Quantifying supine human discomfort in off-road whole-body vibration.

This work presents a new methodology to quantify supine human discomfort during transport when multi-axis whole-body vibration (WBV) and shocks are present. The methodology employs a new scheme to normalise the reported discomfort. Twenty-six human subjects were tested under different off-road conditions and their reported discomforts collected. The paired Wilcoxon signed-rank method was used to investigate the significant differences (p < 0.01) between different track sections on the normalised reported discomfort from the subjects. Analyses based on ISO 2631-1 showed weak correlation with the reported discomfort when significant lateral motions existed. The results with the new formulation showed that discomfort is highly correlated with the vibration dose value at the head of the supine human during WBV (p < 0.001). These results are consistent with previous published work showing that discomfort based on motion at the head-neck region comprises more than 70% of the reported discomfort during supine transport under multiple-axis WBV.Practitioner summary: There are shortcomings in the current approaches to quantifying discomfort of supine humans in multi-axis whole-body vibration where lateral motions are excessive. This study revealed that reported discomfort is strongly related to the vibration dose value at the head of supine subjects rather than the input motion to the body.Abbreviations: WBV: whole-body vibration; RMS: root-mean square; VDV: vibration dose value; PSD: power spectral density; RDn: reported discomfort; NDn: normalized discomfort; : discomfort scaling coefficient; aw(t): frequency-weighted acceleration; wRMS: weighted root-mean square; Aw: weighted root-mean square acceleration; Aw,p: point weighted root-mean square acceleration; Wd: frequency-weighting factor; Wk: frequency-weighting factor; kx: weighed acceleration multiplying factor in x-direction; ky: weighed acceleration multiplying factor in y-direction; kz: weighed acceleration multiplying factor in z-direction; CV: coefficient of variation; VDVp: point vibration dose value; SD: standard deviation; pVTV: point vibration total value.

Low-Frequency Vibrations Enhance Thrombolytic Therapy and Improve Stroke Outcomes.

Background and Purpose- We aim to determine the potential impact on stroke thrombolysis of drip-and-ship helicopter flights and specifically of their low-frequency vibrations (LFVs). Methods- Mice with a middle cerebral artery autologous thromboembolic occlusion were randomized to receive rtPA (recombinant tissue-type plasminogen activator; or saline) 90 minutes later in 3 different settings: (1) a motion platform simulator that reproduced the LFV signature of the helicopter, (2) a standardized actual helicopter flight, and (3) a ground control. Results- Mice assigned to the LFV simulation while receiving tPA had smaller infarctions (31.6 versus 54.9 mm3; P=0.007) and increased favorable neurological outcomes (86% versus 28%; P=0.0001) when compared with ground controls. Surprisingly, mice receiving tPA in the helicopter did not exhibit smaller infarctions (47.8 versus 54.9 mm3; P=0.58) nor improved neurological outcomes (37% versus 28%; P=0.71). This could be due to a causative effect of the 20- to 30-Hz band, which was inadvertently attenuated during actual flights. Mice using saline showed no differences between the LFV simulator and controls with respect to infarct size (80.9 versus 95.3; P=0.81) or neurological outcomes (25% versus 11%; P=0.24), ruling out an effect of LFV alone. There were no differences in blood-brain barrier permeability between LFV simulator or helicopter, compared with controls (2.45-3.02 versus 4.82 mm3; P=0.14). Conclusions- Vibration in the low-frequency range (0.5-120 Hz) is synergistic with rtPA, significantly improving the effectiveness of thrombolysis without impairing blood-brain barrier permeability. Our findings reveal LFV as a novel, safe, and simple-to-deliver intervention that could improve the outcomes of patients. Visual Overview- An online visual overview is available for this article.

Joint Center Estimation Using Single-Frame Optimization: Part 2: Experimentation.

Human motion capture is driven by joint center location estimates, and error in their estimation can be compounded by subsequent kinematic calculations. Soft tissue artifact (STA), the motion of tissue relative to the underlying bones, is a primary cause of error in joint center calculations. A method for mitigating the effects of STA, single-frame optimization (SFO), was introduced and numerically verified in Part 1 of this work, and the purpose of this article (Part 2) is to experimentally compare the results of SFO with a marker-based solution. The experimentation herein employed a single-degree-of-freedom pendulum to simulate human joint motion, and the effects of STA were simulated by affixing the inertial measurement unit to the pendulum indirectly through raw, vacuum-sealed meat. The inertial sensor was outfitted with an optical marker adapter so that its location could be optically determined by a camera-based motion-capture system. During the motion, inertial effects and non-rigid attachment of the inertial sensor caused the simulated STA to manifest via unrestricted motion (six degrees of freedom) relative to the rigid pendulum. The redundant inertial and optical instrumentation allowed a time-varying joint center solution to be determined both by optical markers and by SFO, allowing for comparison. The experimental results suggest that SFO can achieve accuracy comparable to that of state-of-the-art joint center determination methods that use optical skin markers (root mean square error of 7.87⁻37.86 mm), and that the time variances of the SFO solutions are correlated (r =  0.58⁻0.99) with the true, time-varying joint center solutions. This suggests that SFO could potentially help to fill a gap in the existing literature by improving the characterization and mitigation of STA in human motion capture.

Joint Center Estimation Using Single-Frame Optimization: Part 1: Numerical Simulation.

The biomechanical models used to refine and stabilize motion capture processes are almost invariably driven by joint center estimates, and any errors in joint center calculation carry over and can be compounded when calculating joint kinematics. Unfortunately, accurate determination of joint centers is a complex task, primarily due to measurements being contaminated by soft-tissue artifact (STA). This paper proposes a novel approach to joint center estimation implemented via sequential application of single-frame optimization (SFO). First, the method minimizes the variance of individual time frames’ joint center estimations via the developed variance minimization method to obtain accurate overall initial conditions. These initial conditions are used to stabilize an optimization-based linearization of human motion that determines a time-varying joint center estimation. In this manner, the complex and nonlinear behavior of human motion contaminated by STA can be captured as a continuous series of unique rigid-body realizations without requiring a complex analytical model to describe the behavior of STA. This article intends to offer proof of concept, and the presented method must be further developed before it can be reasonably applied to human motion. Numerical simulations were introduced to verify and substantiate the efficacy of the proposed methodology. When directly compared with a state-of-the-art inertial method, SFO reduced the error due to soft-tissue artifact in all cases by more than 45%. Instead of producing a single vector value to describe the joint center location during a motion capture trial as existing methods often do, the proposed method produced time-varying solutions that were highly correlated (r > 0.82) with the true, time-varying joint center solution.

Comparing the Efficacy of Methods for Immobilizing the Thoracic-Lumbar Spine.

OBJECTIVE: The purpose of this study was to compare the relative efficacy of immobilization systems in limiting thoracic-lumbar movements. METHODS: A dynamic simulation system was used to reproduce transport-related shocks and vibration, and involuntary movements of the thoracic-lumbar region were measured using 3 immobilization configurations. RESULTS: The vacuum mattress and the long spine board were generally more effective than the cot alone in reducing thoracic-lumbar rotation and flexion/extension. However, the vacuum mattress reduced these thoracic-lumbar movements to a greater extent than the long spine board. In addition, the vacuum mattress significantly decreased thoracic-lumbar lateral movement relative to the cot alone under all simulated transport conditions. In contrast, the long spine board allowed greater lateral movement than the cot alone in a number of the simulated transport rides. CONCLUSION: Under the study conditions, the vacuum mattress was more effective for limiting involuntary movements of the thoracic-lumbar region than the long spine board. Moreover, the increased lateral bend observed with the long spine board under some conditions suggests it may be inadequate for immobilizing this anatomic region as presently designed. Should emergency medical service providers choose to immobilize patients with suspected injuries of the thoracic-lumbar spine, study results support the use of the vacuum mattress.

Predictive discomfort of supine humans in whole-body vibration and shock environments.

This work presents a predictive model to evaluate discomfort associated with supine humans during transportation, where whole-body vibration and repeated shock are predominant. The proposed model consists of two parts: (i) static discomfort resulting from body posture, joint limits and ambient discomfort; and (ii) dynamic discomfort resulting from the relative motion between the body segments as a result of transmitted vibration. Twelve supine subjects were exposed to single and 3D random vibrations and 3D shocks mixed with vibrations. The subjects' reported discomfort and biodynamic response were analysed under different support conditions, including a rigid surface, a stretcher and a stretcher with a spinal backboard. The results demonstrated good correlations between the predictive discomfort and the reported discomfort for the different conditions under consideration, with R(2) = 0.69-0.94 for individual subjects and R(2) = 0.94 for the group mean. The results also indicated a strong relationship between the head-neck and trunk angular velocities and discomfort during supine transportation. Practitioner Summary: The quantification of discomfort of supine humans under vibration and shocks by using a predictive model is an important contribution to this field, whereby the efficacy of different transport systems can be compared. The predictive discomfort model can be used as design criteria for ergonomic enhancement in supine transportation of humans.

Three-dimensional modeling of supine human and transport system under whole-body vibration.

The development of predictive computer human models in whole-body vibration has shown some success in predicting simple types of motion, mostly for seated positions and in the uniaxial vertical direction. The literature revealed only a handful of papers that tackled supine human modeling in response to vertical vibration. The objective of this work is to develop a predictive, multibody, three-dimensional human model to simulate the supine human and underlying transport system in response to multidirectional whole-body vibration. A three-dimensional dynamic model of a supine human and its underlying transport system is presented in this work to predict supine-human biodynamic response under three-dimensional input random whole-body vibration. The proposed supine-human model consists of three interconnected segments representing the head, torso-arms, and pelvis-legs. The segments are connected via rotational and translational joints that have spring-damper components simulating the three-dimensional muscles and tissuelike connecting elements in the three x, y, and z directions. Two types of transport systems are considered in this work, a rigid support and a long spinal board attached to a standard military litter. The contact surfaces between the supine human and the underlying transport system are modeled using spring-damper components. Eight healthy supine human subjects were tested under combined-axis vibration files with a magnitude of 0.5 m/s2 (rms) and a frequency content of 0.5-16 Hz. The data from seven subjects were used in parameter identification for the dynamic model using optimization schemes in the frequency domain that minimize the differences between the magnitude and phase of the predicted and experimental transmissibility. The predicted accelerations in the time and frequency domains were comparable to those gathered from experiments under different anthropometric, input vibration, and transport conditions under investigation. Based on the results, the proposed dynamic model has the potential to be used to provide motion data to drive a detailed finite element model of a supine human for further investigation of muscle forces and joint dynamics. The predicted kinematics of the supine human and transport system would also benefit patient safety planners and vibration suppression designers in their endeavors.

Predictive discomfort of non-neutral head-neck postures in fore-aft whole-body vibration.

It seems obvious that human head-neck posture in whole-body vibration (WBV) contributes to discomfort and injury risk. While current mechanical measures such as transmissibility have shown good correlation with the subjective-reported discomfort, they showed difficulties in predicting discomfort for non-neutral postures. A new biomechanically based methodology is introduced in this work to predict discomfort due to non-neutral head-neck postures. Altogether, 10 seated subjects with four head-neck postures--neutral, head-up, head-down and head-to-side--were subjected to WBV in the fore-aft direction using discrete sinusoidal frequencies of 2, 3, 4, 5, 6, 7 and 8 Hz and their subjective responses were recorded using the Borg CR-10 scale. All vibrations were run at constant acceleration of 0.8 m/s² and 1.15 m/s². The results have shown that the subjective-reported discomfort increases with head-down and decreases with head-up and head-to-side postures. The proposed predictive discomfort has closely followed the reported discomfort measures for all postures and rides under investigation. STATEMENT OF RELEVANCE: Many occupational studies have shown strong relevance between non-neutral postures, discomfort and injury risk in WBV. With advances in computer human modelling, the proposed predictive discomfort may provide efficient ways for developing reliable biodynamic models. It may also be used to assess discomfort and modify designs inside moving vehicles.

Comparing the Efficacy of Methods for Immobilizing the Cervical Spine.

STUDY DESIGN: This was a prospective simulator study with 16 healthy male subjects. OBJECTIVE: The aim of this study was to compare the relative efficacy of immobilization systems in limiting involuntary movements of the cervical spine using a dynamic simulation model. SUMMARY OF BACKGROUND DATA: Relatively few studies have tested the efficacy of immobilization methods for limiting involuntary cervical movement, and only one of these studies used a dynamic simulation system to do so. METHODS: Immobilization configurations tested were cot alone, cot with cervical collar, long spine board (LSB) with cervical collar and head blocks, and vacuum mattress (VM) with cervical collar. A motion platform reproduced shocks and vibrations from ambulance and helicopter field rides, as well as more severe shocks and vibrations that might be encountered on rougher terrain and in inclement weather (designated as an "augmented" ride). Motion capture technology quantitated involuntary cervical rotation, flexion/extension, and lateral bend. The mean and 95% confidence interval of the mean were calculated for the root mean square of angular changes from the starting position and for the maximum range of motion. RESULTS: All configurations tested decreased cervical rotation and flexion/extension relative to the cot alone. However, the LSB and VM were significantly more effective in decreasing cervical rotation than the cervical collar, and the LSB decreased rotation more than the VM in augmented rides. The LSB and VM, but not the cervical collar, significantly limited cervical lateral bend relative to the cot alone. CONCLUSION: Under the study conditions, the LSB and the VM were more effective in limiting cervical movement than the cervical collar. Under some conditions, the LSB decreased repetitive and acute movements more than the VM. Further studies using simulation and other approaches will be essential for determining the safest, most effective configuration should providers choose to immobilize patients with suspected spinal injuries. LEVEL OF EVIDENCE: 3.

Whole-body vibration transmissibility in supine humans: effects of board litter and neck collar.

Whole-body vibration has been identified as a stressor to supine patients during medical transportation. The transmissibility between the input platform acceleration and the output acceleration of the head, sternum, pelvis, head-sternum, and pelvis-sternum of eight supine subjects were investigated. Vibration files were utilized in the fore-aft, lateral, and vertical directions. The power spectral density across the bandwidth of 0.5-20 Hz was approximately flat for each file. A comparison between a baseline rigid-support and a support with a long spinal board strapped to a litter has shown that the latter has considerable effects on the transmitted motion in all directions with a double magnification in the vertical direction around 5 Hz. The results also showed that the neck-collar has increased the relative head-sternum flexion-extension because of the input fore-aft vibration, but reduced the head-sternum extension-compression due to the input vertical vibration.

Predictive discomfort in single- and combined-axis whole-body vibration considering different seated postures.

OBJECTIVE: The aim of this study was to develop a predictive discomfort model in single-axis, 3-D, and 6-D combined-axis whole-body vibrations of seated occupants considering different postures. BACKGROUND: Non-neutral postures in seated whole-body vibration play a significant role in the resulting level of perceived discomfort and potential long-term injury. The current international standards address contact points but not postures. METHOD: The proposed model computes discomfort on the basis of static deviation of human joints from their neutral positions and how fast humans rotate their joints under vibration. Four seated postures were investigated. For practical implications, the coefficients of the predictive discomfort model were changed into the Borg scale with psychophysical data from 12 volunteers in different vibration conditions (single-axis random fore-aft, lateral, and vertical and two magnitudes of 3-D). The model was tested under two magnitudes of 6-D vibration. RESULTS: Significant correlations (R = .93) were found between the predictive discomfort model and the reported discomfort with different postures and vibrations. The ISO 2631-1 correlated very well with discomfort (R2 = .89) but was not able to predict the effect of posture. CONCLUSION: Human discomfort in seated whole-body vibration with different non-neutral postures can be closely predicted by a combination of static posture and the angular velocities of the joint. APPLICATION: The predictive discomfort model can assist ergonomists and human factors researchers design safer environments for seated operators under vibration. The model can be integrated with advanced computer biomechanical models to investigate the complex interaction between posture and vibration.

Human head-neck models in whole-body vibration: effect of posture.

This work presents passive and muscle-based models to predict the biodynamical response of the human head-neck under fore-aft and combined-axis whole-body vibration considering four head-neck postures: neutral, flexion, lateral flexion, and lateral rotation. The passive model consists of one link, a three-rotational-degrees-of-freedom joint, and traditional spring-mass-damper elements. The muscle-based model is similar to the passive model but has additional muscle components. The additional muscle component comprises spring-mass-damper elements to capture the effects of changes in displacement, velocity, acceleration, and jerk. Eleven male participants were tested under white-noise random vibration input signals at the seat level with a frequency range of 0.5-10Hz and magnitudes of 1.5m/s(2) RMS for the fore-aft condition and 1.0m/s(2) RMS in each direction for the combined-axis condition. The proposed models were able to reasonably predict the frequency content and acceleration of the head-neck for the postures under investigation, with the muscle-based model performing better.

An active head-neck model in whole-body vibration: vibration magnitude and softening.

An active head-neck model is introduced in this work to predict human-dynamic response to different vibration magnitudes during fore-aft whole-body vibration. The proposed model is a rigid-link dynamic system augmented with passive spring-damper tissue-like elements and additional active dampers that resemble the active part of the muscles. The additional active dampers are functions of the input displacement, velocity, and acceleration and are based on active control theories and a kd-tree data-searching scheme. Five human subjects exposed to random fore-aft vibration with frequency content of 0.5-10 Hz were tested under different vibration with magnitudes of 0.46 m/s(2), 1.32 m/s(2), and 1.66 m/s(2) rms. The results showed that the proposed model was able to reasonably capture the softening characteristics of the human head-neck response during fore-aft whole-body vibration of different magnitudes.

A quasi-static discomfort measure in whole-body vibration.

A new methodology for objective evaluation of discomfort in whole-body vibration (WBV) is introduced in this work. The proposed objective discomfort characterizes discomfort based on the relative motion between adjacent segments of the human body from neutral positions. It peaks when the joints reach their limits. The objective discomfort has been tested on five subjects in the fore-aft direction using discrete sinusoidal frequencies of 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 7, 8, 9, 10, 12, 14, and 16 Hz. Each frequency file runs for 15 s with a 3 s resting period as a reference for discomfort comparison. All files run at a constant acceleration of 0.7 m/s(2). The subjects were tested with back support and without back support, and their subjective discomfort was reported based on the Borg CR-10 scale. The proposed objective discomfort has shown significant correlation with the subjective discomfort. The objective discomfort has also been tested on five subjects under multiple-axis random WBV with three common industrial seating configurations (seat-mounted control, floor-mounted control, and steering wheel), and has shown promising results.

Dynamic motion planning of 3D human locomotion using gradient-based optimization.

Since humans can walk with an infinite variety of postures and limb movements, there is no unique solution to the modeling problem to predict human gait motions. Accordingly, we test herein the hypothesis that the redundancy of human walking mechanisms makes solving for human joint profiles and force time histories an indeterminate problem best solved by inverse dynamics and optimization methods. A new optimization-based human-modeling framework is thus described for predicting three-dimensional human gait motions on level and inclined planes. The basic unknowns in the framework are the joint motion time histories of a 25-degree-of-freedom human model and its six global degrees of freedom. The joint motion histories are calculated by minimizing an objective function such as deviation of the trunk from upright posture that relates to the human model's performance. A variety of important constraints are imposed on the optimization problem, including (1) satisfaction of dynamic equilibrium equations by requiring the model's zero moment point (ZMP) to lie within the instantaneous geometrical base of support, (2) foot collision avoidance, (3) limits on ground-foot friction, and (4) vanishing yawing moment. Analytical forms of objective and constraint functions are presented and discussed for the proposed human-modeling framework in which the resulting optimization problems are solved using gradient-based mathematical programming techniques. When the framework is applied to the modeling of bipedal locomotion on level and inclined planes, acyclic human walking motions that are smooth and realistic as opposed to less natural robotic motions are obtained. The aspects of the modeling framework requiring further investigation and refinement, as well as potential applications of the framework in biomechanics, are discussed.