acted upon by some external force."
What are the applications of this law? A sprinter, for example, will not move from the blocks until his legs exert force against them. The high jumper will not take off from his approach run unless a force is applied to change direction.
Newton's Second Law of Motion - Law of Acceleration
"The acceleration of a body is proportional to the force causing it and takes place in the direction the force acts."
More force means more acceleration. A sprinter's acceleration from the blocks is proportional to the force exerted against the blocks. The greater the force exerted, the greater will be the acceleration away from the blocks. In the throwing events, the larger the force exerted on an implement, the greater will be the acceleration and consequently, distance thrown.
Once an implement has been released there are no forces which can act to accelerate it. The same is true in the jumping events. The greater the force the athlete exerts at take-off the greater the acceleration and height or distance achieved. Once the athlete has left the ground nothing he does will accelerate the body. When maximal forces are needed the muscles contract to generate this force and this is why injuries are more likely to occur in the acceleration or deceleration phases of a movement.
Newton's Third Law of Motion - Law of Reaction
"To every action there is an equal and opposite reaction."
A runner exerts a force against the ground. This creates an equal and opposite reaction force which moves the body over the ground.
The law of reaction also applies to movements that occur in the air. In these situations the equal and opposite reaction is shown in movements of other parts of the body. A long jumper, for example, will bring the arms and trunk forward in preparation for landing. The equal and opposite reaction is movement of the legs into a good position for landing.
Center of Gravity
Gravity is a force which is always present and is a pulling force in the direction of the centre of the earth. This force acts on every body through an imaginary point called the centre of gravity (GG). A solid object like a shot or discus has its CG in the centre and this is a fixed point.
The human body is a complex and constantly changing shape. The centre of gravity now moves according to the positioning of the body and limbs. The CG may be inside the body, for example, when standing or it may be outside the body as in the pole vault and flop high jump bar clearances.
When an athlete launches himself or an object into flight gravity will act as a force pulling the athlete or object towards the ground. The flight path of the centre of gravity of a body is a curve called a parabola. The parabolic flight path depends on three factors:
● Speed of take off or release
● Angle of take off or release
● Height of the athlete's CG at take off, or CG of implement at release
Of these, the speed of an athlete at take off, or of an implement at release, is the most important factor. Greater speed means greater distance achieved. Air resistance can also affect the distance travelled by an athlete or implement.
All the principles of movement are based on how forces are made by the athlete
or how they act on the athlete's body. They may appear complex at first but, as
you learn the basics for each event, biomechanics and an analysis of movement
will become an understandable and usable part of your coaching knowledge helping
to make you a better coach.