Back Injury Prevention

Back Injury Prevention: Well and Lesser Known Reasons Why You Should Bend Your Knees

Tim H. Tanaka, Ph.D.

Every winter as the temperature cools down, we see more patients with acute low back injuries.

It is common knowledge that you should bend your knees when lifting heavy objects. The reason behind this recommendation offered by health care professionals, is that by bending your knees, most of the heavy load is shifted to the hips and thighs, instead of directly to the low back. While this biomechanical explanation is valid, it does not fully explain the following facts commonly observed in everyday clinical practice:

  • Stronger back muscles should provide more support against the load. However, acute back injuries during normal daily activities are extremely common even among physically fit individuals.
  • Severe back injuries do not only occur when lifting a very heavy object. More often than not, injuries occur when picking up a light object.

EMG Study Reveals Unique Behavior of Our Back Muscle

The graph on the following page shows low back electromyographic (EMG) activity of a member of our staff who performed the action of bending his trunk forward and returning to the upright position with his knees straight. Muscles emanate an electrical signal upon contraction, which can be recorded using an EMG device. Observation of EMG activity reveals the fine coordination of muscles during action. Notice that lumbar muscle activity not only diminished during the middle of the forward bending movement, but also remained inactive during the initial stage of returning to upright position. Why is this important? When we bend forward with our knees straight and return to upright position, most of the load goes to our low back area. Unfortunately, when our back requires the most help to guard against increased load, lumbar muscles are in inactive resting mode. This behavior of muscle does not easily make sense. It can be parallel to the example of a normally hard working employee (the lumbar erector spinae muscles), who suddenly takes a nap at the most critical moment. This strange phenomenon is observed in most individuals without back problems, regardless of their fitness level. When the muscles are electrically silent without contraction, they are unable to perform their intended role: moving the joints and providing structural support. Just as strong and massive guard dogs do not serve their purpose if they are asleep, the level of our muscle strength does not matter. The spine, disks, and ligaments are exposed to direct force without much needed support. This unique characteristic of the back muscles may be one of the reasons why some physically fit individuals develop acute back injuries quite easily without an obvious trigger of onset.

This sudden silence of muscle activity, a phenomenon known as ‘flexion-relaxation’ was first reported in the medical journal Lancet by British researchers in 1951. Although the phenomenon has been studied by many other researchers since then, this strange but important behavior of lumbar muscles is not widely known, even among back care practitioners today.

Sometimes we do not bother to bend our knees when picking up an object that does not appear to be heavy. This tendency may be truer among physically fit individuals, possibly due to their confidence in their own strength. It is important to note that muscle strength, in addition to endurance, flexibility, and coordination, is very important for injury prevention during many types of movements and activities. My point in this issue however, is that when picking up any object, it is certainly wise to bend your knees, regardless of the size or weight of the object, and your physical strength. As explained, lumbar muscles are electrically silent and unable to contract during the initial return motion from bending forward. It is at this point that the low back spine, disks and ligaments are in their most vulnerable states and the risk of injury is at its greatest.


Low back EMG activity is recorded during trunk forward flexion and return to upright movement. The larger amplitude of signal waves in the graph indicates muscles are being contracted for movement and/or to provide structural support. The flat line or minimum amplitude of wave form fluctuation indicates muscles are less active, lower contraction or relaxed state.

A subject is standing in upright position (a). As the subject initiates trunk flexion, myoelectrical signal appears as a result of contraction of lumbar paraspinal muscles (b). However, the electrical signal spontaneously diminishes at approximately 45 degree flexion (flexion-relaxation). Trunk flexion at the maximum (c). Electrical signal remains silent during the initial phase of extension (d). Returned to upright position (e).

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