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Due to circadian changes caused by fluid loss of the intervertebral disc, body height varies throughout the day. This phenomenon is called spinal shrinkage (Reilly,Tyrrell, & Troup, 1984). Spinal shrinkage differs depending on body positions and mechanical loads during the day. Further, psychological stress (Marras, Davis, Heaney, Maronitis, & Allread, 2000) and time control (Balci & Aghazadeh, 2003) are supposed to impact spinal shrinkage. Therefore, we assume that daily work stressors are positively and daily method/time control are negatively related to daily spinal shrinkage.
Methods
A two-week diary study was conducted on 39 office employees (66.7% women), aged between 17 and 54 (M= 31.37). The DV spinal shrinkage was calculated as body height in the evening minus body height in the morning. Daily physical activities were assessed with an ambulatory device (Sensewear Arband; SWA). We used multilevel analysis with demographic variables as level 2 predictors and accelerometric activity data plus self-reported low effort activities, work stressors, job control, as level 1 predictor variables. At day level our data consist of k=512 days.
Results
During workdays spinal disks shrinked more than during non-work days. After adjustment for sex, age, body weight, time spent on daily physical and leisure activities and biomechanical work strain (sitting, standing, flexed position and lifting), we found, that more daily job control significantly predicted less daily spinal shrinkage. Daily time pressure and social stressors were not significantly related with spinal shrinkage.
Discussion
This is the first diary field study exploring the relationship between daily spinal shrinkage and daily job control. Lack of job control accelerates spinal shrinkage while working. Results are in the line with previous experimental studies and provide an explanation how insufficient job control may increase the risk of musculoskeletal complaints including occupational low back pain.