Stoop or squat: a review of biomechanical studies on lifting technique

Objective. To assess the biomechanical evidence in support of advocating the squat lifting technique as an administrative control to prevent low back pain.

Background. Instruction with respect to lifting technique is commonly employed to prevent low back pain. The squat technique is the most widely advised lifting technique. Intervention studies failed to show health effects of this approach and consequently the rationale behind the advised lifting techniques has been questioned.

Methods. Biomechanical studies comparing the stoop and squat technique were systematically reviewed. The dependent variables used in these studies and the methods by which these were measured or estimated were ranked for validity as indicators of low back load.

Results. Spinal compression as indicated by intra-discal pressure and spinal shrinkage appeared not significantly different between both lifting techniques. Net moments and compression forces based on model estimates were found to be equal or somewhat higher in squat than in stoop lifting. Only when the load could be lifted from a position in between the feet did squat lifting cause lower net moments, although the studies reporting this finding had a marginal validity. Shear force and bending moments acting on the spine appeared lower in squat lifting. Net moments and compression forces during lifting reach magnitudes, that can probably cause injury, whereas shear forces and bending moments remained below injury threshold in both techniques.

Conclusion. The biomechanical literature does not provide support for advocating the squat technique as a means of preventing low back pain.Relevance

Training in lifting technique is widely used in primary and secondary prevention of low back pain, though health effects have not been proven. The present review assesses the biomechanical evidence supporting the most widely advocated lifting technique.     

A biomechanical comparison of lifting techniques between subjects with and without chronic low back pain during freestyle lifting and lowering tasks

OBJECTIVE: To evaluate if chronic low back pain patients perform manual material handling tasks differently from control subjects. DESIGN: Comparative study using a repeated measures design. BACKGROUND: No study evaluated the lifting technique of back pain patients relative to control subjects during free style lifting and lowering tasks. Previous findings suggest that lowering would be more hazardous than lifting to the low back. It would be interesting to evaluate if chronic low back pain patients behave differently than controls when lifting and lowering. METHODS: Thirty-three male subjects (18 controls, 15 suffering from non-specific chronic low back pain) participated. A 12-kg box was lifted (freestyle) from the floor to the hips (1) in front (symmetric task) or (2) to a shelf located at 90 degree on the right (asymmetric task) and was lowered back to the floor. A 3D biomechanical analysis involving the assessment of L5/S1 loading, posture of segments, inertial parameters, and EMG was performed. RESULTS: There was no difference between the groups for postural (trunk and lower limb angles), inertial (trunk velocity and acceleration), and L5/S1 loading (moments and compression) variables. The patients showed abnormally low left lumbar erector spinae (symmetric task, lowering) or high left thoracic erector spinae (all tasks) EMG activation. Significant Group x Action (lifting vs. lowering) interactions were also observed for some inertial and L5/S1 loading variables suggesting that the biomechanical differences detected between lifting and lowering may have a differential influence on the technique used by back pain patients and control subjects. CONCLUSIONS: The gross lifting technique of back pain patients was unaltered relative to controls but the activation of paraspinal muscles differed, suggesting that a more detailed biomechanical analysis, such as the use of EMG driven models, might be required to reveal lumbar impairments during lifting. RELEVANCE: To evaluate if chronic low back pain patients use naturally different lifting techniques to prevent pain exacerbation and damaged lumbar tissue overloading.     

Variation in spinal load and trunk dynamics during repeated lifting exertions.

OBJECTIVES: To quantify the variability in lifting motions, trunk moments, and spinal loads associated with repeated lifting exertions and to identify workplace factors that influence the biomechanical variability.DESIGN: Measurement of trunk dynamics, moments and muscle activities were used as inputs into EMG assisted model of spinal loading.BACKGROUND: Traditional biomechanical models assume repeated performance of a lifting task produces little variability in spinal load because the assessments overlook variability in lifting dynamics and muscle coactivity.METHODS: Five experienced and seven inexperienced manual materials handlers performed 10 repeated lifts at each combination of load weight, task asymmetry and lifting velocity.RESULTS: Box weight, task asymmetry and job experience influenced the magnitude and variability of spinal load during repeated lifting exertions. Surprisingly, experienced subjects demonstrated significantly greater spinal loads and within-subject variability in spinal load than inexperienced subjects. Trial-to-trial variability accounted for 14% of the total variation in compression overall and 32% in lateral shear load. Although the mean spinal load was safely below the NIOSH recommended limit; due to variability about the mean, more than 20% of the lifts exceeded the recommended limit.CONCLUSION: Spinal load changed markedly from one exertion to the next despite identical task requirements. Trial-to-trial variability in kinematics, kinetics, and spinal load were influenced by workplace factors, and may play a role in the risk of low-back pain.RELEVANCE: Ergonomic assessments considering only the mean value of spinal load overlook the fact that a large fraction of the lifts may exceed recommended levels.     

The effect of on-body lift assistive device on the lumbar 3D dynamic moments and EMG during asymmetric freestyle lifting

BACKGROUND: A new on-body personal lift assistive device was developed to reduce force requirements of back muscles during lifting and static holding tasks. METHODS: Nine male subjects participated in the study. Twelve Fastrak sensors were used to record positions and rotations of the segments. Trunk muscles were normalized to maximum and integrated electromyographic amplitudes of the left and right thoracic erector spinae, lumbar erector spinae, external obliques, and rectus abdominalis were compared in asymmetrical lifting for three different loads (5 kg, 15 kg, 25 kg) using free style under two conditions: with and without a lift assistive device. FINDINGS: The assistive device significantly reduced the required muscular effort of the lumbar and thoracic erector spinae (P=0.001) with no significant differences in the level of abdominal muscular activity. Average integrated electromyography amplitudes were reduced across all subjects by 23.9% for lumbar erector spinae, 24.4% for thoracic erector spinae, and 34.9% for the contralateral external oblique muscles. The assistive device had its greatest impact on smaller moments with 30% reduction in lateral bending, and 24% reduction in rotational moments, with only 19.5% a reduction in larger flexion-extension moments. To investigate whether the lift assistive device affected lifting kinematics, the device tensions were zeroed mathematically. No kinematic differences in lifting technique would explain this magnitude of moment reduction. INTERPRETATION: The on-body assistive device reduced the required muscular effort of the lumbar and thoracic erector spinae without adversely affecting the level of abdominal muscle activity. These reductions were mirrored by similar 3D moment reductions.     

Comparison of cumulative low back loads of caregivers when transferring patients using overhead and floor mechanical lifting devices

BACKGROUND: Mechanical lifting devices are recommended as an important intervention for reducing lifting injuries among nursing personnel; previous research suggests that spinal loads are not minimized for all device types. The purpose of this study was to describe the spinal loading pattern while performing a bed to chair transfer comparing Overhead and Floor powered lifting devices. METHODS: A Latin Square design was employed to evaluate five lifting devices while performing a heavy patient transfer. The primary outcomes were spinal compression, and anterior shear (across median and cumulative loading conditions), and ratings of perceived exertion. An inverse dynamic approach was used to calculate the net joint forces and moments about the L5/S1 spinal level. The transfer was partitioned into seven distinct phases for biomechanical analysis. FINDINGS: The proportion of time spent and the mean loads sustained in each phase of the transfer were described. Significant differences in loads were observed between the differing lifting devices, particularly during the transport phases for the Overhead devices. Nurse subjects consistently ranked Overhead lifting devices as most preferred. INTERPRETATION: A large proportion of the time to complete the transfer and cumulative loads occurred during phases that involved only the sling and not the mechanical component of the device. Overhead lifting devices were shown to have lower spinal loads during the transport phases. The results of this study have implications for the use and selection of mechanical lifting devices as part of a strategy to reduce back injuries.     

Balance loss when lifting a heavier-than-expected load: effects of lifting technique.

OBJECTIVES: To compare the lifting techniques of subjects who did and did not maintain their balance with an unexpectedly heavy load, and to examine whether the balance loss increased low back loading. DESIGN: Repeated-measures design. SETTING: A research laboratory. PARTICIPANTS: Fourteen healthy volunteers were assigned to 2 groups in the post hoc analysis. Group 1 (7 men; mean age, 25.6 +/- 4.2yr; height, 1.78 +/-.08m; weight, 83.0 +/- 8.5kg; lifting capacity, 63.2 +/- 8.0kg) maintained balance and was matched to group 2 (7 men; mean age, 26.3 +/- 4.1yr; height, 1.75 +/-.06m; weight, 78.2 +/- 5.3kg; lifting capacity, 64.7 +/- 4.9kg) who lost balance. INTERVENTIONS: Subjects lifted boxes of 5%, 20%, and 35% of their lifting capacity. Load magnitude was expected or unexpected. MAIN OUTCOME MEASURES: Center of mass (COM), lower body mechanics, ground reaction forces, and angular and horizontal momentum. RESULTS: Group 1 consistently showed greater lumbar flexion, less knee flexion, and a higher COM just before and after load liftoff. During the heavier-than-expected 35% lift, the trunk angular velocities lifts indicated that both groups experienced eccentric trunk extensor muscle contractions. CONCLUSIONS: The semisquat technique may protect against balance loss when lifting unexpectedly heavy loads. Eccentric muscle contractions and rapid increases in lumbar joint reaction moments may increase the risk of low back injury when there is a large, unexpected increase in the weight of the lifted load.     

Influence of weight and frequency on thigh and lower-trunk motion during repetitive lifting employing stoop and squat techniques

Changes in kinematics as a function of lifting weight and frequency was investigated in sagittal symmetric repetitive lifting. For every lift cycle (lowering and lifting) the motion range between the upright position (0 degrees ) and the maximum angular displacement of the thigh and lower-trunk body segments was recorded. Ten subjects performed five repetitive lifting bouts with different weight/frequency combinations, using both stoop and squat lifting techniques. In total, 6384 lifts were analysed. The lifting weight or frequency did not influence the motion ranges in stoop lifting. In squat lifting the weight lifted did not appear to have any influence on the motion ranges, while the thigh motion range was significantly smaller at lifting frequency of 20 lifts min(-1) than at a frequency of 10. A significant gradual decrease in the thigh motion range and corresponding increase in the lower-trunk motion range were seen for a majority of the subjects during squat lifting at frequency 20. These changes suggest that quadriceps muscle strength is the limiting factor in repetitive squat lifting. Also the variation in motion ranges was greater in squat lifting than in stoop lifting. RELEVANCE: Forestry work involves frequent lifting. However, compliance in using squat lifting technique, which is recommended for safe lifting, is sometimes poor. Fatigue may be one of the determinants for changes in kinematics and choice of technique in lifting tasks.     

The influence of trunk modelling in 3D biomechanical analysis of simple and complex lifting tasks.

OBJECTIVE: The purpose of this study was to evaluate different methods of estimating the body segment parameters and different methods of partitioning the trunk in order to reduce errors in the inverse dynamic analysis of lifting tasks. DESIGN: The same data set was used to evaluate moment errors associated to five linked models differing by the way the trunk was modelled. BACKGROUND: The inverse dynamic analysis of complex lifting tasks involving significant lower limb displacements requires the use of upper body linked models. However, the estimation of the body segment parameters of trunk segments and the flexible properties of the trunk can lead to errors when using this modelling approach. METHODS: Twenty-one male subjects performed four lifting tasks. Five cameras, two force platforms and a dynamometric box provided inputs to five tridimensional (3D) dynamic linked models. Three modelling parameters of the trunk were tested in these models: (1) the use of a geometric or a proportional anthropometric model to estimate the body segment parameters of the trunk, (2) the location of the antero-posterior position of the centre of mass of the trunk segments (at a percentage of the trunk depth vs on a line between hips and shoulders), and (3) the partitioning of the trunk in two or three segments. The behavior of these linked models was assessed with three different error analyses. RESULTS: The results revealed that all three modelling parameters of the trunk can reduce moment errors, especially when applied to subjects characterized by a larger abdomen. CONCLUSIONS: The inverse dynamic analysis of lifting tasks using an upper body modelling approach should take into consideration the interindividual variability inherent to the trunk morphology and non-rigidity.RELEVANCE: The trunk geometry and flexibility shows considerable variability among individuals. The use of upper body linked models requires adequate modelling of this segment to minimize errors in 3D inverse dynamic analysis of lifting tasks.     

In vivo loads in the lumbar L3–4 disc during a weight lifting extension

Background

Knowledge of in vivo human lumbar loading is critical for understanding the lumbar function and for improving surgical treatments of lumbar pathology. Although numerous experimental measurements and computational simulations have been reported, non-invasive determination of in vivo spinal disc loads is still a challenge in biomedical engineering. The object of the study is to investigate the in vivo human lumbar disc loads using a subject-specific and kinematic driven finite element approach.

Methods

Three dimensional lumbar spine models of three living subjects were created using MR images. Finite element model of the L3–4 disc was built for each subject. The endplate kinematics of the L3–4 segment of each subject during a dynamic weight lifting extension was determined using a dual fluoroscopic imaging technique. The endplate kinematics was used as displacement boundary conditions to calculate the in-vivo disc forces and moments during the weight lifting activity.

Findings

During the weight lifting extension, the L3–4 disc experienced maximum shear load of about 230 N or 0.34 bodyweight at the flexion position and maximum compressive load of 1500 N or 2.28 bodyweight at the upright position. The disc experienced a primary flexion–extension moment during the motion which reached a maximum of 4.2 Nm at upright position with stretched arms holding the weight.

Interpretation

This study provided quantitative data on in vivo disc loading that could help understand intrinsic biomechanics of the spine and improve surgical treatment of pathological discs using fusion or arthroplasty techniques.     

An on-body personal lift augmentation device (PLAD) reduces EMG amplitude of erector spinae during lifting tasks

BACKGROUND: A new on-body personal lift augmentation device was developed to support the back muscles during the repetitive lifting task. METHODS: Nine male subjects participated in the study. Three Fastrak units were used to record positions and rotations of the segments. Trunk muscle normalized and integrated electromyography of the left and right thoracic erector spinae, lumbar erector spinae, external obliques, and rectus abdominis, as well as the kinematic variables of peak lumbar angle, peak pelvis angle, peak trunk acceleration, peak load acceleration were compared in symmetrical lifting for three different loads (5 kg, 15 kg, 25 kg) with three different styles (stooped, squat, free) under two conditions of with and without personal lift assist device. FINDINGS: The lift assist device significantly reduced the required muscular effort of the lumbar (p = 0.001) and thoracic erector spinae with no significant differences in the level of abdominal muscle activity. The amount of integrated electromyography reduction ranged from 14.4% to 27.6% for the lumbar and thoracic erector spinae respectively. Simple measures of trunk posture and accelerations confirmed that there were no differences in lifting technique that would cause the integrated electromyography activity to be reduced. No major kinematic differences were found when the lift assist device was worn indicating that it did not alter these specific technique variables. INTERPRETATION: The lift assist device did reduce the required muscular effort of the lumbar and thoracic erector spinae without adversely affecting the level of abdominal muscle activity. This reduction may help reduce the risk of recurring back injuries or assist in the return to work phase, especially in repetitive tasks.     

Box tilt and knee motions in manual lifting: two differential factors in expert and novice workers

Objective. It was the objective of this study to investigate the kinematic and kinetic effects of two specific handling factors that differentiate expert and novice workers, namely the level of knee flexion and box tilt.

Design. Seven inexperienced subjects were required to lift a 12-kg box in the sagittal plane using three different strategies: (1) reduced knee flexion and a backward box tilt (more typical of experts); (2) large knee flexion and a backward box tilt; (3) large knee flexion and no box tilt (more typical of novices).

Background. The lifting techniques of highly skilled workers differ substantially from those of novices but only limited information is available to compare their biomechanical differences.

Methods. The methods included dynamic segmental analyses to calculate the net moments at all body joints and a planar single-muscle equivalent to estimate compression loadings at L5/S1; total work and joint work distribution were calculated using the integration of joint power. The 3-D kinematic data were acquired with three video cameras and force data were obtained with one AMTI force platform which were synchronized with the film data.

Results. Box tilt reduced the load trajectory and loadings on the lower back and shoulders; a reduced knee flexion affected body posture and reduced mechanical work and loadings on all body joints. The combination of these two factors, box tilt and reduced knee flexion, showed added effects for almost all variables.

Conclusions. These two factors proper to experts were, in this context, biomechanically more advantageous.     

Biomechanical changes in the low back following reduction mammaplasty surgery

Objective. To give evidence of the mechanical consequences of reduction mammaplasty (RM) on the low back.

Design. A repeated-measures analysis was implemented to test the effect of RM on the external loads and angular velocity of the back during both static and dynamic lifting tasks.

Background. Patient follow-up surveys have documented a decrease in the frequency of low back pain following RM, but there is no quantitative data regarding biomechanical changes following surgery.

Methods. Patients were evaluated before and 4–8 weeks following RM. Flexion moment, compression and shear forces at L3-L4 were quantified for isometric flexion angles between 0 and 40 °. External loads and angular velocities of the back were studied during rapid dynamic lifting tasks.

Results. Isometric external flexion moments at L3-L4 decreased following RM. RM did not effect the applied flexion moment in the lumbar spine, but a trend suggested that RM resulted in increased lifting velocity.

Conclusions. RM does act to reduce the loads on the lumbar spine during simple isometric tasks. During dynamic tasks, subjects may be able to lift faster without generating larger loads.     

Load on the shoulder complex during wheelchair propulsion and weight relief lifting

Background

This study focuses on the relationship between overuse in association with wheelchair activities of daily living and risks for osteoarthrosis in the acromioclavicular and sternoclavicular joints. The aim is to quantify the joint moments and joint reaction forces in all three joints of the shoulder complex during wheelchair-related activities of daily living.

Methods

A convenience sample of 17 subjects performed two tasks (wheelchair propulsion and weight relief lifting). Three-dimensional kinematics and kinetics were measured and position and force data were used as input for a musculoskeletal model of the arm and shoulder. Output variables of the model were the moments and the joint reaction forces on the sternoclavicular, acromioclavicular and glenohumeral joints.

Findings

Moments on the sternoclavicular joint were higher than on the acromioclavicular and glenohumeral joint, but the joint reaction forces on the sternoclavicular and acromioclavicular joints were only one third of those on the glenohumeral joint (peak forces around 96 N compared to 315 N for wheelchair propulsion and around 330 N compared to 1288 N for weight relief lifting).

Interpretation

Based on the results found in this study, net joint moments are likely a better measure to describe the load on the acromioclavicular and sternoclavicular joints due to the passive stabilization. Prospective studies on wheelchair overuse injuries should also look at the acromioclavicular and sternoclavicular joints since the load of wheelchair tasks might be a risk factor for osteoarthrosis in these joints.     

Differentiating temporal electromyographic waveforms between those with chronic low back pain and healthy controls

Objectives. Temporal activation patterns from abdominal and lumbar muscles were compared between healthy control subjects and those with chronic low back pain.

Study design. A cross-sectional comparative study.

Background. Synergist and antagonist coactivity has been considered an important neuromuscular control strategy to maintain spinal stability. Differences in onset times and amplitudes have been reported from trunk muscle EMG recordings between healthy subjects and those with low back pain;however, evaluating temporal EMG waveforms should demonstrate whether differences exist in the ability of those with and those without low back pain to respond to changing perturbations.

Methods. The Karhunen–Loève expansion was applied to the ensemble-average EMG profiles recorded from four abdominal and three trunk extensor muscle sites while subjects performed a leg-lifting task aimed at challenging lumbar spine stability. The principal patterns were derived and the weighting coefficients for each pattern were the main dependent variables in a series of two-factor (group and muscle) mixed models.

Results. Three principal patterns explained 96% of the variance in the temporal EMG profiles. The s revealed statistically significant group and muscle main effects (P<0.05) for the principal pattern and significant group by muscle interactions (P<0.05) for patterns two and three. Post hoc analysis showed that patterns were not different among all muscle sites for the healthy controls, but differences were significant for the low back pain group.

Conclusions. The healthy group coactivated all seven sites with the same temporal pattern of activation. The low back pain group used different activation patterns indicative of a lack of synergistic coactivitation among the muscle sites examined.

Relevance

These results provide a foundation for developing a diagnostic classifier of neuromuscular impairment associated with low back pain, that could be used to evaluate the effectiveness of therapeutic interventions to improve muscle coactivation.     

Arm motion and load analysis of sit-to-stand, stand-to-sit, cane walking and lifting

OBJECTIVE: To conduct a pilot study to characterize the hand loads, arm joint angles and external moments corresponding to five activities of daily living demanding of the shoulder, for healthy subjects over 50 years of age. DESIGN: The tasks were sit-to-stand, stand-to-sit, cane walking, lifting a 5 kg box with both hands, and lifting a 10 kg suitcase. BACKGROUND: Arm motion and loading have not been previously studied for functional daily-living tasks involving substantial external loads.Methods. Motion was tracked using an optoelectronic system. Loads were measured using an instrumented chair arm, a force plate, and gravitational and acceleration loads. Six healthy volunteers (3 male, 3 female), mean age 55, with no history of shoulder problems participated in the study. RESULTS: Average peak external moments ranged from 12.3 N m for sitting down into a chair to 27.9 N m for lifting a suitcase. Except for lifting the box, which had much lower loads, average peak hand loads varied from 16% to 19% of body weight (114-134 N). The arcs of motion were larger than for seated activities of daily-living studied previously. CONCLUSIONS: The five tasks studied are commonly performed, yet involve large external moments. Lifting represents the greatest potential loading at the shoulder as it resulted in the highest external moments; furthermore, loads larger than those used in this study might be commonly lifted. RELEVANCE: External moments at the shoulder should not be underestimated, even for activities of daily living.     

Mechanical load on the upper extremity during wheelchair activities

OBJECTIVE: To determine the net moments on the glenohumeral joint and elbow joint during wheelchair activities. DESIGN: Kinematics and external forces were measured during wheelchair activities of daily living (level propulsion, riding on a slope, weight-relief lifting, reaching, negotiating a curb) and processed in an inverse dynamics biomechanic model. SETTING: Biomechanics laboratory. PARTICIPANTS: Five able-bodied subjects, 8 subjects with paraplegia, and 4 subjects with tetraplegia. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURE: Net moments on the glenohumeral joint and elbow joint. RESULTS: Peak shoulder and elbow moments were significantly higher for negotiating a curb and weight-relief lifting than for reaching, level propulsion, and riding on a slope. Overall, the elbow extension moments were significantly lower for subjects with tetraplegia than for those with paraplegia. CONCLUSIONS: The net moments during weight-relief lifting and negotiating a curb were high when compared with wheelchair propulsion tasks. Taking the effect of frequency and duration into account, these loads might imply a considerable risk for joint damage in the long term.     

Principal component analysis of lifting waveforms

BACKGROUND: One limiting factor in lifting research design has been the inability to effectively analyze waveform data, especially when differences in body mass, height, and load magnitude influence the derived kinetic variables. The purpose of this study was to demonstrate the sensitivity of principal component analysis to quantify clinically relevant differences in kinetic lifting waveforms over three load magnitudes and between two separate populations. METHODS: Principal component analysis was applied to five kinetic lifting waveforms. The derived principal component scores were used as the dependent measures in a two-way (clinical status x load magnitude) MANOVA. FINDINGS: Significant low back pain group differences (P<0.05) were found for three of the principal component scores on extension moment generation in the sacral and thoracic regions and for trunk compression. Significant differences were found for each variable with respect to the magnitude across the entire lift time between the three load conditions, as well as four significant differences related to inferred mechanical changes that resulted from lifting increasingly heavier loads. INTERPRETATION: Principal component analysis of kinetic lifting waveforms was shown to be insensitive to a confounding factor of different load magnitudes when attempting to identify previously determined clinically relevant differences in the waveform trajectories. The analysis was able to partition the variability attributed to the direct influence of different external load magnitudes, versus those differences in spinal loading that arose from the variations in the lifting mechanics of increasing loads. The technique could be beneficial for other kinetic analyses where confounding magnitude modifiers like body size are present.     

Reproducibility of intra-abdominal pressure when lifting

Reproducibility of intra-abdominal pressure (IAP) variation was investigated in fifteen young male subjects when lifting three loads, 49, 98 and 147 N, with three speeds of movement, slow, fast and spontaneous, and starting from four standing postures: trunk erect, trunk forward flexed at 30, 60, and 90 degrees of dorsolumbar inclination. Each lift was replicated five times successively and the whole protocol was replicated at one a week interval. The intra-individual variability in IAP response was assessed using the standard deviation of each series of ten lifts. Results showed that variability of peak IAP increased with the load and with the lifting speed. This effect of speed was especially observed for light loads and in forward bending postures. Trunk posture had in itself no significant effect on IAP reproducibility. However, when the variations in the moment acting at lumbar level were taken into account by considering the IAP to moment ratio, the reproducibility of the response was not affected by the load but only by the trunk posture. Reproducibility was lower in the erect posture than in the three flexed positions. It is suggested that this difference relates to the importance in each posture of the moment acting at the shoulder with respect to the total moment acting at lumbar level.     

Lower-limb extensor power and lifting characteristics in disabled elders

Few reports address lifting in disabled elders. Resistance training may facilitate function by improving coordination and muscular recruitment in common lifting tasks. Subjects were considered "functionally limited" if they reported a limitation in at least 1 of 9 possible functional areas listed on the Short-Form Health Survey physical function scale (SF-36), excluding the vigorous activity item. Eighty-nine functionally limited elders (60.3 to 89.8 years old) consented to participate in an intervention trial consisting of a 6-month in-home video-facilitated resistance exercise program using elastic bands. Biomechanical variables (leg extensor power, work, squared jerk), temporal outcomes (lift time and time to peak leg powers), and leg extensor strength were analyzed with the use of analysis of variance (ANOVA) between the (1) experimental group versus control group and the (2) subgroup of the weakest third of subjects (pretest leg extensor strength as percent of body weight [BW]). The experimental group had significant improvements in strength in knee extension (16.7%) and hip extension (20.5%). Resistance-trained weak subjects significantly increased hip extension strength compared to controls. A trend toward improved performance in lifting--decreased total lift time--was noted in the resistance-trained subjects. Significant correlations were found between total leg extension power, total leg extension strength, total work, and lift time. Resistance-trained disabled elders demonstrated strength benefits and several trends consistent with improved coordination and more efficient lifting. Leg-muscle power was related to better functional performance in lifting.