in rise times, durations, energy content and slamming periods. Lab testing is also conducted only up to 10g peak levels, whereas the most severe real impacts at sea can reach nearly 20g. It is also shown that some suspension


seats, as well as standing postures, can amplify and actually more than triple the impacts on humans. Lab trials cannot simulate the human


physiological response, including muscular reflexes. Tey cannot assess the effects of impact on the human posture, which is crucial for the risk of injury. Also, lab trials cannot test the effect of exposure to lateral forces, which are more dangerous than pure vertical forces. Still, measured impact data is reported in units such as Shock Response Spectrum (SRS), Vibration Dose Value (VDV), Daily Equivalent Static Compressive Dose, Normalised for 8 hours (Sed(8)), created with advanced algorithms, based on assumptions regarding dynamic properties of segments of human spines. As the VDV and Sed(8), etc, are defined


based on experiments made on cadaver segments of lumbar spines, fixed in their normal positions(9),


they should not be


used for assessing risk of injuries caused by stochastic multidirectional impacts. Tese impacts, when causing injury, normally also cause significant uncontrolled deflection of the spine. Te dynamic and elastic properties of a severely deflected spine are very different from those of a spine in its normal ‘balanced’ position. Te more a spine is rotated or forward- flexed, the stiffer it gets in the axial impact direction, and the more backward-flexed, the less stable it gets. All extreme positions are generally believed to increase risks of injury. No severe slamming-induced injury has been reported where impact forces are believed to have caused spinal injuries without having also caused deflection of the spine.


Biological relevance By filtering out the frequency content above 20Hz in measured impact data, results from lab testing can lose even more relevance. Historically, filtering was necessary due to limitations of the available hardware and soſtware. With significantly increased processing power and storage media, filtering


Ship & Boat International November/December 2018 45


is no longer required. Justifications for the low-pass filtering include cost efficiency and reduction of noise and structural vibrations, as well as making simulation in the lab easier when not having to replicate the true characteristics of real impacts. With this hard, low-pass filtering, energy with higher frequency content, potentially highly relevant for causing structural failure in hard tissue bone and cartilage, can be hidden away. Low-pass filtering also changes the peak acceleration values and makes rise times of the impacts impossible to identify. To understand why rise time is relevant


for the physiological response and the risk of injury, the following example with a fighter- jet pilot is relevant. When going into a sharp turn, acceleration (g-forces) on the platform and pilot increase gradually, during almost a second, up to 7g or even above. Tis causes significant physiological effects but no structural anatomical failure. In wave-slam events, acceleration can go from 0g to 7g in a few milliseconds. Tere are obvious reasons to believe that this is more challenging to anatomical structures. Sudden experience of pain is one significant reason. A special case of uncontrolled movement


is the deflection of the cervical spine caused by head jolts. Anecdotal


information


suggests that standing subjects exposed to slamming report relatively more neck injuries than low spinal injuries. Tese, just like traditional whiplash injuries, oſten give severe and long-lasting problems, with pain and impaired function, oſten without any


Figure 1


objective findings in currently available diagnostics such as X-Ray, CT or MRI. Claims that energy content above 20Hz in


the impacts lacks relevance for these injuries remain unsubstantiated.


Risks of injury The risk of injury - structural failure in anatomical structures - depends on the forces acting on the human body, and these forces depend on the amount of energy transferred to the human body by the impacts. While injuries occur in all parts of the body, most of the severe and permanent injuries affect the spine. Te risk of injury depends on several factors related to impact forces. Impacts containing horizontal forces


are more dangerous than purely vertical impacts. Oblique and lateral impacts cause not only compression forces, but also shear forces, bending forces, rotational forces and unpredictable and uncontrolled defections of the spine. A contributing cause of cervical spine injuries is the head jolts experienced by both seated and, not least, standing subjects. Documented injuries include vertebral fractures, ranging from wedge-shaped compression to complete shattering of vertebrae, disc ruptures, traumatic olisthesis and ligament ruptures. Some impact-induced injuries result in permanent disabilities, paralysis and disabling chronic pain. Figure 1 shows a vertebra shattered


and fragmented by a slamming impact onboard a high-speed boat. This kind


Feature 4


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