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Effect of muscle tension on non-linearities in the apparent masses of seated subjects exposed to vertical whole-body vibration. (presented at the 2nd International Conference on Whole-body Vibration Injuries)
No full textIn subjects exposed to whole-body vibration, the cause of non-linear dynamic characteristics with changes in vibration magnitude is not understood. The effect of muscle tension on the non-linearity in apparent mass has been investigated in this study. Eight seated male subjects were exposed to random and sinusoidal vertical vibration at five magnitudes (0·35–1·4 m/s2 r.m.s.). The random vibration was presented for 60 s over the frequency range 2·0–20 Hz; the sinusoidal vibration was presented for 10 s at five frequencies (3·15, 4·0, 5·0, 6·3 and 8·0 Hz). Three sitting conditions were adopted such that, in two conditions, muscle tension in the buttocks and the abdomen was controlled. It was assumed that, in these two conditions, involuntary changes in muscle tension would be minimized. The force and acceleration at the seat surface were used to obtain apparent masses of subjects. With both sinusoidal and random vibration, there was statistical support for the hypothesis that non-linear characteristics were less clear when muscle tension in the buttocks and the abdomen was controlled. With increases in the magnitude of random vibration from 0·35 to 1·4 m/s2 r.m.s., the apparent mass resonance frequency decreased from 5·25 to 4·25 Hz with normal muscle tension, from 5·0 to 4·38 Hz with the buttocks muscles tensed, and from 5·13 to 4·5 Hz with the abdominal muscles tensed. Involuntary changes in muscle tension during whole-body vibration may be partly responsible for non-linear biodynamic responses
Effect of muscle tension on non-linearities in the apparent masses of seated subjects exposed to vertical whole-body vibration
No full textIn subjects exposed to whole-body vibration, the cause of non-linear dynamic characteristics with changes in vibration magnitude is not understood. The effect of muscle tension on the non-linearity in apparent mass has been investigated in this study. Eight seated male subjects were exposed to random and sinusoidal vertical vibration at five magnitudes (035–14 m/s2 r.m.s.). The random vibration was presented for 60 s over the frequency range 20–20 Hz; the sinusoidal vibration was presented for 10 s at five frequencies (315, 40, 50, 63 and 80 Hz). Three sitting conditions were adopted such that, in two conditions, muscle tension in the buttocks and the abdomen was controlled. It was assumed that, in these two conditions, involuntary changes in muscle tension would be minimized. The force and acceleration at the seat surface were used to obtain apparent masses of subjects. With both sinusoidal and random vibration, there was statistical support for the hypothesis that non-linear characteristics were less clear when muscle tension in the buttocks and the abdomen was controlled. With increases in the magnitude of random vibration from 035 to 14 m/s2 r.m.s., the apparent mass resonance frequency decreased from 525 to 425 Hz with normal muscle tension, from 50 to 438 Hz with the buttocks muscles tensed, and from 513 to 45 Hz with the abdominal muscles tensed. Involuntary changes in muscle tension during whole-body vibration may be partly responsible for non-linear biodynamic responses
Mathematical models for the apparent masses of standing subjects exposed to vertical whole-body vibration
No full textLinear lumped parameter models of the apparent masses of human subjects in standing positions when exposed to vertical whole-body vibration have been developed. Simple models with a single degree-of-freedom (d.o.f.) and with two (d.o.f.) were considered for practical use. Model parameters were optimised using both the mean apparent mass of 12 male subjects and the apparent masses of individual subjects measured in a previous study. The calculated responses of two (d.o.f.) models with a massless support structure showed best agreement with the measured apparent mass and phase, with errors less than 0.1 in the normalised apparent mass (i.e., corresponding to errors less than 10% of the static mass) and errors less than 5° in the phase for a normal standing posture. The model parameters obtained with the mean measured apparent masses of the 12 subjects were similar to the means of the 12 sets of parameters obtained when fitting to the individual apparent masses. It was found that the effects of vibration magnitude and postural changes on the measured apparent mass could be represented by changes to the stiffness and damping in the two (d.o.f.) models
Effect of phase on human responses to vertical whole-body vibration and shock-analytical investigation
No full textThe effect of the "phase" on human responses to vertical whole-body vibration and shock has been investigated analytically using alternative methods of predicting subjective responses (using r.m.s., VDV and various frequency weightings). Two types of phase have been investigated: the effect of the relative phase between two frequency components in the input stimulus, and the phase response of the human body. Continuous vibrations and shocks, based on half-sine and one-and-a-half-sine accelerations, each of which had two frequency components, were used as input stimuli. For the continuous vibrations, an effect of relative phase was found for the vibration dose value (VDV) when the ratio between two frequency components was three: about 12% variation in the VDV of the unweighted acceleration was possible by changing the relative phase. The effect of the phase response of the body represented by frequency weightings was most significant when the frequencies of two sinusoidal components were about 3 and 9 Hz. With shocks, the effect of relative phase was observed for all stimuli used. The variation in the r.m.s. acceleration and in the VDV caused by variations in the relative phase varied between 3 and 100%, depending on the nature of stimulus and the frequency weighting. The phase of the frequency weightings had a different effect on the r.m.s. and the VDV
A study of the dynamic response of standing and seated persons to vertical whole-body vibration: principal resonance of the body
No full textComparison of biodynamic responses in standing and seated human bodies
No full textThe dynamic responses of the human body in a standing position and in a sitting position have been compared. The apparent mass and transmissibilities to the head, six locations along the spine, and the pelvis were measured with eight male subjects exposed to vertical whole-body vibration. In both postures, the principal resonance in the apparent mass occurred in the range 5–6 Hz, with slightly higher frequencies and lower apparent mass in the standing posture. There was greater transmission of vertical vibration to the pelvis and the lower spine and greater relative motion within the lower spine in the standing posture than in the sitting posture at the principal resonance and at higher frequencies. Transmissibilities from the supporting surface (floor or seat) to the thoracic region had similar magnitudes for both standing and sitting subjects. The lumbar spine has less lordosis and may be more compressed and less flexible in the sitting posture than in the standing posture. This may have reduced the relative motions between lumbar vertebrae and both the supporting vibrating surface and the other vertebrae in the sitting posture. The characteristics of the vibration transmitted to the pelvis may have differed in the two postures due to different transmission paths. Increased forward rotation of the pelvis in the standing posture may have caused the differences in responses of the pelvis and the lower spine that were observed between the two postures
Dynamic response of the standing human body exposed to vertical vibration: influence of posture and vibration magnitude
No full textThe influence of the posture of the legs and the vibration magnitude on the dynamic response of the standing human body exposed to vertical whole-body vibration has been investigated. Motions were measured on the body surface at the first and eighth thoracic and fourth lumbar vertebrae (T1, T8 and L4), at the right and left iliac crests and at the knee. Twelve subjects took part in the experiment with three leg postures (normal, legs bent and one leg), and five magnitudes of random vibration (0·125-2·0 ms-2 r.m.s.) in the frequency range from 0·5-30 Hz. The main resonance frequencies of the apparent masses at 1·0 ms-2 r.m.s. differed between postures: 5·5 Hz in the normal posture, 2·75 Hz in the legs bent posture and 3·75 Hz in the one leg posture. In the normal posture, the transmissibilities to L4 and the iliac crests showed a similar trend to the apparent mass at low frequencies. With the legs straight, no resonance was observed in the legs at frequencies below 15 Hz. In the legs bent posture, a bending motion of the legs at the knee and a pitching or bending motion of the upper-body appeared to contribute to the resonance of the whole body as observed in the apparent mass, with attenuation of vibration transmission to the upper body at high frequencies. In the one leg posture, coupled rotational motion of the whole upper-body about the hip joint may have contributed to the resonance observed in the apparent mass at low frequencies and the attenuation of vertical vibration transmission at high frequencies. The resonance frequency of the apparent mass in the normal posture decreased from 6·75-5·25 Hz with increasing vibration magnitude from 0·125 to 2·0 ms-2 r.m.s. This "softening" effect was also found in the transmissibilities to many parts of the body that showed resonances.</p
Absorption of energy movement of the upper-body of seated subjects exposed to vertical whole-body vibration at the principal resonance frequency
No full textThe dynamic responses of eight male subjects exposed to vertical whole-body vibration have been measured at eight locations of the body in three directions within the sagittal plane: in the vertical, fore-and-aft and pitch axes. The motions were measured on the body surface at the first, fifth and tenth thoracic vertebra (T1, T5, T10), at the first, third and fifth lumbar vertebra (L1, L3, L5) and at the pelvis (the posterior-superior iliac spine), and were corrected so as to estimate the motions of the skeleton. The head motion was measured with a bite-bar. The force at the seat surface was also measured. The subjects were exposed to vertical random vibration in the frequency range from 0·5-20 Hz at a magnitude of 1·0 ms-2 r.m.s. The movement of the upper-body at the principal resonance frequency of the driving-point apparent mass is illustrated by using the transmissibilities from seat vertical vibration to vertical and fore-and-aft vibration at the eight locations on the body. A bending of the lumbar spine, and probably the lowest thoracic spine, possibly coupled with a rocking motion of the upper thoracic spine about the lower thoracic spine, appeared to be dominant. A small bending along the full length of thoracic spine was also found. Pitch motion of the pelvis, possibly accompanied by longitudinal and shear deformations of the tissue underneath the pelvis, was found to occur near the resonance frequency range, but did not appear to make a principal contribution to the resonance observed in the apparent mass. Any significant axial motions along the spine occurred at higher frequencies.</p
Apparent mass and cross-axis apparent mass of standing subjects during exposure to vertical whole-body vibration
No full textThe effects of posture and vibration magnitude on the vertical apparent mass and the fore-and-aft cross-axis apparent mass of the standing human body during exposure to vertical vibration have been investigated. Twelve male subjects were exposed to random vertical vibration over the frequency range 2.0–20 Hz at three vibration magnitudes: 0.125, 0.25 and 0.5 m s?2 rms. Subjects stood in five different postures: upright, lordotic, anterior lean, knees bent and knees more bent. The vertical acceleration at the floor and the forces in the vertical and fore-and-aft directions at the floor were used to obtain the apparent mass and the cross-axis apparent mass.The resonance frequency of the apparent mass was significantly reduced with knees bent and knees more bent postures, but there were only minor effects on the resonance frequency by changing the position of the upper body. Considerable cross-axis apparent mass, up to about 30% of the static mass of subjects, was found. The cross-axis apparent mass was influenced by all postural changes used in the study. In all postures the resonance frequencies of the apparent mass and the cross-axis apparent mass tended to decrease with increasing vibration magnitude. This nonlinear characteristic tended to be less clear in some postures in which subjects increased muscle tensio
Modelling the dynamic mechanisms associated with the principal resonance of the seated human body
No full textObjective. Simple mathematical models have been developed to obtain insights into resonance phenomena observed at about 5 Hz in the dynamic responses of the seated human body exposed to vertical whole-body vibration.Design. Alternative lumped parameter models with a few degrees-of-freedom have been investigated. Rotational degrees-of-freedom, with eccentricity of the centre of gravity of the mass elements, represented responses in the fore-and-aft and pitch axes caused by vertical vibration.Background. The causes of body resonance are not fully understood, but this information is required to develop cause-effect relationships between vibration exposures and effects on human health, comfort and performance.Method. The inertial and geometric parameters for models were based on published anatomical data. Other mechanical parameters were determined by comparing model responses to experimental data.Results. Two models, with four and five degrees-of-freedom, gave more reasonable representations than other models. Mechanical parameters obtained with median and individual experimental data were consistent for vertical degrees-of-freedom but varied for rotational degrees-of-freedom.Conclusions. The resonance of the apparent mass at about 5 Hz may be attributed to a vibration mode consisting of vertical motion of the pelvis and legs and a pitch motion of the pelvis, both of which cause vertical motion of the upper-body above the pelvis, a bending motion of the spine, and vertical motion of the viscera.Relevance. The mathematical models developed in this study may assist understanding of the dynamic mechanisms responsible for resonances in the seated human body. The information is required to represent mechanical responses of the body and assist the development of models for specific effects of vibration
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