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Effective Ergonomics: Addressing Musculoskeletal Fatigue

Ergonomics can prevent musculoskeletal injury by reducing fatigue. Musculoskeletal injuries generally occur in two basic ways: 1) from manual handling (i.e. pushing, pulling, lifting) of objects that require near maximal effort, or 2) from frequent or sustained (held) efforts particularly when performed in awkward positions. Examples of the later would include postures and actions such as: using a computer with the screen too low resulting in a sustained forward head position, or using a screwdriver while reaching high overhead. Repetitive motion injuries (“frequent effort in an awkward position”), are currently receiving a lot of attention in the media. However, the musculoskeletal system is well suited to produce repeated motion at low levels of force. “What is a low level of force?” A low level of force has to do with muscle endurance time. Muscle endurance time is defined as the length of time you can maintain a contraction/effort without fatiguing. In turn, the length of time or duration depends on the amount of blood flow to the muscle. In order to function properly, the muscle must get nutrients and oxygen from the blood, but if flow is compromised, nutrition and oxygen supplies decrease and the muscle fatigues more quickly. Thus the harder and longer the contraction, the quicker the fatigue. Long-term muscle fatigue can lead to injury. In research done by Sjogaard, et al in 1988 and Lee in 1979, they discovered that if the muscle contraction occurred at 50-60% of its maximum, there was no blood flow through the muscle. However if the muscle contraction was less than 10% of its maximum there was little hindrance of blood flow and thus less fatigue and potential injury. How does this effect ergonomic decisions? Let’s return to the example of the low computer screen. With a forward head position, you are asking your neck to sustain a muscular contraction to hold up a head that weighs between 10-14 pounds. Studies show that a forward head position of more than 30 degrees results in decreased endurance time and thus increased muscle fatigue. At 30 degrees of forward head tilt, the contraction force of the neck muscles is greater than 10% and thus blood flow to those muscles is decreased and hence fatigue and possible long-term injury can occur. The ergonomic goal is to get the neck more erect thus returning blood flow to the muscles and decreasing fatigue. Ergonomically we might suggest raising the monitor to allow a horizontal viewing angle. Understanding the effect of sustained and frequent awkward postures and their potential for musculoskeletal injury is the first step toward setting goals and solutions for effective ergonomics. Patty Jorgensen is the Director of Operations for OzP, Inc./Ounce of Prevention, a health, wellness and injury prevention company based in Westlake Village.

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