How to Lift a Weight

Let W be the weight of the upper part of the body, T be the tension in the back muscles, W₀ is the weight to be lifted and R is the reaction force from the hip and z is the perpendicular distance of T from the pivot.The weights create anti-clockwise moments about the hip joint that is equal to Wdsinα+W₀d₀sinα perpendicular to the spine.The moment of the tension in the back muscles, Tz, provides an equal and opposite clockwise moment about the hip joint.Applying the principle of moments:Wdsinα+W₀d₀sinα =TzThe compressive force on the spine is:Tcos θ+Wcosα+W₀cosαWhere θ is the angle between the spine and the line of action of the tension T.If the knees are bent the compressive force on the spine reduces because using the principle of moments Tz=Wdsinα+W₀d₀sinα which is less since α is reduced.