Lower back injuries are a significant global problem. They are particularly common in occupations that require prolonged or repetitive spinal flexion. Sheep shearing is one such occupation and the prevalence of back injuries is severe. Ceiling-supported back harnesses are a commonly used safety device in this occupation but its effectiveness in sheep shearing tasks has yet to be quantified. It is likely that accumulated and time-dependent changes in kinematics and neuromuscular control are relevant in the development of many lower back injuries. This is supported by the literature in sheep shearing, where 68% more injuries occur towards the end of the working day compared to the start. This means that data collected over a full working day is beneficial for measuring the effectiveness of safety interventions. The previous research in safety interventions in shearing have not collected data for more than 15 minutes, and do not adequately address longer term effects. This study compares the effects of wearing a ceiling-supported back harness on shearer kinematics and muscle activity, from the collected data over a full working day and incorporating time-of-day effects. The outcome shows that the use of ceiling-supported back harness results in improvements in kinematic features, but also an increase in muscle activity and fatigue.