Dynamic forces acting on the lumbar spine during manual handling. Can they be estimated using electromyographic techniques alone?

Sports participation plays a relevant role in the relationship between birth weight and bone mineral content in adolescents

The Developmental Origins of Health and Disease hypothesis (DOHaD) proposes that growth during the prenatal period might play a critical role in health, affecting the development of diseases, such as osteoporosis. Bone health is particularly affected by human behaviors when sports participation constitutes the main manifestation of physical exercise. The aim of this study is to analyze the relationship between birth weight (BW) and bone mineral content (BMC) among adolescents, as well as to identify if sports participation and maturity can affect this relationship. The sample was composed of adolescents with ages ranging from 11 to 18 years, stratified according to normal birth weight (n = 331), low birth weight (n = 36), and macrosomia (n = 47), extracted from a wider cross-sectional study (ABCD Growth Study). BW was self-reported by the adolescent's parent. Sports participation was assessed by face-to-face interview. BMC was assessed using dual-energy X-ray absorptiometry. In the multivariate models, the relationships between BW and BMC remained non-significant, while sports participation was significantly related to BMC on lower limbs among boys (r = 0.154; p value = .001) and BMC of upper limbs among girls (r = 0.124; p value = .044). APHV was related to BMC of upper limbs among boys (r = 0.137; p value = .001). In conclusion, BMC was not affected by BW, while this phenomenon seems to be significantly affected by the positive impact of sports participation and maturation on it.

Backstroke-to-Breaststroke Turns Muscular Activity. A Study Conducted in Age Group Swimmers

The aims of this study were to compare surface electromyographic (EMG) activity and kinematic variables among open, somersault, bucket and crossover backstroke-to-breaststroke turning techniques, and identify relationships between the integrated electromyography (iEMG) and kinematics profile focusing on the rotation and push-off efficacy. Following a four-week of systematically increasing contextual interference intervention program, eight 12.38 ± 0.55 years old male swimmers randomly performed twelve repetitions (three in each technique) turns in and out of the wall at maximum speed until the 7.5 m reference mark. Surface EMG values of the right vastus lateralis, biceps femoris, tibialis anterior, gastrocnemius medialis, rectus abdominis, external oblique, erector spinae and latissimus dorsi were recorded and processed using the integrated electromyography (iEMG) and the total integrated electromyography (TiEMG) that was expressed as a percentage of iEMGmax to normalize per unit of time for each rotation and push-off phase. Complementarily, 2D sagittal views from an underwater video camera were digitized to determine rotation and push-off efficacy. The crossover turn presented the highest rotation and push-off iEMG values. Erector spinae and gastrocnemius medialis had the highest activity in the rotation and push-off phases (89 ± 10 and 98 ± 69%, respectively). TiEMG depicted a very high activity of lower limb muscles during push-off activity (222 ± 17 to 247 ± 16%). However, there were no relation between TiEMG and rotation and push-off time, tuck index and final push-off velocity during the rotation and the push-off phases across all the studied turning techniques. The rotation efficacy in age-group swimmers were dependent on rotation time (p = 0.04). The different turning techniques were not distinguishable regarding iEMG activity as a possible determinant of rotation and push-off efficacy. Our study has direct implications for selecting appropriate exercises and designing training programs for optimizing the rotation and push-off phases of backstroke-to-breaststroke turning at young ages.

Why cumulative loading calculated using non-weighted integration may not be suitable for assessing physical stress of the lower back: an empirical investigation of strain during lifting and lowering tasks

When work-related physical stress is assessed using non-weighted integration, it is assumed that different loading conditions have a sufficiently comparable effect on the human body as long as the area under the loading curve is the same. Growing evidence cast doubt on whether this simple calculation can adequately estimate physical work-related strain. This study investigates in vivo, focussing on the lower back, whether the non-weighted method adequately reflects work-related physical strain of the lower back. Strain data resulting from lifting/lowering tasks performed in a laboratory study with an identical area under the loading curve but different load intensities were compared. Results showed that the non-weighted method does not sufficiently reflect the resulting muscular, cardiovascular and perceived strain but underestimates the influence of higher load intensity even in the range of medium physical exposure. Further research is needed regarding the determination of weighting factors and limit values. Practitioner Summary Given the dynamic nature of most physical work activities, the assessment of time-varying loading of the lower back is of particular interest in practice. Results show that the widely used non-weighted calculation method does not accurately reflect the resulting physical strain but underestimates the influence of higher load intensity.Abbreviations: MSD: musculoskeletal disorders; WMSD: work-related musculoskeletal disorders; KIM-LHC: Key Indicator Method Lifting, Holding, Carrying; RES: right erector spinae longissimus; LES: left erector spinae longissimus; HR: heart rate; RPE: rating of perceived exertion; EMG: surface electromyography; ECG: electrocardiography; SENIAM: Surface ElectroMyoGraphy for the Non-Invasive Assessment of Muscles; MVC: maximum voluntary contraction; ANOVA: analysis of variance; Std. error: standard error HIGHLIGHTSResults of this empirical investigation suggest that the widely used non-weighted calculation method is not fully suitable for calculating cumulative loading of the lower back.Even in the range of medium physical exposure the non-weighted calculation method does not accurately reflect the resulting strain on the human body but tends to underestimate the influence of higher load intensity due to higher external weight.Despite the same cumulative loading value obtained when using the non-weighted method, the resulting physical strain values are generally about 20-25% higher.The results may be used to further develop ergonomic assessment methods in order to avoid a misclassification of loading conditions and to prevent the risk of overexertion.

More than Sports Participation: The Role of Ground Reaction Force, Osteocalcin and Lean Soft Tissue on Bone Density Accrual in Adolescents: ABCD Growth Study

The objective of this study was to identify predictors of 12-mo areal bone density accrual in different body segments, lean soft tissue, and osteogenic characteristics attributed to sports participation among adolescent girls and boys. Adolescents (Girls [n = 64], [aged = 14.7]); Boys [n = 129], [aged = 14.6]) were stratified into three groups according to their engagement in different sports (Control [n = 68], Swimming [n = 25], and Weight-bearing sports [n = 100]). Areal bone density (aBMD [g/cm²]) and lean soft tissue (LST) [kg] were measured by dual-energy x-ray absorptiometry (DXA; Lunar DPX-NT; General Electric Healthcare, Little Chalfont, Buckinghamshire, United Kingdom). The ground reaction force (GRF) index attributed to sports participation (Sport-GRF) was created considering the GRF attributed to each sport, body weight of the adolescent, and the amount of time spent in sports participation. Osteocalcin levels (ng/mL) were estimated from a venous blood sample. Multiple regression analysis showed that after adjusting for covariates, the models involving sport-GRF, LST (Δ), and osteocalcin explained 15.8% to 76.2% of the aBMD gains. Specifically in girls, OC was only associated with lower limb aBMD accrual. In boys, however, sport ground reaction forces were positively associated with total spine aBMD accrual. Furthermore, the LST (Δ) was positively associated with aBMD accrual in all body sites (β = 0.003 to 0.011) in both sexes. Increases in LST contributed significantly to gains in aBMD accrual in both sexes, being a more important predictor of changes in bone outcomes than ground reaction forces and osteocalcin.

Effects of workers' Body Mass Index and task conditions on exertion psychophysics during Vertical Handling Tasks

BACKGROUND

Obesity prevalence in the workforce is clearly increasing. Simultaneously, manual lifting/lowering loads, referred to as Vertical Handling Tasks (VHT) in this paper, are common in industries and services. Performing VHT exposes workers to physical overload, which can be measured using a psychophysical approach. Various risk factors can increase this overload, including individual factors such as workers' Body Mass Index (BMI).

OBJECTIVE

To study the possible effects of workers' BMI and some task conditions on physical overload during VHT.

METHODS

Psychophysical data were collected from 51 participants having different body constitutions (including non-obese, overweight and obese). The participants performed 6 VHT (3 different loads ×2 workstation configurations), during which they lifted and lowered a test-box between their knees and shoulders. For each task, they reported their perceived exertion using the Borg Category Ratio-10 (CR-10) scale.

RESULTS

The results showed that the CR-10 scale is sensitive to the variation of the task conditions tested. However, the psychophysical data pointed to a tendency to decrease the perception of physical overload as workers' BMI increases.

CONCLUSIONS

This may compromise the validity of the application of psychophysical data as an ergonomic approach for Work-Related Musculoskeletal Disorders (WRMSD) prevention in obese workers.

Variation in lifting kinematics related to individual intrinsic lumbar curvature: an investigation in healthy adults

Objective

Lifting postures are frequently implicated in back pain. We previously related responses to a static load with intrinsic spine shape, and here we investigate the role of lumbar spine shape in lifting kinematics.

Methods

Thirty healthy adults (18-65 years) performed freestyle, stoop and squat lifts with a weighted box (6-15 kg, self-selected) while being recorded by Vicon motion capture. Internal spine shape was characterised using statistical shape modelling (SSM) from standing mid-sagittal MRIs. Associations were investigated between spine shapes quantified by SSM and peak flexion angles.

Results

Two SSM modes described variations in overall lumbar curvature (mode 1 (M1), 55% variance) and the evenness of curvature distribution (mode 2 (M2), 12% variance). M1 was associated with greater peak pelvis (r=0.38, p=0.04) and smaller knee flexion (r=-0.40, p=0.03) angles; individuals with greater curviness preferred to lift with a stooped lifting posture. This was confirmed by analysis of those individuals with very curvy or very straight spines (|M1|>1 SD). There were no associations between peak flexion angles and mode scores in stoop or squat trials (p>0.05). Peak flexion angles were positively correlated between freestyle and squat trials but not between freestyle and stoop or squat and stoop, indicating that individuals adjusted knee flexion while maintaining their preferred range of lumbar flexion and that 'squatters' adapted better to different techniques than 'stoopers'.

Conclusion

Spinal curvature affects preferred lifting styles, and individuals with curvier spines adapt more easily to different lifting techniques. Lifting tasks may need to be tailored to an individual's lumbar spine shape.

Wearable technology for biomechanics: e-textile or micromechanical sensors?

The possibility of gathering reliable information about movement characteristics during activities of daily living holds particular appeal for researchers. Data such as this could be used to analyze the performance of individuals undergoing rehabilitation and to provide vital information on whether or not there is an improvement during a neurorehabilitation protocol. Wearable devices are particularly promising toward this aim, because they can be used in unstructured environments (e.g., at home). Recently, two different approaches in this area have become very popular and show promising performance: the use of inertial sensors together with advanced algorithms (e.g., Kalman filters) and the development of e-textile, in which the sensing technology is directly embroidered into the garment worn by the user.

Validity of estimates of spinal compression forces obtained from worksite measurements

Estimates of peak spinal compression in manual materials handling were compared between a state-of-the-art laboratory technique and a method applicable at the worksite. Nine experienced masons performed seven simulated tasks in a mock-up in the laboratory and nine matched masons were studied during actual performance of the same tasks at the worksite. From kinematic and kinetic data obtained in the laboratory, compression forces on the L5S1 joint were calculated. In addition, compression forces were estimated from the horizontal and vertical position of the blocks handled relative to the subject measured at the worksite. Comparison of group-averaged values showed that the worksite method underestimated peak compression by about 20%. Rank ordering of tasks for back load was, however, consistent between methods, supporting validity of the worksite method to compare different tasks or to determine the effects of ergonomic interventions with regard to mechanical back load. STATEMENT OF RELEVANCE: This study validated a method that can be used by ergonomists to determine the effects of (characteristics of) manual materials handling tasks on back load at the worksite.

The effect of backpack heaviness on trunk-lower extremity muscle activities and trunk posture

The purpose of the present study is to analyze trunk-lower extremity muscle activities and trunk postural changes during the carriage of different backpacks. Nineteen male university students (21+/-3 years) participated in the experiment's four standing modes: (1) unloaded standing, (2) 10% body weight (BW) load (in the form of a backpack), (3) 15% BW load and (4) 20% BW load. Bilateral rectus abdominis, erector spinae, vastus medialis and biceps femoris muscle activities were recorded using surface electromyography (SEMG), while trunk inclination, side flexion and rotation were measured by using VICON 250 during all standing modes. The results showed that rectus abdominis muscle activities increased progressively and disproportionably as the backpack load increased. As for the trunk posture, almost the same backward inclination was adapted even with increasing backpack heaviness. Twenty percent BW backpack causes the most significant muscular and postural changes so it should be avoided. However, it is recommended to study other backpack factors such as frequency of usage, usage time, type of the backpack and age to come up with a complete usage recommendation.

Computation of trunk equilibrium and stability in free flexion–extension movements at different velocities

Velocity of movement has been suggested as a risk factor for low-back disorders. The effect of changes in velocity during unconstrained flexion-extension movements on muscle activations, spinal loads, base reaction forces and system stability was computed. In vivo measurements of kinematics and ground reaction forces were initially carried out on young asymptomatic subjects. The collected kinematics of three subjects representing maximum, mean and minimum lumbar rotations were subsequently used in the kinematics-driven model to compute results during the entire movements at three different velocities. Estimated spinal loads and muscle forces were significantly larger in fastest pace as compared to slower ones indicating the effect of inertial forces. Spinal stability was improved in larger trunk flexion angles and fastest movement. Partial or full flexion relaxation of global extensor muscles occurred only in slower movements. Some local lumbar muscles, especially in subjects with larger lumbar flexion and at slower paces, also demonstrated flexion relaxation. Results confirmed the crucial role of movement velocity on spinal biomechanics. Predictions also demonstrated the important role on response of the magnitude of peak lumbar rotation and its temporal variation.

Analysis of squat and stoop dynamic liftings: muscle forces and internal spinal loads

Despite the well-recognized role of lifting in back injuries, the relative biomechanical merits of squat versus stoop lifting remain controversial. In vivo kinematics measurements and model studies are combined to estimate trunk muscle forces and internal spinal loads under dynamic squat and stoop lifts with and without load in hands. Measurements were performed on healthy subjects to collect segmental rotations during lifts needed as input data in subsequent model studies. The model accounted for nonlinear properties of the ligamentous spine, wrapping of thoracic extensor muscles to take curved paths in flexion and trunk dynamic characteristics (inertia and damping) while subject to measured kinematics and gravity/external loads. A dynamic kinematics-driven approach was employed accounting for the spinal synergy by simultaneous consideration of passive structures and muscle forces under given posture and loads. Results satisfied kinematics and dynamic equilibrium conditions at all levels and directions. Net moments, muscle forces at different levels, passive (muscle or ligamentous) forces and internal compression/shear forces were larger in stoop lifts than in squat ones. These were due to significantly larger thorax, lumbar and pelvis rotations in stoop lifts. For the relatively slow lifting tasks performed in this study with the lowering and lifting phases each lasting approximately 2 s, the effect of inertia and damping was not, in general, important. Moreover, posterior shift in the position of the external load in stoop lift reaching the same lever arm with respect to the S1 as that in squat lift did not influence the conclusion of this study on the merits of squat lifts over stoop ones. Results, for the tasks considered, advocate squat lifting over stoop lifting as the technique of choice in reducing net moments, muscle forces and internal spinal loads (i.e., moment, compression and shear force).

Partitioning the contributing role of biomechanical, psychosocial, and individual risk factors in the development of spine loads

BACKGROUND CONTEXT

The role of biomechanical workplace factors in spine loading has been well documented. However, our understanding of the role of psychosocial and individual factors in producing spine loads is poorly understood. Even less is understood about the relative contribution of these factors with respect to kinematic, kinetic and muscle activity responses, as well as spine loading.

PURPOSE

To explore the relative contribution of biomechanical and psychosocial workplace factors and individual characteristics on the biomechanical responses and spine loading.

STUDY DESIGN/SETTING

The contribution of various levels of workplace factors to spine loading was monitored under laboratory conditions.

PATIENT SAMPLE

Sixty (30 male and 30 female) college-age individuals who were asymptomatic to low back pain.

OUTCOME MEASURES

Trunk kinematics and kinetics, muscle activity and the three-dimensional spinal loads.

METHODS

The subjects performed lifting tasks while being exposed to varying levels of biomechanical (lift rate, load weight and task asymmetry) and psychosocial (social support and mental concentration) workplace factors as well as an unexplored (load placement) workplace factor.

RESULTS

The workplace job demands that had the largest contribution were load placement (4% to 30%) and load weight (15% to 55%). Mental concentration and social environment had a relatively small contribution to the spinal loads (up to 0.2%). Anthropometry played a large role in the shears (about 12% to 58%) but a relatively minor role in the compressive forces (about 3%).

CONCLUSIONS

Under the given experimental conditions, load weight is the most important factor when controlling compression forces associated with lifting, but other factors, such as individual characteristics, significantly contribute to the shear loads. Thus, one must account for the weight lifted and the anthropometric dimensions when designing the workplace. For the first time, the relative contribution of workplace job demands and individual factors in the development of spine loading have been identified.

The impact of mental processing and pacing on spine loading: 2002 Volvo Award in biomechanics

STUDY DESIGN

The impact of various levels of mental processing and pacing (during lifting) on spine loading was monitored under laboratory conditions.

OBJECTIVES

To explore how mental demands and pacing influence the biomechanical response and subsequent spine loading and, to determine whether individual characteristics have a modifying role in the responses.

SUMMARY OF BACKGROUND DATA

Modern work often requires rapid physical exertions along with demands of mental processing (both psychosocial stressors). While the effect of physical workplace factors on spine loading has been widely documented, few studies have investigated the impact that interaction of psychosocial factors and individual factors has on spine loads.

METHODS

For this study, 60 subjects lifted boxes while completing two types of mental processing tasks: 1) series tasks with decisions occurring before the act of lifting, and 2) simultaneous tasks with decisions occurring concurrently with the lift. For both of these mental processing conditions, two intensities of mental load were evaluated: simple and complex. Task pacing was also adjusted under slow and fast conditions. Finally, individual characteristics (personality and gender) were evaluated as potential modifiers. An electromyographically assisted model evaluated the three-dimensional spine loads under the experimental conditions.

RESULTS

Simultaneous mental processing had the largest impact on the spine loads, with the complex intensity resulting in increases of 160 N with lateral shear, 80 N with anteroposterior shear, and 700 N with compression. Increased task pace produced greater lateral shear (by 20 N), anteroposterior shear (by 60 N), and compression loads (by 410 N). Gender and personality also influenced loadings by as much as 17%.

CONCLUSIONS

Mental processing stress acted as a catalyst for the biomechanical responses, leading to intensified spine loading. Mental stress appeared to occur as a function of time pressures on task performance and resulted in less controlled movements and increases in trunk muscle coactivation. These adjustments significantly increased spine loading. These results suggest a potential mechanism for the increase in low back pain risk resulting from psychosocial stress caused by modern work demands.

An EMG technique for measuring spinal loading during asymmetric lifting

OBJECTIVES

To compare two methods of calibrating the erector spinae electromyographic signal against moment generation in order to predict extensor moments during asymmetric lifting tasks, and to compare the predicted moments with those obtained using a linked-segment model.

METHODS

Eight men lifted loads of 6.7 and 15.7 kg at two speeds, in varying amounts of trunk rotation. For each lift, the following were recorded at 60 Hz; the rectified and averaged surface electromyographic signal, bilaterally at T10 and L3, lumbar curvature using the 3-Space Isotrak, movement of body segments using a 4-camera Vicon system, and ground reaction forces using a Kistler force-plate. Electromyographic (EMG) and Isotrak data were used to calculate lumbosacral extensor moments using the electromyographic model, whereas movement analysis data and ground reaction forces were used to estimate net moments using the linked-segment model. For the electromyographic technique, predictions of extensor moment were based on two different sets of EMG-extensor moment calibrations: one performed in pure sagittal flexion and the other in flexion combined with 45 degrees of trunk rotation.

RESULTS

Extensor moments predicted by the electromyographic technique increased significantly with load and speed of lifting but were not influenced by the method of calibration. These moments were 7-40%greater than the net moments obtained with the linked-segment model, the difference increasing with load and speed.

CONCLUSIONS

The calibration method does not influence extensor moments predicted by the electromyographic technique in asymmetric lifting, suggesting that simple, sagittal-plane calibrations are adequate for this purpose. Differences in predicted moments between the electromyographic technique and linked-segment model may be partly due to different anthropometric assumptions and different amounts of smoothing and filtering in the two models, and partly due to antagonistic muscle forces, the effects of which cannot be measured by linked-segment models. RelevanceAsymmetric lifting is a significant risk factor for occupationally-related low back pain. Improved techniques for measuring spinal loading during such complex lifting tasks may help to identify work practices which place the spine at risk of injury.

Lumbar loading during lifting: a comparative study of three measurement techniques

Low back loading during occupational lifting is thought to be an important causative factor in the development of low back pain. In order to regulate spinal loading in the workplace, it is necessary to measure it accurately. Various methods have been developed to do this, but each has its own limitations, and none can be considered a "gold standard". The purpose of the current study was to compare the results of three contrasting techniques in order to gain insight into possible sources of error to which each is susceptible. The three techniques were a linked segment model (LSM), an electromyographic (EMG)-based model, and a neural network (NN) that used both EMG and inertial sensing techniques. All three techniques were applied simultaneously to calculate spinal loading when eight volunteers performed a total of eight lifts in a laboratory setting. Averaged results showed that, in comparison with the LSM, the EMG technique calculated a 25.5+/-33.4% higher peak torque and the NN technique a 17.3+/-10.5% lower peak torque. Differences between the techniques varied with lifting speed and method of lifting, and could be attributed to differences in anthropometric assumptions, antagonistic muscle activity, damping of transient force peaks by body tissues, and, specific to the NN, underestimation of trunk flexion. The results of the current study urge to reconsider the validity of other models by independent comparisons.

Peak stresses observed in the posterior lateral anulus

STUDY DESIGN

The stress distributions within cadaveric lumbar intervertebral discs were measured for a range of loading conditions.

OBJECTIVES

To examine the distribution of stress across the area of the intervertebral disc and to compare regional variations in peak stress during compression loading with various flexion angles.

SUMMARY OF BACKGROUND DATA

The rate of disc degeneration and the occurrence of low back disorders increase with higher mechanical loading of the spine. The largest peak stresses occur in the anulus.

METHODS

Human lumbar L2--L3 and L4--L5 cadaver functional spinal units were obtained and tested. The distribution of disc stress was measured using a pressure probe with loads applied, pure compression and compression with 5 degrees of either flexion or extension.

RESULTS

Stress profiles were recorded across the intervertebral disc at a compressive force of 1000 N and each of the three flexion-extension angles. The highest values (2.99 +/- 1.31 MPa) were measured during extension-compression lateral to the midline of the disc in the posterior anulus. The pressure in the nucleus was relatively unchanged by flexion angle remaining about 1.00 MPa for a 1000-N compression.

CONCLUSIONS

Pressure measurements of the cadaveric nucleus have been used to validate models of lumbar spine loading and to evaluate the risk of low back injury and disc herniation. Previous observations limited to midsagittal measurements of the nucleus did not identify the regions of highest stress. The highest values observed here within the posterolateral anulus correspond to common sites of disc degeneration and herniation.

The effects of motion on trunk biomechanics

OBJECTIVE

To review the literature that evaluates the influence of trunk motion on trunk strength and structural loading.

BACKGROUND

In recent years, trunk dynamics have been identified as potential risk factors for developing low-back disorders. Consequently, a better understanding of the underlying mechanisms involved in trunk motion is needed.

METHODS

This review summarizes the results of 53 studies that have evaluated trunk motion and its impact on several biomechanical outcome measures. The biomechanical measures consisted of trunk strength, intra-abdominal pressure, muscle activity, imposed trunk moments, and spinal loads. Each of these biomechanical measures was discussed in relation to the existing knowledge within each plane of motion (extension, flexion, lateral flexion, twisting, and asymmetric extension).

RESULTS

Trunk strength was drastically reduced as dynamic motion increased, and males were impacted more than females. Intra-abdominal pressure seemed to only be affected by trunk dynamics at high levels of force. Trunk moments were found to increase monotonically with increased trunk motion. Both agonistic and antagonistic muscle activities were greater as dynamic characteristics increased. As a result, the three-dimensional spinal loads increase significantly for dynamic exertions as compared to isometric conditions.

CONCLUSIONS

Trunk motion has a dramatic affect on the muscle coactivity, which seems to be the underlying source for the decrease strength capability as well as the increased muscle force, IAP, and spinal loads. This review suggests that the ability of the individual to perform a task "safely" might be significantly compromised by the muscle coactivity that accompanies dynamic exertions. It is also important to consider various workplace and individual factors when attempting to reduce the impact of trunk motions during dynamic exertions. Relevance This review provides insight as to why trunk motions are important risk factors to consider when attempting to control low-back disorders in the workplace. It is apparent that trunk motion increases the risk of low-back disorders. To better control low-back disorders in industry, more comprehensive knowledge about the impact of trunk motion is needed. A better understanding of muscle coactivity may ultimately lead to reducing the risk associated with dynamic exertions.

Sudden and unexpected loading generates high forces on the lumbar spine

STUDY DESIGN

A cross-sectional study of spinal loading in healthy volunteers.

OBJECTIVES

To measure the bending and compressive forces acting on the lumbar spine, in a range of postures, when unknown loads are delivered unexpectedly to the hands.

SUMMARY OF BACKGROUND DATA

Epidemiologic studies suggest that sudden and unexpected loading events often lead to back injuries. Such incidents have been shown to increase back muscle activity, but their effects on the compressive force and bending moment acting on the spine have not been fully quantified. Furthermore, previous investigations have focused on the upright posture only.

METHODS

In this study, 12 volunteers each stood on a force plate while weights of 0, 2, 4, and 6 kg (for men, 40% less for women) were delivered into their hands in one of three ways: 1) by the volunteer holding an empty box with handles, into which an unknown weight was dropped; 2) by the same way as in 1, but with volunteer wearing a blindfold and earphones to eliminate sensory cues; or 3) by the volunteer sliding a box of unknown weight off a smooth table. Experiments were carried out with participants standing in upright, partially flexed, and moderately flexed postures. Spinal compression resulting from muscular activity was quantified using electromyographic signals recorded from the back and abdominal muscles. The axial inertial force acting up the long axis of the spine was calculated from the vertical ground reaction force. The bending moment acting on the osteoligamentous spine was quantified by comparing measurements of lumbar curvature with the bending stiffness properties of cadaveric lumbar spines.

RESULTS

The contribution from abdominal muscle contraction to overall spinal compression was small (average, 8%), as was the axial inertial force (average, 2.5%), and both were highest in the upright posture. Peak bending moments were higher in flexed postures, but did not increase much at the moment of load delivery in any posture. Peak spinal compressive forces were increased by 30% to 70% when loads were suddenly and unexpectedly dropped into the box, and by 20% to 30% when they were slid off the table, as compared with loads simply held statically in the same posture (P < 0.001). The removal of audiovisual cues had little effect.

CONCLUSIONS

Sudden and alarming events associated with manual handling cause a reflex overreaction of the back muscles, which substantially increases spine compressive loading. Manual handling regulations should aim to prevent such events and limit the weight of objects to be lifted.