The influence of seat backrest angle on perceived discomfort during exposure to vertical whole-body vibration.

National and International Standards (e.g. BS 6841 and ISO 2631-1) provide methodologies for the measurement and assessment of whole-body vibration in terms of comfort and health. The EU Physical Agents (Vibration) Directive (PAVD) provides criteria by which vibration magnitudes can be assessed. However, these standards only consider upright seated (90°) and recumbent (0°) backrest angles, and do not provide guidance for semi-recumbent postures. This article reports an experimental programme that investigated the effects of backrest angle on comfort during vertical whole-body vibration. The series of experiments showed that a relationship exists between seat backrest angle, whole-body vibration frequency and perceived levels of discomfort. The recumbent position (0°) was the most uncomfortable and the semi-recumbent positions of 67.5° and 45° were the least uncomfortable. A new set of frequency weighting curves are proposed which use the same topology as the existing BS and ISO standards. These curves could be applied to those exposed to whole-body vibration in semi-recumbent postures to augment the existing standardised methods. PRACTITIONER SUMMARY: Current vibration standards provide guidance for assessing exposures for seated, standing and recumbent positions, but not for semi-recumbent postures. This article reports new experimental data systematically investigating the effect of backrest angle on discomfort experienced. It demonstrates that most discomfort is caused in a recumbent posture and that least was caused in a semi-recumbent posture.

The influence of seat backrest angle on human performance during whole-body vibration.

This study investigated the effects of reclined backrest angles on cognitive and psycho-motor tasks during exposure to vertical whole-body vibration. Twenty participants were each exposed to three test stimuli of vertical vibration: 2-8 Hz; 8-14 Hz and 14-20 Hz, plus a stationary control condition whilst seated on a vibration platform at five backrest angles: 0° (recumbent, supine) to 90° (upright). The vibration magnitude was 2.0 ms(-2) root-mean-square. The participants were seated at one of the backrest angles and exposed to each of the three vibration stimuli while performing a tracking and choice reaction time tasks; then they completed the NASA-TLX workload scales. Apart from 22.5° seat backrest angle for the tracking task, backrest angle did not adversely affect the performance during vibration. However, participants required increased effort to maintain performance during vibration relative to the stationary condition. These results suggest that undertaking tasks in an environment with vibration could increase workload and risk earlier onset of fatigue. PRACTITIONER SUMMARY: Current vibration standards provide guidance for assessing exposures for seated, standing and recumbent positions, but not for semi-recumbent postures. This paper reports new experimental data systematically investigating the effect of backrest angle on human performance. It demonstrates how workload is elevated with whole-body vibration, without getting affected by backrest angle.

The influence of content, task and sensory interaction on multimedia quality perception.

Human sensory interaction plays an important (but not yet fully understood) role in determining how individuals interact with the world around them. There are numerous types of sensory interaction and this paper examines the interaction of the auditory and visual senses for viewers of multimedia systems. This paper addresses two questions: first, does perception of quality in one modality affect the perception of quality in the other modality and, second, does focusing attention towards one modality affect the viewer's ability to detect errors in the other modality? The perception of audio quality and video quality are closely linked for certain multimedia content. To investigate this relationship, two experiments were conducted where participants were presented with multimedia content where varying distortion had been introduced into both the auditory and visual streams. Participants were asked to state their opinion of the audio, video or overall quality using a standardized scale. Results and subsequent statistical analysis showed that subjective audio quality varied with the video quality and vice versa. Furthermore, when a participant was attending to just one modality, they were less sensitive to reduced quality in the other modality.

Self-reported musculoskeletal problems amongst professional truck drivers.

Occupational driving has often been associated with a high prevalence of back pain. The factors that contribute to cause the pain are diverse and might include prolonged sitting, poor postures, exposure to whole-body vibration and other non-driving factors such as heavy lifting, poor diet or other psychosocial factors. In Europe, truck drivers are likely to be considered an 'at risk' group according to the Physical Agents (Vibration) Directive and therefore risks will need to be reduced. This questionnaire-based study set out to examine the relationship between musculoskeletal problems and possible risk factors for HGV truck drivers to help prioritize action aimed at risk reduction. Truck drivers (n = 192) completed an occupational questionnaire with two measures of vibration exposure (weekly hours and distance driven). Items on manual handling, relevant ergonomics factors and musculoskeletal problems were also included. Reported exposures to vibration ranged from 12 to 85 h per week, with a mean of 43.8 h. Distances driven ranged from 256 to 6400 km (mean 2469 km). Most of the respondents (81%) reported some musculoskeletal pain during the previous 12 months and 60% reported low back pain. Contrary to expectations, vibration exposures were significantly lower among those who suffered musculoskeletal symptoms when distance was used as an exposure measure. Manual handling and subjective ratings of seat discomfort were associated with reported musculoskeletal problems.

Effect of vibration magnitude, vibration spectrum and muscle tension on apparent mass and cross axis transfer functions during whole-body vibration exposure.

Twelve seated male subjects were exposed to 15 vibration conditions to investigate the nature and mechanisms of the non-linearity in biomechanical response. Subjects were exposed to three groups of stimuli: Group A comprised three repeats of random vertical vibration at 0.5, 1.0 and 1.5 ms(-2) r.m.s. with subjects sitting in a relaxed upright posture. Group B used the same vibration stimuli as Group A, but with subjects sitting in a 'tense' posture. Group C used vibration where the vibration spectrum was dominated by either low-frequency motion (2-7 Hz), high-frequency motion (7-20 Hz) or a 1.0 ms(-2) r.m.s. sinusoid at the frequency of the second peak in apparent mass (about 10-14 Hz) added to 0.5 ms(-2) r.m.s. random vibration. In the relaxed posture, frequencies of the primary peak in apparent mass decreased with increased vibration magnitude. In the tense posture, the extent of the non-linearity was reduced. For the low-frequency dominated stimulus, the primary peak frequency was lower than that for the high-frequency dominated stimulus indicating that the frequency of the primary peak in the apparent mass is dominated by the magnitude of the vibration encompassing the peak. Cross-axis transfer functions showed peaks of about 15-20% and 5% of the magnitudes of the peaks in the apparent mass for x- and y-direction transfer functions, respectively, in the relaxed posture. In the tense posture, cross-axis transfer functions reduced in magnitude with increased vibration, likely indicating a reduced fore-aft pitching of the body with increased tension, supporting the hypothesis that pitching contributes to the non-linearity in apparent mass.

The European vibration directive--how will it affect the dental profession?

On 6 July 2005, the EU Physical Agents (Vibration) Directive (2002) came into force across all member states. This will mean that legally enforceable limits on hand-arm vibration exposures will be introduced and that risk management must be set in place at work. This article briefly describes the content of the Directive, how this will affect the dental profession and what measures will be required to ensure compliance.

Symptoms of musculoskeletal disorders in stage rally drivers and co-drivers.

BACKGROUND: During stage rallying, musculoskeletal injuries may be provoked by the high magnitude of vibration and shock to which the driver and co-driver are exposed. Drivers and co-drivers experience similar exposure to whole body mechanical shocks and vibration but different exposure to hand/wrist stressors. OBJECTIVES: To investigate by a questionnaire study the prevalence of symptoms of musculoskeletal injuries after rallying in 13 professional and 105 amateur stage rally competitors. METHODS: The self administered questionnaire investigated whole body and hand/wrist symptoms of musculoskeletal injury. It was loosely based on the Nordic design. RESULTS: 91% of participants who competed or tested for more than 10 days a year (n=90) reported discomfort in at least one body area after rallying. Problems in the lumbar spine (70%), cervical spine (54%), shoulders (47%), and thoracic spine (36%) were the most common. There was a higher prevalence of cervical spine discomfort for co-drivers (62%) than for drivers (46%). Conversely, there was higher prevalence of discomfort in the hands and wrists for drivers (32%) than co-drivers (9%). The prevalence of low back pain in rally participants is higher than that generally reported for workers exposed to whole body vibration. The prevalence of discomfort in the hand and wrist for rally drivers is similar to that previously reported for Formula 1 drivers. CONCLUSIONS: Most stage rally drivers and co-drivers report symptoms of musculoskeletal injury. It is logical to relate the high prevalence of symptoms of injury to the extreme environment of the rally car.

Non-linearities in apparent mass and transmissibility during exposure to whole-body vertical vibration.

The causes of low back pain associated with prolonged exposure to whole-body vibration are not understood. An understanding of non-linearities in the biomechanical responses is required to identify the mechanisms responsible for the dynamic characteristics of the body, to allow for the non-linearities when predicting the influence of seating dynamics, and to predict the adverse effects caused by various magnitudes of vibration. Twelve subjects were exposed to six magnitudes, 0.25-2.5ms(-2) rms, of vertical random vibration in the frequency range 0.2-20Hz. The apparent masses of the subjects were determined together with transmissibilities measured from the seat to various locations on the body surface: the upper and lower abdominal wall, at L3, over the posterior superior iliac spine and the iliac crest. There were significant reductions in resonance frequencies for both the apparent mass and the transmissibilities to the lower abdomen with increases in vibration magnitude. The apparent mass resonance frequency reduced from 5.4-4. 2Hz as the magnitude of the vibration increased from 0.25-2.5ms(-2) rms. Vertical motion of the lumbar spine and pelvis showed resonances at about 4Hz and between 8 and 10Hz. When exposed to vertical vibration, the human body shows appreciable non-linearities in its biodynamic responses. Biodynamic models should be developed to reflect the non-linearity.

Difference thresholds for automobile seat vibration.

Reductions in vehicle vibration that may contribute to improvements in overall vehicle ride could individually be too small to be detected by drivers or passengers. This study investigated the 'difference threshold' (the difference in magnitude between two stimuli which is just sufficient for their difference to be detected) required for a change in vehicle ride to be perceived and whether this was consistent with Weber's Law. Ten male and 10 female subjects sat in a car seat and were exposed to four different reproductions of the vertical vibration recorded on the seat of a car. Three of the stimuli had the same waveform recorded while the car traversed a tarmac surface. This waveform was reproduced using three different magnitudes of vibration at the seat: 0.2, 0.4 and 0.8 m s(-2) r.m.s. (Wb weighted). The other stimulus was recorded with the car traversing a 'pavé' surface that gave a different waveform that was reproduced at a magnitude of 0.4 m s(-2) r.m.s. (Wb weighted). There were significant differences in the absolute difference thresholds measured using the same waveform at the three different magnitudes. When the difference thresholds were expressed in relative terms (the proportion by which two stimuli must differ in magnitude to be discriminated), the relative difference thresholds were approximately 13%, and independent of both the vibration magnitude and the vibration waveform. The results are therefore consistent with Weber's Law. No consistent differences were observed between the responses of male and female subjects.

Models of the apparent mass of the seated human body exposed to horizontal whole-body vibration.

BACKGROUND: Many environments contain vibration with simultaneous vertical and horizontal components. Mathematical lumped parameter models of the mechanical impedance of the seated human body have previously been defined for exposure to vertical vibration. This paper proposes models for the response of the seated body when exposed to horizontal vibration. METHODS: Four target functions were derived from previously reported measurements of the apparent masses of seated subjects exposed to fore-and-aft and lateral vibration at both 0.5 and 1.0 ms(-2) r.m.s. Parameters were optimized for six different three degree-of-freedom models to fit the modulus of the model responses to the four target functions. RESULTS: The modulus and phase of the apparent masses optimized for all combinations of vibration magnitude and direction were close to the responses previously measured and reported in the literature. Fitted parameters for all models with elements in series showed at least one element with a parameter that tended to zero. CONCLUSIONS: Models with three parallel single degree-of-freedom systems with a rigid support generally gave the closest representation of the apparent mass of the seated body exposed to horizontal vibration. More experimental data on the effect of gender, posture and magnitude of vibration on the apparent masses of seated subjects would be useful to enable these models to be improved.

The apparent mass of the human body exposed to non-orthogonal horizontal vibration.

Apparent masses of 15 male and 15 female subjects have been measured during exposure to various directions of horizontal vibration. Twenty vibration conditions were used in the experiment. In each of five directions (0, 22.5, 45, 67.5 and 90 degrees to the mid-sagittal plane) subjects were exposed to random vibration in the frequency range of 1.5-20 Hz at 0.25, 0.5 and 1.0 m s(-2) r.m.s. The five remaining conditions were selected to give measurements whereby the magnitude of the x-component of the vibration was fixed and the gamma-component changed and vice-versa. Two peaks were observed in the apparent masses. The first peak occurred at about 3 Hz and reduced in frequency with increases in vibration magnitude. The frequency of the first peak also reduced as the direction of vibration changed from 0 to 90 degrees. The magnitude of the peak increased as the vibration magnitude and direction increased. The second peak occurred at about 5 Hz and decreased in both frequency and magnitude with increases in vibration magnitude. There was no change in the frequency of the second peak with vibration direction, although the magnitude of the peak decreased as the angle of vibration to the mid-sagittal plane increased. Increasing the magnitude of the x-component of vibration whilst using a fixed y-component changed the magnitude of the first peak but did not change the frequency of the first or any characteristics of the second peak. In contrast, increasing the y-component of vibration whilst using a fixed x-component changed the frequencies and magnitudes of both peaks. Predictions of the response at 45 degrees by applying the principle of superposition to data measured at 0 and 90 degrees showed that the response of the body with direction was not linear. This implies that the apparent mass in non-orthogonal axes cannot be predicted from the apparent masses measured in orthogonal directions.