The Biomechanics of Impact Injury by Albert I. King

Author:Albert I. King
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

9.3.2.5 Tennyson’s Research on In Vivo Muscular Response to +G z Acceleration

The last study to use the vertical accelerator was performed by Tennyson (1976) who wanted to investigate the effect of muscular response during caudocephalad acceleration (See also Tennyson et al. (1977). Unanesthetized dogs were used to determine the delay in response of paraspinal extensor muscles because the sled was not man rated to test volunteers. A protocol to subject unanesthetized dogs to vertical acceleration was submitted and approved by the Wayne State University Institutional Review Board. Low g-levels of 3–5 g were used to ensure that the dogs would not suffer any pain or sustain any injury. The tool used to determine muscular delay was electromyography (EMG) but at the time there were no commercially impact-resistant EMG amplifiers available. Figure 9.22 shows a bank of “homemade” EMG amplifiers designed and built with the latest available semiconductor technology. They did not contain any of the now obsolete vacuum tubes and were therefore impact resistant. These amplifiers were placed on board the vertical accelerator to amplify the EMG signals. Before testing the dogs, the EMG amplifiers and a pair of leads suspended in air were subjected to a vertical acceleration to ensure that the leads would not pick up any spurious signals. Figure 9.23 shows the sled acceleration and zero output from the EMG amplifiers, ensuring that any output from the animals would be genuine EMG signals. Two dogs (beagles) destined for sacrifice after participating in a medical experiment were acquired from the research team that used them, with the promise that they would be found a home after the vertical accelerator experiments . They were anesthetized on the morning of the test day for the insertion of needle electrodes into several of their paraspinal muscles. A specially made jacket was used to protect the needles from being pulled out. The leads from the EMG needles were firmly connected to a terminal junction in the jacket, as shown in Fig. 9.24. EMG amplifiers were connected to this junction so that the signals could be amplified and sent onto an analog tape recorder. Figure 9.25 shows an anesthetized animal ready for testing. By the time the sled was prepared for launch, the dog would have awakened from the anesthetic and was fully aware of its surroundings. Figure 9.26 shows the beagle in the accelerator seat, waiting for the test to begin. Each animal was tested multiple times but not once did either animal try to get off the sled. Examples of EMG data collected are shown in Figs. 9.27 and 9.28 for the lumbar multifidus muscle and the spinalis cervicis muscle. The superimposed vertical acceleration pulse was used to determine the delay between onset of acceleration and appearance of EMG. The EMG onset delay is summarized in Table 9.3 for the six muscles that were monitored. The delay times varied from 22 to 36 ms. The delay in humans is expected to be longer because the nerves involved are longer than those in a dog.

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