LS2lecture28Biomech

LS2lecture28Biomech - LS 2 Lecture 28 Principles of...

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LS 2 Lecture 28 Principles of Biomechanics How are animals designed to transmit & bear forces? (with a focus on locomotion) De Motu Animalium Giovanni Borelli (1608-1679) Eadweard Muybridge (1830-1904) Prof Lieberman (danlieb@fas.harvard.edu) Office Hours Tuesday 1-2 and by appt Text: 364-377 392-415
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Biomechanics : study of how internal forces (e.g. muscles) & external forces (e.g. gravity) act on the body structural properties = shapes of organs and other structures material properties = stiffness, failure stress & strain of tissues osteons, neurovascular canals, and osteocyte lacunae Kinematics = motion Kinetics = forces
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Today : Monday : -Human gaits -Forces and moments -How do you move with legs? -How does the skeleton cope with stresses?
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A reminder of Newton s 3 Laws 3. Reaction : For every action there is an equal & opposite reaction Forces: non-contact (gravity: 1 kg = 9.8 m/sec 2 ) contact : Ground Reaction Force (GRF) Joint Reaction Force (JRF) friction, resistance, elastic, etc 1 N = force to accelerate 1 kg 1 m/s 2 2. Acceleration : F = mass • acceleration 1. Inertia : a body stays at rest unless acted upon by a force
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Force Platform (not available to Muybridge) A key force in locomotion = Ground Reaction Force ( GRF, or F G ) F G F G = reaction force between foot and ground (how hard foot is pushing against ground and vice versa) Represents force transmitted via the limb during stance phase of gait (when limb is on ground)
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F G Forces are vectors with a magnitude and an orientation F G can be resolved into orthogonal component vectors ( F G here is the resultant of F V and F H ) F V F H
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Key point : forces exert a torque (or moment , M) around joints F G M = F G R R= moment arm = perpendicular distance between joint and line of action of force (also a called lever arm, load arm, in-lever, out-lever) R r = tendency of force to cause rotation about an axis
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Muscle forces (F m ) support body weight and generate movement by producing opposing moments Knee F G R = F m r (shown at ankle) R F G F m r r = muscle ‘moment arm’ or ‘lever arm’ At any moment in time: musculoskeletal system consists of a series of jointed levers R = GRF ‘moment arm’ or ‘lever arm’
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R r F G F m Mechanical advantage = ratio of moment arms ( r/ R ) Note well: Smaller moment arm ‘ r ’ requires a
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LS2lecture28Biomech - LS 2 Lecture 28 Principles of...

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