H.M. Toussaint

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.

Validation of a dynamic linked segment model to calculate joint moments in lifting.

A two-dimensional dynamic linked segment model was constructed and applied to a lifting activity. Reactive forces and moments were calculated by an instantaneous approach involving the application of Newtonian mechanics to individual adjacent rigid segments in succession. The analysis started once at the feet and once at a hands/load segment. The model was validated by comparing predicted external forces and moments at the feet or at a hands/load segment to actual values, which were simultaneously measured (ground reaction force at the feet) or assumed to be zero (external moments at feet and hands/load and external forces, beside gravitation, at hands/load). In addition, results of both procedures, in terms of joint moments, including the moment at the intervertebral disc between the fifth lumbar and first sacral vertebra (L5-S1), were compared. A correlation of r = 0.88 between calculated and measured vertical ground reaction forces was found. The calculated external forces and moments at the hands showed only minor deviations from the expected zero level. The moments at L5-S1, calculated starting from feet compared to starting from hands/load, yielded a coefficient of correlation of r = 0.99. However, moments calculated from hands/load were 3.6% (averaged values) and 10.9% (peak values) higher. This difference is assumed to be due mainly to erroneous estimations of the positions of centres of gravity and joint rotation centres. The estimation of the location of L5-S1 rotation axis can affect the results significantly. Despite the numerous studies estimating the load on the low back during lifting on the basis of linked segment models, only a few attempts to validate these models have been made. This study is concerned with the validity of the presented linked segment model. The results support the models validity. Effects of several sources of error threatening the validity are discussed.

Segment inertial parameter evaluation in two anthropometric models by application of a dynamic linked segment model

The estimation of segment inertial parameters (SIPs) is an important source of error in inverse dynamic analysis. In most individual cases SIPs are derived from extrapolation of known SIPs of a certain population through regression equations (proportional models). Another well-known method is the use of mathematical approximation of the shape of human body segments combined with estimations of segment densities (geometric models). In the current study five males and five females performed four different lifting movements in the sagittal plane. A full body linked segment model was applied twice to the same data set, once using a proportional and once using a geometric anthropometric model. As a full body linked segment model is an overdetermined system of equations, four equations could be formed to test the summed effect of SIP errors on the inverse dynamic analysis. The overall performance in terms of coefficients of correlation was better for the geometric model as compared to the proportional model. When a back lifting movement was performed, the equations indicated systematic errors in the proportional model. However, when a leg lifting movement was performed, the equations indicated systematic errors in the geometric model. Therefore, analyzing only one kind of movement does not suffice to draw conclusions with respect to the reliability of an anthropometric model.

Energetics of Competitive Swimming

SummaryAn analysis of the mechanics and energetics of swimming reveals that different factors play key roles in success in competitive swimming events. Knowledge of these performance factors will help the development of optimal training programmes, especially when their relative importance can be identified. One approach to doing this is to evaluate the energy cost of swimming and the energy generating systems that cover these costs.It appears that the rate of energy expenditure is related to the velocity, the gross efficiency, the propelling efficiency and a drag factor. Energy is generated by aerobic and anaerobic processes. A balance should exist between the energy necessary to swim a distance in a certain time and the total energy available in this time from the energy producing system. This balance was used to predict the performance times over difference distances and to predict the effect of a 10% increase in the aerobic capacity, the anaerobic capacity or the propelling efficiency on the performance times, while keeping all other factors constant. The 0% increase in propelling efficiency resulted in both a reduction in time over the short distance as well as an improvement in performance over the long distance which was superior to the gains found when increasing the maximal aerobic or anaerobic power by 10%.It is concluded that for an optimal use of training time and for an optimal use of the capacities of the swimmer, it seems important to determine both the mechanical parameters (technique, drag) and the parameters describing the energy production. By determining the weak and strong points of competitive swimmers, the optimal training distances and what performance factors are the weakest and most likely to improve with training can be determined.

JOINT MOMENTS AND MUSCLE ACTIVITY IN THE LOWER EXTREMITIES AND LOWER BACK IN LIFTING AND LOWERING TASKS

The mechanical loading on the body during the act of lifting has been estimated frequently. The opposite act of lowering has received much less attention. The aim of the present study was to compare the mechanical loading of the musculoskeletal system in lifting and lowering. Eight subjects repetitively lifted and lowered a load, using two different techniques (a leg and a back technique). The ankle, knee, hip and lumbosacral joint moments were estimated and the myoelectrical (EMG) activity of seven (leg and back) muscles was recorded. The differences between the lifting and lowering phase for the leg technique were similar to those observed when the back technique was applied. The joint moment curves in lifting showed a high level of agreement with the (time-reversed) moment curves in lowering. Peak moments in lowering were only slightly lower than in lifting (peak lumbar moments were 5.4% lower). These small differences were related to different acceleration profiles at the centre of gravity of the body/load complex. The EMG activity was considerably lower in lowering than in lifting. The mean EMG in lowering (average for seven muscles) was only about 69% of the EMG in lifting. This was attributed to the different types of muscle actions involved in lifting (mainly concentric) and lowering (mainly eccentric). Furthermore, the EMG results suggest that similar inter-muscular coordination is involved in lowering and lifting. The results give rise to the assumption that in lifting and lowering similar muscle forces are produced to meet the (nearly) equal joint moments, but in lowering these forces are distributed over a smaller cross-sectional area of active muscle, which might imply a higher risk of injury.

Trunk extensor endurance and its relationship to electromyogram parameters

SummaryThe present study was designed to investigate the relationship between muscle performance and electromyogram (EMG) parameters of the trunk extensor muscles in the development of fatigue. Nine subjects performed continuous isometric trunk extensions at 25% and 40% maximal voluntary contraction. The EMG signals of the longissimus thoracis, iliocostalis lumborum, multifidus and latissimus dorsi muscles were recorded. The EMG amplitude (RA-EMG) appeared to increase consistently during the contractions in all muscles, whereas the mean power frequency (MPF) showed a fairly consistent decrease during the contractions. The time constants of the exponential change of the RA-EMG and of the MPF were related to the endurance time. The prediction of endurance based on both EMG parameters appeared to yield better results than the prediction based on the relative force. In particular the time constants of the MPF changes of the multifidus and longissimus muscles appeared to be good predictors of endurance time. The consistency of the spectrum shift of EMG appeared to coincide with a reduced variability of the activation of the muscle involved.

The evaluation of a practical biomechanical model estimating lumbar moments in occupational activities

To estimate the mechanical load on the low back in manual materials handling, the Static Strength Prediction Model (SSPM, University of Michigan) is widely used in the occupational field. It requires (for practical reasons) only a small number of input variables (five body segment angles, standing height, total body mass, external load on the hands) on which basis the moment at the lumbo-sacral intervertebral joint (beside other parameters) is computed. The dynamic character of the activities is ignored in the calculations. To evaluate the validity of the SSPM in various situations, lumbar moments in lifting/lowering activities at different lifting techniques and speeds obtained by the SSPM, were compared with those obtained by a more comprehensive dynamic model (DM). An analysis of variance showed significant effects (p = 0.001) of the biomechanical model applied and the lifting speed used on the peak lumbar moment values. No effects of lifting technique were found. The differences in results from the SSPM and DM were dependent on the lifting speed: the SSPM peak lumbar moments were on average 9% (not significant), 21% (significant at p = 0.005) and 42% (p = 0.0001) smaller compared to the DM moments in the slow (mean velocity in a complete lifting/lowering cycle, 0.2 m s-1), normal (0.4 m s-1) and fast (0.8 m s-1) speed condition respectively. The results indicate that the static/dynamic difference between the models is a major source for the different lumbar moments, while other differences between the SSPM and DM are of minor importance.

Load knowledge affects low-back loading and control of balance in lifting tasks

This study investigated the effect of the presence or absence of load knowledge on the low-back loading and the control of balance in lifting tasks. Low-back loading was quantified by the net sagittal plane torque at the lumbo-sacral joint. The control of balance was studied by the position of the centre of gravity relative to the base of support, the horizontal and vertical momentum of the centre of gravity and the angular momentum of the whole body. In a first experiment, 8 male subjects lifted a rather heavy load (22% of body mass), using a leglift and a backlift, while they were familiar with the load mass. To counteract the threat to balance, imposed by picking up a load in front of the body, the subjects performed specific preparations, based upon the known load mass; prior to load pick-up, profound changes in the horizontal and angular momentum were found. The preparations were technique specific. Preserving balance seemed easier while picking up a load with a backlift than with a leglift. In the second experiment, 25 male subjects lifted a 6 kg box, which they expected to be 16 kg, because, in a series of lifts, the load mass was changed from 16 to 6 kg without their knowledge. Despite the 10 kg difference in actual load mass, the net torque at the lumbo-sacral joint was not different between lifting 6 and 16 kg, until 150 ms after box lift-off. Moreover, lifting of the overestimated load mass caused a disturbance of balance in 92% of the trials. The postural reactions aimed at regaining balance were not accompanied by an increased low-back loading. It was concluded that the absence of load knowledge, and the following overestimation of the load mass to be lifted, lead to an increased mechanical load on the lumbar spine and to an increased risk of losing balance in lifting tasks. Both events may contribute to a higher risk of low-back injury in manual materials handling tasks.

Flexion relaxation during lifting: Implications for torque production by muscle activity and tissue strain at the lumbo-sacral joint

During the full flexion phase of the back lift movement the lumbar part of the erector spinae muscle exhibits a reduced activity level (flexion relaxation). This study addresses the question how the required extension torque in the lumbo-sacral joint (L5/S1 joint) is balanced during the period in which apparently the lumbar erector spinae ceases to take its share. Six subjects participated in the experiment in which they performed seven lifting tasks. The load, the range of movement, and the phase in which the load was handled (lifting or lowering) were varied. A dynamic linked segment model was applied to determine the momentary torques acting at the L5/S1 joint, while the EMGs of the lumbar and thoracic part of the erector spinae muscle were measured. Furthermore, the lengths between markers on the lumbar and thoracic part of the trunk were determined to reveal changes in length during the movement. The dynamic EMGs were normalized to trunk angle-dependent maximal levels. The L5/S1 joint torques were analysed and combined with the normalized EMG data and the kinematics of the trunk, which are assumed to indicate the elongation of passive tissues. Although in the normalization procedure the change of the length-force relationship of the erector spinae was taken into account, the dynamic lumbar EMG activity decreased to a low-activity level (the phenomenon of flexion relaxation). This coincided with a 25% increase in lumbar length suggesting that passive tissue strain provided part of the required extension torque. In the tasks where a barbell was handled a significant increase in EMG level of the thoracic part of the erector spinae occurred just before the flexion relaxation at the lumbar level. Apparently, the extensor function of the lumbar part is then taken over by the thoracic part of the erector spinae muscle. This suggests that an intricate coordinating mechanism is operative that apportions the load to be balanced over active--(lumbar and thoracic part of the erector spinae) and passive structures (post vertebral ligaments).

An investigation into the relevance of the pattern of temporal activation with respect to erector spinae muscle endurance.

SummaryThe aim of the present study was to evaluate the viability of a relationship between the temporal activation pattern of parts of the erector spinae muscle and endurance. Seven subjects performed intermittent isometric contractions [4 s at 7007o maximal voluntary contraction (MVC), 2 s rest] until exhaustion, during which the electromyographical (EMG) activity of the multifidus, iliocostalis thoracis and longissimus muscle segments was recorded. Endurance was defined as the time until exhaustion. Subjects were divided into a high and a low endurance group. The high endurance group showed significantly more variability of EMG amplitude over succeeding contractions. This group demonstrated significantly more alternations of EMG activity between parts of the muscle also. Variability of the EMG amplitude within the contractions did not differ between the groups, nor did MVC. The results indicated that alternating activity between different parts of the erector spinae muscle may function to postpone exhaustion of this muscle as a whole.