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Citation
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Kingma H, Patijn J, de Jong I, Slangen R, Gosens HW, Stevens J, Dekker A, Lansbergen M, van der Horst M, Wismans J, van Mameren H. Int. Musculoskelet. Med. 2009; 31(3): 101-114. |
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Abstract
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Introduction: The impact of muscle contraction upon head movements induced by a forward acceleration was studied as a model for low impact car-accidents. The purpose of our research is to examine: (i) whether and by which mechanism neck muscle contraction is induced after onset of the movement; and (ii) if voluntary neck muscle contraction prior to the movement affects head motion.
Subjects and methods: Eight healthy volunteers (mean age 21.4 years) were included in this study. Volunteers were sitting on a standard car seat mounted on a sled. The computer-controlled, motor-driven sled was forward accelerated with 0.6 g. In test series 1, head and body movement were detected with a standard video camera (20 ms per frame) and accelerometers mounted on the head without interference with muscle activity. Muscle activation was detected by simultaneous measurement of surface EMG of m. splenius capitis, m. trapezius (descending part), m. scalenis medius, m. sternocleido mastoideus, m. digastricus (anterior belly), m. mylohyoideus, m. pectoralis major, m. quadriceps, tibialis anterior and triceps surae. The subject's movements and muscle activity were examined during forward sled acceleration when: (i) the subjects were relaxed and could not anticipate the precise start of sled acceleration; (ii) when they contracted all body muscles for about 4 s prior to and during the sled acceleration; and (iii) when head and trunk were fixed to the chair. The same subjects were accelerated in test series 2, but now head and trunk movements were measured with a better time resolution by use of a high-speed video camera (500 Hz) and accelerometers mounted on the head (bite board), body and sled. Head and body movements were measured under conditions 1 and 2 as described above. Reproducibility was tested by repeating each test condition once, including repositioning of the subjects, the accelerometers and body fixed sell spot markers.
Results and conclusions: The current experimental set-up allows a low-cost evaluation of head and body movements induced by low impact velocity impulses. We observed that sudden whole body acceleration induces head accelerations that exceed the acceleration of the impact by a factor of 2-3-fold. After the impact, the head first remains stationary in space while the sled and trunk move forward (relative head and upper trunk translation). Subsequently, head and trunk rotate backwards, ultimately followed by an additional retroflexion of the head alone. Precontraction and anticipation of the impact leads to a faster increase of general muscle tone. It does not affect the initial translation but leads to a reduction of about 30-35% of head rotations and head angular velocities. The muscle contraction is most like a generalised alerting response. It should be noted that the current results were obtained at a loading level of 0.7 g, which is far below the 5-12 g levels observed in rear-end collisions. Since the current study shows only small influences of the muscle contraction, it is assumed that the contraction will not be strong enough to limit the larger head and trunk motion with greater impacts and does not reduce the probability of getting a whiplash injury. |