Biomechanics: Human Machine System

produces force

Motor (Mover):

• Changes magnitude and direction of the forces

• Generates target motion

Machine Body

provides resistance

Resistor

Skeletal muscles

Motor example

bones & joints

Motion body example

Body or environment

Resistor Example

Force applied to the wheel

Type 1: Wheel and Axle

Force applied to the Axle

Type 2: Wheel & Axle

= 2nd class lever

Type 1: Wheel and Axle

= 3rd class lever

Type 2: Wheel & Axle

Turning a car steering wheel, opening a doorknob, or using a screwdriver.

Type 1: Wheel and Axle Example

When you rotate your upper arm inward or outward, your forearm swings like a steering wheel. Because the "wheel" (your forearm) is wider than the "axle" (your upper arm bone), it gives your muscles extra leverage. This leverage makes it much easier for your shoulder to generate a powerful twisting force to throw a ball, swing a racket, or push something heavy.

Type 1: Wheel and Axle Human Body Example

A bicycle drivetrain works just like a lever that helps you go fast.When you pedaling, your feet turn the front gears. The chain carries that power to a small gear on the back wheel.Because that back gear is small, it is easy for the chain to spin it around quickly. Since that small gear is locked to the center of the much larger wheel, every time the small gear makes one full turn, the big wheel makes one full turn too. Because the outside of the wheel is so large, it covers a lot of ground very quickly, turning your leg power into high speed.

Type 2: Wheel & Axle Example

When a short muscle sits very close to a joint (like your shoulder or hip), it only needs to shrink a tiny bit to make the rest of your long arm or leg swing through a massive, fast-moving arc.

Type 2: Wheel and Axle Human Body example

• MA > 1

• a wheel and axle makes work easier by multiplying your force.

  It means you can turn a heavy load at the center (the axle) by using much less effort on the outside (the wheel).

Type 1: Wheel and Axle Advantage

• MA < 1

• a speed and range of motion (ROM) advantage means that a small, powerful movement at the center of a wheel creates a much larger and faster movement at the outside edge.

Type 2: Wheel and Axle Advantage

• changes direction of force

• forces act downward

Pulley

• When muscles contract, they act as pulleys.

• With the patella sitting at the front of the knee it elongates the moment arm ​

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Pulley Example

• Simple machine w/ bar-like (or simply rigid) body that rotates about an axis (fulcrum-supporting the lever)​

• Forces: resistance, force, fulcrum force​

• Any rigid structure that rotates around an axis can be a lever

Lever

(force)(force arm)

Force torque =

(resistance)(resistance arm)

Resistance torque =

• (F) (FA)= (R)(RA)

net torque = 0

• No net torque acting on the object/No rotation​ / All the torque cancel each other out.

Angular:

• F + R + F_fulcrum = 0

• net force = 0

• Each force is a vector ​

• All the forces cancel each other out

Linear:

Gain of a system = output (resistance you overcome) / input (your effort)

Mechanical advantage (MA)

resistance / force 

MA =

MA > 1: mechanically advantageous (less force required) (you use less force to overcome the resistance)

Mechanical advantage (MA)

MA < 1: mechanically disadvantageous (more force) (you use more force to overcome the resistance)

Mechanical disadvantage

MA = 1: neutral (same force) (force and resistance cancel out each other)

Mechanically Neutral

• Fulcrum located between

• R & F in the same direction​

• Fulcrum acts in the opposite direction​

• The forces cancel each other out to maintain equilibrium ​

• The largest force acting on the seesaw is at the fulcrum​

• MA = 1​

• MA depends on the location of fulcrum (MA > 1, = 1, < 1)

1st class lever (similar to a seesaw)

atlanto-occipital joint

1st class lever Example

• Fulcrum is located at one end​

• The resistance is located between the fulcrum and force arm​

• Resistance is the largest force ​

• Force arm and the fulcrum force travel in the same direction

• Advantage in terms of force (MA > 1)​

• Most levers used in daily life

2nd class lever

• ankle in tiptoeing

  • Tip toeing (resistance is the weight of the body) (force is generated by the muscles)

2nd class lever Example

F_fulcrum + F = Resistance

2nd class lever Equation

• Your force is located between the fulcrum and resistance arm​

• Your force is the largest force ​

• Resistance arm and the fulcrum force travel in the same direction

• Disadvantage in terms of force (MA < 1)

• RA > FA (F > R)

• Advantage in displacement & speed​

• Main source of human mobility

3rd-class lever

elbow joint

3rd-class lever example

F_fulcrum + R = F

3rd class lever Equation

• torque produced by individual muscles

• Cause joint motions​

• Muscle generates force

Muscle torques

•  perpendicular line from the joint center to muscle’s line of action

• Varies as the joint angle changes​

• Moment arm becomes the Longest when the angle of pull = 90°

Moment arm

• Angle of pull changes FA (moment arm) & MA​

• FA (moment arm) is max when angle of pull = 90°​

• Joint angle of max MA varies from one muscle to another​

• FA is moment arm of the muscle ​

• MA depends on the joint angel, it is not constant ​

• MA does not have a unit it is a ratio

MA of the muscle

MA= R/F=FA/RA

MA of the muscle equation

Sum of the torques produced by the muscles about a joint​

Joint torque

• Elbow flexors: Biceps Brachii​ (BB), Brachioradialis​ (BR), Brachialis​ (B), pronator teres​

• Elbow extensors: TB, anconeus​

Agonist & antagonist torques