Sports Biomechanics

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Sports and Exercise Biomechanics

course outline

1. Introduction to sports biomechanics concepts and their application to movement
2. Importance and functions of sports biomechanics
3. Axes and planes of the body
4. Range of body motions (ROM)
5. Forces and their application in sports
6. Linear and angular types of Motion and their determinants
7. Human muscle
8. Weight and movement
9. Friction
10. MOMENTUM, IMPACT AND IMPULSE
11. Energy and uses of energy to perform work
12. Concepts of body power
13. Strength
14. Efficiency in sports movement

Definition

Definition

The science that deals with an athlete’s movement whilst also considering the internal and external forces that are in effect whilst performing any desired movement

Introduction

You are probably planning a career as a PE teacher, coach, or some other physical activity specialist, and you probably are or have been active as a participant in one or more sports or fitness activities. Suppose a student or athlete asks you why should I do this skill this way or why isn’t this technique better?. Perhaps you even asked such questions when you were a student or athlete. Was the coach or teacher able to answer your question? Were you asked these questions? Could you answer them?. Traditional teaching and coaching tell you what techniques to teach or coach, where as biomechanics tells you why those techniques are the best to teach or coach. Biomechanics in sports, can be stated as the muscular, joint and skeletal actions of the body during the execution of a given task, skill and/or technique. Proper understanding of biomechanics relating to sports skill has the greatest implications on: sport's performance, rehabilitation and injury prevention, along with sport mastery.

Importance of Biomechanics

Biomechanics includes the study of all living things, plant, animal; animal biomechanics includes only animals as subjects of study; human biomechanics includes only humans; and sport biomechanics includes only humans involved in exercise and sport. We might define exercise and sport biomechanics as the study of forces and their effects on humans in exercise and sport.

   Performance Improvement'

The ultimate goal of exercise and sport biomechanics is performance improvement in exercise or sport. A secondary goal is injury prevention and rehabilitation. This secondary goal is closely related to the first and could almost be considered part of the primary goal, because an uninjured athlete will perform better than an injured athlete.

   Technique Improvement

The most common method for improving performance in many sports is to improve an athletes technique. This is one of the motivating factors for studying biomechanics. The application of biomechanics to improve techniques may occur in two ways; Teachers and coaches may use their knowledge of mechanics to correct actions of a student or athlete in order to improve the execution of a skill, or a biomechanics researcher may discover a new and more effective technique for performing a sports skill. In the first instance, teachers and coaches use qualitative biomechanical analysis methods in their everyday teaching and coaching to effect changes in technique. In the second instance a biomechics researcher uses quantitative biomechanical analysis methods to discover new techniques, which are then communicated to teachers and coaches to implement them

   Equipment Improvement

How else can biomechanics contribute to performance improvement? What about improved designs for the equipment used in various sports? Shoes and apparel(sports cloth) constitute the equipment used in almost every sport. The equipment worn may have an effect on the performance, either directly or through injury prevention. Besides shoes and apparel, many sports require the use of some sort of implement. Think of sports in which an implement is used in your institution. How have changes in sports implements changed performances in these sports? What about bicycling, swimming, tennis, golf, hockey, high jump, javelin throwing, soccer, basketball, etc. Lighter and better-designed implements have not only contributed to improved performances by elite athletes in these sports, they have contributed to improved performances by recreational participants as well.

    Training Improvement

Biomechanics has the potential of leading to modifications in training and thus improvements in performance. This application of biomechanics can occur in several ways. An analysis of the technique deficiencies of an athlete can assist the coach or teacher in identifying the type of training the athlete requires to improve in performance. The athlete may be limited by the strength or endurance of certain muscle groups, by speed of movement, or by one specific aspect of his/her technique. Sometimes, the limitation may be obvious. For example a gymnast attempting an iron cross maneuver requires tremendous strength in the adductor muscles of the shoulder. A mechanical analysis of the maneuver would reveal this, but it is already obvious to gymnastics coaches and observers. In other sport skills, the strength requirements may not be so obvious.

    Injury prevention and rehabilitation 

Some believe that injury prevention and rehabilitation should be the primary goal of exercise and sport biomechanics. Biomechanics is useful to sports medicine professionals in identifying what forces may have caused an injury, how to prevent the injury from recurring (or occurring in the first place), and what exercises may assist with rehabilitation from the injury. Biomechanics can be used to provide the basis for alterations in technique, equipment, or training to prevent or

   rehabilitate injuries.
       *Techniques to reduce injury

*Equipment designs to reduce injury

Axes and planes of the body

Axes and planes of the body

Range of motions (ROM)

1

Friction

Friction can either be dry friction also referred to as coulomb friction or fluid friction, which is developed between two layers of fluids and occurs when dry surfaces are lubricated. The behaviour of fluid friction is complicated, and because it occurs less frequently in sports, we will limit our discussion to dry friction. Dry friction acts between the non-lubricated surfaces of solid objects or rigid bodies in contact and acts parallel to the contact surfaces. Consider playing basketball on a rough court, friction generated between court surface and the shoe sole is dry friction. Friction arises as a result of interactions between molecules of the surfaces in contact. When dry friction acts between two surfaces that are not moving relative to each other, it is referred to as static friction (also referred to as limiting friction when describing the maximum amount of friction that develops just before two surfaces begin to slide). Consider stepping on the court and just before sliding/moving there is frictional force generated and holds the two surfaces static. When dry friction acts between two surfaces that are moving relative to each other, it is referred to as dynamic friction (also referred to as sliding friction or kinetic friction).

Friction and Normal Contact Force

Adding books to a pile increases the inertia of the pile by increasing its mass. This shouldn’t affect the static friction force, though, because there is no apparent way an increase in mass could affect interactions of the molecules of the contacting surfaces, it is these interactions that are responsible for friction. The weight of the pile was also increased by the added books, increasing the weight increases the normal contact force acting between the two surfaces. This would increase the interactions of the molecules of the contacting surfaces, because they would be pushed together harder, so it is not the weight of the books that caused the increase in static friction force, but the increase in the normal contact force. If the normal contact force and the friction force are measured, the friction force is proportional to the normal contact force, as one increase, the other increases proportionally. This is true for both static and dynamic friction. In this case, the friction force is horizontal, and the normal contact force is a vertical force influenced by the weight of books.

Activity

Let’s do some experimentation to learn more about friction. Place a book on a flat horizontal surface such as a desk or tabletop. Now push sideways against the book and feel how much force you can exert before the book begins to move. What force resists the force that you exert on the book and prevents the book from sliding? The resisting force is static friction, which is exerted on the book by the table or desk. If the book does not slide, then the static friction acting on the book is the same size as the force you exert on the book, but in the opposite direction. So, the effects of these forces are cancelled, and the net force acting on the book is zero. Put another book on top of the original book and push again. Can you push with a greater force before the books begin to move? Add another book and push again. Can you with an even greater force now? As you add books to the pile, the magnitude (size) of the force you exert before the books slide becomes bigger and so does the static friction force

Activity

Now try holding the book against a vertical surface, such as a wall. Can you do this if you only push against the book with a horizontal force? How hard must you push against the book to keep if from sliding down the wall? What force opposes the weight of the book and prevents it from falling? The force of your hand pressing against the book is acting horizontally, so it can’t oppose the vertical force of gravity pulling down on the book. The force acting upward on the book is friction between the book and the wall (and possibly between the book and your hand). The force you exert against the book affects friction since the book will slide and fall if you don’t push hard enough. Again we see that the friction is affected by the normal contact force-the contact force acting perpendicular to the friction force and the contact surfaces

Friction and Surface Area

Does surface area affect friction? Let’s try out an experiment to see if increasing or decreasing surface area in contact affects friction force. In adding books to a pile, surface areas in contact between the book and table varied dramatically, but friction did not change noticeably. In fact, dry friction, both static and dynamic, is not affected by the size of surface area in contact. This statement is probably not in agreement with your previous notions about friction, but you have just demonstrated it to yourself. If that isn’t enough to convince you that the friction is unaffected by surface area, let’s try to explain it. Dry friction arises due to the interaction of the molecules at the surface areas in contact. We have seen that if we press these surfaces together with greater force, the interactions of the molecules will be greater and friction will increase. It makes sense to say that if we increase the area of the surfaces in contact, we also increase the number of molecules that can interact with each other, and thus we create more friction. but if the force pushing the surfaces together remains the same, with the greater surface area in contact, this force is spread over a greater area, and the pressure between the surfaces will be less( pressure is force divided by area). So the individual forces pushing each of the molecules together at the contact surfaces will be smaller, thus decreasing the interactions between the molecules and decreasing friction. It looks like a trade-off. The increase in surface area increases the number of molecular interactions, but the decrease in pressure decreases the magnitude of these interactions. So the net effect of increasing surface area is zero, and friction is unchanged.

Activity

take a hardback book and lay it on the table or desk. (it is important that you use a hardback book). Push the book back and forth across the table and get a feeling for how large the dynamic and static friction forces are. Try to only exert horizontal forces on the book. Now, try the same thing with the book on its ends and sides. Use a rubber band to hold the book closed, but don’t let the rubber band touch the table as you’re sliding the book. Are there any noticeable differences between the frictional forces you feel with the book in its different orientations? Try it with another hardback book

Friction and Contacting Materials

Friction is affected by the size of the normal contact force, but is unaffected by the area of contact. What about the nature of the materials they are in contact? Is the friction force on rubber-soled shoes different from the friction on leather-soled shoes? Let’s try an experiment to investigate how the nature of the materials in contact affects the friction force between them. Let’s observe the difference between the frictions of a book on a table and a shoe on the table. Place the book on the table and put an athlete’s shoe on top of it, push the book back and forth across the table and get a feeling of how large the dynamic and static friction forces are. Now put the shoe on the table, sole down and place the book on top of it and get a feeling of how large the dynamic and static friction forces are. Which produce larger friction forces? In both conditions the weight and mass of objects being moved remains the same, the surface area of contact changed but we determined that friction is unaffected by that. The variable that must be responsible for the changes in the observed frictional force is the difference in the type of material that was in contact with the table. Greater existed between the table and the softer and rougher sole of the shoe than between the table and the smoother and harder book cover.

Fluid forces

Fluid forces are those offered by air and water. When a body or object moves through air or water, it is affected by fluid friction which acts in the opposite direction to the motion of the moving body. The amount of air resistance or fluid friction experienced depends upon the shape of the object and the speed at which the object is moving.

Air resistance

Air resistance is prevalent in most sporting activity, although its effects on performance can vary greatly.

Example

Consider the examples below. How might air resistance affect each? Long jump Projectiles such as balls, shuttlecocks and javelins Cyclists Sprinters

Air resistance and projectile motion

Air resistance offered to a projectile while in flight may change the parabolic flight path. Flight paths can be categorized as:

• Parabolic (a uniform symmetrical shape)
• Nearly parabolic
• Asymmetric

To discuss the effect of air resistance on flight paths, we will examine more closely the flight of a shot put and badminton shuttlecock.

Shot put – a study of projectile motion Flight is governed by the ratio of weight to air resistance. Since air resistance is dependent upon the size, shape and speed of an object, all slow moving objects have little air resistance. The weight of the object will be the determining factor and will almost form a parabolic arc.

Faster moving objects have greater air resistance. This causes rapid deceleration and slowing down of the projectile until a point is reached where once again weight becomes the determining factor, leading to a symmetric flight path. Observe the flight of a badminton shuttlecock from a high serve: it will decelerate rapidly and drop vertically- hopefully on the back baseline.

Motion and types

We might define motion as the action or process of change in position. Movement is a change in position. Moving involves a change in position from one point to another. Two things necessary for motion to occur;

• Space (space to move in e.g. field of play)
• Time (time during which to move)

Movements are classified into linear, angular or both (general).

LINEAR MOTION

Linear motion is also referred to as translation. It occurs when all points on a body or object move the same distance, in the same direction. This can happen in two ways;

• Rectilinear translation
• Curvilinear translation

Rectilinear translation is the motion you would probably link to linear motion. It occurs when all points on the body or object move in a straight line so the direction of motion does not change, the orientation of the object does not change, and all points on the object move the same distance. Examples in sports or human movement include;

• skater gliding across the ice in a static position,
• sailboarder zipping across the lake in a steady breeze,
• bicyclist coasting along a flat section of the road etc

Curvilinear translation is very similar to rectilinear translation. It occurs when all points on the body or object move so that the orientation of the object does not change and all points on the object move the same distance. Examples include;

• a gymnast on a trampoline,
• a diver,
• a ski jumper.
• The skateboarder and in-line skater can achieve both types of linear motion.

The difference between rectilinear and curvilinear translation is that the paths followed by the points on the object in curvilinear translation are curved, so the direction of motion of the object is constantly changing, even though the orientation of the object does not change.

ANGULAR MOTION

It is also referred to as rotary motion or rotation. It occurs when all points on a body or object move in circles (or parts of circles) about the same fixed central line or axis. Angular motion can occur about an axis within the body or outside the body. A child on a swing is an example of angular motion about an axis of rotation within the body. Examples in sports and human movements include; individual movements of our limbs (if isolated, almost all our limbs are examples of angular motion), a giant swing on a horizontal bar.

GENERAL MOTION

General motion is a combination of linear and angular motions. It is the most common type of motion exhibited in sports and human movement. Combining the angular motions of our limbs can produce linear motions of one or more body parts. When both the knee and hip joints extend, you can produce a linear motion of your foot, extension of the elbow and horizontal adduction at the shoulder can produce a linear motion of the hand. Running and walking are also good examples of general motion.

MOMENTUM, IMPACT AND IMPULSE

Momentum

Momentum is the amount of motion a moving object has and is the product of its mass and velocity:

Mo = MxV, where Mo is momentum, M is mass and V is velocity

A sprinter with a mass of 75 kg and a velocity of 10m/s has a momentum of 750kgm/s. From the above equation it can be seen that a body’s momentum can be changed by altering either mass or velocity. However, in sporting activity the mass of a body or object generally remains constant, so any change in momentum must be due to a change in velocity (acceleration). For example a long jumper may increase velocity by changing their approach run, in order to increase their momentum before takeoff. Once in the air, the velocity and mass of the jumper remains constant, so momentum is said to be conserved. This extends Newton’s first law of motion; In any system of bodies that exert forces on each other, the total momentum in any direction remains constant unless some external force acts on the system in that direction

Impact

Momentum becomes more important in sporting situations where collisions or impacts occur. The outcome of the collision depends largely upon the amount of momentum each of the bodies possessed before the collision took place. The body with greater momentum will be more difficult to stop. For

Example

If a prop forward weighing 90kg and a scrum half weighing 60kg were both traveling at the same velocity, the prop forward would have a greater momentum and would need to apply a larger force to stop his path to the try line. The speed at which the squash ball is struck is determined by the momentum of the racket head at the time of impact.

A change in momentum is synonymous with a change in acceleration and as such relates to Newton’s second law of motion. This is expressed as: F = ma In order to work out the acceleration (a), the following equations must be used:

   a = V-U/t

Thus: F = m(v-u)/t

F = mv-mu/t

Ft = mv-mu

Ft = change in momentum

Impulse

The final equation above suggests that any change in momentum is dependent upon the product of the force and the time that force is applied to an object, known as impulse. It therefore follows that any increase in the force applied- or the time over which the force is applied, the outgoing momentum of the object will increase. This has important implications for sporting situations where acceleration of a body or object is essential.

Example

A follow through a racket or hockey stick will ensure that the time over which the force has been applied is at its maximum; the change in momentum or acceleration of the ball will be greater than if a follow through of the stick had not been performed. The shot put: over time the technique of this event has been transformed. Originally a sideways stance was adopted before the put. However, the O’Brien technique aims to apply a force over a longer period of time by incorporating a one and three quarter turn and increasing the acceleration of the shot. Again by performing a follow through the athlete can ensure that the time of contact has been maximized. Of course in addition to developing this technique, the athlete still requires the necessary physical attributes in order to apply a large force to the implement. Water polo; a player catching the ball with one hand will aim to decrease the velocity over a long period of time and so decrease the force exerted by the ball on the hands. S/he does this by meeting the ball early and withdrawing the catching hand in the direction of the ball’s motion, thus cushioning the impact; this will prevent the ball hitting the hand and bouncing off uncontrollably. Cricket: when catching a fast moving cricket ball, the same principles applies, which can prevent injury to the hands. Other examples include the use of crash mats in activities such as gymnastics and high jump.