The impact of office chair features on lumbar lordosis,intervertebral joint and sacral tilt angles: a radiographic assessment

Abstract

Background: The purpose of this study was to determine which office chair feature is better at improving spine posture in sitting. Method: Participants (n = 28) were radiographed in standing, maximum flexion and seated in four chair conditions: control, lumbar support, seat pan tilt and backrest with scapular relief. Measures of lumbar lordosis, intervertebral joint angles and sacral tilt were compared between conditions and sex. Results: Sitting consisted of approximately 70% of maximum range of spine flexion. No differences in lumbar flexion were found between the chair features or control. Significantly more anterior pelvic rotation was found with the lumbar support (p = 0.0028) and seat pan tilt (p < 0.0001). Males had significantly more anterior pelvic rotation and extended intervertebral joint angles through L1–L3 in all conditions (p < 0.0001). Conclusion: No one feature was statistically superior with respect to minimising spine flexion, however, seat pan tilt resulted in significantly improved pelvic posture.

Practitioner Summary: Seat pan tilt, and to some extent lumbar supports, appear to improve seated postures. However, sitting, regardless of chair features used, still involves near end range flexion of the spine. This will increase stresses to the spine and could be a potential injury generator during prolonged seated exposures.     

Passive stiffness changes in the lumbar spine and effect of gender during prolonged simulated driving

Gender differences in lumbar and pelvic posture have been reported previously in prolonged sitting, both in an office chair and automobile seat. To date, it is not known whether these postural exposures during prolonged driving affect the passive lumbar spine flexion stiffness. The purpose of this study was to examine time-varying responses of passive lumbar spine stiffness, lumbar spine and pelvic postures during a 2 h simulated driving trial. Secondary goals investigated the influence of gender on lumbar spine stiffness, discomfort scores and seat pressure profiles. Twenty (10 males, 10 females) subjects were recruited to complete a 2 h simulated driving task. Passive lumbar range of motion was measured on a customized frictionless jig before, halfway through and at the end of 2 h. During driving there was a time-varying difference in the lumbar flexion angles adopted by the gender groups. A significant interaction (p = 0.0458) was found for gender and time with women being found to sit significantly different than males in the second hour of driving exhibiting greater maximum lumbar flexion (60.0% ROM (±1.27) than men 50.0% ROM (±1.5). Both men and women demonstrated similar passive stiffness changes characterized by an initial increase in transitional zone stiffness after 1 h (+0.1 Nm/degree for males and +0.3 Nm/degree for females, p = 0.2372). Over 2 h of driving there was a non-significant trend of genders to respond differently to the seated exposure. Specifically transitional zone stiffness was found to increase in males (0.86 (SD 0.31) to 0.92 (SD 0.31) Nm/degree) and decrease in females (0.81 (SD0.88) to 0.73 (SD 0.52) Nm/degree) (p = 0.1178). Differences in lumbar posture and passive stiffness over 2 h of simulated driving were demonstrated between genders in this study.Relevance to industryGender specific ergonomic interventions should be investigated for the automobile seat. Additionally, the changes in passive stiffness induced by prolonged seated exposures could introduce altered low back kinematics in activities performed after a long car ride. Lifting scenarios such as luggage unloading or parcel delivery are common activities immediately after driving. The altered stiffness of the lumbar spine in these activities could have potential ergonomics and injury related implications for both the general population and professional drivers.     

Using sitting as a component of job rotation strategies: Are lifting/lowering kinetics and kinematics altered following prolonged sitting

Workers are often required to perform manual materials handling tasks immediately following periods of prolonged sitting either as a secondary job component of as different tasks in a job rotation strategy. The goal of this investigation was to determine if changes to low-back kinetics and/or kinematics occurred during repetitive lifting/lowering exertions following extended seated exposures. Upper body kinematics, lumbar spine flexion angle, pelvic orientation and bilateral muscle activity from the external abdominal obliques and lumbar erector spinae were recorded for 8 males and 8 females while they alternated between sessions of repetitive lifting/lowering and prolonged sitting. Upper body kinematics were used as inputs to a linked segment model to compute low-back flexion/extension moments, compression, and shear. Peak lumbar flexion was reduced by 1.8° during the lifting/lowering exertions following the first hour of sitting which consequently led to a reduction of approximately 50 N in the reaction anteroposterior shear forces. Sitting postures were consistent with previously reported data. The reduced shear loads during repetitive lift/lower exertions following prolonged sitting may be a consequence of alterations in passive tissue properties which could alter the risk of low-back injury, although future research is required to examine the biomechanical significance of this finding. Changes to both kinematics and kinetics were minimal suggesting that using prolonged sitting as a component of a task series in job rotation does not alter the risk present when combined with repetitive lifting tasks.     

The ratio of thoracic to lumbar compression force is posture dependent

Despite the evidence suggesting that between 8% and 55% of manual labourers experience thoracic pain, research on spinal loading during occupational tasks has been almost invariably limited to the lumbar spine. In this study, we determined the ratio of thoracic to lumbar compression force and the relative risk of injury to each region in various postures. Compressive forces on the spine were calculated based on previously reported thoracic and lumbar intradiscal pressures and disc cross-sectional areas. Flexion postures were associated with an approximate doubling in lumbar compression force but only small increases (or even decreases) in thoracic compression. The ratio of thoracic to lumbar compression was above the tolerance ratio (i.e. the ratio of thoracic to lumbar compressive strength) during upright postures and below the tolerance ratio during flexion postures, indicating that upright postures may pose a greater relative risk of injury to the thoracic spine than to the lumbar spine.

Practitioner summary: Previously reported thoracic and lumbar in vivo disc pressures during various postures were compared. The ratio of thoracic and lumbar compression increased during upright postures and decreased in flexed postures, indicating that upright postures may pose a greater risk of injury to the thoracic spine than to the lumbar spine.     

Low back joint loading and kinematics during standing and unsupported sitting

The aim was to examine lumbar spine kinematics, spinal joint loads and trunk muscle activation patterns during a prolonged (2 h) period of sitting. This information is necessary to assist the ergonomist in designing work where posture variation is possible -- particularly between standing and various styles of sitting. Joint loads were predicted with a highly detailed anatomical biomechanical model (that incorporated 104 muscles, passive ligaments and intervertebral discs), which utilized biological signals of spine posture and muscle electromyograms (EMG) from each trial of each subject. Sitting resulted in significantly higher (p<0.001) low back compressive loads (mean +/- SD 1698 +/- 467 N) than those experienced by the lumbar spine during standing (1076 +/- 243 N). Subjects were equally divided into adopting one of two sitting strategies: a single 'static' or a 'dynamic' multiple posture approach. Within each individual, standing produced a distinctly different spine posture compared with sitting, and standing spine postures did not overlap with flexion postures adopted in sitting when spine postures were averaged across all eight subjects. A rest component (as noted in an amplitude probability distribution function from the EMG) was present for all muscles monitored in both sitting and standing tasks. The upper and lower erector spinae muscle groups exhibited a shifting to higher levels of activation during sitting. There were no clear muscle activation level differences in the individuals who adopted different sitting strategies. Standing appears to be a good rest from sitting given the reduction in passive tissue forces. However, the constant loading with little dynamic movement which characterizes both standing and sitting would provide little rest/change for muscular activation levels or low back loading.     

Comparison of intradiscal pressures and spinal fixator loads for different body positions and exercises

Loading of the spine is still not well understood. The most reliable results seemed to come from the intradiscal pressure measurements from studies by Nachemson, 1966. A new similar study by Wilke et al. (1999) complemented the present study and confirmed some of the earlier data, although it contradicted others. The new data did not confirm that the load on the spine is higher in sitting compared with standing and did not find distinct differences between positions in which subjects were lying down. The objective of this paper was to compare results from two independent in vivo studies (applying different methods) to provide information about spinal loading. In one of these studies (Wilke 1999), intradiscal pressure was measured in one volunteer in different postures and exercises, and in the other study (Rohlmann et al. 1994) the loads on an internal spinal fixation device (an implant for stabilising unstable spines) were determined in 10 patients. The absolute values of the results from both studies were normalized and compared for many body positions and dynamic exercises. The relative differences in intradiscal pressure and flexion bending moments in the fixators corresponded in most cases. Both studies showed slightly lower loads for sitting than for standing and comparatively low loads in all lying positions. High loads were measured for jogging, jumping on a trampoline and skipping. Differences between trends for intradiscal pressure and for flexion bending moments in the fixators were found when the load was predominantly carried by the anterior spinal column, as during flexion of the upper part of the body or when lifting and carrying weights. The combination of the results from these two methods may improve the understanding of the biomechanical behaviour of the lumbar spine and may be used to validate models and theories of spinal loading.     

Predicting the vertebral inclination of the lumbar spine

The objectives of this study were to examine the accuracy of the external stick marker method in the assessment of sagittal plane vertebral inclination (L₁ to S₁) during trunk flexion and to develop regression equations for predicting vertebral inclinations of the lumbar spine. Lateral radiographs of 16 subjects were taken from the upright position to a trunk flexion of 90°, in 30° increments. Each subject was radiographed in only three of the four torso positions to minimize the risks of radiation. The inclinations of the vertebrae in the radiographic view were then obtained. The results show that the stick marker technique is a poor protocol for measuring vertebral inclination of the lumbar spine. During trunk flexion, the upper vertebrae incline linearly and the lower vertebrae incline exponentially. This is verified by the finding that the best-fit equations selected by regression techniques were linear at the upper vertebrae (L₁, L₂ and L₃) and non-linear at the lower ones (L₄, L₅ and S₁), with a mean R ² value of 0.964. The inherent difference in motion pattern between the vertebrae of the lumbar spine during trunk flexion is discussed for clinical and ergonomic purposes.     

Predicting the vertebral inclination of the lumbar spine

The objectives of this study were to examine the accuracy of the external stick marker method in the assessment of sagittal plane vertebral inclination (L1 to S1) during trunk flexion and to develop regression equations for predicting vertebral inclinations of the lumbar spine. Lateral radiographs of 16 subjects were taken from the upright position to a trunk flexion of 90 degrees, in 30 degrees increments. Each subject was radiographed in only three of the four torso positions to minimize the risks of radiation. The inclinations of the vertebrae in the radiographic view were then obtained. The results show that the stick marker technique is a poor protocol for measuring vertebral inclination of the lumbar spine. During trunk flexion, the upper vertebrae incline linearly and the lower vertebrae incline exponentially. This is verified by the finding that the best-fit equations selected by regression techniques were linear at the upper vertebrae (L1, L2 and L3) and non-linear at the lower ones (L4, L5 and S1), with a mean R2 value of 0.964. The inherent difference in motion pattern between the vertebrae of the lumbar spine during trunk flexion is discussed for clinical and ergonomic purposes.     

Effects of seated lumbar extension postures on spinal height and lumbar range of motion during prolonged sitting

Prolonged sitting during sedentary work has been reported as a potential risk factor for low back pain. Furthermore, prolonged sitting can result in both reduced spinal height (SH) and lumbar range of motion (LROM). This study compared the effects of no intervention (control) with two recovery postures on SH and LROM (flexion and extension) during prolonged sitting. Twenty-four participants were randomly assigned to three interventions for three consecutive days. The interventions comprised two seated lumbar extension recovery postures (unsupported sustained and supported dynamic lumbar extension postures) and a control. Both interventions facilitated a relatively short recovery period for both SH and LROM. Supported dynamic lumbar extension conditions significantly helped SH recovery, as compared with control condition, after the first recovery posture intervention, and both postures have potential to maintain LROM. However, both postures failed to induce SH recovery over an extended time.     

Use of visual perception in estimating static postural stresses: magnitudes and sources of errors

Very little is known about the magnitudes and sources of errors associated with the visual estimation of postural classification displayed on TV screens. This study was conducted to address this issue. Sixty-three subjects participated in the experiments. The findings indicate that: (1) subjects found it difficult to evaluate upper extremity postures (particularly the elbow and the wrist), while the postures around the lower back were the easiest to evaluate; (2) the lower extremity positions affected the ability of the subjects to accurately classify postures around the wrist, elbow, shoulder, neck, and lower back, with the estimates being > 70% for sitting and > 60% for standing (except for the elbow); and (3) in general, flexion and extension are easier to evaluate than neutral and non-neutral postures.     

Load moments and myoelectric activity when the cervical spine is held in full flexion and extension

Abstract

Sustained joint load in extreme positions (namely maximally flexed or extended positions) has been described as causing pain. The aim of the present study is to analyse eight different sitting work postures with respect to extreme positions, and to assess the mechanical load and the levels of muscular activity arising in defined extreme positions of the cervical spine. Ten healthy female workers from an electronics plant took part in laboratory experiments. For seven of these, levels of neck and shoulder muscular activity in sitting postures with the cervical spine in different manually-adjusted extreme positions were recorded using surface electrodes. Loading moments of force about the bilateral motion axis of the atlanto-occipital joint (Occ-C1) and the spinal cervico-thoractc motion segments (C7-T1) were calculated. Extreme or almost extreme positions occurred in sitting postures with the thoracolumbar back inclined slightly backwards or with the whole spine flexed. Electromyographic (EMG) activity levels were very low in the manually-adjusted extreme positions. The load moment for the Occ-Cl joint when the whole neck was flexed was only 1·2 times the value for the neutral position of the head, but for C7-T1 it increased to 3·6 times. It is concluded that extreme positions of the cervical spine do occur in sitting work postures, and that the levels of muscular activity in such positions are low. Thus, recordings of muscle activity and calculations of load moment alone are not a sufficient basis for evaluating work postures: thorough recordings of spine positions should be included.     

Hamstring stretching significantly changes the sitting biomechanics

The tightened hamstring connecting the tibia to the pelvis provides more posterior pelvic tilting and consequent backward curvature of the sacral and lumbar vertebrae. Therefore, the study goal was to quantitatively investigate the effect of hamstring and low back stretching on the trunk biomechanics in a system-level perspective. Twelve healthy subjects performed two stretching interventions (hamstring only (HS); hamstring + low back (HLS)) for 40 s on two separate days. They sat on a stool before and after the intervention while capturing trunk kinematics and EMG. In addition, the lumbar flexion angle at which the L4 paraspinals deactivate (i.e., flexion-relaxation phenomenon; FRP) was monitored while trunk flexion-extension trials, performed before and after the protocol. The FRP onset angle was captured to verify the biomechanical changes in the lower extremity and trunk systems. In the results, the stretching intervention significantly increased the reaching distance by 6.3 cm in the sit-and-reach test performed immediately before and after the intervention. The flexible hamstring improved the lumbar flexion angle and head postures in both the HS and HLS. However, the HLS induced laxity in lumbar passive tissues, as confirmed by changes in the FRP, and significantly increased co-activation in the low back. The stress-relaxation of the hamstring and surrounding passive tissues could help to maintain better lumbar flexion angle (i.e., lumbar lordosis) while sitting. Periodic HS for 40 s without any significant lumbar flexion may be recommendable for office workers who sit for long periods.     

Influence of automobile seat lumbar support prominence on spine and pelvic postures: a radiological investigation

Background

The use of lumbar supports has been associated with decreased reports of low back pain during driving exposures. However, there has been limited work investigating whether lumbar supports actually change spine and pelvic postures at the level of the vertebrae.

Purpose

To investigate the effectiveness of a lumbar support in changing radiological measures of lumbar spine and pelvic postures and to examine the impact of support excursion magnitudes on these postures.

Methods

Eight male subjects were recruited with no history of back injury, pathologies or low back pain within the past 6 months. Radiographs were taken in four postures: standing, and sitting with 0 cm, 2 cm and 4 cm lumbar support prominence (LSP).

Results

Lumbar lordosis angle increased from 20° with no support to 25° with 2 cm support and 30° with 4 cm support. Lumbar lordosis angles were significantly different between 0 cm support and 4 cm support (p < 0.0001) and between 2 cm support and 4 cm support (p = 0.0256). Increasing lumbar support reduced the flexion at intervertebral disc joints throughout the lumbar spine, however, these remained significantly different from upright standing (p > 0.001) with the exception of L1/L2 in 4 cm support (p = 0.1381) and L5/S1 for all seated postures (p = 0.0687). All measures of pelvic posture were significantly different in sitting compared to standing (p < 0.0001), however, the lumbar support had no significant impact on seated pelvic posture.

Conclusions

Lumbar supports were shown to impact the vertebral rotations of the lumbar spine yet had no effect on pelvis postures. Increasing support from the current maximum of 2 cm–4 cm resulted in increased lumbar lordosis. The changes were mostly imparted at the upper lumbar spine joints with the most marked change being exhibited at the approximate level of the lumbar support apex: in the L2/L3 joint.     

Comparison of intradiscal pressures and spinal fixator loads for different body positions and exercises.

Loading of the spine is still not well understood. The most reliable results seemed to come from the intradiscal pressure measurements from studies by Nachemson, 1966. A new similar study by Wilke et al. (1999) complemented the present study and confirmed some of the earlier data, although it contradicted others. The new data did not confirm that the load on the spine is higher in sitting compared with standing and did not find distinct differences between positions in which subjects were lying down. The objective of this paper was to compare results from two independent in vivo studies (applying different methods) to provide information about spinal loading. In one of these studies (Wilke 1999), intradiscal pressure was measured in one volunteer in different postures and exercises, and in the other study (Rohlmann et al. 1994) the loads on an internal spinal fixation device (an implant for stabilising unstable spines) were determined in 10 patients. The absolute values of the results from both studies were normalized and compared for many body positions and dynamic exercises. The relative differences in intradiscal pressure and flexion bending moments in the fixators corresponded in most cases. Both studies showed slightly lower loads for sitting than for standing and comparatively low loads in all lying positions. High loads were measured for jogging, jumping on a trampoline and skipping. Differences between trends for intradiscal pressure and for flexion bending moments in the fixators were found when the load was predominantly carried by the anterior spinal column, as during flexion of the upper part of the body or when lifting and carrying weights. The combination of the results from these two methods may improve the understanding of the biomechanical behaviour of the lumbar spine and may be used to validate models and theories of spinal loading.     

About the impact of repetitive spine flexions due to labour on passive mechanics of the lumbar spine

Repetitive flexion of the lumbar spine during labour may have an impact on the development of low back pain, but current evidence is sparse. This study focuses on the effect of palletising on the lumbar spine's passive mechanics (flexion characteristics). As other passive structures begin to creep from long-term loading, changes in lumbar passive flexion characteristics have been hypothesised. The lumbar spine's passive flexion characteristics were investigated in 22 volunteers before and after palletising. For comparison, measurements were performed also during relaxed upright standing and while palletising with breaks for exercise. Measurement of passive flexion characteristics was done by a custom-made machine, and posture of the lumbar spine was captured by a kinematographic device. The torque acting within the body on lumbar level L4/5 was analysed. To exclude active forces by lumbar muscles during measurements, lumbar muscle activation was monitored by surface electromyography. Lumbar spine passive stiffness increased significantly (almost 50%) due to palletising. After relaxed standing and palletising with exercise breaks, this change could not be generally verified. The increased stiffness of passive structures of the lumbar spine provoked by palletising over half an hour suggests a degree of tissue fatigue. Even though it is unclear which passive structures change their passive mechanics and how this happens, this highlights the importance of break management during repetitive flexion of the lumbar spine.     

Dynamic biomechanical modelling of symmetric and asymmetric lifting tasks in restricted postures

This article describes investigations of dynamic biomechanical stresses associated with lifting in stooping and kneeling postures. Twelve subjects volunteered to participate in two lifting experiments each having two levels of posture (stooped or kneeling), two levels of lifting height (350 or 700 mm), and three levels of weight (15,20, or 25 kg). One study examined sagitally symmetric lifting, the other examined an asymmetric task. In each study, subjects lifted and lowered a box every 10 s for a period of 2 min in each treatment combination. Electromyography (EMG) of eight trunk muscles was collected during a specified lift. The EMG data, normalized to maximum extension and flexion exertions in each posture, was used to predict compression and shear forces at the L3 level of the lumbar spine. A comparison of symmetric and asymmetric lifting indicated that the average lumbar compression was greater in sagittal plane tasks; however, both anterior-posterior and lateral shear forces acting on the lumbar spine were increased with asymmetric lifts. Analysis of muscle recruitment indicated that the demands of lifting asymmetrically are shifted to ancillary muscles possessing smaller cross-sectional areas, which may be at greater risk of injury during manual materials handling (MMH) tasks. Model estimates indicated increased compression when kneeling, but increased shear forces when stooping. Increasing box weight and lifting height both significantly increased compressive and shear loading on the lumbar spine. A multivariate analysis of variance (MANOVA) indicated complex muscle recruitment schemes—each treatment combination elicited a unique pattern of muscle recruitment. The results of this investigation will help to evaluate safe loads for lifting in these restricted postures.     

Lumbar posture and individual flexibility influence back muscle flexion-relaxation phenomenon while sitting

Although numerous studies have documented back muscle flexion-relaxation phenomenon (FRP) in standing postures, few studies have examined the FRP in various seated lumbar postures and individual flexibilities. This study, therefore, recruited 18 male students and assigned to low- and high-flexibility groups (9 in each). Activation of thoracic and lumbar erector spinae (ES) and lumbosacral angles were examined while participants sat in two postures (lordosis and kyphosis) and flexed their trunks at 15°, 30°, 45°, 60°, and maximum flexion. Results showed that kyphotic lumbar posture caused relatively low and unchangeable thoracic and lumbar ES activations, whereas lordotic lumbar posture engendered more contractive and varying thoracic and lumbar ES activations. Flexible participants exhibited higher thoracic ES activation than less flexible participants during lordotic sitting. Thoracic ES seemed to play a compensative role to stabilize the spine in the lordotic sitting posture, especially when the trunk was flexed over 45°. In lordotic lumbar posture, FRP occurred only in the lumbar ES; however, the activation and lumbosacral angles were still higher than those in kyphotic posture. The increased back muscle activation associated with lumbar lordosis may partially share the load on passive interspinous tissues, which are close to the discs during these flexed trunk positions.Relevance to industryThis study suggests that various lumbar postures and individual flexibilities may cause different FRP patterns when sitting. While performing seated tasks, people should exercise caution about the lumbar posture.     

Low back joint loading and kinematics during standing and unsupported sitting

The aim was to examine lumbar spine kinematics, spinal joint loads and trunk muscle activation patterns during a prolonged (2 h) period of sitting. This information is necessary to assist the ergonomist in designing work where posture variation is possible—particularly between standing and various styles of sitting. Joint loads were predicted with a highly detailed anatomical biomechanical model (that incorporated 104 muscles, passive ligaments and intervertebral discs), which utilized biological signals of spine posture and muscle electromyograms (EMG) from each trial of each subject. Sitting resulted in significantly higher (p< 0.001) low back compressive loads (mean±SD 1698±467 N) than those experienced by the lumbar spine during standing (1076±243 N). Subjects were equally divided into adopting one of two sitting strategies: a single ‘static’ or a ‘dynamic’ multiple posture approach. Within each individual, standing produced a distinctly diVerent spine posture compared with sitting, and standing spine postures did not overlap with flexion postures adopted in sitting when spine postures were averaged across all eight subjects. A rest component (as noted in an amplitude probability distribution function from the EMG) was present for all muscles monitored in both sitting and standing tasks. The upper and lower erector spinae muscle groups exhibited a shifting to higher levels of activation during sitting. There were no clear muscle activation level diVerences in the individuals who adopted diVerent sitting strategies. Standing appears to be a good rest from sitting given the reduction in passive tissue forces. However, the constant loading with little dynamic movement which characterizes both standing and sitting would provide little rest/change for muscular activation levels or low back loading.     

The load on the lumbar spine during asymmetrical bi-manual materials handling

Previous biomechanical analyses of typical load manipulation tasks were mainly limited to sagittal-plane activities or to static cases. This paper includes the biomechanical determination and assessment of lumbar load during asymmetrical bi-manual materials handling tasks which involve lateral turning of the body, trunk inclination, and sagittal flexion and lateral bending of the spine. Diagonal lifting tasks were analysed for different values for load weight (0-40 kg) and task duration (0·75-1·5 s). Whereas a constant grasp height of 15 cm was assumed, the height for releasing the load differed (50, 100, 150 cm). A dynamic spatial human model (‘The Dortmunder’) was used for calculating the torque in the sagittal, frontal, and transversal planes through the lumbosacral joint and for determining the compressive and the sagittal and lateral shear force at the L5-S1 disc. The trajectories of body segments and load are computer-simulated on the basis of postures adopted during the movement. During diagonal lifting of loads, lumbosacral torque in the sagittal plane is considerably larger than the lateral bending and torsional torque components. Dynamic analyses result in higher maximum values in the lumbar-load time curves than static analyses. The shorter the time for task execution, the higher the resultant dynamic effects and, in consequence, the higher the lumbar load. Lumbosacral compression and shear increase with increasing load-release heights due to higher acceleration and retardation of body and load when the same grasp position and task duration are assumed. The maximum load-bearing capacity of the lumbar spine was determined on the basis of strength data for isolated lumbar segments provided in the literature. The compressive strength falls within the same range as the compressive forces calculated for asymmetrical lifting of loads up to 40 kg. On account of the wide scattering of the compressive strength values, the main influences were determined (age and gender). At an age of 40 years, strength is approx. 6·7 kN for males and 4·7 kN for females (decrease with age per decade: 1·0 kN males, 0·6 kN females). In order to avoid overestimating an individual's lumbar compressive strength, predicted values should be reduced, e.g., by the standard deviation in the male or female samples (2·6 kN or 1·5 kN). Although only a few maximum shear force values are available in the literature, comparison with the calculated values for diagonal lifting leads to the conclusion that sagittal and lateral shear should not be ignored in the assessment of lumbar load during asymmetrical handling tasks.     

Stability of sitting postures: the influence of degrees of freedom

Observational studies of sitting have shown that, during spontaneous sitting, people adopt a variety of postures. Various researchers have formulated theories to explain why people adopt their sitting postures. Branton (1969) hypothesized that there is continual need for postural stability while sitting. Dempster (1955) stated that additional stability could be obtained through temporarily closing chains of body segments, or, in other words, through decreasing the number of degrees of freedom of the body. The present study elaborates on Dempster's theory. The aim of this study was to determine the influence of the degrees of freedom of the body on postural stability in sitting postures. For 21 different sitting postures, the total number of degrees of freedom was determined. Postural sway, a measure for postural stability, was determined using a 3D motion and position measurement system with ten healthy subjects. This study shows that the mean path length at the level of the second thoracic vertebra (PL0.05), a measure derived from postural sway, increases significantly (p < 0.0001) with an increase of the number of degrees of freedom of the body (DoFB). Closer examination of the data showed that a model taking into account only the degrees of freedom of the lumbar and thoracic spine and pelvis seems to be a better predictor of postural sway than the total number of degrees of freedom of the body.