Photograph-based ergonomic evaluations using the Rapid Office Strain Assessment (ROSA).

The Rapid Office Strain Assessment (ROSA) was developed to assess musculoskeletal disorder (MSD) risk factors for computer workstations. This study examined the validity and reliability of remotely conducted, photo-based assessments using ROSA. Twenty-three office workstations were assessed on-site by an ergonomist, and 5 photos were obtained. Photo-based assessments were conducted by three ergonomists. The sensitivity and specificity of the photo-based assessors' ability to correctly classify workstations was 79% and 55%, respectively. The moderate specificity associated with false positive errors committed by the assessors could lead to unnecessary costs to the employer. Error between on-site and photo-based final scores was a considerable ∼2 points on the 10-point ROSA scale (RMSE = 2.3), with a moderate relationship (ρ = 0.33). Interrater reliability ranged from fairly good to excellent (ICC = 0.667-0.856) and was comparable to previous results. Sources of error include the parallax effect, poor estimations of small joint (e.g. hand/wrist) angles, and boundary errors in postural binning. While this method demonstrated potential validity, further improvements should be made with respect to photo-collection and other protocols for remotely-based ROSA assessments.

Less is more: high pass filtering, to remove up to 99% of the surface EMG signal power, improves EMG-based biceps brachii muscle force estimates.

It is generally assumed that raw surface EMG (sEMG) should be high pass filtered with cutoffs of 10-30 Hz to remove motion artifact before subsequent processing to estimate muscle force. The purpose of the current study was to explore the benefits of filtering out much of the raw sEMG signal when attempting to estimate accurate muscle forces. Twenty-five subjects were studied as they performed rapid static, anisotonic contractions of the biceps brachii. Biceps force was estimated (as a percentage of maximum) based on forces recorded at the wrist. An iterative approach was used to process the sEMG from the biceps brachii, using progressively greater high pass cutoff frequencies (20-440 Hz in steps of 30 Hz) with first and sixth order filters, as well as signal whitening, to determine the effects on the accuracy of EMG-based biceps force estimates. The results indicate that removing up to 99% of the raw sEMG signal power resulted in significant and substantial improvements in biceps force estimates. These findings challenge previous assumptions that the raw sEMG signal power between about 20 and 500 Hz should used when estimating muscle force. For the purposes of force prediction, it appears that a much smaller, high band of sEMG frequencies may be associated with force and the remainder of the spectrum has little relevance for force estimation.

The responses of leg and trunk muscles to sudden unloading of the hands: implications for balance and spine stability.

OBJECTIVE: To examine the anticipatory and responsive actions of leg and trunk muscles, and their role in whole-body and spine control in situations of sudden unloading of the hands in the sagittal plane. DESIGN: EMG and force plate measures were used to determine the baseline, anticipatory responses and post unloading responses of selected trunk and leg muscles under different conditions of unload timing knowledge. BACKGROUND: Postural muscles have been observed to increase activation in anticipation of a known loading situation to decrease the overall effect of an impulsive load delivered to the spine. It is thought that this increased activation places the spine in a more stable state, thereby reducing the likelihood of injury. Comparisons have not been made previously of the responses of postural muscles to unloading conditions where the certainty of unload timing is varied. METHODS: Eleven male subjects, holding a 6.8 kg load in the hands, were subjected to three different unloading conditions: (1) voluntary load drop; (2) known timing of load release; (3) unknown timing of load release. Anterior-posterior center of pressure data, as well as EMG activity on 8 right side muscles, were collected for 10 trials in each condition. RESULTS: Anterior-posterior center of pressure responses were significantly different (P<0.05) between each of the three conditions. Lumbar erector spinae and thoracic erector spinae significantly decreased anticipatory activity as knowledge of the unload timing increased. Five of the eight monitored muscles demonstrated significantly decreased response levels as knowledge of the timing of unloading increased. CONCLUSIONS: When an unload is self-triggered, preparatory adjustments can be made which reduce the overall postural perturbation to the body, and the spine in particular, while minimizing the responsive activity of trunk muscles. RELEVANCE: Spinal instability has been identified as a risk factor for low back injury during trunk loading. This study demonstrates that, in situations of sudden unloading, knowledge of the timing of the unloading may lead to anticipatory actions of postural muscles which actually decrease spinal stability, thereby increasing the risk of injury were an unexpected perturbation to occur.

The influence of horizontal velocity on interlimb symmetry in normal walking.

Changes in horizontal velocity (HV) are known to influence many biomechanical characteristics of human locomotion. The purpose of the present study was to investigate this phenomenon with respect to the interlimb symmetry of walking in a normal population. Peak and temporal ground reaction force data from both feet of 20 able-bodied males were collected at each of three relative velocity conditions (slow, normal and fast). These data were analyzed using of a series of 2 x 3 repeated measures ANOVAs, which revealed a high degree of interlimb (bilateral) symmetry across HV conditions despite significant intralimb (unilateral) changes. In contrast to this primary finding were two significant interaction effects for the stance time and peak vertical force at push-off measures respectively. These interactions indicated greater asymmetries at the slow HV condition with a trend toward improved symmetry at higher velocities. Although these results may provide some theoretical insight into the underlying nature of symmetry in gait, their overall magnitude does not seem to invalidate the current widespread use of symmetry assumptions in clinical and research settings today.

Medio-lateral balance adjustments preceding reflexive limb withdrawal are modified by postural demands.

We have recently observed medio-lateral balance adjustments (BA) preceding reflexive stepping elicited by noxious stimulation. While task specific modulation is evident for BA prior to voluntary leg movement, it is unclear whether rapid BA reactions (prior to 'reflexive' stepping) represent a generic response to evoked limb withdrawal or can be modified to suit task-conditions. This study was designed to establish whether the CNS is able to modify rapid onset latency BAs to match task conditions. Reflexive stepping was evoked by applying a noxious stimulus (50 ms stimulus train, 1 ms pulses, 300 Hz, 4 x perceptual threshold) to the plantar surface of the either the left or right foot. Task conditions were varied prior to stimulation by having subjects maintain one of three different static positions: (1) lean left (70% body weight (BW) on left), (2) neutral (50% BW both sides), (3) lean right (70% BW on right). BAs were denoted by centre-of-pressure (CoP) excursions towards the swing foot after the onset of noxious stimulation (average onset latency of 128 ms). There was a significant increase in frequency of occurrence and a significant increase in magnitude of CoP shift when the stimulation was applied to a loaded limb (leaning with 70% BW on the stimulated foot) as compared to an unloaded limb (30% BW). In addition, 78% of loaded trials featured steps taken with the unstimulated foot, which delayed removal of the stimulated foot. Collectively, the results indicate modifiability of the very rapid onset balance adjustments that precede the onset of limb withdrawal revealing complex control of balance exists even over very brief latencies.

The in vivo dynamic response of the human spine to rapid lateral bend perturbation: effects of preload and step input magnitude.

STUDY DESIGN: A repeated measures design was used to determine the effects that combinations of two preloads and two added loads have on spine mechanics both before and during the response to the added load. OBJECTIVE: To investigate the effects of varying initial isometric and added step input load magnitudes on mechanical and electromyographic responses of the trunk during sudden loading that causes lateral bending moments. SUMMARY OF BACKGROUND DATA: Cocontractions of the antagonistic and agonistic muscles of the trunk are required for stability during loading of the spine. In several in vivo studies, it was observed that trunk muscle cocontraction serves a functional role before the application of unexpected or sudden loads. The response of agonistic and antagonistic trunk muscles to rapid lateral bend moments would provide further insight into the dynamic stability mechanisms of the spine. METHODS: In this study, 13 men maintained an upright standing posture while resisting the application of lateral bend moments produced by four different loading conditions comprising combinations of two preloads (5% or 15% of the maximum isometric lateral bend moment) and two added loads (20% or 30%). The preloading was used to develop different initial levels of trunk stiffness before the application of the added loads. The lateral bend moment and angular rotation of the trunk were measured, as well as the surface electromyogram amplitudes of the bilateral internal oblique, external oblique, rectus abdominus, lumbar erector spinae, and thoracic erector spinae muscles. Dependent measures were recorded during the steady state preload conditions, and peak values were recorded after the load was added. RESULTS: Higher added loads resulted in higher peak lateral bend rotations, and higher preloads resulted in lower rotations. The patterns of response were similar for the peak lateral bend moments and the electromyogram amplitudes from four of the five agonistic muscles. The thoracic erector spinae excepted, each of the other four muscles demonstrated larger responses in the agonistic muscles. However, all of the antagonistic muscles showed some increase in electromyogram activity in response to the added load. The thoracic erector spinae appeared to have the role of counteracting the flexor moments created by the abdominal muscles and the maintenance of spine stability. The agonistic external obliques and lumbar erector spinae had the largest responses to the added load. A comparison of the 35% loading conditions showed an increased response of the trunk to the 5% + 30% condition (with lower initial trunk stiffness), as compared with the 15% + 20% condition. CONCLUSIONS: The findings from this study show that higher levels of preactivation can serve to increase spine compression and trunk muscle stiffness, thereby attenuating the lateral displacements caused by rapid loading. Furthermore, antagonistic muscles were observed to respond rapidly to such perturbations with large increases in activation when preactivation and spine stability were low. The trunk muscles monitored all were larger, multisegmental muscles. The results from this study lend support to previous studies suggesting that the larger multisegmental muscles make a significant contribution to spinal stability.

Preparatory balance adjustments precede withdrawal response to noxious stimulation in standing humans.

Self-initiated leg movement in standing humans is preceded by a medio-lateral preparatory balance adjustment (PBA); however, such preparatory balance control is often absent in reflex-like stepping responses evoked by whole-body instability. The presence or absence of the PBA may reflect a task-dependent modulation of the response serving to preserve lateral stability (PBA present) or avoid delay in the lifting of the foot (PBA absent). To examine whether such task-dependent modulation can occur during more stereotypical limb movements, we examined spinally-mediated withdrawal responses evoked by noxious stimulation of the foot. Results showed that rapid limb withdrawal was preceded by a large PBA when subjects were standing but not when they were supine. The PBA caused limb withdrawal to the noxious stimulation to be delayed. However, the onset of the PBA in the standing trials was equivalent in timing to the onset latency of the classic withdrawal responses recorded during the supine trials. Evidence of a preparatory balance adjustment evoked, in advance of a delayed withdrawal response, at very rapid latencies (underlying muscle activation at 70-120 ms) may raise new questions about the neural mechanisms underlying the co-ordination of balance and movement.

The in vivo dynamic response of the spine to perturbations causing rapid flexion: effects of pre-load and step input magnitude.

OBJECTIVE: To evaluate the impact of muscle pre-activation levels and load magnitude on the response of the trunk to loading conditions causing rapid flexion. DESIGN: Eight male subjects were asked to maintain an upright standing posture while resisting the application of forward flexion moments produced by four different loading conditions consisting of combinations of two pre-loads (4% or 16% of the maximum extensor moment) and two added loads (12% or 24%). Pre-loading was used to develop different initial levels of trunk muscle activity prior to the application of the added loads. Of special interest were the two conditions that resulted in total final loads of 28%. BACKGROUND: Cocontraction of the antagonistic and agonistic muscles of the trunk are required to provide stability during normal physiological loading conditions. In several in vivo studies, levels of trunk muscle cocontraction have been observed prior to the application of unexpected or sudden loads. Forces from the abdominal muscles have been proposed to provide stability when extensor moments are generated. The response of trunk muscles to rapid flexor moments would provide further insight into the dynamic stability mechanisms of the spine. METHODS: Measurements were made of the trunk extensor moments, angular displacement of the trunk and unilateral surface EMG amplitudes of three abdominal and three trunk extensor muscles. Values were recorded during the isometric pre-load and for the maximum magnitude of each variable in response to the added load. RESULTS: Higher pre-loads resulted in lower flexion rotations of the spine and higher added loads caused larger rotations. With increasing magnitudes of final loads, a corresponding increase in trunk extensor moments and trunk muscle cocontraction was observed. The largest activations were observed in the lumbar erector spinae and thoracic erector spinae muscles, while smaller yet substantial EMG activity was observed in the internal oblique and external oblique. A comparison of the 28% loading conditions showed an increased response of the trunk to the [4 + 24] loading condition (with lower initial trunk stiffness) when compared to the [16 + 12] loading condition. CONCLUSIONS: Pre-activation of trunk extensor muscles can serve to reduce the flexion displacements caused by rapid loading. The abdominal oblique muscles, especially external oblique, will rapidly increase their activation levels in response to rapid loading. These changes are more pronounced when pre-activation levels are low, resulting in lower initial trunk stiffness and spine compression force. It is proposed that these factors will ultimately affect spine stability and the risk of injury. RELEVANCE: The results of this study provide insight into several mechanisms involved in the dynamic stability of the spine. Injuries can be caused by unexpected and rapid loading of the spine. A study of the mechanisms available to respond to such perturbations is important to an understanding of spine mechanics and the etiology of low back injury.

Trunk muscle co-contraction increases during fatiguing, isometric, lateral bend exertions. Possible implications for spine stability.

STUDY DESIGN: Surface electromyographic activities were recorded from bilateral trunk muscles as test participants maintained a 50% maximum, voluntary, isometric, lateral bend contraction to volitional exhaustion. OBJECTIVES: To challenge the trunk with a prolonged lateral bend task and observe the responses of the agonist and antagonist muscles to the resulting fatigue. SUMMARY OF BACKGROUND DATA: Lateral bend exertions of the trunk have been identified as a risk factor for injury to spine tissues. However, little is known about the response of spine muscles to fatigue and the subsequent implications for spine stability, joint loading, and tissue injury. Surface electromyography provides a window on muscle loading and progressive fatigue. METHODS: Eleven male participants performed a set of maximum lateral bend exertions at the start of the trial, then maintained an upright standing posture while resisting 50% of the maximum moment until volitional exhaustion, then performed another set of maximum contractions. All contractions were isometric. Measurements were made, throughout each contraction, of the lateral bend moment and abdominal and trunk extensor electromyographic activities at six bilateral surface electromyography sites. Electromyographic amplitude and mean power frequency were calculated with 500-millisecond segments recorded serially every 800 milliseconds. Mean values were calculated with data from the first and last 5% of the trial durations. Statistics determined if fatigue had a significant effect on maximum moment and the electromyographic statistics for each muscle site. RESULTS: Fatigue resulted in a significant decrease in maximum lateral bend moment and increase in moment variability. Decreases in mean power frequency, indicating fatigue, were observed in three agonists (the trunk extensors) and one antagonist muscle. Unfatigued agonist electromyographic amplitudes correlated well with the mechanical advantage of muscles to generate lateral bend moments. Unfatigued antagonist activities were low except for the thoracic erector spinae. The agonists and antagonists demonstrated average increases of 17% and 8%, respectively, when pooled across muscles. Much of this change was predicted to have been associated with increases in co-contracting muscle forces. CONCLUSIONS: The trunk responded to a prolonged, lateral bend contraction by increasing co-contraction as agonist trunk muscles fatigued. It was proposed that the fatigue compromised neural coordination and that the co-contraction served to maintain spine stability.

Fatigue-related EMG responses of trunk muscles to a prolonged, isometric twist exertion.

OBJECTIVE: To determine the responses of the trunk muscles to fatigue during sustained, isometric axial torque exertions. DESIGN: Electromyographic (EMG) amplitude and frequency parameters were used to investigate the effects of prolonged contractions on the recruitment patterns of trunk muscles, with special emphasis on those antagonistic and stabilizing muscles not contributing directly to the required axial torque. BACKGROUND: High levels of muscle coactivity have been observed during axial torque generation in the trunk. It has been suggested that this serves to enhance postural stability of the spine during potentially injurious twist exertions. Muscle fatigue decreases force-generating capacity but the resulting effects on muscle recruitment, and the subsequent consequences for postural stability, have not been explored previously during the production of axial torques in the spine. METHODS: Eleven male and 11 female subjects maintained an isometric axial torque exertion to the left (40% of maximum) until volitional exhaustion in an upright standing posture. Maximum axial torques were measured before and after the trial. The average EMG amplitude (AEMG) and mean power frequency (MPF) of seven bilateral trunk muscles (representing agonists, antagonists and trunk stabilizers) were continuously monitored throughout the trial. RESULTS: The average decrease in maximum torque was 18.5% and the average endurance time was 163 +/- 52 s. Evidence of muscle fatigue was provided by significant MPF decreases in 12 of the 14 muscles monitored (P < 0.05) with a overall average decrease of 20.4%. There was a significant increase in AEMG with time for 11 of the 14 muscles monitored (P < 0.05). These increases in activation were linked to increases in muscle force for most of the antagonistic and stabilizing muscles. Gender effects were rarely observed. CONCLUSIONS: Fatigue results in an change in the recruitment patterns of trunk muscles. Muscles serving as antagonists and trunk stabilizers during prolonged axial twist exertions increased their force levels in response to fatigue.

NIOSH equation horizontal distances associated with the Liberty Mutual (Snook) lifting table box widths.

A study was conducted to determine the NIOSH equation horizontal distances (dH) associated with the three different box widths (34, 49 and 75 cm) and lift starting heights (floor, knuckle and shoulder) used in the psychophysically based Liberty Mutual lifting tables (Snook 1978, Snook and Ciriello 1991). Data were collected with 12 male and 12 female subjects and three repetitions were performed for each of the nine lifting conditions. No gender effects were observed so male and female data were pooled. The value of dH was positively related to box width but there was also a significant interaction between box width and starting height. When pooled across lift heights the average values of dH were 44, 49 and 57 cm for the 34, 49 and 75 cm box widths, respectively. When pooled across box widths the average values of dH were 52, 45 and 52 cm for the floor, knuckle and shoulder height lifts, respectively. A knowledge of the dH associated with each box width will allow for direct comparisons to be made between the NIOSH and Liberty Mutual outputs. This will facilitate further validation of the NIOSH equations. A variable (GAP) was calculated to indicate the horizontal distance from the ankles to the edge of the box. Previously, this GAP has been assumed to remain constant and values of 15, 20 and 25 cm have been proposed. The GAP was observed to have an overall mean of 23.6 cm with individual condition means ranging from 14.6 cm to 31.2 cm. When pooled across conditions the mean GAP values were equal to dH minus half the box width.

Effects of muscle kinematics on surface EMG amplitude and frequency during fatiguing dynamic contractions.

Fifteen male subjects performed a repetitive elbow flexion/extension task with a 7-kg mass until exhaustion. Average joint angle, angular velocity, and biceps brachii surface electromyographic (EMG) amplitude (aEMG) and mean power frequency (MPF) were calculated with each consecutive 250-ms segment of data during the entire trial. Data were separated into concentric or eccentric phases and into seven 20 degree-ranges from 0 to 140 degrees of elbow flexion. A regression analysis was used to estimate the rested and fatigued aEMG and MPF values. aEMG values were expressed as a percentage of amplitudes from maximum voluntary contractions (MVC). Under rested dynamic conditions, the average concentric aEMG amplitude was 10% MVC higher than average eccentric values. Rested MPF values were similar for concentric and eccentric phases, although values increased approximately 20 Hz from the most extended to flexed joint angles. Fatigue resulted in an average increase in concentric and eccentric aEMG of 35 and 10% MVC, respectively. The largest concentric aEMG increases (up to 58% MVC) were observed at higher joint velocities, whereas eccentric increases appeared to be related to decreases in velocity. Fatigue had a similar effect on MPF during both concentric and eccentric phases. Larger MPF decreases were observed at shorter muscle lengths such that values within each angle range were very similar by the end of the trial. It was hypothesized that this finding may reflect a biological minimum in conduction velocity before propagation failure occurs.

A validation of techniques using surface EMG signals from dynamic contractions to quantify muscle fatigue during repetitive tasks.

The purpose of the current study was to determine the validity of quantifying biceps brachii fatigue with dynamic measures of surface electromyo-graphic (EMG) mean power frequency (MPF) through comparisons with the well-established isometric methodology. Subjects performed repetitive elbow flexion-extension movements with a hand held load of 7 kg until volitional exhaustion. Elbow joint angle and biceps brachii EMG signals were recorded continuously during the fatiguing movement (in 250-ms segments) and during isometric, isotonic contractions (in 1000-ms segments) performed at a 90 degrees flexion angle before and after the trial. The MPF and average EMG amplitude (AEMG) were also calculated with each sample, and a polynomial regression analysis was used to characterize the time history of changes and to determine the rested and fatigued values for the dynamic EMG with: (a) all dynamic samples above 5% MVC and (b) only samples where the elbow joint was between 80 degrees and 100 degrees of flexion. There was a significant increase in AEMG and a decrease in MPF for the isometric contractions and both dynamic methods. When compared to dynamic values at rest and fatigue, the isometric AEMG and MPF were substantially lower and slightly higher, respectively. No significant differences were observed between the AEMG or MPF results from the two methods of processing the dynamic EMG. The decreases in MPF ranged from 25% to 29% and did not differ between methods. The absolute and relative increases in isometric AEMG were substantially lower than with both dynamic methods. The current results support the use of MPF values from surface EMG signals recorded during dynamic contractions to quantify fatigue of the biceps brachii muscle. The proposed methodology can be used to monitor fatigue continuously throughout a dynamic movement with minimal disturbance to the task being performed and without the need to monitor joint angles.

Mechanically corrected EMG for the continuous estimation of erector spinae muscle loading during repetitive lifting.

Few studies have been carried out on the changes in biomechanical loading on low-back tissues during prolonged lifting. The purpose of this paper was to develop a model for continuously estimating erector spinae muscle loads during repetitive lifting and lowering tasks. The model was based on spine kinematics and bilateral lumbar and thoracic erector spinae electromyogram (EMG) signals and was developed with the data from eight male subjects. Each subject performed a series of isometric contractions to develop extensor moments about the low back. Maximum voluntary contractions (MVCs) were used to normalize all recorded EMG and moment time-histories. Ramp contractions were used to determine the non-linear relationship between extensor moments and EMG amplitudes. In addition, the most appropriate low-pass filter cut-off frequencies were calculated for matching the rectified EMG signals with the moment patterns. The mean low-pass cut-off frequency was 2.7 (0.4) Hz. The accuracy of the non-linear EMG-based estimates of isometric extensor moment were tested with data from a series of six rapid contractions by each subject. The mean error over the duration of these contractions was 9.2 (2.6)% MVC. During prolonged lifting sessions of 20 min and of 2 h, a model was used to calculate changes in muscle length based on monitored spine kinematics. EMG signals were first processed according to the parameters determined from the isometric contractions and then further processed to account for the effects of instantaneous muscle length and velocity. Simple EMG estimates were found to underestimate peak loading by 9.1 (4.0) and 25.7 (11.6)% MVC for eccentric and concentric phases of lifting respectively, when compared to load estimates based on the mechanically corrected EMG. To date, the model has been used to analyze over 5300 lifts.

Quantification of erector spinae muscle fatigue during prolonged, dynamic lifting tasks.

A method was developed to quantify erector spinae fatigue resulting from repetitive dynamic lifting in the sagittal plane. This method was tested with the data from eight male subjects lifting inertial loads of 19 kg and 17 kg during sessions of 20 min and 2 h, respectively. Surface EMG electrodes were applied over sites representing the bilateral lumbar and thoracic erector spinae and external oblique muscles. Maximal and submaximal isometric trunk extensor contractions were performed at the start, intermittently throughout, and at the end of the dynamic lifting trials, within an apparatus designed to control spine posture in the upright standing position. These exertions were used to assess the decreases in strength, endurance and EMG mean power frequency (fw) as well as the increases in EMG amplitude that have been shown to accompany muscle fatigue. The average of the group for extensor strength decreased 17% and 21% (P < 0.05) and the endurance times decreased 60% and 62% (P < 0.01) for the 2-h and the 20-min session, respectively. The average endurance time decreased at least 10% for each subject in each session. Strength decreased at least 10% in all but 2 of 16 cases (both in the 20-min session). The average decreases in fw were 12% (lumbar) and 17% (thoracic; P < 0.05) in the 2-h sessions and 20% (lumbar; P < 0.05) and 14% (thoracic) in the 20-min sessions. There was also a significant increase in EMG amplitude (P < 0.05) for both muscle group in both sessions.(ABSTRACT TRUNCATED AT 250 WORDS)

Trunk muscle and lumbar ligament contributions to dynamic lifts with varying degrees of trunk flexion.

This study was done to assess the interplay between muscular and ligamentous sources of extensor moment during dynamic lifting with various loads and flexion angles of the trunk segment for 15 subjects lifting a total of 150 loads. Ligament forces predicted from an anatomically detailed biomechanical model did not generally contribute more than 60 Nm for most of the lifts because the lumbar spine was only flexed to a moderate and constant degree for each load condition. In contrast, additional moment demands associated with increases in hand load were supported by muscle. Although the compression forces on the L4-5 intervertebral disc were fairly insensitive to the interplay between the recruitment of muscle and ligament, the shear force was significantly higher with a greater degree of lumbar flexion. The risk of injury may be influenced more by the degree of lumbar flexion than the choice of stoop or squat technique.

Reduction in anterior shear forces on the L 4L 5 disc by the lumbar musculature.

The purpose of this study was to assess the possible role of muscles in offsetting the anterior shear forces caused by the load and upper body mass and their accelerations that act on the L 4L 5, intervertebral joint during dynamic squat lifts. Fifteen males lifted five loads from 5.8 to 32.4 kg. Anterior shear forces estimated to be acting on the lumber spine, based on model output, ranged from 492 N at 5.8 kg to 736 N at 32.4 kg. However, the peak shear force that had to be supported by the facets and possibly the disc remained relatively constant at approximately 200 N, regardless of the load mass. The posteriorly directed fascicles of the lumbar portions of the iliocostalis lumborum and longissimus thoracis muscles increased their force output, as estimated from an EMG driven model, in proportion to the anterior load shear force demands, thereby sharing the load on the intervertebral joint. It appears that the combination of anatomical design and neural control of the musculature leads to a situation where the resultant shear force on the joint can be maintained at a relatively constant and safe level in the types of lifts studied. This 'safety' mechanism is useful only with the preservation of lordosis during lifting, when the muscles must provide the majority of the support moment.