KIN 147: Biomechanical Assessment of Movement

KIN 147: BIOMECHANICAL ASSESSMENT OF MOVEMENT

Moments of Force

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Biomechanics Review

What is Torque?

Synonyms

– Moment

– Moment of Force

Biomechanics Review

Biceps Force

Biceps Moment Arm

Biomechanics Review

Torque = F * ma

Torque = I * α

I = Moment of Inertia

α = Angular Acceleration

Moment of Inertia = a quantity expressing a body's tendency to resist angular acceleration

Force = m * a

Question

How can we tell how much force or torque is acting on each joint?

Option 1: Embed Force Sensor

Option 2: Inverse Dynamics

1. Kinematics of each segment a) Position b) Velocity c) Acceleration

2. Ground Reaction Forces a) Anterior/Posterior b) Medial/Lateral c) Vertical

3. Anthropometrics a) Length b) Mass c) Center of Mass Location d) Moment of Inertia

Estimation of force and torque acting on a joint based on several measurements

Anthropometric Estimates

Inverse Dynamics

ΣF = ma

ΣT = Iα

Scary Math

Inverse Dynamics

So, what do we end up with?

At each lower extremity joint (ankle, knee, hip)

• Internal joint force

• Internal joint moment

Units for torque are N*m.

Generally we normalize by body mass, so N*m / kg

“Internal” Force & Moments

Internal forces and moments are mainly caused by:

• Muscle contraction

• Passive soft tissue stretch/tension

• Articular reaction forces

ΣF (internal) + ΣF (external) = ma

ΣM (internal) + ΣM (external) = Iα

External Moments

• The external moment is often reported in biomechanics literature

• The external moment represents the torque applied to the joint by external forces

• Largely related to the interaction between the GRF and the joint

– Other joint forces & moments will also play a factor

Internal / External Moment Example

• In this case, there is an external knee adduction (i.e. varus) moment

– Largely from vGRF, but other joint forces & moments will also play a factor

• The muscles & other soft tissue structures will counteract this external moment to produce the internal moment

Sagittal Plane Joint Moments - Walking

Sagittal Plane Joint Moments - Walking

Sagittal Plane Joint Moments - Walking

Knee Varus Moment & Knee OA

High external knee varus moments have been linked to increased pain and rate of disease progression in patients with knee osteoarthritis

Frontal Plane Knee Moments

Normal Valgus Varus