Biomechanical Principles

Biomechanics:Application of mechanics to the bodyStudy of internal and external forces on the body.

Newton’s 1st Law

• Law of Inertia– Every body persists in a state of rest or uniform

motion unless other forces cause it to change.

Newton’s 2nd Law

• Law of Acceleration– The acceleration of a body is proportional to

the magnitude of the resultant forces and inversely proportional to the mass of the body.

– a=F/m or Acceleration equals force divided by mass (Force = mass x acceleration)

• More force, then more acceleration• More mass, then less acceleration

Newton’s 3rd Law

• Law of Action, Reaction– For every action there is an equal and opposite

reaction

Body Movement

• Achieved biomechanically by contraction of the muscles within the system of levers and pulleys

• Assumption made that there is an intact nervous system

Control of Movement• Central nervous system and peripheral nervous

system• Bones • Articulations or joints• Muscles

– Smooth– Cardiac– Skeletal (striated)*

Primary Sources of Force

• Gravity - acting on body parts or weights of attached objects; External, constant, always a downward force

• Internal - active contractions or passive stretching of muscles

• External – force exerted on the body (from outside the body)– manual or mechanical resistance

Center of Gravity• Stability dependent on:

– 1) height of the center of gravity above the base of support

– 2) the size of the base of support– 3) the location of the gravity line within the

base of support– 4) the weight of the body

Center of GravityClinical Application

Center of GravityClinical Application

Center of GravityClinical Application

Center of GravityClinical Application

Waitress• Sarah Jessica Parker is waiting tables at Bennigan’s. She needs to balance a tray with a burger and a coke. The large coke weighs 0.5 pounds. The burger weighs 0.3 pounds. The distance from the middle of her hand to the center of the coke glass is 6 inches. How far away from the center of the hand does the center of the burger need to be placed? What type of lever system is this?

Crowbar

• Tom Hanks is using a crowbar to move a large rock. The distance from the fulcrum to the rock is 10 inches and the distance between from the fulcrum to Tom’s hands is 25 inches. The rock weighs 50 pounds. How much force does Tom need to put on the crowbar to lift the rock?

Wheelbarrow

• Arnold S. is cleaning up his back yard. He is gathering the yard waste in his wheelbarrow. The wheelbarrow weighs 20 lbs. The yard waste weighs 60 lbs. The center of gravity of the wheelbarrow and yard waste is located a distance of 2 feet from the wheel center. The handles are 4 feet from the wheel center. How much force does Arnold need to lift the wheelbarrow so he can start moving it?

Purse

• Jennifer Anniston is holding a 6 pound purse on her hand which exerts an external torque (along the weight of her forearm and hand) that equals the internal torque of her biceps acting on the forearm, causing her elbow to be maintained in a position of 90 degrees of flexion. The weight of her forearm and hand is 5 pounds, the center of gravity of the forearm and hand is located 8 inches distal to the elbow, and the insertion of the biceps is 2 inches distal to the elbow. If the internal torque is 100 foot-pounds, what must be the distance from her hand to her elbow?

Purse• Jennifer Anniston wants to use less force to hold the purse (she’s not that strong). If she wants to use only 88 foot-pounds of internal torque, what must she do?