Gait Analysis and Biomechanics

Chapter 12

Overview

• Defining the gait cycle• Guessing game: sagittal joint kinematics• Sprinting kinematics• Ground reaction forces– Walking at different speeds– Running– Implications on Joint Moments

• One pathology

Basic Vocabulary

• The gait cycle: initial contact of one leg to initial contact of the same leg– E.G. right heelstrike right heelstrike

• Includes two phases– Stance Phase: when the foot is on the ground– Swing Phase: when the leg is swinging forwards

SWING SWINGSWING

Basic Vocabulary

• Includes important events:– Initial contact (heelstrike)– Toe-off– Opposite toe-off (e.g. when the left leg leaves the

ground)– Opposite initial contact (e.g. when the left leg

finishes swinging and hits the ground again)

Basic Vocabulary

• Base of support– At heelstrike, you are in double limb support– In the middle of stance phase, you are in single

limb support– After opposite heelstrike, you are once again in

double limb support

SWING SWINGSWING

Cadence and Step Length

• Step Length– distance from one foot strike to the next (left to

right or right to left)– about 0.75 m for normal adults

• Stride Length (one gait cycle)– two successive steps (by both left and right feet)– about 1.5 m for normal adults

• Cadence– number of steps (left and right) taken per minute– about 110 st/min for normal adults– Like a pendulum, lower-limb swings at a

frequency (cadence) inversely proportional to it’s length, so shorter people have a higher cadence.

• Velocity– about 1.5 m/s or 5 km/hr in normal adults

• Velocity = stride length x cadence 120

• and therefore: • Stride length = 120 x velocity

cadence

Children

• Children have shorter legs, so cadence is increased – 170 st/min at age 1 yr to 140 st/min at 7y

• Stride length is roughly the same as height (stature), so a child 0.5 m tall will have an expected stride length of about 0.5 m

• Velocity is roughly 1 stature/s, so a child 0.5 m tall will have an expected walking velocity of about 0.5 m/s

Guessing Game

• Sagittal plane joint angle

• Graphed for one side from heelstrike to heelstrike

• Vertical line separates stance phase from swing phase

Joint Kinematics: sample

• Ankle

Plantarflexion

Dorsiflexion

Heelstrike Toe-off Heelstrike

Joint Kinematics

• Ankle

Plantarflexion

Dorsiflexion

Heelstrike Toe-off Heelstrike

Ankle Rockers (Perry)

• First rocker: ankle platarflexion after heelstrike

• Second Rocker: ankle dorsiflexion– Foot is stationary– Tibia is rotating over the foot

• Third rocker: Forefoot dorsiflexion as heel rises (foot rocker)

Knee

Extension

Flexion

Heelstrike Toe-off Heelstrike

Hip

Extension

Flexion

Heelstrike Toe-off Heelstrike

Sprinting looks quite different

• Note femur parallel to ground• Note clearance

Ground Reaction Force

• (Newton’s 3rd Law of motion)• Walking– Heel strike transient– Slow– Fast

• Running– No double-limb support

One key to understanding gait

• Moments!

External Moments

• Hip

• Knee

• Ankle

External Moments

• Hip

• Knee

• Ankle

External Moments

• Hip

• Knee

• Ankle

External Moments

• Hip

• Knee

• Ankle

External Moments

• Hip

• Knee

• Ankle

Key to Understanding Pathological Gait

• MOMENTS!

• Example: what if quadriceps is weak?

What’s the external moment on the knee?

Flexor or extensor?

Model

• What muscles must resist an external knee flexor moment?

Resisting a moment

• The knee extensors must resist an external knee flexion moment

• What are the knee extensors?

• What if they are weak?

Adaptations

• What can be done to protect quads?– Reduce flexion moment• How?

– Affect moment arm– How?

» Affect vector direction» Affect joint center

Adaptations

• What can be done to protect quads?– Reduce flexion moment• Move knee center back• Move GRF direction forward

Anterior Trunk Lean

Other “solutions”

• Knee hyperextension• Dynamic Limb Retraction• Hand on thigh

Applications of Net Moments

• Related to walking

• Frontal Plane

• Identify:– Direction of moment

about right hip when left leg is off the ground

– Muscles that must fire to resist that moment

Question

• What happens if right abductors are weak?

• Trendelenberg Sign

What could a cane do at the hip?

• Construct a free body diagram that demonstrates how the use of a cane can alleviate compression on the hip

Weight on one leg –

large moment

resisted by abductors

Summary

• Walking and running are complex cyclic motions that involve interaction of both limbs and large sagittal plane motion

• Ground reaction forces and joint moments improve understanding of normal and pathological gait