Common advice for individuals lifting objects from a standing position includes “keep the back straight” and “lift with the legs.”
As a fitness professional, what if your client was to ask for an explanation of this advice? Is this advice correct from a biomechanical perspective?
Figure A shows an incorrect method of lifting an object from a standing position. For this example consider the most-often injured area of the human body during lifting: the lower back.
The load (weight) being lifted does not change due to improper lifting technique. So, if the force (load) applied to the body does not increase with improper lifting technique, how does the risk of injury increase?
The answer is that the magnitude of a load being lifted is not equal to the load placed on the body. If a load (force) is applied at a distance relative to a point of rotation (fulcrum), measure the effect using a moment, not a force.
In the example illustrated in Figure A, the load (100 lb) is transmitted through the lifter’s arms to the upper body. The perpendicular distance from the arms to the fulcrum point (the lower back) is given as 2.0 ft for this example.
The flexion moment at the low back would be, Mflexion = 100 lb · 2 ft, Mflexion = 200 ft · lb.
To counter the flexion moment, the extensor muscles of the lower back would have to produce an equal and opposite (Newton’s Third Law) moment 5 200 ft · lb of extension, significantly larger than the 100-lb load being lifted.
Figure A shows that the longer the anatomical moment arm due to improper lifting technique becomes, the larger the moment imposed on the muscles and structures of the low back.
Proper biomechanical lifting technique from the standing position is illustrated in Figure B. The weight of the load to be lifted, which acts vertically due to the force of gravity, intersects the articulations of the low back, hip, knee and ankle as close as possible in order to minimise the length of any moment arm.
If the moment arm of any joint of the body is equal to zero while performing the lift, the moment would also be equal to zero.
This means that only a linear force would be applied to that joint, specifically, a compressive force.
The anatomical design of the articulations of the spine is well suited for compressive forces, whereas rotational moments create potentially harmful stresses to these same structures.