Prolonged sitting is problematic for the back. Unfortunately, this fact seems to be rather unknown in the occupational world. Those recovering from back injuries who return to modified work are often given "light duties" that involve prolonged sitting. While such duties are perceived as being easy on the back, they can be far from that. Even though the returning worker states that she cannot tolerate sitting, that in fact she would be more comfortable walking and even lifting, she is accused of malingering. This is the result of a misunderstanding of sitting mechanics.
Epidemiological evidence presented by Videman, Nurminen, and Troup (1990) documented the increased risk of disc herniation in those who perform sedentary jobs characterized by sitting. Known mechanical changes associated with the seated posture include the following:
• Increase in intradiscal pressure when compared to standing postures (Nachemson, 1966)
• Increases in posterior annulus strain (Pope et al., 1977)
• Creep in posterior passive tissues (McGill and Brown, 1992), which decreases anterior-posterior stiffness and increases shearing movement (Schultz et al., 1979)
• Posterior migration of the mechanical fulcrum (Wilder et al., 1988), which reduces the mechanical advantage of the extensor musculature (resulting in increased compressive loading)
These changes caused by prolonged sitting have motivated occupational biomechanists attempting to reduce the risk of injury to consider the duration of sitting as a risk factor when designing seated work. A recently proposed guideline suggested a sitting limit of 50 minutes without a break, although this proposal will be tested and evaluated in the future.
We have developed a three-point approach for reducing back troubles associated with prolonged sitting:
1. Use an ergonomic chair, but use it properly (very few actually do). Many people think that they should adjust their chair to create the ideal sitting posture. Typically, they adjust the chair so that the hips and knees are bent to 90° and the torso is upright (see figure 8.24). In fact, this is often shown as the ideal posture in many ergonomic texts. This may be the ideal sitting posture, but for no longer than 10 minutes! Tissue loads must be migrated from tissue to tissue to minimize the risk of any single tissue’s accumulating microtrauma. This is accomplished by changing posture. Thus, an ergonomic chair is one that facilitates easy posture changes over a variety of joint angles (see figure 8.25). Callaghan and McGill (2001a) documented the range of spine postures that people typically adopt to avoid fatigue. Some have three or four preferred angles. The primary recommendation is to continually change the settings on the chair. Many workers continue to believe that there is a single best posture for sitting and are reluctant to try others. This is, of course, unfortunate, as the ideal sitting posture is a variable one. Many employees need to be educated as to how to change their chairs and the variety of postures that are possible.