Neuromuscular and postural responses to sudden loading and high frequency lifting: effects of posture and fatigue
Sudden unexpected loading and high frequency repetitive lifting have been identified as risk factors for low back injury. Despite the risk that such factors pose, there is a dearth of literature concerning the influence of posture and fatigue on muscular, postural, subjective and physiological responses in these situations. The primary aim of this thesis was to examine the effects of posture and lifting-induced fatigue on neuromuscular, postural, psychophysical and physiological responses to sudden loading and repetitive lifting. The first two studies in this thesis focused on muscular and postural responses to sudden loading of a hand-held box in an upright and stoop standing posture. Trials were performed with and without visual warning and following lifting-induced fatigue. Sudden loading in an upright posture resulted in co-activation of oblique abdominal and erector spinae musculature, with relatively simultaneous joint initiation of the lower limb and lumbar spine. In contrast, sudden loading in the stoop posture produced minimal abdominal muscle contribution. Maximal angular joint excursion was significantly less in the stoop than that in the upright posture. Prior exposure and warning primarily affected postural responses in the upright posture with earlier activation of the trunk and lower limb musculature resulting in a reduction in joint excursion. Such a mechanism may enhance postural stability. However, the influence of warning on reducing joint motion was compromised when fatigued. Studies three and four of the thesis focused on the effects of self-selected lifting posture (squat, mixed and stoop) and fatigue during repetitive lifting on psychophysical, physiological, neuromuscular and lumbar spine kinematic responses. Study three compared lifting postures using ratings of perceived exertion and heart rate response patterns. Similar subjective and physiological responses were found for different lifting postures, with a progressive rise in ratings of perceived exertion and heart rate measures throughout the task. All postures exhibited evidence of erector spinae muscle fatigue at the end of the task. Study four examined the effect of self-selected lifting posture on lumbosacral kinematics and associated trunk muscle activation patterns during lifting and lowering, pre and post fatigued. When compared to squat lifting, individuals who self-selected a stoop lifting technique displayed different lower erector spinae activation patterns. The stoop lifters also flexed their lumbar spines at faster velocities and adopted greater lumbosacral flexion than squat lifters, which could potentially place greater strain on the passive structures of the spine. Fatigue influenced lumbosacral motion patterns of stoop and mixed lifters to a greater extent than squat lifters, resulting in a decreased range and velocity of lumbar extension. Findings from this thesis provide insight into the underlying neuromuscular control of the trunk during high risk manual handling activities. It provides important information concerning the possible mechanism of postural and fatigue related musculoskeletal injury. This will assist the development of training and rehabilitation programmes targeted at neuromuscular, physiological, biomechanical and psychophysical requirements of manual handling tasks.