Asymmetric lateral loading of the human trunk

Sammanfattning: The overall aim of this thesis was to characterize the biomechanics and neuromuscular control of the human trunk in response to asymmetric lateral loading. The pulling force and strength capacities as well as muscle activation patterns, trunk kinematics and postural balance in relation to lateral trunk flexion and loading were given special consideration. The experiments were carried out on 39 healthy volunteers. Asymmetric lateral loading was provided by an isokinetic dynamometer or by free weights, either attached to a handle and pulley system or held directly in one hand along the side. Lifting, lowering and eccentric and concentric muscle strength was studied as well as compensations to a sudden lateral translation of the support surface, simulating a slip. Measurements included movement recordings with goniometers, video or optoelectronic systems, balance evaluation via centre of pressure calculations from force plate recordings, abdominal pressure measured intra-gastrically with a pressure transducer, and muscle activities obtained via intra-muscular fine-wire techniques from both deep and superficial trunk muscles potentially contributing to the movements and stabilization of the lumbar spine. Lowering and eccentric muscle actions were always accompanied by higher maximal voluntary force and strength outputs than lifting and concentric muscle actions, irrespective of movement speed And position in the range of motion. Extrapolating the strength measurements and relating them to the balance requirements and differences in the utilization of frictional forces could explain differences in position for optimal force output observed between lowering and lifting. Most of the trunk muscles on the side opposite to the load, including the transversus abdominis, psoas, and quadratus lumborum, showed an increased activation with increased load and degree of lateral flexion to the loaded side. The simultaneous activation of muscles on the same side as the load was lower and decreased with load. This bilateral co-activation was greater for the ventral than for the dorsal muscles, particularly in the lowering situation. The range of motion of the spine in lateral flexion was largest in the upper lumbar and lower thoracic regions. The response to a sudden sideways translation of the support surface during asymmetric loading of the trunk with a hand-held load differed depending on the direction of the perturbation. When the translation occurred towards the unloaded side the postural correction was delayed and associated with multiple changes in direction of movement of the trunk and the centre of pressure. This reduced accuracy could be explained either by a conflict between motor commands for the ongoing voluntary task and the postural response or by a mechanical effect of the addition of the asymmetric load to the trunk. These studies have presented new knowledge about the movements and stabilization of the human trunk, and spine, under different asymmetric lateral loading conditions in healthy adult subjects. The results elucidate basic interactions between biomechanical parameters and motor control in the steering of the human trunk. In addition, the data obtained should contribute to forming a better basis for understanding the mechanisms underlying malfunction and pain in the region of the lumbar spine and thereby also to improving methods for diagnosing, curing and, ultimately, preventing these types of disorders.