The Science of Biomechanics How Do I Move? Chapter 8 The Science of Biomechanics

Objectives: Identify the external forces acting on the human body. Describe the resulting motion. Describe the expected path and motion of any projectile. Differentiate between throw-like and push-like skills. Determine the degree of stability possessed by an athlete. Understand the causes and effects of somersaulting. Qualitatively analyze simple sport skills that involve throwing, striking, or hitting an object. Sport Books Publisher

Biomechanics Examines the internal and external forces acting on the human body and the effects produced by these forces. Aids in technique analysis and the development of innovative equipment designs. Draws on knowledge from sports medicine, physical therapy, kinesiology, and biomechanical engineering. Sport Books Publisher

Quantitative Analyses Involves measurement of variables that are thought to optimize or maximize performance golf swing For Example: Pattern of forces using a force platform Sequence of muscle activity using electromyography (EMG) Three-dimensional (3D) movements of each body segment using high speed cinematography Sport Books Publisher

Qualitative Analyses Involves obtaining visual or aural information to asses performance It requires: A framework within which skilled performance can be observed A set of principles with which movement can be analyzed A checklist to use when identifying errors Techniques to use for error detection and correction Sport Books Publisher

Kinematics Study of Motion The study of time and space factors of a body in motion. The variables used to describe motion are time, displacement, velocity, and acceleration. These variables are used to describe both linear and angular motion (angular displacement, angular velocity, and angular acceleration). Sport Books Publisher

Kinematics Variables Time: temporal characteristics of a performance, either of the total skill or its phases Displacement: length and direction of the path an athlete takes from start to finish Angular Displacement: direction of, and smallest angular change between, the rotating body’s initial and final position Velocity: displacement per unit of time Angular Velocity: angular displacement per unit of time Acceleration: rate of change of velocity Angular Acceleration: angular velocity per unit of time Sport Books Publisher

Kinetics Study of Motion Focuses on the various forces that are associated with a movement Internal Forces: Generated by muscles pulling via their tendons on bones, and to bone-on-bone forces exerted across joint surfaces External Forces: Acting from without, such as the force of gravity or the force from any body contact with the ground, environment, sport equipment, or opponent Sport Books Publisher

Human Body Models Particle Model: Used when the object of interest (the human body or an object) is airborne after being thrown, struck, or kicked Stick Figure Model: Used when the object is in contact with its environment Rigid Segment Model: Used for more sophisticated quantitative analyses Sport Books Publisher

Three models used to represent the diver Particle Model Rigid Segment Model Stick Figure Model Sport Books Publisher

Particle Model The drawing consists of a single point, representing the body’s center of mass The body is isolated from its surroundings The only external forces acting on the body are gravity and air resistance (when a large velocity is involved) Limited to bodies that are in flight (projectile motion) Sport Books Publisher

Preliminary Steps for Analyzing Human Motion Identify the system to be studied, which is to separate the object of interest from its surroundings. Step 2 Identify the frame of reference in which the movement takes place. Step 3 Identify the type of motion that is occurring, the body planes in which movement takes place (sagittal, frontal, or transverse). Identify the axes of rotation about which rotational motion occurs (sagittal, frontal, or vertical). Sport Books Publisher

Types of Motion Angular motion General motion Linear motion Sport Books Publisher

Curvilinear Motion When all parts of the body move the same distance, in the same direction, at the same time Translation refers to movement of the body as a unit without individual segment parts of the body moving relative to one another Rectilinear motion occurs when movement follows a straight line Curvilinear motion occurs when the movement path is curved Sport Books Publisher

Angular and General Motion Angular Motion (rotation) Occurs when a body moves along a circular path, through the same angle, in the same direction, and at the same time The axis of rotation is the point about which movement occurs All joint motions are angular motions General Motion A combination of linear and angular motion Includes most athletic and many everyday activities Sport Books Publisher

Causes of motion The only cause of motion of the human body is the application of an external force Force is any action, a push or pull, which tends to cause an object to change its state of motion by experiencing an acceleration Constant Velocity occurs when an object is not accelerating Linear Motion is caused by forces which act through a body’s center of mass Angular motion is caused by forces that do not go through the center of mass Sport Books Publisher

Linear motion results when the forces are applied through the center of mass Angular motion results when the forces are applied away from the center of mass Center of Mass Center of Mass

Scalar and Vector Quantities Scalar quantities have only magnitude (time) Vector quantities have magnitude and direction (force) - vectors are straight-line segments with one end defined as the tail and the arrow tip defined as the head. tail head Sport Books Publisher

Adding Vectors a b + b a = Resultant Resultant The head of a vector points in the direction of the quantity the vector represents Vectors can be added together using the head-to-tail method - to add vector B to vector A, an identical vector is drawn (same length and direction) as vector B beginning at the head of vector A - the resultant vector is directed from the tail of vector A to the head of vector B a b + b a = Resultant Resultant Sport Books Publisher

Levers Simple machines that augment the amount of work done by an applied force A rigid body (i.e., long bone) that rotates about a fixed point (i.e., joint) called a fulcrum (F) Acting on the lever is a resistive force (R, i.e., weight of a limb segment) an applied force (AF, i.e., muscle contraction). Three classes of levers: a. First class (e.g., teeter-totter) b. Second class (e.g., wheelbarrow) c. Third class (e.g., screen door with spring closing) Sport Books Publisher

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Factors affecting the moment of force D A. Balanced teeter-totter B. Increasing the moment arm by leaning backwards C. Increasing the applied force by adding a friend