All-terrain vehicles (ATVs) are used widely throughout the United States both for recreational activities, such as hunting and riding, and in occupational settings, such as agriculture. In both such activities, ATVs can pose a risk of injury and death resulting from a number of factors, including operator error, operating in an altered state, and misuse. In a study by Milosavlijevic et al. (2011), a survey of agricultural workers in New Zealand displayed that roughly 60% of workers who operated an ATV experienced a loss of control event. The same study showed that nearly 50% of those occurred while riding on a steep slope. Despite these high levels of incidence, little research has been performed to understand the mechanisms that lead to loss of control or instability.;In order to study loss of control and instability, a four-degree-of-freedom human-in-the-loop ATV simulator was developed and integrated with a shaking table in a real-time interaction. The simulator allowed for operators to approach conditions by their own means and to study mechanisms of each individual's approach to traversing a terrain. Inertial measurement units were used to measure the acceleration and angular velocity of each subject at the S1, T10, C7, and head. Inertial measurement units were also mounted on the ATV and table. The ATV was instrumented with 32 force sensors to sense reaction forces applied to the machine at the feet, hands, saddle, and seat. A five-camera motion capture system was used to capture posture throughout the time series of each terrain. Data were reviewed and analyzed at a finer resolution of roughly three seconds in select subjects at moments when there were peaks in both force and acceleration.;Analysis showed that a cycle of instability and stability existed as moments of sudden input accelerations caused reactions in the spine and head. Further, component accelerations were analyzed to identify the acceleration of the subject relative to the ATV. The cycle and the time from input acceleration to bracing on the ATV to regain stability serve as a baseline for the subject's reaction to input acceleration and the time between input acceleration and regaining stability.
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