Biomechanics

The effort, that is, the insertion of the muscle tendon, is close to the fulcrum (the joint) but between the joint and the load (i.e. the rest of the limb) being moved. The load and resistance are the same force.

Unless the bed is tilted, there is no component of the patient’s weight that is parallel to the bed. The force of gravity and the force exerted by the hanging masses do not change.

Minimising the separation between the centres of gravity also minimises the load arms of the levers involved. This will decrease the muscular effort required.

Weight is a force, while pressure is a force divided by the area upon which it is acting.

As long as the centre of gravity is above the base of support, there will be no unbalanced force tending to tip you over. Try standing with you back against the wall, and then bend over to touch your toes.

Newton’s second law states F = m × a. This may be rearranged to m = F ÷ a or in words mass = weight ÷ acceleration.

Pulleys do change the direction of the traction force so that the downward pull of the hanging mass can be redirected to a direction parallel (say) to a leg.

Traction and counter-traction are both forces so are both vector quantities.

They are balanced or stable. That is, they will not need to shift their feet to prevent them from “overbalancing”.

Newton’s third law, F₁ = -F₂. When an object (the hanging mass) exerts a force on another object (the patient in traction), the second object exerts an equal but opposite force on the first object.