Effects of activity rest schedules on physiological strain and spinal loadin hospital-based porters

Workers in physically demanding occupations require rest breaks to recover from physiological stress and biomechanical loading. Physiological stress c an increase the risk of developing musculoskeletal disorders and repeated l oading of the spine may increase the potential for incurring back pain. The aim of the study was to assess the impact of an altered activity-rest sche dule on physiological and spinal loading in hospital-based porters. An exis ting 4-h activity-rest schedule was obtained from observations on eight mal e porters. This schedule formed the normal trial, which included two 5- and one 15-min breaks. An alternative 4-h schedule was proposed (experimental condition) that had two breaks each of 12.5 min. It was hypothesized that t he experimental trial is more effective in promoting recovery from physiolo gical strain and spinal shrinkage than the normal trial, due to the 5- min breaks being insufficient to allow physiological variables to return to res ting levels or the intervertebral discs to reabsorb fluid. Ten males perfor med both test conditions and oxygen uptake (V) over dot O-2, heart rate, mi nute ventilation (V) over dot E, perceived exertion and spinal shrinkage we re recorded. There were no significant differences in any of the measured v ariables between the two trials (p >0.05). Median heart rates were 78 (rang e 71-93) and 82 (71-90) beats.min(-1) for the normal trial and the experime ntal trial respectively, indicating that the activity was of low intensity. The light intensity was corroborated by the oxygen uptakes (0.75, range 0. 65-0.94 l.min(-1)). Spinal shrinkage occurred to the same extent in the two trials (2.12 +/- 3.16 mm and 2.88 +/- 2.92 mm in the normal trial and the experimental trial respectively). Varying the length and positioning of the rest breaks did not significantly affect the physiological responses or ma gnitude of spinal shrinkage between the two trials. More physically demandi ng work than the porters' schedule should induce greater physiological fati gue and spinal shrinkage. The ratio between activity and rest breaks would then become more important.