PubMed · 1821545
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E Sánchez Villares. 1991. [Adolescence: historic evolution].. https://pubmed.ncbi.nlm.nih.gov/1821545/
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STUDY DESIGN: Experimental analysis was performed to determine the effects of low-velocity rear-end impacts on phasic and magnitudinal activity of the cervical muscles in expected and unexpected conditions. OBJECTIVES: To determine the phasic response of the cervical muscles to increasing low-velocity rear-end impacts, and to compare the quantitative effects of awareness and unawareness of impending impact. SUMMARY OF BACKGROUND DATA: The literature contains little information on the etiology of whiplash injuries. Animal and cadaver studies have yielded some insight into the phenomenon. However, in vivo studies of the cervical muscular response and head-neck kinematics to low-velocity impacts are rare. METHODS: Seven healthy volunteers were subjected to rear-end impacts of 4.9, 8.8, 10.8, and 13.7 m/s2 acceleration at two levels of expectation: expected and unexpected. Bilateral electromyograms of the sternocleidomastoids, trapezii, and splenii capitis were recorded. Triaxial accelerometers recorded the acceleration of the chair, torso at the shoulder level, and head of the participant. RESULTS: At an acceleration of 13.7 m/s2, the sternocleidomastoids generated up to 179% of their maximal voluntary contraction electromyogram, whereas the splenii and trapezii did not exceed 35% of their maximal voluntary contraction in most of the experimental conditions. Electromyographic variables were significantly affected by the levels of acceleration and expectation (P < 0.001). The onset time and peak electromyogram time for the sternocleidomastoid progressively decreased with increasing levels of acceleration. The kinetic variables and the electromyographic variables regressed significantly on the acceleration (P < 0.01), explaining from 96% to 100% of the variability. CONCLUSIONS: Muscle responses were greater with higher levels of acceleration. Because the muscular component of the head-neck complex plays a central role in the abatement of higher acceleration levels, it may be a primary site of injury in the whiplash phenomenon.
The aim of the study was to understand: (i), how far anticipatory postural adjustments (APA) participate in the sequences of a complex task; (ii), how, in the sagittal plane, horizontal and vertical progressions of the center of gravity (CG) are managed to achieve the prescribed goal. A posturo-kinetics sequence is identified during which forward CG progression is controlled in terms of a center of pressure (CP)-CG interaction. CP shifts and horizontal CG accelerations are correlated in timing and amplitude. There exists a control of whole body progression during the impulse, implying a braking of forwards movement. Vertical and horizontal CG accelerations are inter-dependent, and backward CP shift duration is related to its forward displacement. During the vertical jump initiation, whole body vertical propulsion and forward fall are linked. To increase impulse amplitude, initial whole body disequilibrium is lengthened and forward CG acceleration is greater. APA programming is probably elaborated on the basis of cognitive evaluation of the antero-posterior disturbance.
Control of isometric grip forces during manipulation of objects is an essential feature of all skilled manual performances. Recent studies suggested that the anticipation of movement-induced loads may be a cerebellar function. We analysed grip force adjustments to fluctuations of inertial loads during discrete vertical movements with a grasped object in five patients with cerebellar atrophy and five healthy control subjects. Normally grip force is precisely adapted to the load fluctuations, in particular to the maximum load force, which occurs early in upward and late in downward movements. Both groups produced similar accelerations of the grasped object and consequently similar maximum loads. However, cerebellar patients established increased static grip forces during stationary holding of the object and increased force ratios between grip and load force at the time of maximum acceleration. These findings are congruent with earlier studies analysing grip and load force coupling in patients with cerebellar lesions. In contrast to earlier studies, we found no significant differences in the timing of grip force onset and grip force maximum relative to the onset of movement and maximum acceleration, respectively, between normal controls and four of five cerebellar patients. However, a regression analysis between grip and load forces during the load increase and decrease phases of the movement suggested deficits in the close temporospatial coupling between the two forces in all cerebellar patients. Our findings give further support to the notion that the cerebellum plays a crucial role in the forward control of grip force magnitude and timing during voluntary object manipulation. Compared to earlier studies, the increase in grip forces may be interpreted as a general control strategy to compensate for motor deficits, whereas impairments of temporal grip force regulation may occur at different degrees of dysfunction during the progression of cerebellar atrophy.