A moment arm is the distance between the joint axis and the line of force acting on the joint; an example is using a wrench to loosen a nut, the wrench acts as a moment arm and produces more force than trying to free the nut by hand. In physics, the product of force applied multiplied by the moment arm results in torque.
The moment arm is used in everyday tools, machines and the human body. Hammers and nail pullers use the length of the handle or arms to offset the force required to drive in or pull out nails. The body's joints are examples of axis points and the limbs of moment arms. Cars use axis points and moment arms in abundance, with both being found in the wheels, doors and inner workings of the vehicle.
In construction and building, doors and windows are an example of a moment arm; the distance between the hinge and the door knob reduces the force needed to open or close the door.
Many moment arms are found in the human body and skeleton. Joints such as the knees, elbows, fingers and toes are effected by force applied by motor arms, such as legs, arms, feet and hands.