The response of posteroanterior lumbar stiffness to repeated loading.

Lumbar posteroanterior (PA) responses are determined by manual examination and are used to guide treatment decisions and interpret changes in symptoms within and between treatments. Mechanical devices that simulate manual assessment have been developed to measure lumbar PA responses. The two variables used to describe lumbar PA responses to mechanical loading are stiffness coefficient K and displacement D30. The purpose of this study was to investigate the behaviour of lumbar PA responses with repeated loading over time. Lumbar PA responses at L4 were measured in 18 pain-free subjects using a mechanical device. Measurements were made for five consecutive loading cycles on three test occasions. The responses were compared between the five cycles within a single test occasion and between three test occasions. An identical procedure was also used to test a set of elastic springs for comparison. There was a significant increase in both stiffness coefficient K and displacement D30 between the first cycle and subsequent cycles of a single test occasion on human subjects. This response which demonstrates an increase in stiffness and displacement between the first and subsequent cycles can be considered a normal response to PA loading. PA stiffness remains constant over several tests both within one day and between days.