Erector Spinae Muscle Forces

A realistic model of the forces in the erector spinae muscles that support the back when bent over.

What it Shows:

This is an analysis of the tensions in muscles and forces on joints as an application of torques in static equilibrium situations. The muscles involved in supporting the human torso in a stooped position are realistically modeled, and the extraordinary large forces calculated and demonstrated.

How it Works:

When bending over into a stooped position, the principal muscles which support the back are the erector spinae (or sacrospinal) muscles. These muscles attach between the ilium and the lower part of the sacrum, with their upper parts attached to the spinous processes of all the lumbar and four thoracic vertebrae. The gross mechanical effect of the erector spinae muscles can be approximated by a single cord, which acts on the spine at a point 48 cm from the lumbosacral joint. The question to be answered is, "What is the tension in this cord representing the erector spinae?" The demonstration makes it dramatically clear that the tension is enormous.

The total torque about the lumbosacral joint (for a 140 lb man) is 151 N m, as determined below:

stoop lift

45 lb dumbbells are readily available, so that is the weight we use in the model to provide the torque. One also needs to know the distance between the erector spinae muscles and the spinal column:

cross section

In the model we've taken the distance between the ilium (represented by the pulley in the model) and the lumbosacral joint to be approximately 10 cm. Given this information, the full-scale model becomes the following:

ers model

Balancing the torques and solving for the tension gives a value of 1513 N. That kind of tension force can be supplied by hanging 345 lbs from the cable representing the spinae erector muscles:


Setting it Up:

One 45-lb plus three 100-lb barbell plates are hung from the steel cable to balance the 45-lb dumbbell. Obviously you do not want the model all assembled beforehand, as this would give away the answer to the problem and spoil the drama. The 1000-lb capacity "Handy Hoister" is used to hold the dumbbells in place while attaching them to the cable.


This problem is found in many of the introductory physics textbooks geared to the life sciences. The demonstration not only livens up what would otherwise be a ho-hum calculation on the blackboard, but also makes a strong visual impression. Note that 345 pounds of tension is about 2.5 times the weight of a 140-lb person, and the tension scales with the weight of the person. The model can be used discuss the large compressional forces in the lumbar spine in this stooped position. The demonstration is also an opportunity to talk about the proper lifting of a load. Lifting a load in a stooped position very clearly increases the torque and the forces on the back can rise to dangerously high levels. A crouch-lift greatly lessens the torques.


We wish to mention the importance of engaging the abdominal muscles (core strength) when lifting. Additionally, one should either hold the breath, or breath out with resistance while lifting. Since muscles can only contract when engaged, it might seem counter-intuitive that this action will assist in lifting in the stooped position—one would think that the contracting abdominal muscles pull the torso down and only add to the torque. The abdominals actually run diagonally, horizontally, as well as vertically, and engaging these core muscles has the effect of increasing the inter abdominal pressure (IAP)—the IAP and muscles are indicated in figure 3.47 above. Below is a ball supporting the weight of the torso model without the erector spinae. Note though that the torso weight has been reduced by using a 10-lb dumbbell.


The ball represents the abdomen. Even though it's quite a beer belly, the abdominal walls are clearly under tension. The inter abdominal pressure (pressure inside the ball) is also elevated due to the load. The lesson here is that the inter abdominal pressure aids in supporting a load. Of course, if the person has weak abs and is trying to lift a heavy load, the increased abdominal pressure could cause a hernia, pushing the intestines out between the weak abdominal muscles.