Postural and trunk muscle response to sudden release during stoop lifting tasks before and after fatigue of the trunk erector muscles

The effect of fatigue on the muscular and postural response to sudden release of different stoop lifting loads was studied. Ten male volunteers performed a series of stoop lifting trials before and after fatigue of the erector spinae. Trials were performed using loads of 20, 40, 60, and 80 N, and sudden release of load was triggered randomly on one of the repetitions using an electromagnetic release. The onset of release was registered by an accelerometer, centre of pressure (COP) motion was recorded via a forceplate, and EMG activities of the latissimus dorsi (LD), erector spinae (ES), rectus abdominus (RA), external oblique (EO) and internal oblique (IO) muscles were recorded. A slightly reduced lifting speed was seen after fatigue, particularly at the higher loads, but this had little effect on the perturbing force at release, which was dominated by the release load. A significant effect of fatigue was seen on the antero-posterior COP motion, with the postural disturbance being decreased after fatigue. Fatigue resulted in a significant increase in ES (p = 0.029) and LD (p = 0.015) relaxation times and, while the response patterns (relaxation, contraction or no response) of the anterior trunk muscles (RA, EO, IO) were not always consistent, the proportion of response by relaxation was greater after fatigue. This resulted in a lower incidence but longer duration of co-contraction of the ES-RA, ES-EO and ES-EO muscle groups following fatigue, such that the mean co-contraction duration of these groups showed no significant differences before and after fatigue. The response to sudden release is a balance between maintaining postural stability and at the same time preventing the trunk musculature from overloading the spine and risking tissue injury. While fatigue of the trunk extensors does not appear to increase either the risk of fall or stumble or the incidence of co-contraction following sudden release of stoop lifting tasks, the duration of co-contraction appears to increase following fatigue. Further study is required to quantify the loading on the spine during sudden release of different lifting tasks before and after more realistic fatigue conditions.     

The effects of release height on center of pressure and trunk muscle response following sudden release of stoop lifting tasks

Background and objectives: Sudden release of load during lifting threatens postural stability and is countered by trunk muscle response, which can generate high loads on the spine, and may be a cause of tissue injury. The postural threat following sudden release and the consequent muscular response are likely to depend on the posture at the time of release. This study investigates the effects of sudden release of load at two release heights of one- and three-quarters of the knee to shoulder distance during stoop lifting.

Methods: Ten normal southern Chinese male volunteers were subject to sudden release of 20, 40, 60 and 80 N loads during stoop lifting trials. The release was randomly selected to be on the third, fourth or fifth cycle of a trial and was triggered at heights of one- and three-quarters of the total knee to shoulder lifting distance. The subjects stood on a force platform to allow the postural disturbance to be recorded by monitoring the center of pressure (COP), and electromyographic (EMG) data were collected from the rectus abdominus, internal oblique, external oblique, erector spinae and latissimus dorsi muscle groups.

Results: The COP excursion moved closer to the posterior limit of stability with increasing release load, and this effect was significantly more marked for release from the lower of the two heights. The minimum posterior COP separation from the posterior limit of stability was significantly less for the lower release height at all loads (p<0.001 in all cases). EMG data showed that the agonist–antagonist co-contraction durations were higher for the lower release height, and unlike sudden release from the higher level, showed a significant increase with increasing load.

Conclusions: Sudden release at lower release height during stoop lifting results in significantly greater postural disturbance and spinal loading. The mean load predicted to result in fall or stumble at the lower release height (133 N) is significantly less than that predicted at the higher of the two release heights (245 N). A more marked effect of release load is also seen in the postural disturbance and trunk muscle co-contraction time for the lower release height, and particular care should therefore be taken when handling potentially unstable loads under these conditions. If the security of the load cannot be guaranteed, storage at a higher level may reduce the risk of injury due to sudden release of the load.     

Postural perturbation and muscular response following sudden release during symmetric squat and stoop lifting

Squat and stoop lifting have been examined in some detail, but limited data exist regarding the sudden release of load during such lifting. Ten participants performed squat and stoop lifting trials with loads of 20, 40, 60 and 80N, and sudden release was randomly included in one of the lifting cycles. Postural perturbation was recorded via centre of pressure displacement using a force platform and the electromyographic response of trunk and lower limb muscles was recorded.

Results indicated that irrespective of lifting posture, an ‘ankle’ response strategy to sudden release was elicited, where the anterior muscles of the lower limb contracted first, followed by the anterior trunk muscles, relaxation of the posterior trunk muscles and, finally, relaxation of the posterior lower limb muscles. The latency of muscles responding by contraction tended to decrease slightly with increasing load for both postures, while the latency of muscles responding by relaxation increased, resulting in increased trunk muscle co-contraction durations. The postural disturbance appeared to be greater for squat lifting than stoop lifting at the higher loads of 60 and 80N, as the centre of pressure moves significantly closer to the posterior limit of static stability (the line joining the heels).

In terms of stability and muscular response, squat lifting may not be the most appropriate strategy if a sudden release of loads greater than approximately 50N is likely.     

Muscular and centre of pressure response to sudden release of load in symmetric and asymmetric stoop lifting tasks

Sudden changes in load during asymmetric lifting may be associated with a particularly high risk of loss of balance and spinal injury. Centre of pressure (COP) motions and electromyographic responses of trunk and lower limb muscles were studied in 10 normal male volunteers during sudden release of 20, 40, 60 and 80N stoop lifting loads in symmetric and asymmetric postures. Similar overall COP responses and muscular response strategies to sudden release of load were seen in both postures, although the asymmetric posture showed a larger medio-lateral COP displacements and greater co-contraction asymmetries. While sudden release of load in asymmetric stoop lifting does not seem to involve a greater risk of fall than symmetric lifting, the muscular response results in more complex and asymmetric loading of the trunk, indicating greater localised segmental loading and therefore increased risk of tissue injury.     

Postural perturbation and muscular response following sudden release during symmetric squat and stoop lifting

Squat and stoop lifting have been examined in some detail, but limited data exist regarding the sudden release of load during such lifting. Ten participants performed squat and stoop lifting trials with loads of 20, 40, 60 and 80N, and sudden release was randomly included in one of the lifting cycles. Postural perturbation was recorded via centre of pressure displacement using a force platform and the electromyographic response of trunk and lower limb muscles was recorded.Results indicated that irrespective of lifting posture, an 'ankle' response strategy to sudden release was elicited, where the anterior muscles of the lower limb contracted first, followed by the anterior trunk muscles, relaxation of the posterior trunk muscles and, finally, relaxation of the posterior lower limb muscles. The latency of muscles responding by contraction tended to decrease slightly with increasing load for both postures, while the latency of muscles responding by relaxation increased, resulting in increased trunk muscle co-contraction durations. The postural disturbance appeared to be greater for squat lifting than stoop lifting at the higher loads of 60 and 80N, as the centre of pressure moves significantly closer to the posterior limit of static stability (the line joining the heels).In terms of stability and muscular response, squat lifting may not be the most appropriate strategy if a sudden release of loads greater than approximately 50N is likely.     

Effect of lifting belts on trunk muscle activation during a suddenly applied load

The National Institute for Occupational Safety and Health suggests there is insufficient biomechanical or epidemiological evidence to recommend the use of back belts in industry. From a biomechanical perspective, previous work suggests that lifting belts stiffen the torso, particularly in the frontal and transverse planes. To determine whether lifting belts stiffen the torso and alter the trunk muscle response during a sudden loading event, we tested the hypotheses that (a) lifting belts alter peak muscle activity recorded with electromyography (EMG) during sudden loading and (b) lifting belts have a larger impact on trunk muscle response when sudden loads are applied asymmetric to the torso's midsagittal plane. A sudden load was delivered to 10 men and 10 women without history of low back disorder via a cable attached to a thoracic harness; motion was restricted to the lumbar spine. Results indicate that gender was not a significant factor in this study. The lifting belt reduced the peak normalized EMG of the erector spinae muscles on average by 3% during asymmetric loading, though peak normalized EMG was increased by 2% during symmetric loading. Lifting belts have been shown to slightly reduce peak erector spinae activity during asymmetric sudden loading events in a constrained paradigm; however, the effects of lifting belts are too small to provide effective protection of workers. Actual or potential applications include the assessment of lifting belts as protective devices in workers based on the effects of lifting belts on the trunk muscle activity.     

Quantitative trunk muscle electromyography during lifting at different speeds

This study was designed to investigate the effects of trunk motion under lifting conditions described by the Work Practices Guide for Manual Lifting (NIOSH, 1981). Eight male volunteers were used as subjects in this study. Three independent variables; lift style, load location and subjective lift velocity, were controlled under sagittally symmetric lifting conditions. Dependent variables consisted of trunk muscle electromyographic (EMG) activity, actual trunk velocity and load acceleration. There was no effect of lift style. However, as the trunk velocity increased, EMG activity increased within the lastissimus dorsi and rectus abdominus muscles but not within the erector spinae muscles. The erector spinae muscles, unlike the other muscles, was also unaffected by load location and load acceleration. These findings suggest ways in which lifting guides should be adjusted to account for the effects of dynamic motion.     

The effect of stance width on trunk kinematics and trunk kinetics during sagitally symmetric lifting

Lifting technique can have a significant impact on spine loading during lifting. The sports biomechanics literature has documented changes in trunk and lower extremity kinematics and muscle coactivation patterns as a function of stance width during high force dead lift and squat exercises. The focus of the current study was to explore whether these lifting stance width effects might translate into the occupational setting under more moderate load level conditions. Twelve subjects performed repetitions of a sagittally symmetric lifting and lowering task (10 kg load) under three stance width conditions: narrow (feet together), moderate (feet shoulder width) and wide (feet 150% of shoulder width). As they performed these exertions, trunk kinematics were captured using the lumbar motion monitor while the activity of the trunk muscles (erector spinae, rectus abdominis) and lower extremity muscles (gluteus maximus, vastus lateralis and vastus medialis) were evaluated using normalized electromyography. The results showed that both the range of motion and peak acceleration in the sagittal plane were significantly affected by the stance width. The muscle activation levels, however, were not significantly affected by the stance width. These results collectively would indicate that the stance width effects seen in power lifting activities do not translate well into the occupational environment where more moderate loads are typically lifted.

Relevance to industry

Exploring alternative lifting strategies may provide an opportunity to reduce the incidence of low back disorders. Lifting stance width is one variable that has not been explored in the ergonomics literature.     

A study by EMG stick diagrams of the muscular activities in the trunk flexion and extension movement

The activity of the leg and abdominal muscles in trunk flexion and extension was investigated with reference to the sudden decreases and increases of the erectores spinae activity. The movement was performed under conditions with and without an additional load, and with and without fatigue. Surface EMGs were recorded from the erectores spinae, the gluteus maximus, the semitendinosus, the rectus abdominis and the external oblique. The pattern of activity was analysed using EMG stick diagrams. Under the condition without fatigue, the semitendinosus activity increased during the sudden changes of the erectores spinae activity, but the abdominal muscles were not activated during the movement. However, the rectus abdominis was activated whenever the semitendinosus activity did not increase during the changes of the erectores spinae activity. Under the condition of fatigue, the leg muscle was vigorously active during the movement, and the abdominal muscles were activated before and after the erectores spinae activity changed suddenly. The results suggest that the leg muscle plays some important part during the sudden changes of the erectores spinae activity.     

Short-duration fatigue alters neuromuscular coordination of trunk musculature: implications for injury

The aim of this investigation was to determine the effect of muscle fatigue, produced by two different fatigue protocols, on the coordination of trunk and thigh muscles during the performance of a manual-handling task (e.g. a weighted stoop lift). The two fatigue protocols were designed to produce either (a) a non-specific widespread fatigue of trunk and limb muscles (e.g. rowing fatigue protocol), or (b) a specific fatigue of the trunk extensor musculature (e.g. back extension fatigue protocol). Specifically, we wished to determine whether the coordination of trunk muscles during a stoop lift was compromised more, or less, by either of these two fatigue protocols. Ten male subjects (20-24 years) were tested utilising an electromyographic technique which collected electromyograms from trunk flexor and extensor muscles, as well as the Hamstring muscle group, during a pre- and a post-fatigue performance of a weighted stoop lift. The results showed that the back extension fatigue protocol, but not the rowing fatigue protocol, produced significant (p<0.05) changes in the timing of trunk muscle activation during a stoop lift. The longer periods of muscle activation seen only after the back extension fatigue protocol, suggested that fatigue of these muscles had required the CNS to alter their periods of activation to a pattern similar to that previously seen in elderly populations. The results also suggested that intense short-duration motor tasks, which may differentially target the back and its musculature, could leave the spine susceptible to increased risk of injury even though worker perceptions of general fatigue are low. Risk assessment guidelines for manual handling should consider not only the weight and frequency of the lift, but lift duration as well to maintain worker safety.     

Muscle activity and range of motion during active trunk rotation in a sitting posture

Twisted trunk postures during tractor driving are associated with low-back pain. The purposes of this study were to quantify the muscle activity as a function of twisting angle, to quantify the range of motion (ROM) during active trunk rotation and to determine whether there were any differences between tractor drivers and office workers and between twisting direction for these variables. The subjects performed exertions in a seated position, twisting from the neutral position to the end of the ROM. The results showed that external oblique and erector spinae had significantly different activation patterns depending on twisting direction. For the contralateral external oblique and the ipsilateral erector spinae, the muscle effort required to twist the trunk was low up to about 20° twisting angle, then the muscle effort needed to twist the trunk increased progressively. No significant differences due to occupation or twisting direction were found. The result implies that work in twisted trunk postures might be a risk factor for low-back pain.     

Effect of wearing jeans on the back muscle flexion-relaxation phenomenon

The flexion–relaxation phenomenon (FRP) in back muscles is related to the lower back load. Although the FRP has been widely discussed, the effect of wearing jeans—a common attire in workplaces—on the FRP is unexplored. This study recruited 20 male participants with high and low flexibilities (10 each) and measured the related muscle activity and lumbopelvic movement at five trunk flexion angles (0°–90°) when wearing two types of bottom wears (swim trunks and fit jeans). Results revealed that the bottom wear type and trunk angle significantly affected all responses, whereas flexibility affected only erector spinae and hamstring activities. Participants with high flexibility and wearing swim trunks exhibited greater erector spinae activity than did those with low flexibility and wearing jeans. Thus, those who have low flexibility or are wearing jeans are likely to experience more activity reductions in erector spinae. However, the result with hamstring activity was the opposite. Wearing jeans limited participants’ pelvic rotation, forcing them to bend their lumbar spine further to complete trunk flexion, thus reducing erector spinae activity. A looser attire should be chosen to avoid constraining the pelvic rotation when work involves a deep trunk flexion.Relevance to industryPeople wearing jeans have limited pelvic movement and decreased erector spinae activity during deep trunk flexion, and this may further strain their interspinous ligaments. Thus, people should not wear fit jeans when in workplaces where their work involves stooping or trunk flexion exceeding 45° to minimize the load on their lower back.     

Comparison of three strategies of trunk support during asymmetric two-handed reach in standing

No trunk support (NTS) was compared to a lower trunk support (LTS) of leaning against a worktable and a dynamic upper trunk support (UTS) using postural kinematics, trunk extensor muscle activity and subjective rating of both comfort and effort. Ten females completed 3 repetitions where they lifted 0 and 5 kg load from a symmetrical position at hip-height to a 45° asymmetric position at: i) hip-height and ii) shoulder-height. Human motion capture showed trunk flexion decreased by 12° ± 10 with trunk support with hip-height reach. The table blocked axial rotation of the pelvis which was compensated by an additional 8° ± 6 rotation of the thoracic segment. Surface EMG of the lumbar erector spinae, contralateral to reach, showed the UTS to be almost twice as effective as the LTS with shoulder-height reach with a 30% ± 18 reduction. With hip-height reach, UTS resulted in a smaller reduction equal to 23% ± 27 while the LTS had no effect. Further investigation is needed to determine optimal performance parameters for trunk support with complex, dynamic trunk postures and whether altered kinematics arising from LTS have higher risk of upper back discomfort.     

Task rotation effects on upper extremity and back muscle activity

Job rotation is an intuitive approach to distributing work to minimize muscular fatigue. The purpose of the current study was to evaluate rotation between lifting and gripping on muscle activity and effort. Ten male participants performed all 4 combinations of two 15 min tasks in 30 min trials split between separate days to prevent fatigue. The tasks of lifting a 12 kg box and gripping at 20% of maximum were performed 6 times per minute (5 s work: 5 s rest). Muscle activity (percentiles, gaps) and perceived effort were significantly affected by the task combinations. The forearm and upper erector spinae muscles did not benefit as greatly from rotating between lifting and gripping tasks as the lower erector spinae, deltoid or trapezius. In addition to gross task differences, overlaps in muscle activity between “low back” and “upper extremity” tasks must be considered when creating effective job rotation schemes.     

The effects of prior warning and lifting-induced fatigue on trunk muscle and postural responses to sudden loading during manual handling

This study investigated the effects of warning and lifting-induced fatigue on trunk muscle activity and postural responses to sudden loading. Thirty-one male subjects were subjected to sudden loading of a hand-held box with and without prior warning, before and after either lifting-induced fatigue or light callisthenic exercises. Results showed that warning did not alter the level of trunk muscle activity prior to sudden loading. Following warning, there was a reduction in all muscle and joint onset latencies and the magnitude of hip and knee flexion. Although fatigue did not influence muscle and joint initiation, it did negate the effects that warning had on reducing joint displacement. These findings indicate that warning prior to sudden loading may enhance postural responses, reduce ranges of joint motion and increase stability. However, the benefits of prior warning for reducing ranges of joint motion may not be present when a person is fatigued. Sudden unexpected loading and fatigue arising from manual handling practices in the workplace have been identified as contributing factors to the risk of low back injury. Findings from this study provide information that is important for the design of interventions intended to reduce the incidence of manual handling-related back injuries.     

The effects of prior warning and lifting-induced fatigue on trunk muscle and postural responses to sudden loading during manual handling

This study investigated the effects of warning and lifting-induced fatigue on trunk muscle activity and postural responses to sudden loading. Thirty-one male subjects were subjected to sudden loading of a hand-held box with and without prior warning, before and after either lifting-induced fatigue or light callisthenic exercises. Results showed that warning did not alter the level of trunk muscle activity prior to sudden loading. Following warning, there was a reduction in all muscle and joint onset latencies and the magnitude of hip and knee flexion. Although fatigue did not influence muscle and joint initiation, it did negate the effects that warning had on reducing joint displacement. These findings indicate that warning prior to sudden loading may enhance postural responses, reduce ranges of joint motion and increase stability. However, the benefits of prior warning for reducing ranges of joint motion may not be present when a person is fatigued. Sudden unexpected loading and fatigue arising from manual handling practices in the workplace have been identified as contributing factors to the risk of low back injury. Findings from this study provide information that is important for the design of interventions intended to reduce the incidence of manual handling-related back injuries.     

Maximum isometric trunk muscle strength and activity at trunk axial rotation during sitting

This study was performed to provide information relating to the twisted posture being characteristic of the driver of an agricultural tractor working in the field. The relationship of trunk axial strength and muscle activity to trunk twisting angle of prerotation was determined and quantified. Differences between tractor drivers and office workers, and between the two directions of twisting action were also studied. Nine male tractor drivers and nine male office workers performed isometric maximum efforts at about −40, −20, 0, 20 and 40° of pre-set trunk twisting angles in both the clockwise and counterclockwise directions. Exerted torque, true angle of prerotation and muscle activity from left and right side of each of obliquus externus, rectus abdominis and erector spinae were measured simultaneously. The results showed that the subjects could exert the greatest torques when being prerotated in the opposite direction and the lowest torques when being prerotated in the same direction to the direction of exertion. The exerted torques were within the range of 65 – 145 Nm. There were large differences in obliquus externus and erector spinae activity due to the twisting direction. There were also changes in muscle activity from obliquus externus and rectus abdominis due to prerotation angle. The results raised questions concerning the involvement of the passive tissues and the use of deeper muscles during trunk axial rotation, which should be further investigated.     

Progressive fatigue effects on manual lifting factors

The purpose of this study was to evaluate the effect of progressive fatigue on factors that previously have been associated with increased risk of low back pain in various occupational settings, during a repetitive lifting task where a freestyle lifting technique was used. A laboratory experiment was conducted to evaluate electromyography amplitude, kinematic, and kinetic parameters of repetitive freestyle lifting during a 2‐hour lifting period. Subjective fatigue rating increased over time, indicating that the participant “felt” increasingly fatigued as the experiment progressed. Static composite strength decreased an average of 20% from the beginning to the end of the experiment. Effect of lifting posture (semi‐squat, semi‐stoop, and stoop) was observed on peak trunk flexion angle, trunk flexion angle at initiation of the lift, and knee angle at initiation of the lift indicating that, in freestyle lifting, participants assume quantitatively different lifting techniques. A significant effect of the time–posture interaction was observed on the dynamic leg lift floor to knuckle height strength, indicating that dynamic strength may change over time depending on lifting posture selected. © 2009 Wiley Periodicals, Inc.     

A novel optimization model for predicting trunk muscle forces during asymmetric lifting tasks

Low back pain (LBP) is a social and economic problem throughout industry. Repetitive asymmetric postures frequently occur in manual materials handling tasks and such asymmetric lifting has been epidemiologically linked to LBP. Therefore, biomechanical lifting models must be developed to predict muscle forces during asymmetric lifting tasks. This paper proposes an optimization model that was revised to take into account the activities of trunk muscles during asymmetric lifting tasks. Also, three optimization models (minimize maximum muscle intensity: MIN_I_MAX, minimize sum of magnitudes of the muscle forces raised to power 3: MIN_F³, and minimize sum of the muscle intensities raised to power 3: MIN_I³) are compared under various asymmetric lifting conditions. The revised model not only reflects the twisting effect of muscle force vectors for eight primary trunk muscles when trunk rotation is involved, but also accurately predicts the forces of left erector spinae, left latissimus dorsi and left external oblique muscles when compared with EMG signals obtained from experiments. Furthermore, MIN_I_MAX exhibits the best prediction capability among the three optimization models.Relevance to industryA novel optimization model proposed herein considers the twisting effect of muscle force vectors for eight trunk muscles when trunk rotation is involved. An accurate biomechanical model which reflects the asymmetric lifting conditions would significantly facilitate the evaluation of job and workplace design as well as provide a practical clinical evaluation technique.     

How to lift a box that is too large to fit between the knees

Many studies compared lifting techniques such as stoop and squat lifting. Results thus far show that when lifting a wide load, high back loads result, irrespective of the lifting technique applied. This study compared four lifting techniques in 11 male subjects lifting wide loads. One of these techniques, denoted as the weight lifters' technique (WLT), is characterised by a wide foot placement, moderate knee flexion and a straight but not upright trunk. Net moments were calculated with a 3-D linked segment model and spinal forces with an electromyographic-driven trunk model. When lifting the wide box at handles that allow a high grip position, the WLT resulted in over 20% lower compression forces than the free, squat and stoop lifting technique, mainly due to a smaller horizontal distance between the l5S1 joint and the load. When lifting the wide box at the bottom, none of the lifting techniques was clearly superior to the others.

Statement of Relevance: Lifting low-lying and large objects results in high back loads and may therefore result in a high risk of developing low back pain. This study compares the utility of a WLT, in terms of back load and lumbar flexion, to more familiar techniques in these high-risk lifting tasks.