Cumulative creep response of viscoelastic lumbar tissue as a function of work-rest schedule.

We explore the effect of stress-recovery schedule on the cumulative creep response of lumbar tissues. Twelve participants performed a 48-minute protocol that consisted of 12 min of full trunk flexion and 36 min of upright standing. Two stress-recovery (work-rest) schedules were considered: a) three minutes of full trunk flexion followed by twelve minutes of upright standing (3:12), and b) one minute of full trunk flexion followed by four minutes of upright standing (1:4). Lumbar kinematics and EMG activity of erector spinae muscles were collected. Cumulative creep deformation was explored by considering the changes in peak lumbar flexion angles during full flexion and changes in the angles of flexion-relaxation (EMG-off) of the lumbar extensor musculature after the 48-minute protocol. The results of time-dependent lumbar flexion angle during full flexion revealed a noticeable creep response in both work-rest schedules, but the cumulative creep response was significantly greater in the 3:12 schedule (Δ3.5°) than in the 1:4 schedule (Δ1.6°). Similarly, the change in the EMG-off lumbar flexion angle in the 3:12 schedule was significantly greater than in the 1:4 schedule (Δ2.5° vs -Δ0.2°, respectively). These results indicate that the passive lumbar tissues recover their force producing capability more rapidly with shorter cycle times.

Effects of a Passive Back-Support Exosuit on Postural Control and Cognitive Performance During a Fatigue-Inducing Posture Maintenance Task.

OBJECTIVE: To evaluate the effectiveness of passive back-support exosuit on postural control and cognitive performance during a fatigue-inducing posture maintenance task. BACKGROUND: Wearable support systems (exoskeletons/exosuits) reduce physical demands but may also influence postural control and cognitive performance by reducing muscular fatigue. METHOD: Eighteen participants visited on two different days to test an exosuit system and performed dual-task cognitive assessments based on human information processing (information acquisition, information integration, and action implementation) while maintaining a 35° trunk flexion posture for 16 minutes. Center-of-pressure (CoP), cognitive performance, and perceived workload were recorded, while erector spinae muscle activity was captured to quantify muscle fatigue. RESULTS: The exosuit was effective in reducing erector spinae muscle fatigue during the static posture maintenance task (61% less in Δmedian frequency: -9.5 Hz (EXO-Off) versus -3.7 Hz (EXO-On)). The fatigue-inducing task increased CoP velocity as a function of time (29% greater: 9.3 mm/sec (pre) versus 12.0 mm/sec (post)), and exosuit use decreased CoP velocity (23% less: 12.1 mm/sec (EXO-Off) versus 9.4 mm/sec (EXO-On)). The exosuit was also effective at mitigating cognitive degradation, as evidenced by a higher hit-to-signal ratio (8% greater: 81.3 (EXO-Off) versus 87.9 (EXO-On)) in the information integration task and reducing perceived workload in all stages of human information processing. CONCLUSION: Exosuit provided benefits of postural control and information integration processing during a 16-min static posture maintenance task. APPLICATION: Torso exoskeletons/suits can have positive implications for occupations with concurrent physical and cognitive demands.

Quantifying the effectiveness of a passive trunk-support exosuit at reducing erector spinae muscle fatigue during a quasi-static posture maintenance task.

The objective of this study was to explore the effectiveness of a passive back-support exosuit at reducing low back muscle fatigue during an 18-minute trunk posture maintenance task. On two separate days sixteen participants performed an 18-minute trunk posture profile that reflected trunk flexion postures observed during a challenging vascular surgery procedure. On one day they performed the procedure with the support of the exosuit, on the other day without. Test contractions were performed every three minutes to capture the time-dependent electromyographic activity of the bilateral erector spinae muscles. Time domain (amplitude) and frequency domain (median frequency) measures of erector spinae muscle fatigue were assessed. Results revealed that the exosuit significantly reduced the measures of erector spinae muscle fatigue in terms of both amplitude (6.1%) and median frequency (5.3%), demonstrating a fatigue reduction benefit of the exosuit in a realistic surgical posture maintenance task.

To examine the potential adoption of a back-support exosuit system in the surgical environment, this study used an 18-minute posture maintenance task that reflected trunk flexion postures observed during a vascular surgery procedure and suggests that the exosuit system can effectively reduce low back muscle fatigue during a vascular surgical procedure.

Effects of a Passive Back-Support Exosuit on Erector Spinae and Abdominal Muscle Activity During Short-Duration, Asymmetric Trunk Posture Maintenance Tasks.

OBJECTIVE: To examine the effects of asymmetry and lower extremity mobility restrictions on the effectiveness of a passive back-support exosuit in short-duration, static trunk flexion postures. BACKGROUND: The effectiveness of trunk exoskeletons/suits for sagittally symmetric trunk posture maintenance has been investigated, but there has been limited study of the effects of asymmetric trunk postures or lower extremity motion restriction. METHOD: Sixteen participants held trunk flexion postures involving trunk flexion (20°, 40°, 60°), asymmetry (0°, 30°), and lower extremity mobility (Free, Restricted) for 3 s. Participants held these postures with and without an exosuit while erector spinae and abdominal muscle activities were collected. RESULTS: There were no significant interactions between exosuit and asymmetry or exosuit and lower extremity motion restrictions, indicating no significant effects of these factors on the effectiveness of the exosuit at reducing trunk muscle activity. The exosuit was found to be effective at reducing erector spinae muscle activity regardless of asymmetry of posture or lower extremity restrictions (average 21%, from 11.2% MVC to 8.8% MVC). The magnitude of the erector spinae activity at 60° of trunk flexion with the exosuit was similar to that seen at 20° without the exosuit. CONCLUSION: The exosuit consistently provided biomechanical benefit through reduced activation of the erector spinae muscles and neither asymmetry of trunk posture nor lower extremity restriction influenced this effectiveness. APPLICATION: Trunk exoskeletons/suits can reduce trunk muscle activation and understanding how characteristics of the trunk postures assumed impact these responses may help target tasks wherein these devices may be effective.

Creep deformation of viscoelastic lumbar tissue during sustained submaximal trunk flexion postures.

Most in-vivo human experiments exploring creep deformation of viscoelastic lumbar tissue have used a maximum trunk flexion posture to engage the lumbar passive tissues. Recent evidence suggests that static trunk flexion tasks requiring submaximal trunk flexion can lead to gradual changes in the lumbar lordosis and this leads to our hypothesis that maintaining submaximal trunk flexion postures may lead to significant creep deformation of the viscoelastic lumbar tissues. Sixteen participants maintained a trunk flexion posture that was ten degrees less than the trunk flexion posture eliciting flexion-relaxation phenomenon for 12 min with breaks for a maximal trunk flexion protocol every three minutes. Trunk kinematic and extensor EMG measures were captured during the static, submaximal trunk flexion protocol as well as during the maximal trunk flexion protocol to provide evidence of creep development in the lumbar passive tissues. Results revealed that 12-minutes of submaximal trunk flexion led to significant increases in peak lumbar flexion angle (1.3°) and EMG-off lumbar flexion angle for L3/L4 paraspinals (2.9°). During the submaximal trunk flexion protocol, the changes in the lumbar flexion angle at 3-6 min and 6-9 min (average Δ5.4°) were significantly greater than at 0-3 min (Δ2.0°). The contribution of this study is the demonstration that sustained submaximal trunk flexion posture (i.e., constant global system) can lead to creep deformation of the viscoelastic lumbar tissue due to the increased lumbar flexion (i.e., altered local system) and may be attributed to a reduction in lumbar lordosis as the extensor muscles fatigue.

Effect of trunk flexion angle and time on lumbar and abdominal muscle activity while wearing a passive back-support exosuit device during simple posture-maintenance tasks.

Quantifying the trunk flexion angles at which wearable support systems (exoskeletons/exosuits) provide substantial trunk extension moment during posture maintenance tasks (such as those seen in surgical environments) can provide a deeper understanding of this potential intervention strategy. Understanding how time (i.e. adaptation/learning) might impact the reliance on wearable support is also of value. Sixteen participants were asked to maintain specific trunk flexion angles (range 0-60°) with and without an exosuit system while erector spinae and rectus abdominis muscle activity were captured using surface electromyography. The effects of the exosuit showed a statistically significant (p < 0.007) effect on the activity of the erector spinae muscles at 10-60°-an effect that became 'large' (Cohen's d = 0.84) after 20° of trunk flexion. There were no meaningful time-dependent trends in the levels of muscle activation indicating there was no adaptation/learning effect of the exosuit in the task studied.Practitioner summary: This study examined the effectiveness of a back-support exosuit as a function of trunk flexion angle and time of use. The results revealed that the exosuit significantly reduced erector spinae muscle activity beyond 20° of trunk flexion but did not show a meaningful adaption/learning effect.Abbreviations: LBP: low back pain; EMG: electromyography; NEMG: normalized electromyography; IMU: inertial measurement unit; ES: erector spinae; RA: rectus abdominis; MVC: maximum voluntary contraction; FFT: Fast Fourier Transform.

Development and Assessment of a Method to Estimate the Value of a Maximum Voluntary Isometric Contraction Electromyogram from Submaximal Electromyographic Data.

The electromyographic (EMG) normalization (often to maximum voluntary isometric contraction [MVIC]) is used to control for interparticipant and day-to-day variations. Repeated MVIC exertions may be inadvisable from participants' safety perspective. This study developed a technique to predict the MVIC EMG from submaximal isometric voluntary contraction EMG. On day 1, 10 participants executed moment exertions of 100%, 60%, 40%, and 20% of the maximum (biceps brachii, rectus femoris, neck flexors, and neck extensors) as the EMG data were collected. On day 2, the participants replicated the joint moment values from day 1 (60%, 40%, and 20%) and also performed MVIC exertions. Using the ratios between the MVIC EMGs and submaximal isometric voluntary contraction EMG data values established on day 1, and the day 2 submaximal isometric voluntary contraction EMG data values, the day 2 MVIC EMGs were predicted. The average absolute percentage error between the predicted and actual MVIC EMG values for day 2 were calculated: biceps brachii, 45%; rectus femoris, 27%; right and left neck flexors, 27% and 33%, respectively; and right and left neck extensors, both 29%. There will be a trade-off between the required accuracy of the MVIC EMG and the risk of injury due to exerting actual MVIC. Thus, using the developed predictive technique may depend on the study circumstances.

Effects of passive exoskeleton support on EMG measures of the neck, shoulder and trunk muscles while holding simulated surgical postures and performing a simulated surgical procedure.

Exoskeletons have shown significant impact at reducing the biomechanical demand on muscles during repetitive lifting and overhead tasks in non-healthcare industries. However, the benefits of exoskeletons are yet to be realized in the operating room, particularly as work-related musculoskeletal disorders continue to be a concern for surgeons. This study quantified the effect of using neck, arm, and trunk exoskeletons on muscle activity while assuming typical postures held in the operating room. Fourteen participants were recruited to participate in this study. In this two-part experiment participants were asked to 1) hold a series of neck flexion, arm abduction and trunk flexion postures seen in surgical procedures, and 2) perform a simulated surgical task requiring five different trunk flexion posture levels. Participants were required to complete these tasks with and without passive exoskeleton(s). This study showed that even for postures held short time periods, exoskeletons are beneficial at reducing the demand on muscles; however, the reduction in muscle demand depends on body segment and postural angle, as intended with these passive exoskeletons. Furthermore, for the simulated surgical task with awkward trunk flexion postures (10-65°), the trunk exoskeletons showed a significant reduction in the rate of rise in back muscle sEMG (+1.365%MVC/min vs. +0.769%MVC/min for non-dominant lumbar extensor muscles, p = 0.0108; +1.377%MVC/min vs. +0.770%MVC/min for the dominant lumbar extensor muscles, p = 0.0196) over 25 min, consequently resulting in improved trunk subjective discomfort scores (7.34 vs. 4.30, p < 0.05), with no impact on the neck and shoulder biomechanical demand. The results from this study indicate that exoskeletons may be a potential intervention to reduce biomechanical loading during surgery.

Exploring the relationship between neck flexion and neck problems in occupational populations: a systematic review of the literature.

A systematic review was conducted to evaluate the relationship between occupational neck flexion angles and neck problems. The synthesised findings were used to answer three research questions: (1) Is there a positive/negative relationship between neck flexion and neck problems? (2) What is the appropriate angular threshold for neck flexion as a risk factor for neck problems? (3) What are the gaps in our current knowledge? A review of 21 papers revealed (1) a consistent positive correlation between neck flexion and neck problems, and (2) a neck flexion angle of 20° as the most evidence-based (not necessarily the best) cut-off angle separating high- and low-risk neck flexion postures. Future research should focus on the (1) continuous collection of three-dimensional neck postures through longitudinal studies to quantify cumulative exposures of neck postures, and (2) development of standard descriptions of 'neck problems' and 'neck flexion' to facilitate the development of a dose-response relationship. Practitioner summary: Practitioners depend on thresholds for evaluating neck postural exposure using work assessment tools; however, the scientific basis for this is unclear. This systematic review investigated the angular threshold for neck flexion and found 20° of neck flexion with the greatest evidence-based support as the threshold for high-risk neck postural exposure.

Impact of a Neck Strap Intervention on Perceived Effort, Thumb Force, and Muscle Activity of Clarinetists.

OBJECTIVE: Clarinetists often report discomfort of the right wrist and thumb and note that it is likely the result of the constant force applied to the area from the instrument's weight and the musician's technique. One preventative measure to reduce this discomfort is the use of a neck strap. The objective of the current study was to document the biomechanical impacts of this intervention. METHODS: Eight experienced clarinetists played a series of three etude pieces while playing both with and without a neck strap. For each condition, the force between the right thumb and clarinet was measured, electromyographic (EMG) data were collected from seven muscle groups, and subjective assessment of perceived effort was obtained. RESULTS: The results showed that when the neck strap was used, there was a significant decrease in the average force between the thumb and clarinet (p<0.05) and a decrease in the average perceived effort required for the right shoulder and thumb of the participants (p<0.05). Importantly, there were no statistically significant increases in the muscle activity of any of the neck and shoulder muscles with the introduction of the neck strap intervention. CONCLUSION: A neck strap intervention had positive effects on the right thumb while not causing any known adverse effects to other areas such as the neck, upper back, and shoulders.

The effects of repetitive bouts of a fatiguing exertion (with breaks) on the slope of EMG measures of localized muscle fatigue.

Decreases in the median frequency of the power spectrum and increases in amplitude measures of an electromyographic signal have been used to assess localized muscle fatigue. How these responses are affected by repetitive bouts of exertions - separated by rest breaks - is not well understood. It was hypothesized that repetitive bouts of a fatiguing, isometric exertion, separated by periods of rest, would have cumulative effects (across bouts) on the slope of these EMG-based variables, with an expectation of a steeper rate of decline in the median frequency and a steeper rate of increase in amplitude measures in subsequent bouts. To test these hypotheses, 24 participants performed four bouts of an isometric (15% MVC) elbow flexion exertion. Each exertion lasted for four minutes and then a 15-minute break was provided between bouts. Surface electromyography was used to capture the activity of the biceps brachii at twenty-second intervals during the exertions. The median frequency and average rectified value were calculated, as were the slopes of these variables within each of the four-minute bouts. Contrary to the original hypotheses, the results showed that there were no statistically significant differences in the slopes of these EMG-based measures across bouts. One direction for future work is to explore different combinations of work-rest durations to refine this response while another may be to explore alternate EMG-based measures of muscular fatigue that may be more sensitive to this cumulative effect.

Effect of surgical radiation personal protective equipment on EMG-based measures of back and shoulder muscle fatigue: A laboratory study of novices.

Interventional radiologists are at increased risk for musculoskeletal discomfort/disorders and this has been linked to the use of radiation personal protective equipment (rPPE). This study examined the effects of rPPE on the development of fatigue of the erector spinae and trapezius muscles. Surface electromyography (EMG) was used to capture muscle activity, and both time domain (average rectified value) and frequency domain (median frequency) measures were considered in the assessment of localized muscle fatigue. Sixteen participants performed a simulated surgical procedure requiring intermittent 30° flexed static trunk posture with and without rPPE on separate days. The results showed that the rPPE condition demonstrated significantly greater (p < 0.05) downward shift in median frequency in the left lumbar erector spinae and left lower thoracic erector spinae consistent with task-induced localized muscle fatigue. Ergonomic intervention strategies are discussed.

Combined effect of low back muscle fatigue and passive tissue elongation on the flexion-relaxation response.

Previous literature has documented the alterations in the flexion-relaxation response of the lumbar extensor musculature to passive tissue elongation (PTE) and muscle fatigue (MF). There is no study, however, that has explored this response as a function of the combined effect of both PTE and MF, which is often seen in occupational settings. Twelve participants performed three experimental protocols on three different days to achieve (1) PTE, (2) MF and (3) PTE&MF (combined). Trunk kinematics and muscle activities were monitored to assess the effects of these protocols on the peak lumbar flexion angle and the lumbar angle of the flexion-relaxation of the trunk extensor muscles. Results showed responses to the uni-dimensional stresses (PTE and MF) consistent with those seen in the previous literature, while the combined protocol elicited responses that more closely matched the PTE protocol.

Medial knee joint loading during stair ambulation and walking while carrying loads.

Carrying loads while walking or using stairs is a common activity of daily living. Knee osteoarthritis is associated with increased external knee adduction moment (KAM) during walking, so understanding how the additional challenges of stairs and carrying loads impact these moments is of value. Sixteen healthy individuals performed three types of MOTION (walking, stair ascent, stair descent) under three LOAD conditions (no load, carrying a 13.6kg front load, carrying 13.6kg load in a backpack). Three-dimensional gait analysis was used to measure KAM. Results of ANOVA showed a significant main effect of both MOTION and LOAD on peak KAM (p<0.001), but no significant MOTION×LOAD interaction (p=0.250). Peak KAM during stair ascent was about two-times those seen in stair descent (p<0.001) and was significantly higher than those seen in walking (p<0.001). Conditions with LOAD generated significantly greater KAM as compared to the no-LOAD conditions (p<0.001). These findings suggest that carrying a load of moderate magnitude while climbing stairs significantly increases the peak KAM - a risk factor associated with knee osteoarthritis.

An algorithm for defining the onset and cessation of the flexion-relaxation phenomenon in the low back musculature.

The flexion-relaxation phenomenon (FRP) in the low back provides insights into the interplay between the active and passive tissues. Establishing a reliable algorithm for defining the lumbar angle at which the muscles deactivate and reactivate was the focus of the current paper. First, the EMG data were processed using six different smoothing techniques (no smoothing, moving average, moving standard deviation, Butterworth low pass filter at 0.5 Hz, 5 Hz, and 50 Hz) herein called the processed EMG (pEMG). The FRP points were then defined using four thresholds (pEMG less than 3% MVC, pEMG less than 5% MVC, pEMG less than 2 times FRP pEMG, and pEMG less than 3 times FRP pEMG). Finally, a duration requirement was tested (no duration requirement, pEMG data must maintain threshold requirement for 50 data points). Each combination of smoothing, threshold, and duration were applied through a computer program to each muscle for all trials and established an EMG-off and EMG-on angle for each muscle. These estimates were compared to the gold standard of expert-identified EMG-off and EMG-on angles and the root mean square error (RMSE) between this gold standard and the predictions of the algorithms served as the dependent variable. The results showed that the most important factor to produce low values of RMSE is to utilize a Butterworth low pass filter of 5 Hz or less and, if this is employed, there is no value to a duration requirement. The results also suggest that using the "3 times FRP pEMG" threshold technique may provide further improvements in these predictions.

Describing the active region boundary of EMG-assisted biomechanical models of the low back.

BACKGROUND: Electromyography-assisted (EMG-assisted) biomechanical models are used to characterize the muscle and joint reaction forces in the lumbar region. However, during a full-range trunk flexion, there is a transition of extension moment from the trunk extensor muscles to the passive tissues of the low back, indicating that the empirical EMG data used to drive these EMG-assisted models becomes less correlated with the extensor moment. The objectives of this study were to establish the trunk flexion angles at which the passive tissues generate substantial trunk extension moment and to document how these angles change with asymmetry. METHODS: Participants performed controlled trunk flexion-extension motions in three asymmetric postures. The trunk kinematics data and the electromyographic activity from L3- and L4-level paraspinals and rectus abdominis were captured. The time-dependent net internal active moment (from an EMG-assisted model) and the net external moment were calculated. The trunk and lumbar angles at which the net internal active moment was less than 70% of the external moment were found. FINDINGS: The trunk flexion angle at which the net internal moment reaches the stated criteria varied as a function of asymmetry of trunk flexion motion with the sagittally symmetric case providing the deepest flexion angle of 38° (asymmetry 15°: 33°; asymmetry 30°: 26°). INTERPRETATION: These results indicate that EMG-assisted biomechanical models need to consider the role of passive tissues at trunk flexion angles significantly less than previously thought and these flexion angles vary as a function of the asymmetry and direction of motion.

Medial longitudinal arch deformation during walking and stair navigation while carrying loads.

BACKGROUND: Understanding the biomechanics of the medial longitudinal arch (MLA) may provide insights into injury risk and prevention, as well as function of the arch-supporting structures. Our understanding of MLA deformation is currently limited to sit-to-stand, walking, and running. MATERIAL AND METHODS: Three-dimensional deformation of the MLA of the right foot was characterized in 17 healthy participants during several simulated activities of daily living. MLA deformation was quantified by both changes in arch length and navicular displacement during the stance phase of three motions: walking, stair ascent, and stair descent. Three levels of load were also evaluated: no load, a front load (13.6 kg), and a backpack load (13.6 kg). Force platforms and an eight-camera motion capture system were used to collect relevant lower extremity kinetic and kinematic data. RESULTS: Motion type had a significant (p < 0.05) effect on navicular displacement and arch length elongation with navicular displacement being greatest during stair descent, while the walking and stair descent conditions showed the greatest increase in arch length. External load did not significantly affect either of these two measures (p > 0.05). CONCLUSION: Differences in the MLA deformation variables resulting from varied dynamic activities of daily living can be greater than those during walking and should be considered. CLINICAL RELEVANCE: Detailing the mechanics of the MLA may aid in further understanding injuries associated with the MLA, and the results of the current study indicate that these mechanics change based on activity.

The effect of a lower extremity kinematic constraint on lifting biomechanics.

Leaning against a stationary barrier during manual materials handling tasks is observed in many industrial environments, but the effects of this kinematic constraint on low back mechanics are unknown. Thirteen participants performed two-handed lifting tasks using both a leaning posture and no leaning posture while trunk kinematics, muscle activity and ground reaction force were monitored. Results revealed that lifting with the leaning posture required significantly less activity in erector spinae (26% vs. 36% MVC) and latissimus dorsi (8% vs. 14% MVC), and less passive tissue moment compared with the no leaning posture. Peak sagittal accelerations were lower when leaning, but the leaning posture also had significantly higher slip potential as measured by required coefficient of friction (0.05 vs. 0.36). The results suggested that the leaning lifting strategy provides reduced low back stress, but does so at the cost of increased slip potential.

Influence of asymmetry on the flexion relaxation response of the low back musculature.

BACKGROUND: the flexion relaxation phenomenon has been extensively studied in sagittally symmetric postures. Knowledge about this phenomenon in asymmetric trunk postures is less well understood, and may help to reveal the underlying physiology of the passive tissue/active tissue load-sharing mechanism in the lumbar region. METHODS: twelve participants performed fifteen controlled, full range trunk flexion-extension motions toward three asymmetric lifting postures (0° (sagittally symmetric), 15°, and 30° from the mid-sagittal plane). The electromyographic activity data from the paraspinals at the L3 and L4 levels and trunk kinematics data from motion sensors over the C7, T12 and S1 vertebrae were recorded. The lumbar flexion angles at which these muscles' activities were reduced to resting levels during forward flexion provided quantitative data describing the effects of asymmetry on the passive tissue/active tissue interaction. FINDINGS: flexion relaxation was observed in the muscles contralateral to the direction of the asymmetric trunk flexion motion. The response of the ipsilateral extensor musculature was much less consistent, with many trials indicating that flexion relaxation was never achieved. Increasing asymmetry from 0° to 30° led to a 10% reduction in the maximum lumbar flexion. Lumbar flexion angles necessary to achieve flexion relaxation in the contralateral muscles also decreased (L4 paraspinal-related angle decreasing by 15% and the L3 paraspinal-related angle decreasing by 21%). INTERPRETATION: under asymmetric conditions the lumbar flexion angle at which the transition from active muscle to passive ligamentous extension moment is altered from that seen in symmetric motions and this transition can have implications for the loading of the spine in full flexion (or near full flexion) postures.

The effects of horizontal load speed and lifting frequency on lifting technique and biomechanics.

Lifting loads that have a horizontal velocity (e.g. lifting from a conveyor) is often seen in industry and it was hypothesised that the inertial characteristics of these loads may influence lifting technique and low back stress. Seventeen male participants were asked to perform lifting tasks under conditions of four horizontal load speeds (0 m/s, 0.7 m/s, 1.3 m/s and 2.4 m/s) and two lifting frequencies (10 and 20 lifts/min) while trunk motions and trunk muscle activation levels were monitored. Results revealed that increasing horizontal load speed from 0 m/s to 2.4 m/s resulted in an increase in peak sagittal angle (73 degrees vs. 81 degrees ) but lower levels of peak sagittal plane angular acceleration (480 degrees /s(2) vs. 420 [corrected] degrees /s(2)) and peak transverse plane angular acceleration (200 degrees /s per s vs. 140 degrees /s per s) and a consistent increase in trunk muscle co-activation. Participants used the inertia of the load to reduce the peak dynamics of the lifting motion at a cost of increased trunk flexion and higher muscle activity. STATEMENT OF RELEVANCE: Conveyors are ubiquitous in industry and understanding the effects of horizontal load speed on the lifting motions performed by workers lifting items from these conveyors may provide some insight into low back injury risk posed by these tasks.