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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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