THE DEVELOPMENT OF RESPONSE STRATEGIES IN PREPARATION FOR SUDDEN LOADING TO THE TORSO

Sudden and unexpected loading to the torso has been reported in the li terature as a potential cause of low-back disorders. When such loading s occur, it is hypothesized that the body's response is designed to mi nimize the destabilizing postural disturbance, and to minimize the mec hanical loading of the musculoskeletal system. This study tested hypot heses regarding the role of task experience in the development of prep aratory strategies that potentially minimize the postural disturbance to the body and minimize the mechanical loading of the spine. These st rategies were hypothesized to consist of muscle pretensioning, postura l changes, and intra-abdominal pressure (IAP). Four subjects participa ted in five to six experimental sessions in which a sudden load was ap plied by dropping a weight once a minute for 30 minutes, Electromyogra phic (EMG) data from 10 trunk muscles, IAP data, and postural data wer e collected during the initial session and final sessions for each sub ject. The results indicate where each subject developed a unique prepa ratory strategy. The preparation always involved the pretensioning of the erector spinae muscles, although the coactivation of the other tru nk muscles was quite variable across subjects. During the sudden loadi ng the overall postural disturbance was not consistently reduced; howe ver, the trunk flexion was significantly reduced in most subjects. Fur thermore, the estimated spinal compression due to muscle loading was s ignificantly reduced in all subjects.