Trunk muscle activation and low back loading in lifting in the absence of load knowledge

People who know the actual mass of an object to be lifted normally prepare themselves before attempting a lift to control the movement and to minimize low back loading. In this study, the trunk muscular reactions and low back torque were investigated in the situation in which the individual did not know the actual mass but only had some idea of the range within which the m ass lay. Nine males lifted boxes weighing 6.5 or 16.5 kg under the conditio n in which they knew the actual mass before attempting a lift (the 'known' condition) and the condition in which they only had the information that th e mass would be within the range of 6.5-16.5 kg (the 'unknown' condition). The ground reaction forces and body movements were measured in the trials a nd, from these, the L5/S1 torques were calculated. The activation of back a nd abdominal muscles was also measured. For the 6.5 kg weight, a higher (16 %) back muscle activation in grasping the box and a higher (10%) peak L5/S1 torque in actual lifting were observed in the 'unknown' compared with the 'known' weight condition. For the 16.5 kg weight, the back muscle activatio n was lower (10%) during grasping, and higher (10%) during lifting in the ' unknown' compared with the 'known' weight condition. Knowledge of the load had no effect on the activation of the abdominal muscles. It was concluded that in the so-called 'unknown' conditions, the risks of low back injury we re increased in comparison with the conditions where the actual weight was known in advance.