The Lower Limb as a System of Levers

Andy HorwoodIndependent Researcher & Lecturer.

Product Research & Design HealthyStep LtdVisiting Lecturer & Fellow Staffordshire University

● Why do levers matter?● What is stress?● What is strain?● What is angular moment?● What are the classes of levers?● Where are these lever classes in the lower limb?

What influence does this piffle have on pathology?

What Is This Lecture All About?

ENERGETICS (the study of energy in systems)

Efficient Energetic = Efficient Biomechanics = Survival = Evolution = Efficient Energetics

Why Do Levers Matter?

Levers affect mechanical efficiency = Effect our energetics.Efficient Biomechanics + Healthy MSK Tissues = Efficient Energetics

Levers are ONE of the methods used to achieve mechanical efficiency

Force applied a structure

● Connective Tissue Tethering● Muscle Contraction● Extrinsic Factors● GRF

● Newton’s 1st ○ Law Of Inertia/Momentum

● Newton’s 2nd ○ Law (F =M X A)

● Newton’s 3rd Law○ (Every Action Has Equal & Opposite Reaction)

What is stress?

GRF over time

● Connective Tissue Tethering● Muscle Contraction● Extrinsic Factors● GRF

● Newton’s 1st ○ Law Of Inertia/Momentum

● Newton’s 2nd ○ Law (F =M X A)

● Newton’s 3rd Law○ (Every Action Has Equal & Opposite Reaction)

What is stress?

Force Applied A Structure

Large Movements of CoMPotential Energy

CoM athighest point

Kinetic Energy

Kinetic Energy

HS MS TS

Ankle Plantarflexion

Foot Flat

Strain = Deformity = Work Done

WORK CAN BE POSITIVE OR NEGATIVE

MOMENTUM ROTATING AROUND AN AXIS(remember joint axes are instantaneous)

•Moment of Inertia x Angular Velocity I x ω = L

•What is moment of inertia? •Like mass but considers where the mass is distributed to the axis of rotation. •I = Σ (mn . r 2/n).

WHAT IS ANGULAR MOMENTUM?

Angular velocity number of degrees (or rotations) per second

● If forces either side of an axis = rotation A● If forces are over axis = no rotation B

Angular momentum

A B

If forces equal on same side forces furthest away from axis create the direction of rotation. AIf unequal forces are equal distance from axis the larger force creates the direction of rotation. B

A B

Simple Foot-ankle Model

LATERALMEDIAL

Why Shoes & Leg Shape Matter

CoM & Gravity = CoG

BWV BWV

GRF GRF

Right

The nearer the forces to the axis the easier for muscles to control rotations. Shoes matter!

With The Right Shoe

Linear Momentum

FLEXION / EXTENSION (sagittal plane)

Joint motion is ANGULAR MOMENTUM, created by GRF and CoM interaction or

muscle contraction

Most muscle resists angular moment in closed chain.(eccentric contraction)

Heel Contact

CoM

CoG

Early Loading Response

plantarflexion moment

flexion

End Loading Response

Foot flat● CoM lowest● Kinetic energy used up

Early Midstance

True Midstance

● CoM at maximum height● High Potential energy

Late Mid-Stance

● Potential turns to kinetic energy

Terminal Stance

E

Pre-Swing

Classes of Levers

FR or E

E or R

F

ER

ER F

● 1st CLASS LEVER○ Effort on opposite sides of

fulcrum ■ (inefficient or efficient)

● 2nd CLASS LEVER○ Effort furthest away from

fulcrum ■ (efficient)

● 3rd CLASS LEVER○ Resistance furthest away

from fulcrum ■ (inefficient)

Classes of Levers

● 1st CLASS LEVER○ Effort on opposite sides of

fulcrum ■ (inefficient or efficient)

● 2nd CLASS LEVER○ Effort furthest away from

fulcrum ■ (efficient)

● 3rd CLASS LEVER○ Resistance furthest away

from fulcrum ■ (inefficient)

ER F

E/RE/R

F

R

F

E

Hip Abductors in Single limb support

Lower Limb Example - 1st Class

RE E F

RF

E RER

XX

Swing limb

Swing limb

● Achilles at heel lift● MTP joints are the

fulcrum(s)

E

R

F

CoM

E R

Lower Limb Example - 2nd Class

R

F

E

Lower Limb Example - 3rd Class Lever

FE

R

F

E

R

FE

R

Mechanical efficient or physiologically efficient/speed

Why Choose Inefficient 3rd Class?

E

E

R

R

Muscle Contraction - Levers Principles

ER

F

R

R

R

effort is > than resistance

effort is = to the resistance

effort is < than resistance

Hip Abductor Dysfunction

Class 1 Lever Pathomechanics

ISOMETRIC ECCENTRIC

F F

E ERR

E

R

Isometric Eccentric

Class 2 Lever Pathomechanics.

F F F

E

R

E

R R

E

Wheelbarrow Foot

E

RR

F

●○ Loading rate○ Stride length

Class 3 lever dysfunction

R

BWV

GRF

● Tibialis Anterior Tendinopathy● Anterior shin splint

E R

● Tibial anterior stresses acceptable ● Angular momentum controlled ● Loading rate acceptable

● Tibial anterior stresses to high● Angular momentum uncontrolled● Loading rate too high

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