Low back joint loading and kinematics during standing and unsupported sitting

The aim was to examine lumbar spine kinematics, spinal joint loads and trun k muscle activation patterns during a prolonged (2 h) period of sitting. Th is information is necessary to assist the ergonomist in designing work wher e posture variation is possible-particularly between standing and various s tyles of sitting. Joint loads were predicted with a highly detailed anatomi cal biomechanical model (that incorporated 104 muscles, passive ligaments a nd intervertebral discs), which utilized biological signals of spine postur e and muscle electromyograms (EMG) from each trial of each subject. Sitting resulted in significantly higher (p < 0.001) low back compressive loads (m ean+/-SD 1698+/-467 N) than those experienced by the lumbar spine during st anding (1076+/-243 N). Subjects were equally divided into adopting one of t wo sitting strategies: a single 'static' or a 'dynamic' multiple posture ap proach. Within each individual, standing produced a distinctly different sp ine posture compared with sitting, and standing spine postures did not over lap with flexion postures adopted in sitting when spine postures were avera ged across all eight subjects. A rest component (as noted in an amplitude p robability distribution function from the EMG) was present for all muscles monitored in both sitting and standing tasks. The upper and lower erector s pinae muscle groups exhibited a shifting to higher levels of activation dur ing sitting. There were no clear muscle activation level differences in the individuals who adopted different sitting strategies. Standing appears to be a good rest from sitting given the reduction in passive tissue forces. H owever, the constant loading with little dynamic movement which characteriz es both standing and sitting would provide little rest/change for muscular activation levels or low back loading.