Design and Development of an Earthquake-Simulated Environment for the Study of Postural Stability

Much is still unknown about the control of posture and postural stability within the unstable environment of an earthquake. Hence, the objectives behind this study were to investigate the lower-extremity muscular response and head acceleration of human subjects in maintaining postural stability during the event of a simulated seismic activity. In order to achieve these goals, an earthquake simulator was designed and developed to reproduce seismic activities compliant with the Richter magnitude and the Mercalli intensity scales. Since the Love wave is the most accountable for causing damage, it was selected from among the four existing types of earthquake activities in the simulator design. Validation tests of the developed simulator demonstrated high levels of safety, accuracy, and repeatability. A four-channel commercial biopotential data-acquisition system was used in conjunction with the earthquake simulator to collect dynamic electromyographic (EMG) data from the gastrocnemius-soleus and tibialis anterior muscles in the lower extremities, and to measure the anterior-posterior head acceleration using a 50-g linear axial accelerometer. Five healthy adult volunteers, free from any musculoskeletal or neurological disorders, participated in the validation of the system. Test trials were comprised of 10 stages, beginning with quiet standing and followed by an increasing simulated earthquake magnitude from 3.5 to 6.5 degrees on the Richter´s scale. Data analysis revealed notable variations in lower extremity muscular activity and head acceleration, in the control of postural stability, as a result of the increased simulated seismic activities. Repeated trials showed marked reductions in the recorded data suggesting an adaptive learning about the perturbed environment. The results obtained from this study could be useful in determining the threshold of falling within different subject populations during an earthquake, and in improving the interior safety and ergonomics- - of elderly care and rehabilitation centers found on seismic belts.