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Biomechanics Lecture 2

Introduction to measurements in biomechanics

Biomechanical Problems

•  Locomotion patterns •  Clinical Research •  Occupational Research •  Sport Performance Enhancement •  Mechanical Analysis of Specific Movements •  Safety and Injury Reduction •  Energy cost with specific movements •  Transitional changes •  NASA, microgravity and musculoskeletal system. •  Osteoporosis •  Mobility Impairment

Basic Biomechanics, 4th edition Susan J. Hall

Biomechanics so far:

•  Biomechanics Lab: •  The kinematics and EMG of walking (the effect

of incline/decline or speed manipulation)

•  Lecture 1: •  Introduction to basic measurements in

biomechanics

BASIC BIOMECHANICS: OBJECTIVES

•  To describe movement precisely, using well-defined terms

•  To consider the role of force in movement

•  To demonstrate techniques in biomechanics that are used to analyse movement

•  Movement is not in isolation from the external environment

•  There is a complex interaction of forces acting on the body :gravity (constant); frictional forces (changing)

•  Forces from wind, water, objects all influence movement

•  We need to understand physical laws of external environment to understand the prediction of these forces and compensation for these forces

•  All human movement involves the rotation of body segments about their joint axes

•  These actions are produced by the interaction of forces associated with external loads and muscle activity

•  Human movement is the consequence of an imbalance between components of these forces to produce rotation

•  The capability of a force to produce a rotation: torque

Standard Reference Terminology���Directional Terms

•  Superior •  Inferior •  Anterior •  Posterior •  Medial •  Lateral

•  Proximal •  Distal •  Superficial •  Deep

Body Planes & Axes

Coronal or Frontal Plane

Horizontal Plane

Sagittal Plane

Vertical Axis

Lateral Axis

Anterior-Posterior Axis

Posture : various definitions

•  The position of the body

•  The maintenance for a period of time of a position in space

•  The intrinsic mechanisms of the body that counteract gravity

•  The position held when locomotion ceases

Lying on back

Lying on front

Lying on right side

Lying on left side

Lying supine with head higher than feet

Sitting straight up or leaning slightly back

Body Postures

Flexion: to bend at a joint, or reduce the angle

Extension: to straighten at a joint, or increase the angle

Abduction: movement away from the midline

Adduction: movement towards the middle

Medial rotation: to turn inward

Lateral rotation: to turn outward

Supination: to rotate the forearm, palm outwards

Pronation: to rotate the forearm so palm faces backwards

Movements

TERMINOLOGY

Statics is concerned with bodies which are in equilibrium

Dynamics is concerned with unbalanced bodies which undergo motion

•  Define and identify common units of measurement for force, weight, pressure, volume, density, torque, and impulse

•  Identify and describe the different types of mechanical loads that act on the human body

•  Identify and describe the uses of available instrumentation for measuring kinetic quantities

•  Distinguish between vector and scalar quantities

Objectives

TERMINOLOGY

Kinematics is concerned with describing and analysing motion in terms of time, displacement, velocity & acceleration

Kinetics is concerned with the analysis of forces which produce movement

A position-time graph

Movement involves the shift from one position to another

Reprinted by permission from R.M. Enoka 1988.

TERMINOLOGY

Scalar quantities have only magnitude

(eg Movement Speed)

Vector quantities have both magnitude and direction

(eg Movement Velocity)

Reprinted by permission from R.M. Enoka 1988.

Position-Velocity-Acceleration-���(Jerk – Snap – Crackle - Pop)

Graphing Changing Positions Reprinted by permission from R.M. Enoka 1988.

Reprinted by permission from R.M. Enoka 1988.

Kinematics of a 100 m sprint

•  Videotape of runner, measuring displacement of hip

•  Deviations in the linearity of displacement more obvious from velocity and acceleration traces

Reprinted by permission from R.M. Enoka 1988.

Muybridge

High-speed photography for the understanding of animal locomotion

Leland Stanford and Eadweard Muybridge

When a horse trots, are all four hooves off the ground at the same time?

(1872-1879)

Different types of motion capture

Trajectories of Leg Movements in Acoustically Steering Crickets

Sound

• Requirements for 3D motion capture:

2+ cameras

Usually 6-12 cameras

Movement Kinematics

Calibrating the 3D space���

•  Static L frame calibration of space

• Dynamic ‘wand’ calibration

Consider the necessary frame rate human movement studies often 50-150 Hz: 60 Hz for lab walking study

2D or 3D Kinematics •  2D is simple and fast •  2D is often cheap (1 video camera) •  With 2D there can be projection errors •  2D can over-simplify

•  3D better, but not always necessary •  3D difficult if subject is moving through a large

capture volume eg running etc •  Particularly important not to move cameras during

capture

Kinematic descriptions in 3D

•  Description of an object in space

•  Three sets of coordinates can describe the pose of an object (position + orientation)

Measuring Angles Relative Angles (joint rotations) The angle between the longitudinal axis of two adjacent segments.

Absolute Angles (segment rotations) The angle between a segment and the right horizontal of the distal end.

Measure on the same side of joint

A straight fully extended position is generally defined as 0 degrees

Measure from the same direction from a single reference - either horizontal or vertical

Reprinted by permission from R.M. Enoka 1988.

Looking at changes in angle-angle relationships during an action

Basic Concepts ���Related to Kinetics

•  Inertia •  Mass •  Force

– Free body diagram •  Center of Gravity •  Weight (W=mg)

•  Pressure •  Volume •  Density •  Torque •  Impulse

Basic Biomechanics, 4th edition Susan J. Hall

Movement Kinetics

•  Ground Reaction Force: The reaction force provided by the support surface on which the movement is performed.

•  Newton’s Law of Action-Reaction (The reaction of the ground to the accelerations of all body segments)

Sample Problem 1

1. If a scale shows that an individual has a mass of 68 kg, what is that individual’s weight?

Known: m = 68 kg Solution Answer Wanted: weight wt = 667 N Formulas: wt = mag wt = 150 lbs 1 kg = 2.2 lbs Basic Biomechanics, 4th edition

Susan J. Hall

Sample Problem 2 Is it better to be stepped on by a women wearing a

spike or by a court shoe? Known: wt = 556 N As = 4 cm2 Ac = 175 cm2 Solution Wanted: Answer Pressure exerted by the spike heel p = 139N/cm2 Pressure exerted by the court shoe p = 3.8 N/cm2 Formulas: p = F/A 43.75 times more pressure Basic Biomechanics, 4th edition Susan J. Hall

Centre of Pressure

•  The location of the ground reaction force under the foot

Eg in runners, differences in position of

ground strike

Force or Torque may be generated in several different ways:

Internal - Muscle Elasticity

External - Gravity Momentum Resistance (air, ground etc)

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