Seat Design Principles to Reduce Neck Injuries in Rear Impacts
From: Traffic Inj Prev. 2008 Dec;9(6):552-560
In the 1990s, research was conducted at General Motors R&D Center on seat safety in rear impacts. It led to the development of high retention seats and an active head restraint to improve occupant safety. This article provides an overview of the design principles found from that research and focuses on seat characteristics that lower whiplash risks.
Sled and quasistatic seat testing showed how occupants interact with the seat in rear impacts and what seat characteristics improve occupant retention, energy management, and support of the head and neck, lowering injury risks. Neck displacements, moments, and forces were used to assess whiplash and more severe injury risks. A QST test was developed to quasi-statically push a dummy rearward into the seat to determine seat stiffness, frame strength, and peak bending moment. These parameters were related to neck displacements associated with whiplash. Sled tests were run with in-position and out-of-position male and female Hybrid III dummies to assess performance. A high retention seat and active head restraint were developed and put into production in 1997.
High retention seats have 2.3 times greater moment, develop 2.2 times greater load, but have the same stiffness as earlier yielding seats. Seat stiffness was found to be a principle characteristic related to neck displacements associated with whiplash. The combination of a stronger frame, yielding seatback, and high-forward head restraint in the high retention seat provides early head support and low neck displacements in rear impacts. Larger reductions in neck displacement were obtained by adding an active head restraint that moves the head restraint forward and upward by occupant penetration into the seatback. This substantially reduces head contact time, neck displacements, and loads.
Whiplash risks are related to seat stiffness, the position of the head restraint, and frame strength. Low seat stiffness allows the occupant to move into the seatback without high loads on the torso until the head and neck is supported by the head restraint. A strong seat frame reduces early seatback rotation that increases the gap to the head restraint and drops it in relation to the occupant’s head. A high and forward head restraint provides support of the head and neck. Large forces can be applied to the occupant once the head, neck, and torso are supported by the seat and head restraint without adverse loading of the spine. The addition of an active head restraint closes the gap behind the head before significant load develops on the neck. The movement provides a more upward trajectory of the head restraint. Low-speed rear crashes are not just a matter of whiplash; older occupants, some with cervical stenosis, are at risk for paralyzing spinal cord injury.
Cervical Range of Motion Associations With Subclinical Neck Pain
From: Spine. 2004 Jan 1;29(1):33-40
The problem of neck pain is common in the general population, with 70% of individuals affected at some time in their lives, and about 5% to 10% of adults suffering a disabling neck problem. A recent random population-based study suggested that, in any group of young adults, approximately one third wake up with neck pain or stiffness once per week. Despite the high prevalence of neck problems, very few studies are available indicating any physical associations with the development of neck pain. Accordingly, there is a lack of knowledge concerning early signs of pathology for neck pain, such as which movements are affected when early neck symptoms appear.
It is well understood that cervical structures can be affected by specific causes such as degenerative disease, trauma, and/or inflammatory disorders, and that neck pain can result. Another group of neck pain cases with mainly mechanical disorders, including those thought to arise from habitual postures and degenerative involvement, have been referred as nonspecific neck pain. However, most cases of nonspecific neck pain are similar in presentation to that seen when cervical structures are injured by disease or trauma, even though systemic conditions cannot be found as the underlying cause of neck complaints.
It has been proposed that nonspecific neck pain problems result from poor posture, in terms of sustained, long-term, abnormal physiologic loads on the neck. These loads compromise pain-sensitive structures and thereby affect the function of the cervical spine, causing a musculoskeletal imbalance in the upper quarter of the body. For example, a habitual excessively forward head posture has been suggested to be pain provoking, with a consequential reduction in muscle strength.
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Changes in EMG activity during clenching in chronic pain patients with unilateral temporomandibular disorders
From: J Electromyogr Kinesiol. 2008 Nov 26; [Epub ahead of print]
The study assessed the differences in electromyographic (EMG) activity recorded during clenching in women with chronic unilateral temporomandibular joint disorders as compared to control subjects. Seventy-five full dentate, normo-occlusion, right handed, similarly aged female subjects were recruited. Twenty five subjects presented with right side temporomandibular joint disorder, 25 presented with left side temporomandibular joint disorder and 25 pain free control subjects participated. Using integrated surface EMG over a 1 s contraction, the anterior temporalis and masseter muscles were evaluated bilaterally while subjects performed maximum voluntary clenching. Lower EMG activation was observed in patients with temporomandibular joint disorders as compared to control subjects (temporalis: 195.74+/-18.57 vs. 275.74+/-22.11; masseters: 151.09+/-17.37 vs. 283.29+/-31.87. An asymmetry index was calculated to determine ratios of right to left sided activation. Patients with right sided temporomandibular joint disorders demonstrated preferential use of their left sided muscles (asymmetry index -5.35+/-4.02) whereas patients with left sided temporomandibular joint disorder demonstrated preferential use of their right sided muscles (asymmetry index 6.95+/-2.82). This unilateral reduction in temporalis and masseter activity could be considered as a specific protective functional adaptation of the neuromuscular system due to nociceptive input. The asymmetry index may be a useful measure in discriminating patients with right vs. left sided temporomandibular joint disorders.
Sleep posture and unilateral renal stone formation
From: Scand J Urol Nephrol. 2008;42(6):551-4
Until now, there has been no hypothesis to explain the aetiology of recurrent unilateral urolithiasis in stone formers. Previous studies suggested that sleep posture may lead to alterations of renal haemodynamics. The probable association between sleep posture and unilateral urolithiasis is reported here. In this prospective cohort study, 38 patients with recurrent unilateral renal stones and 42 healthy controls were studied. Background variables were evaluated using a questionnaire. Sleep posture was recorded with a sleep recorder. Unpaired t test, Mann-Whitney test, and chi-squared test were performed as needed and was considered significant.
Of 17 patients with right kidney stones, right side down sleep posture was seen in 15 (88.2%) and supine/prone in two (11.8%). Of 21 patients with left kidney stone, sleep posture was left side down in 13 (61.9%), right side down in two (9.5%) and supine/prone in six (28.6%). That is, a significant association was found between lateral sleep posture and ipsilateral kidney stone formation [odds ratio (OR)=21.13 for left side, 95% confidence interval (CI) 4.16-123.8; and OR=21.14 for right side, CI 95% 3.6-160.4]. Although the impact of sleep posture in recurrent unilateral stone formation remains unclear, sleep posture may play a causative role in promotion of urolithiasis. Although this observation needs further investigation, further studies should be conducted on the efficacy of appropriate education and learning to increase patients’ capability to change routine sleep posture.
