Chapter 13 Equilibrium and Human Movement KNES 361

What is torque?

• The rotary effect of a force about an axis of rotation

• Measured as the product of force and the force moment arm (AKA torque arm, force arm)

• The shortest (perpendicular) distance between a force line of action and an axis of rotation

Where do torques occur within the human body?

The product of muscle tension and muscle moment arm produces a torque at the joint crossed by the muscle

What is a lever?

• A simple machine consisting of a relatively rigid bar-like body that can be made to rotate about an axis or on a fulcrum.

• There is first-, second- and third-class levers

What is mechanical advantage?

The ratio of the moment arm of the force (force arm) to the moment arm of the resistance (resistance arm) for a given lever

MA= FA / RA

What is static equilibrium? 

A motionless state in which there is no net force or net torque acting 

The conditions of static equilibrum are:

What is the center of gravity? 

• The point around which a body’s weight is equally balanced in all directions 

• Also referred to as the centered of mass or mass centroid 

• (Need not be physically located inside of a body) 

Why is the center of gravity of interests in the study of human biomechanics? 

• It serves as an index of total body motion 

• Path of the center of gravity of a runner 

• The body responds to external forces as if all mass were concentrated at the CG 

• This is consequently the point at which the weight vector is shown to act in a free-body diagram 

What is Stability?

Resistance to disruption of equilibrium

What is balance?

Ability to control equilibrium

What can increase a body’s stability?

• Increasing body mass

• Increasing friction between the body and the surfaces of contact