The Basic Physics of Lifting; Hips or Knees?

Why is it better for us to bend at our knees and not our hips when lifting things? Before answering this we need to understand the words torque and couple.  Torque is force applied perpendicular to a lever a certain distance from an axis of rotation. This distance is called the lever arm. A clockwise torque is arbitrarily deemed negative, and vice versa. A couple is two equal forces coming from opposite directions whose lines are displaced by a certain distance. Couples produce torque, calculated using either force (because they are equal) and the perpendicular distance between them which acts as a lever arm. A calculation can also be done where each force uses a portion of the perpendicular distance as a moment arm, and the overall torques are added.

Having looked at that, imagine you are lifting a 10kg object, which due to gravity weighs 100N, by bending at your hips. Adding on the weight of your torso, which we shall generously assume is 30kg; your upper body now weighs 400N. Now imagine we use a whizz-bang machine to calculate that the centre of mass of your torso-object complex is exactly 40cm in front of your hips. This means that your torso is producing 400N x 0.4m = -160Nm of Torque. Now imagine you pick this weight up by bending at your knees. You torso still weighs 400N in total, but now our whizz-bang machine is telling us that the centre of mass of your torso (while holding your weight) is only 10cm in front of your hips. This means that your torso is only producing 400N x 0.1m = -40Nm of torque at your hips.

This clockwise rotation is resisted by the contraction of the back muscles to prevent the torso from rotating forward. As the back muscles contract, the spine is pulled straight and compressed along its length. This creates a couple where the back muscles are resisting being stretched, and the spine is resisting being compressed. Now, because we can find the lever arm of this couple by assuming the distance between the muscles and the spine is 3cm (which it roughly is), and we already know that this couple needs to produce 40Nm or 160Nm of torque to lift our torso (depending on if we are bending at the hips or the knees), we are able to determine the force compressing the spine and being produced by the back muscles in each case.

Torque (Nm) = Force (N) x Moment arm (m)

Force (N) = Torque (Nm) / Moment arm (m)

Bend at Knees             Force = 40Nm/0.03m = 1333.33N

Bend at Hips                Force = 160Nm/0.03m = 5333.33N

As we can see, an apocalyptically large amount of force is travelling through both your spine and your back muscles when lifting from your hips, particularly when compared to the forces travelling through each if you lift with your knees. As lifting with your hips near quadruples the force required, the chances of injuring a muscle are significantly higher. Furthermore, if the back arches at all throughout the lift, then the vertebral bodies of the vertebrae along the spine will be pushed together along one side and prised open on the other. This has the effect of squeezing the intervertebral discs out of place, causing “slipped discs”. It is for these reasons that we have all been instructed time and again to lift with our knees, and not with our hips.