The effects of motion on trunk biomechanics

Objective. To review the literature that evaluates the influence of trunk m otion on trunk strength and structural loading. Background. In recent years, trunk dynamics have been identified as potenti al risk factors for developing low-back disorders. Consequently, a better u nderstanding of the underlying mechanisms involved in trunk motion is neede d. Methods. This review summarizes the results of 53 studies that have evaluat ed trunk motion and its impact on several biomechanical outcome measures. T he biomechanical measures consisted of trunk strength, intra-abdominal pres sure, muscle activity, imposed trunk moments, and spinal loads. Each of the se biomechanical measures was discussed in relation to the existing knowled ge within each plane of motion (extension, flexion. lateral flexion, twisti ng, and asymmetric extension). Results. Trunk strength was drastically reduced as dynamic motion increased , and males were impacted more than females. Intra-abdominal pressure seeme d to only be affected by trunk dynamics at high levels of force. Trunk mome nts were found to increase monotonically with increased trunk motion. Both agonistic and antagonistic muscle activities were greater as dynamic charac teristics increased. As a result, the three-dimensional spinal loads increa se significantly for dynamic exertions as compared to isometric conditions. Conclusions. Trunk motion has a dramatic affect on the muscle coactivity, w hich seems to be the underlying source for the decrease strength capability as well as the increased muscle force, IAP, and spinal loads. This review suggests that the ability of the individual to perform a task 'safely" migh t be significantly compromised by the muscle coactivity that accompanies dy namic exertions. It is also important to consider various workplace and ind ividual factors when attempting to reduce the impact of trunk motions durin g dynamic exertions.