The influence of biodynamic factors on the mechanical impedance of the hand and arm

Abstract

The purpose of this study was to investigate the mechanical impedance of the human hand-arm system during exposure to random vibration under various experimental conditions and to evaluate statistically whether these experimental conditions have any influence on magnitude and phase of the mechanical impedance. A further aim was to compare the obtained results with other investigations where sinusoidal excitation has been used. The mechanical impedance was estimated in ten healthy subjects during exposure to random vibration, with a constant velocity spectrum within the frequency range 4-2000 Hz, by use of a specially designed laboratory handle. In the study, the influence of various conditions, such as vibration direction (Xh, Yh, Zh), grip force (25-75 N), feed force (20-60 N), frequency-weighted acceleration level (3, 6, 9, 12 m/s2) and hand and arm posture (five flexions, two abductions) were studied. The outcome showed that the vibration direction and the frequency of the vibration stimuli have a strong significant influence on the impedance of the hand. An increased vibration level resulted in a significantly lower impedance for frequencies over 100 Hz. Increase grip and feed forced led on the other hand to an increased impedance for all frequencies. With regard to hand and arm posture, the results show that the flexion and abduction had a significant contribution for frequencies below 30 Hz. Furthermore, the influence of some of the studied variables had a non-linear effect on the impedance but also differed between different exposure directions. It was concluded, moreover, that the vibration response characteristics of the hand and arm differ, depending whether the signal is a discrete frequency signal or a signal consisting of several frequencies.

Recent Advances in Biodynamics of Human Hand-Arm System

“…Although the absolute error in the estimation of the handle mass could be small, it could result in a large percentage of error. The impedance phase angles reported by a few investigators 64,76) were close to 90° at high frequencies (>100 Hz), which implies that the hand and fingers could respond almost rigidly in the high frequency range. Dong et al 10) pointed out that this is not a natural phenomenon because the vibration at such frequencies can only be effectively transmitted to the soft tissues of the hand which can effectively absorb or isolate high frequency vibration; otherwise, the high frequency vibration would be effectively transmitted to the arm.…”

“…There are considerable differences among reported data 1,62) . Some of them may be caused from the multitudes of vibration excitations, vibration magnitudes, applied forces, hand and arm postures, handle shapes and dimensions, and individual differences 12,13,47,[63][64][65][66][67][68][69][70] . However, the data measured under similar experimental conditions also show significant differences not only in magnitude but also in trend 1,62) .…”

Mechanical Impedance and Absorbed Power of Hand-Arm under xh-Axis Vibration and Role of Hand Forces and Posture

“…The DPMI magnitude was observed to increase slightly with an increase in intensity of swept harmonic vibration in one study by Burström 4) , while another study by the same author reported a slight decrease in the DPMI magnitude under higher levels of random excitations 3) . Gurram et al 2) observed insignificant effect of the vibration intensity on the DPMI response.…”

“…These are mostly evaluated in terms of driving-point mechanical impedance (DPMI) and absorbed power characteristics, under vibration along the three translational axes of the basicentric coordinate system (x h , y h and z h ) that are applied independently [1][2][3][4][5][6][7][8][9][10][11][12] . The biodynamic response, whether expressed in terms of DPMI or absorbed power, is known to depend upon many intrinsic and extrinsic variables, such as the frequency, magnitude and direction of vibration, the hand-handle coupling forces, handle size, hand-arm posture and individual characteristics.…”

“…Although the handhandle coupling force is directly influenced by both the grip and the push forces 14) , only a few studies have attempted to characterize the effect of push force on the hand-arm biodynamic response along the x h -axis. Burström 3) reported that a higher push force causes higher DPMI modulus at frequencies below 25 Hz and above 125 Hz. Another study, however, has reported negligible effects of grip and push forces on the power absorbed by the hand and arm system under excitations due to 4 different tools 15) .…”