The operation of vehicles, e.g. trucks, has been demonstrated to be highly correlated with the occurrence of low back pain and herniated discs (Kelsey et al. [1]). Truck drivers and other vehicle operators typically report two to four times the number of low back pain problems and disabilities as the normal population (Sandover [2]). Vehicle-related lower back injuries have been attributed in a large part to vibration-induced stresses in the lumbar spine. In particular, many vehicles have vibration resonances at frequencies that coincide with the 4-5 Hz fundamental resonance of seated individuals. Vibration at resonance routinely damages mechanical structures and could surely be a cause of lumbar spine damage. Recently, much effort has gone into the design and development of seat suspensions for trucks and other vehicles in an attempt to reduce most of the injury-causing vibrations. Such seats would have the added benefit of reducing operator fatigue and possibly accident rates.
Unfortunately, methods with which to accurately test these seats are sub-optimal at best. Using human subjects has many inherent problems. Foremost are concerns regarding subject safety. To what level and duration of vibration exposure can subjects be tested safely is not completely known. Additionally, although mechanical test platforms are designed to be ''failsafe'' with redundant safety features and motion limits, 100% failsafe is, of course, not a reality. Servo-hydraulic test platforms have the capability of severely injuring a subject in the event of a malfunction which, for whatever reason, is not contained through failsafe operations. Another problem associated with human subject testing is the lack of repeatability between and even within subjects. Different body masses as well as different body types contribute to inter-subject variability. Differences in seated posture between subjects and overtime within subjects also contribute to the large variability found using human subjects.
In order to address the problems associated with using human subjects for seat vibration testing, SAE J1384 [3] recommends that a 75 kg inert mass be used in lieu of a human subject. This is the method most widely used in the industry to test new seat designs. Recently, testing in the authors' lab using this method has shown that using an inert mass does not reproduce the seat response when compared to testing with a human subject. The difference is greatest near the resonant frequency of humans, somewhere between 4 and 5 Hz. Since this is thought to be the frequency range most detrimental to humans, a better device was felt could be used which would more realistically simulate the human during seat testing. The device would ideally be: simple to construct, able to reproduce the resonant frequency of a human, and highly repeatable. No device meeting these criteria was found in the literature.