| dc.description | Several ergonomic studies have examined how neuromuscular recruitment strategies reflect activation amplitudes from trunk muscles for different occupational demands. It has been suggested that trunk muscles must be recruited in a highly coordinated manner to account for the external load and to stabilize the spine. Antagonist and synergistic co-activation and differential and selective recruitment of trunk muscle sites are common neuromuscular strategies observed during dynamic occupational lifting. However, to date, these strategies are presented as the mean activations from groups of muscle sites and include a small subset of muscle sites. Therefore, the main purpose of this thesis was to quantify the principal activation amplitude patterns of 24-trunk muscle sites in response to simulated occupational tasks with minimal trunk motion. Twenty-nine right-hand dominant male (15) and female (14) subjects aged 20-50 years with no history of LBP participated in the study. The subjects were required to handle a 3.0 kg load in 2 reach positions (normal, maximum) during 3 lifting tasks (symmetrical, asymmetrical, transfer) using one or both hands while maintaining a constant upright trunk posture. Multi-lead surface EMG records were obtained from 6 bilateral abdominal muscle sites and 6 bilateral back extensor muscle sites. Three-dimensional angular displacement of the trunk and pelvis was monitored. The RMS amplitude for each muscle site within movement phases was normalized to the site-specific maximum voluntary activation level. All normalized values of 24-trunk muscle sites were used as the activation amplitude pattern. Pattern recognition techniques transformed the original variables (24) to a reduced number of uncorrelated variables (scores ) while maintaining the salient features of the measured amplitude patterns. For each task, the scores were tested using separate ANOVAs to determine the effects of the experimental conditions (reach, movement or hand). Pattern recognition techniques quantified the activation amplitude pattern using only 3 to 4 principal patterns, which described close to 95% of the variance in the neuromuscular strategies. The results showed statistically greater activation amplitudes for the back sites during the more demanding work conditions across the experimental tasks. Selective recruitment was observed for the medial back sites during the symmetrical task and lateral back sites for the asymmetrical conditions. The left and right-handed lifting tasks produced similar activation amplitude patterns, but opposite in direction, for the back sites. This suggested that there was no handedness effect. Across the 3 tasks, antagonist co-activation between the internal oblique and back sites to similar amplitudes characterized the lighter physical demands, whereas differential recruitment among the back sites characterized the more physically demanding conditions. While a specific abdominal muscle recruitment strategy was exhibited across the tasks, the external oblique sites showed differential recruitment during the asymmetrical conditions. Overall, this study characterized the relationships among a large number of muscle sites and documented the specific recruitment strategies utilized for different work demands. The ability to capture the neuromuscular demands from a large number of muscles will offer new insights into ergonomic initiatives and workstation layout to optimize the work method and the physical demands placed on the worker. | en_US |
