JOINT MOMENTS AND MUSCLE-ACTIVITY IN THE LOWER-EXTREMITIES AND LOWER BACK IN LIFTING AND LOWERING TASKS

The mechanical loading on the body during the act of lifting has been estimated frequently. The opposite act of lowering has received much l ess attention. The aim of the present study was to compare the mechani cal loading of the musculoskeletal system in lifting and lowering. Eig ht subjects repetitively lifted and lowered a load, using two differen t techniques (a leg and a back technique). The ankle, knee, hip and lu mbosacral joint moments were estimated and the myoelectrical (EMG) act ivity of seven (leg and back) muscles was recorded. The differences be tween the lifting and lowering phase for the leg technique were simila r to those observed when the back technique was 'applied. The joint mo ment curves in lifting showed a high level of agreement with the (time -reversed) moment curves in lowering. Peak moments in lowering were on ly slightly lower than in lifting (peak lumbar moments were 5.4% lower ). These small differences were related to different acceleration prof iles at the centre of gravity of the body/load complex. The EMG activi ty was considerably lower in lowering than in lifting. The mean EMG in lowering (average for seven muscles) was only about 69% of the EMG in lifting. This was attributed to the different types of muscle actions involved in lifting (mainly concentric) and lowering (mainly eccentri c). Furthermore, the EMG results suggest that similar inter-muscular c oordination is involved in lowering and lifting. The results give rise to the assumption that in lifting and lowering similar muscle forces are produced to meet the (nearly) equal joint moments, but in lowering these forces are distributed over a smaller cross-sectional area of a ctive muscle, which might imply a higher risk of injury.