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Exploring the Effects of Seated Whole Body Vibration Exposure on Repetitive Asymmetric Lifting Tasks a
ab
c
Jay P. Mehta , Steven A. Lavender , Richard J. Jagacinski & Carolyn M. Sommerich a
Integrated Systems Engineering, The Ohio State University, Columbus, Ohio
b
Department of Orthopaedics, The Ohio State University, Columbus, Ohio
a
c
Department of Psychology, The Ohio State University, Columbus, Ohio Accepted author version posted online: 29 Sep 2014.Published online: 13 Jan 2015.
Click for updates To cite this article: Jay P. Mehta, Steven A. Lavender, Richard J. Jagacinski & Carolyn M. Sommerich (2015) Exploring the Effects of Seated Whole Body Vibration Exposure on Repetitive Asymmetric Lifting Tasks, Journal of Occupational and Environmental Hygiene, 12:3, 172-181, DOI: 10.1080/15459624.2014.960573 To link to this article: http://dx.doi.org/10.1080/15459624.2014.960573
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Journal of Occupational and Environmental Hygiene, 12: 172–181 ISSN: 1545-9624 print / 1545-9632 online c 2015 JOEH, LLC Copyright DOI: 10.1080/15459624.2014.960573
Exploring the Effects of Seated Whole Body Vibration Exposure on Repetitive Asymmetric Lifting Tasks Jay P. Mehta,1 Steven A. Lavender,1,2 Richard J. Jagacinski,3 and Carolyn M. Sommerich1 1
Integrated Systems Engineering, The Ohio State University, Columbus, Ohio Department of Orthopaedics, The Ohio State University, Columbus, Ohio 3 Department of Psychology, The Ohio State University, Columbus, Ohio
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2
This study investigated changes in the physiological and behavioral responses to repetitive asymmetric lifting activity after exposure to whole body vibrations. Seventeen healthy volunteers repeatedly lifted a box (15% of lifter’s capacity) positioned in front of them at ankle level to a location on their left side at waist level at the rate of 10 lifts/min for a period of 60 minutes. Prior to lifting, participants were seated on a vibrating platform for 60 minutes; in one of the two sessions the platform did not vibrate. Overall, the physiological responses assessed using near-infrared spectroscopy signals for the erector spinae muscles decreased significantly over time during the seating and the lifting tasks (p < 0.001). During repetitive asymmetric lifting, behavioral changes included increases in peak forward bending motion, twisting movement, and three-dimensional movement velocities of the spine. The lateral bending movement of the spine and the duration of each lift decreased significantly over the 60 minutes of repetitive lifting. With exposure to whole body vibration, participants twisted farther (p = 0.046) and twisted faster (p = 0.025). These behavioral changes would suggest an increase in back injury risk when repetitive lifting tasks are preceded by whole body vibration exposure. Keywords
biomechanics, lifting behavior, musculoskeletal disorders, near-infrared spectroscopy, repetitive lifting, whole body vibration
Address correspondence to: Steven A. Lavender, Integrated Systems Engineering, 246 Baker Systems Engineering, 1971 Neil Avenue, Columbus, OH; e-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/uoeh.
INTRODUCTION
I
n 2012, the Bureau of Labor Statistics (BLS) reported employment in transportation and material moving occupations may increase by 20% from 2010 to 2020.(1) The predicted increase comes with the advancement of technology and 172
e-commerce where there is an increasing demand for goods to be distributed at a faster rate. Individually, exposure to physical factors such as repetitive lifting and whole body vibration (WBV) seen in these occupations have been associated with low back pain (LBP).(2,3) Occupations such as beverage delivery sequentially expose workers to WBV and repetitive lifting tasks. The question is whether these combined exposures further elevate the risk of back injury. Research has shown that WBV exposure to vibration frequencies between 4.5 and 8 Hz increases the risk of low back disorders (LBD) and causes degenerative changes in the spinal system.(4) Epidemiological studies have identified a strong association between occupational driving and risk of low back pain.(5–7) Specifically, truck drivers, tractor drivers, bus drivers, taxi drivers, and earth equipment movers are at greater risk for experiencing LBP.(5–7) In animal studies, there is direct evidence which supports the link between exposure to vibrations and disc degeneration. (8–10) In humans, exposure to WBV can affect neuromuscular control of the human spine. This has been reported as creep in the passive structures of the spinal system(11,12) and increased delay in the feedback control of the neuromuscular response. (13,14) As compared to seating without vibration, studies have also reported development of back muscle fatigue with exposure to 5 Hz WBV. (15,16) These changes in the neuromuscular control due to seated WBV exposure can have implications for spinal stability(17) and thereby increase the risk of back injury. Epidemiological studies have also indicated that exposure to WBV is often accompanied by manual handling tasks. (7,18) The association between manual material handling tasks and LBP is well established. (19–22) More specifically, manual handling tasks that involve repetitive bending, twisting, carrying, or lifting movements have been associated with LBP. (22–25) In addition, repetitive lifting during manual handling tasks has been associated with muscle fatigue. (26) Muscle fatigue induced by repetitive trunk movements can affect neuromuscular control of the spine. (27–30) Thus, the increased rate of muscle fatigue with WBV exposure is thought to increase the
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rate of fatigue during the lifting components of these jobs and therein, by the yet to be determined mechanism, increase LBD risk. In compensating for muscle fatigue during a repetitive lifting task, studies have shown that people adapt their working strategy resulting in larger behavioral changes. During repetitive symmetric lifting tasks, behavioral changes have been reported as alternation between stoop and squat postures(31–34) and between trunk and hip kinematics. (35,36) Behavioral adaptations during a repetitive asymmetric lifting task have been seen as an increase in the amount of forward bending of the spine when initiating lifts and an increase in the lateral bending velocity, (37) both of which are behaviors identified by the Lumbar Motion Monitor model to increase LBD risk. (23) Recently, near-infrared spectroscopy (NIRS) has been used to quantify changes in the muscle physiology. (38,39) There is direct evidence to suggest the link between tissue oxygenation levels (as measured by NIRS) and localized muscle fatigue. (40) Specifically, studies have shown high correlations between changes in the median frequency response of electromyographic signals and physiological measures obtained from the NIRS system. (40,41) Decrease in tissue oxygenation levels during seated WBV exposure has been demonstrated by Maikala and Bhambhani. (42,43) In our prior work, we have shown changes in erector spinae oxygenation levels with the same type of repetitive asymmetric lifting task that was used for this study. (37) The aim of this research was to determine if the behavioral and physiological changes associated with repetitive asymmetric lifting tasks are more pronounced when the lifting is preceded by WBV exposure. Relative to lifting tasks that are not preceded by WBV, repetitive asymmetric lifting activity preceded by WBV exposure is hypothesized to (1) accelerate the decrease in tissue oxygenation in the back muscles; (2) lead to a more rapid increase in sagittal plane range of motion while performing a controlled spine flexion-extension motion protocol; (3) result in more frequent and pronounced compensatory behaviors during a defined lifting task as measured by increases in spine kinematic measures; and (4) increase biomechanical loading of the spine. METHODS Experimental Design This research is part of a larger study investigating the effects of vibration exposure and lifting task precision demands on biomechanical changes experienced during a repetitive lifting task. Since our initial analyses indicated that there were no significant interactions between lifting precision demands and vibration exposure, this article is focused on analyzing the effects of repetitive lifting under high task precision demands that is preceded by a simulated driving task which was performed with and without WBV. The repeated measures design counterbalanced the sequences of WBV exposure, and was scheduled at least a week apart. For each of the experimental conditions, participants lifted a box (15% of their maximum
capacity) repetitively over a 60-minute period using a stoop posture. The dependent variables were selected to assess changes in the physiological and behavioral responses. During the 60minute simulated driving task and the 60-minute repetitive asymmetric lifting task, changes in muscle physiology were assessed using hemoglobin levels derived from the NIRS signals. Subjective work load was evaluated using a self-report measure (Borg CR-10 scale).(45) Behavioral and biomechanical changes during the repetitive lifting task were assessed using measures of lift duration, three-dimensional spine kinematics between (T1 and S1), and three-dimensional spine movements. In addition, three-dimensional peak motions were evaluated during a flexion-extension motion assessment protocol (FEMAP) to evaluate changes in motor performance during the simulated driving and repetitive lifting tasks. Sample Seventeen healthy volunteers, 13 males and 4 females between the ages of 18 and 32 (mean = 21.3 years, s.d. = 4.1 years) participated in the study. Mean height and weight of the participants were 1.76 m (s.d. = 0.07 m) and 79.7 kg (s.d. = 11.2 kg). Participants had no prior history of musculoskeletal disorders of the back, neck, shoulder, arms, or legs within the past six months. All participants were recruited from a university student population and had no experience in manual material handling jobs. All participants signed an institutional review board (IRB) approved consent document prior to participating. Apparatus A custom-built WBV platform was designed and developed for this study (Figure 1a). The seated participants were vibrated at a frequency of 5 Hz (sinusoidal vibrations) and vertical acceleration levels of 0.1 g as they performed a simulated driving task. (44) The seat did not have a back support. A passive structure was constructed from Creform materials to create a circular conveyor system. Lifts originated from a height of 0.25 m above the floor and 0.43 m in front of the participants, and terminated 0.86 m above floor level, 0.43 m from the spine to the participant’s left side. The origin and destination conveyors provided 90 degrees of asymmetry to the participants left side (Figure 1b). The destination conveyor had 0.006 m side-to-side space between the box and the guide rails, and a vertical clearance of 0.02 m. The wooden box (0.4 × 0.3 × 0.25 m) lifted by the participants had handles and was filled with reams of paper to the desired weight. Self-report measures (Borg CR-10 scale)(45) were obtained to quantify changes in the overall subjective work load during the seating and the repetitive lifting task. NIRS was used to evaluate changes in muscle physiology during the 60 minutes of simulated driving and the 60 minutes of lifting activity. The NIRS system provides a direct measurement of oxygen delivered and utilized at the muscle site. Sensors from the two-channel INVOS 4100 Cerebral Oximeter (Somanetics Corporation, Troy, MI) were attached over the erector spinae
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The steering wheel and pedal assembly was adjusted to each participants’ desired driving position.
FIGURE 1. Experimental setup during the simulated driving task (a) and the lifting task (b).
muscles at the L3 level. Changes in total, oxygenated, and deoxygenated hemoglobin levels were continuously monitored at 85 Hz. Previously, studies have demonstrated oxygenated hemoglobin levels to be more sensitive in detecting physiological changes during repetitive work(37,40); thus, this is the NIRS measure used in this article. Three-dimensional spine motions were captured (at 120 Hz) using a magnetic motion capture system (The Motion Monitor, Chicago, IL) to assess spine kinematic changes associated with the repetitive asymmetric lifting task and performance changes during the FEMAP performed periodically throughout the data collection process. Bertec force plates (Bertec Corp, Columbus, OH) were used to measure ground reaction forces that provided input into a three-dimensional dynamic linked-segment model within the Motion Monitor System that computes the three-dimensional movements acting at L5/S1. Two Inscale (Terra Haute, IN) force scales, positioned beneath the conveyors, were used to identify the timing of the initial lift and the box placement in the data stream collected during the lifting task. Maximum lifting strength, used to scale the box weight for each individual participant’s capability, was measured using a dynamometer. During 60 minutes of the simulated driving task, participants interacted with the Scania Truck Driving Simulator (SCS Software, Prague, Czech Republic). The driving controls included a steering wheel with integrated pedals mounted in front of the vibrating platform (Racin’ Pro, Subsonic). 174
Procedures At the beginning of each participant’s first experimental session, lifting strength was assessed using a maximum isometric assessment in which the participant attempted to pull upwards on a handle located at the same height and distance from the ankles as the handles on the box during initial part of the lifting task. The box weight which was adjusted to 15% of each individual subject’s maximum lifting strength (indicated by the larger of the two exertions). On average, the box weighed 11.06 kg (s.d. = 3.53 kg). Eleven motion capture system sensors were attached to the participants at the following locations: on the back of the head, at the top of the thoracic spine (T1), over the top of the sacrum (S1), and bilaterally between the elbow and shoulder joint, between hip and knee joint, between knee and ankle joint, and on the wrist using velcro straps and tape. The location of the upper and lower arm and legs sensors was approximately halfway between the adjacent joints; a calibration procedure was used to measure the distance of the sensor from each adjacent joint. Additionally, the two NIRS sensors were placed bilaterally on the erector spinae muscles at the L3 level. Prior to beginning each session, the participant’s maximum range of motion (ROM) in the sagittal plane was measured. With eyes closed, participants were asked to bend forward maximally to obtain each individual’s maximum trunk flexion capacity. 60-minute Simulated Driving Period Prior to the lifting task in each session, the participants performed the simulated driving task during which they were seated on the vibration platform for 60 minutes without back support. Prior to one driving session, the platform was turned on, therein exposing the participants to vibration; in the other session, the participants sat on the same platform for the same amount of time, but it did not vibrate. The participants were instructed that they were not judged on their driving ability. Before beginning the 60-minute driving period, and every 20 minutes thereafter during the 60 minutes of simulated driving, the participants performed the Flexion-Extension Motion Assessment Protocol (FEMAP) described below. The Lifting Task Using the handles, participants repetitively lifted the box from a conveyor in front of them (0.25 m above the floor) to the conveyor located to their left side (0.86 m above floor) without moving their feet and while using primarily a stoop posture. The task was paced so that the participants performed 10 lifts/min for 10 minutes. An audio signal provided every 6 sec indicated when each lift should be initiated. At the end of each 10-minute lifting period, the FEMAP (described below) was performed and a Borg scale rating was obtained, after which another 10-minutes of lifting was initiated. This process of lifting for 10 minutes followed by the FEMAP
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continued until 60 minutes of lifting was completed or until the participant indicated he or she was fatigued and no longer able to continue. Where participants terminated their lifting before the end of the 60-minute period, a final FEMAP was performed and a Borg rating was obtained. Participants were given an opportunity to practice the lifting task before starting the first 10-minute block of lifts. Flexion-Extension Motion Assessment Protocol (FEMAP) Given that Parnianpour and colleagues (1988) reported out-of-plane motions became more prevalent with localized back muscle fatigue, a protocol was developed to periodically evaluate trunk kinematic performance at selected points during the subject’s participation in the experiment. This assessment took approximately 30 seconds and was conducted at the beginning of the 60-minute driving task and every 20 minutes thereafter. Once the lifting task was initiated, the FEMAP was conducted in between each 10 minutes of the lifting activity. (37) In this procedure, the participant with eyes closed repeatedly flexed forward to a target position that was defined as two-thirds of their maximum trunk flexion capacity as fast as they could and returned to an upright standing posture. These motions were repeated 10 times, during which spine kinematic data were collected. Spine position information was provided via auditory feedback; an auditory tone signaled when participants reached the target position and when they returned back to their upright standing posture. Prior to the first experimental session, participants were allowed to practice the flexion-extension task until they felt comfortable with this procedure. Data Analysis Self-report scores obtained from Borg scale ratings were used to identify changes in the overall subjective work load during the 60 minutes of simulated driving and the 60 minutes of repetitive lifting activity. For the muscle physiology measure, the average over each 20-minute period of the driving task and each 10-minute period of the lifting task was calculated. While the simulated driving data were normalized to baseline values, (37) the lifting data were normalized to the initial 10 minutes of the lifting activity. To assess behavioral and biomechanical changes during the prolonged lifting task, the peak three-dimensional spine kinematics and movements were obtained for each lift. The timing of the individual lifts was calculated based on the force scale data. Means of the duration data and peak kinematic and movement measures for each 10 minutes of lifting activity were computed to identify parameters indicative of behavioral and biomechanical adaptations during lifting. For each flexion-extension movement task during the FEMAP, three-dimensional peak displacements and movement velocities were obtained. The resulting data from the NIRS and motion monitor system measures were analyzed using a within subjects repeated measures ANOVA procedure using IBM SPSS, version 19
FIGURE 2. Borg rating - The Borg rating measure during the lifting task as a function of vibration and time. Vertical bars indicate standard error of the mean.
(IBM Corporation, Armonk, NY). Due to the difference in the nature of the task between seating and lifting, the Borg, NIRS, and the FEMAP measures were separately analyzed for the 60 minutes of simulated driving and the 60 minutes of repetitive lifting. For the participants who were not able to complete 60 minutes of repetitive lifting (N = 3, one session each), we used a conservative imputation approach of carrying their last measured value forward so they could be included in the ANOVA procedures. A paired t-test comparison was conducted when significant interactions were obtained between vibration exposure and time. In all statistical tests, a p-value 0.05). Muscle Physiology Measures Oxygenated hemoglobin levels were used to quantify changes in the erector spinae muscle physiology. Oxygenated hemoglobin levels from the left and right erector spinae muscles were highly correlated (r > .92) in both the vibration and no-vibration conditions. Considering the high correlations between the left and right erector spinae muscles, further results focus on the changes oxygenated hemoglobin levels measured on the right side, which was contralateral to the load placement. Figure 3 shows the percentage change of oxygenated hemoglobin derived from the NIRS system decreased significantly over time (p < 0.001) during the lifting task. More specifically, the oxygenated hemoglobin levels decreased significantly for the initial 30 minutes of the repetitive lifting task after which no significant change was observed for this
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Changes in three-dimensional spine movements during the repetitive asymmetric lifting task are shown in Figure 5. Overall, the forward bending movement on the spine (Figure 5a) was unaffected by the repetitive lifting activity over time (p > 0.05). While the twisting movement on the spine (Figure 5b) increased significantly over time (p = 0.012), the lateral bending movement on the spine (Figure 5c) decreased significantly over time (p < 0.001). Exposure to WBV showed no effect on the movement measures. The three-dimensional movement velocities of the spine increased significantly (p < 0.01) over time (Figure 6). In addition to the time-dependent effects, twisting velocities were larger when the lifting followed WBV (62.0 vs 56.9 degrees/ sec, p = 0.025).
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FIGURE 3. NIRS - Oxygenated hemoglobin levels obtained from the NIRS system during the lifting task as a function of vibration and time. Vertical bars indicate standard error of the mean.
physiological measure. WBV exposure prior to the lifting task did not affect these changes in oxygenated hemoglobin levels (p > 0.05). Behavioral Changes with Repetitive Lifting The overall lift duration during the repetitive asymmetric lifting task decreased significantly over time (p < 0.001). This trend was linear (r2 = 0.72) with an average decrease of 0.02 seconds with every 10 minutes of repetitive lifting activity. Exposure to WBV did not change the overall lift duration. A summary of the ANOVA results based on the analysis of the data obtained during the lifting task are shown in Table I. Figure 4a shows the amount of forward bending increased significantly over time (p = 0.016). A post hoc analysis showed that the magnitude of the forward bending during the last 10 minutes of repetitive lifting task was significantly greater than that observed during the initial 10 minutes. WBV exposure showed no effect on this measure. The amount of spine twisting during the repetitive lifting task (Figure 4b) was significantly greater after exposure to whole body vibration (28.8o vs 25.4o, p = 0.046). However, there were no temporal trends in the overall twisting motions across the 60 minutes of lifting. Lateral bending motion of the spine (Figure 4c) was not affected by exposure to WBV (p > 0.05) and did not significantly change over the 60 minutes of lifting.
Flexion-Extension Motion Assessment Protocol (FEMAP) FEMAP During the Driving Task None of the three-dimensional spine kinematics variables showed any change due to the vibration exposure. However, the 60 minutes of simulated driving did significantly affect the peak sagittal plane range of motion (p = 0.047), but not the twisting or lateral bending ROM. (p > 0.05). Peak forward bending, extension, and lateral bending velocities of the spine assessed during the FEMAP also increased significantly across the simulated driving period (p < 0.05). Likewise, there was a similar trend for the twisting velocity (p = 0.061). FEMAP results obtained During the Lifting Task WBV exposure prior to lifting did not affect the spine ROM in the sagittal, frontal, or transverse planes, and showed no significant interactions with time (Table II). Across the 10-minute blocks of lifting, the peak sagittal plane range of motion increased significantly over time (p = 0.003). However, the peak deviations in the axial and coronal planes were not affected by the repetitive lifting activity. The FEMAP spine movement velocities increased significantly across the samples obtained between each 10-minute block of lifting (p < 0.05). For the lateral bending velocity, there was a significant interaction between the vibration exposure and the amount of time spent lifting (Table II). Post hoc analysis showed that this measure increased after 20 minutes of repetitive lifting when lifting was preceded by quiet seating,
TABLE I. The p-Values From the ANOVA Results for Peak Values Averaged Across the 100 Lifts Within Each 10-min Block of Lifting for the Three-Dimensional Spine Kinematic and Movement Measures Independent Variables Vibration Exposure (VE) Time (T) VE x T
Flexion
Twist
Lateral Bend
0.335 0.016 0.895
0.046 0.572 0.575
0.582 0.551 0.625
Forward Bending Moment
Twisting Moment
Lateral movement
Extension Velocity
Twisting Velocity
Lateral Bending Velocity
0.509 0.204 0.299
0.420 0.012 0.604
0.523 < 0.001 0.472
0.440 0.005 0.508
0.025 < 0.001 0.587
0.392 0.001 0.833
Notes: All p-values < 0.05 are indicated by the bold font.
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FIGURE 4. Lifting - The amount of (a) forward bending, (b) twisting, and (c) lateral bending as a function of vibration and time. Vertical bars indicate standard error of the mean.
whereas, only marginal increase (p = 0.057) was observed when lifting was preceded by WBV exposure (Figure 7d). For the twisting velocity, there was a marginally significant interaction (p = 0.069) between vibration exposure and time spent lifting (Figure 7c). No significant interaction between vibration exposure and time spent lifting was observed for the flexion and extension movement velocities of the spine (Figures 7a and 7b). DISCUSSION
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he purpose of this study was to understand the sequential effects of seated WBV exposure on repetitive asymmetric lifting activity. Overall, exposure to WBV showed no significant effect on the ratings of perceived exertion and muscle physiology measures during the 60 minutes of repetitive asymmetric lifting. As for behavioral measures, repetitive asymmetric lifting activity preceded by WBV exposure resulted in
FIGURE 5. Lifting - (a)forward bending movement, (b) twisting movement, and (c) lateral bending movement as a function of vibration and time. Vertical bars indicate standard error of the mean.
significantly larger twisting motions and twisting velocities of the spine. Changes in the Overall Subjective Workload Our results showed that the overall subjective work load increased significantly over time during the 60 minutes of repetitive asymmetric lifting. These results are similar to other studies in the literature that have shown increases in Borg ratings of perceived exertion during repetitive lifting tasks. (34,45–47) Increases in perceived physical effort over time have been shown previously to be associated with the development of muscle fatigue. (48–51) However, exposure to WBV showed no effect on the overall subjective work load, indicating that this subjective measure of fatigue is largely affected by the prolonged seating and lifting activity independent of the vibration exposure. Changes in Muscle Physiology Measures Tissue oxygenation measures obtained from the NIRS system were used to quantify changes in the erector spinae
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activity is indicative of fatigue development in the erector spinae muscles. Our hypothesis that the exposure to WBV would lead to a larger drop in the tissue oxygenation measures was not supported. As such, our results are in disagreement with the prior studies that have shown erector spinae fatigue with seated WBV exposure. (15,16) In these prior studies, participants were holding an extra weight and were seated with trunk flexed forward. (15,16) These variations would further engage the back extensor muscles and potentially enhance fatigue development. Studies that have looked at a more realistic scenario have found similar back muscle fatigue for WBV exposure and quiet sitting without WBV exposure as seen in the current study. (52,53) It is possible that exposure to WBV increased the amount of co-contraction from the trunk musculature, thereby fatiguing the abdominal muscles; however, we did not measure abdominal muscle physiology.
FIGURE 6. Lifting - (a) Extension, (b) twisting, and (c) lateral bending velocities of the spine as a function of vibration and time. Vertical bars indicate standard error of the mean.
muscle physiology. During the repetitive lifting activity, the oxygenated hemoglobin levels decreased significantly over time. Previously, studies have shown development of muscle fatigue, as measured with EMG spectral analysis, is associated with decrease in oxygenated hemoglobin levels.(40,41) Thus, the decrease in oxygenated hemoglobin levels during the simulated driving and the repetitive asymmetric lifting
Behavioral Changes after Exposure to WBV The amount of twisting and the twisting velocity of the spine during the repetitive asymmetric lifting tasks were significantly higher when participants were exposed to WBV. Exposure to WBV has been known to affect neuromuscular control of the spine. Specifically, studies have reported increased latency in the feedback response after exposure to seated WBV. (13,14,54) Thus, the larger twisting motions and movement velocity after exposure to WBV could be a coping strategy to compensate for the poor feedback response in controlling precise motions during box placement. Twisting motions increase facet contact pressures and tensile stresses on selected annulus disc fibers, (55) thereby increasing the risk of injury. In their lumbar motion monitor model, Marras et al. (23) identified twisting velocity as one of the key factors used to determine LBD risk. Both the amount of twisting and the velocity of twisting have been reported to increase coactivity from the trunk musculature, (36,56) and increased co-contraction has been demonstrated to increase LBD risk. (57) Behavioral Changes Associated with Time The behavioral changes reported here are similar to the results obtained in our prior work. (37) In this study, twisting movements typically peaked while picking the boxes in a flexed posture. One of the tissues under strain in the stooped posture from which the lifts originated is the annulus fibrosis,
TABLE II. P-Values From the ANOVA Results for the Three-Dimensional Spine Kinematics Measures During the FEMAP Collected in Between Lifting Activity Independent Variables Time (T) VE x T
ROM (Sagittal)
ROM (axial)
ROM (coronal)
Forward Bending Velocity
Extension Velocity
Twisting Velocity
Lateral Bending Velocity
0.944 0.003 0.932
0.840 0.323 0.580
0.442 0.752 0.306
0.673 < 0.001 0.439
0.882 0.001 0.969
0.239 0.024 0.069
0.368 0.033 0.008
p < 0.05 are indicated by bold values.
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FEMAP
T
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he flexion-extension motion assessment protocol was developed to test whether prolonged repetitive asymmetric lifting that was preceded by whole body vibration exposure leads to deterioration in motor performance when performing a simple flexion-extension task. Our hypothesis was partially supported in that participants showed an increase in their sagittal plane range of motion during the simulated driving and the lifting tasks. Larger deviations of the spine in the sagittal plane suggest creep in the passive structures of the spine (59) or a change in the neuromuscular response. However, the lack of differences between the vibration conditions suggest that it is the lifting task rather than the vibration exposure that is contributing to this increased motion in the sagittal plane. There were some limitations concerning this study. First, we should recognize that all our participants were novice manual handlers from a university population, and previous studies have shown differences between experienced and inexperienced lifters. (60–,62) One should be cautious in generalizing the results obtained from this study to experienced lifters. Second, the participants were not provided a backrest during the seated driving task. The addition of a backrest would substantially change the transmission of vibrations to the human body. (63) Third, the workplace layout was fixed in our study. Due to this, the relative work height and reach distance to the box would have varied with the anthropometric diversity in the sample. Fourth, the participants were restricted from lifting with their legs. This was done to limit the movement degrees of freedom to the spine and upper extremities. Clearly, the ability to increase the use of lower extremities in lifting could significantly alter the results found in this study. CONCLUSION
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ith a repetitive asymmetric lifting activity, behavioral changes that increase the risk of back injury included more forward bending, larger twisting movements, and larger three-dimensional movement velocities. This risk was further elevated when lifting activity was preceded by WBV exposure as there was more spine twisting and larger twisting velocities observed with exposure to WBV.
FIGURE 7. (a) Forward bending, (b) extension, (c) twisting, and (d) lateral bending velocities of the spine sampled during the FEMAP which was run every 10 minutes during the lifting task. Vertical bars indicate standard error of the mean.
which is not well suited to resist large twisting movements. Thus, the increase in twisting movements along with simultaneous forward bending motions would increase the risk of low back injury. (58) Further, three of the five factors that changed over time with the repetitive lifting activity (amount of forward bending, twisting velocity, and lateral bending velocity) have also been shown to increase the risk of back injury. (23)
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48. Dedering, A., G. N´emeth, and K. Harms-Ringdahl: Correlation between electromyographic spectral changes and subjective assessment of lumbar muscle fatigue in subjects without pain from the lower back. Clin. Biomech. (Bristol, Avon) 14(2):103 (1999). 49. Kimura, M., H. Sato, M. Ochi, et al.: Electromyogram and perceived fatigue changes in the trapezius muscle during typewriting and recovery. Eur. J. Appl. Physiol. 100(1):89–96 (2007). 50. Hummel, A., T. L¨aubli, M. Pozzo, et al.: Relationship between perceived exertion and mean power frequency of the EMG signal from the upper trapezius muscle during isometric shoulder elevation. Eur. J. Appl. Physiol. 95(4):321–326 (2005). 51. Looze, M.D., T. Bosch, and J.V. Die¨en: Manifestations of shoulder fatigue in prolonged activities involving low-force contractions. Ergonomics 52(4):428–437 (2009). 52. El Falou, W., J. Duchˆene, M. Grabisch, et al.: Evaluation of driver discomfort during long-duration car driving. Appl. Ergonomics 34(3):249–255 (2003). 53. Santos, B.R., C. Lariviere, A. Delisle, et al.: A laboratory study to quantify the biomechanical responses to whole-body vibration: The influence on balance, reflex response, muscular activity and fatigue. Int. J. Indus. Ergonomics 38:626–639 (2008). 54. Arora, N., and S.G. Grenier: Acute effects of whole body vibration on directionality and reaction time latency of trunk muscles: The importance of rest and implications for spine stability. J. Electromyog. Kinesiol. 23(2):394–401 (2012).
55. Natarajan, R.N., S.A. Lavender, H.A. An, et al.: Biomechanical response of a lumbar intervertebral disc to manual lifting activities: A poroelastic finite element study. Spine 33: 1958–1965 (2008). 56. Marras, W.S., and K.P. Granata: A biomechanical assessment and model of axial twisting in the thoracolumbar spine. Spine 20(13):1440–1451 (1995). 57. Granata, K.P., and W.S. Marras: The influence of trunk muscle coactivity on dynamic spinal loads. Spine 20(8):913–919 (1995). 58. Duncan, N.A., and A.M. Ahmed: The role of axial rotation in the etiology of unilateral disc prolapse: An experimental and finite-element analysis. Spine 16(9):1089–1098 (1991). 59. Adams, M.A., and P. Dolan: Time-dependent changes in the lumbar spine’s resistance to bending. Clin. Biomech. 11(4):194–200 (1996). 60. Marras, W.S., J. Parakkat, A.M. Chany, et al.: Spine loading as a function of lift frequency, exposure duration and work experience. Clin. Biomech. 21(4):345–352 (2006). 61. Plamondon, A., C. Lariviere, A. Delisle, et al.: Relative importance of expertise, lifting height and weight lifted on posture and lumbar external loading during a transfer task in manual handling. Ergonomics 55(1):85–102 (2012). 62. Lee, J., and M.A. Nussbaum: Experienced workers exhibit distinct torso kinematics/kinetics and patterns of task dependency during repetitive lifts and lowers. Ergonomics 55(12):1535–1547 (2012). 63. Hinz, B., H. Seidel, G. Menzel, et al.: Effects related to random wholebody vibration and posture on a suspended seat with and without backrest. J. Sound Vibrat. 253(1):265–282 (2002).
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Journal of Occupational and Environmental Hygiene Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/uoeh20
Exploring the Effects of Seated Whole Body Vibration Exposure on Repetitive Asymmetric Lifting Tasks a
ab
c
Jay P. Mehta , Steven A. Lavender , Richard J. Jagacinski & Carolyn M. Sommerich a
Integrated Systems Engineering, The Ohio State University, Columbus, Ohio
b
Department of Orthopaedics, The Ohio State University, Columbus, Ohio
a
c
Department of Psychology, The Ohio State University, Columbus, Ohio Accepted author version posted online: 29 Sep 2014.Published online: 13 Jan 2015.
Click for updates To cite this article: Jay P. Mehta, Steven A. Lavender, Richard J. Jagacinski & Carolyn M. Sommerich (2015) Exploring the Effects of Seated Whole Body Vibration Exposure on Repetitive Asymmetric Lifting Tasks, Journal of Occupational and Environmental Hygiene, 12:3, 172-181, DOI: 10.1080/15459624.2014.960573 To link to this article: http://dx.doi.org/10.1080/15459624.2014.960573
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Journal of Occupational and Environmental Hygiene, 12: 172–181 ISSN: 1545-9624 print / 1545-9632 online c 2015 JOEH, LLC Copyright DOI: 10.1080/15459624.2014.960573
Exploring the Effects of Seated Whole Body Vibration Exposure on Repetitive Asymmetric Lifting Tasks Jay P. Mehta,1 Steven A. Lavender,1,2 Richard J. Jagacinski,3 and Carolyn M. Sommerich1 1
Integrated Systems Engineering, The Ohio State University, Columbus, Ohio Department of Orthopaedics, The Ohio State University, Columbus, Ohio 3 Department of Psychology, The Ohio State University, Columbus, Ohio
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2
This study investigated changes in the physiological and behavioral responses to repetitive asymmetric lifting activity after exposure to whole body vibrations. Seventeen healthy volunteers repeatedly lifted a box (15% of lifter’s capacity) positioned in front of them at ankle level to a location on their left side at waist level at the rate of 10 lifts/min for a period of 60 minutes. Prior to lifting, participants were seated on a vibrating platform for 60 minutes; in one of the two sessions the platform did not vibrate. Overall, the physiological responses assessed using near-infrared spectroscopy signals for the erector spinae muscles decreased significantly over time during the seating and the lifting tasks (p < 0.001). During repetitive asymmetric lifting, behavioral changes included increases in peak forward bending motion, twisting movement, and three-dimensional movement velocities of the spine. The lateral bending movement of the spine and the duration of each lift decreased significantly over the 60 minutes of repetitive lifting. With exposure to whole body vibration, participants twisted farther (p = 0.046) and twisted faster (p = 0.025). These behavioral changes would suggest an increase in back injury risk when repetitive lifting tasks are preceded by whole body vibration exposure. Keywords
biomechanics, lifting behavior, musculoskeletal disorders, near-infrared spectroscopy, repetitive lifting, whole body vibration
Address correspondence to: Steven A. Lavender, Integrated Systems Engineering, 246 Baker Systems Engineering, 1971 Neil Avenue, Columbus, OH; e-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/uoeh.
INTRODUCTION
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n 2012, the Bureau of Labor Statistics (BLS) reported employment in transportation and material moving occupations may increase by 20% from 2010 to 2020.(1) The predicted increase comes with the advancement of technology and 172
e-commerce where there is an increasing demand for goods to be distributed at a faster rate. Individually, exposure to physical factors such as repetitive lifting and whole body vibration (WBV) seen in these occupations have been associated with low back pain (LBP).(2,3) Occupations such as beverage delivery sequentially expose workers to WBV and repetitive lifting tasks. The question is whether these combined exposures further elevate the risk of back injury. Research has shown that WBV exposure to vibration frequencies between 4.5 and 8 Hz increases the risk of low back disorders (LBD) and causes degenerative changes in the spinal system.(4) Epidemiological studies have identified a strong association between occupational driving and risk of low back pain.(5–7) Specifically, truck drivers, tractor drivers, bus drivers, taxi drivers, and earth equipment movers are at greater risk for experiencing LBP.(5–7) In animal studies, there is direct evidence which supports the link between exposure to vibrations and disc degeneration. (8–10) In humans, exposure to WBV can affect neuromuscular control of the human spine. This has been reported as creep in the passive structures of the spinal system(11,12) and increased delay in the feedback control of the neuromuscular response. (13,14) As compared to seating without vibration, studies have also reported development of back muscle fatigue with exposure to 5 Hz WBV. (15,16) These changes in the neuromuscular control due to seated WBV exposure can have implications for spinal stability(17) and thereby increase the risk of back injury. Epidemiological studies have also indicated that exposure to WBV is often accompanied by manual handling tasks. (7,18) The association between manual material handling tasks and LBP is well established. (19–22) More specifically, manual handling tasks that involve repetitive bending, twisting, carrying, or lifting movements have been associated with LBP. (22–25) In addition, repetitive lifting during manual handling tasks has been associated with muscle fatigue. (26) Muscle fatigue induced by repetitive trunk movements can affect neuromuscular control of the spine. (27–30) Thus, the increased rate of muscle fatigue with WBV exposure is thought to increase the
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rate of fatigue during the lifting components of these jobs and therein, by the yet to be determined mechanism, increase LBD risk. In compensating for muscle fatigue during a repetitive lifting task, studies have shown that people adapt their working strategy resulting in larger behavioral changes. During repetitive symmetric lifting tasks, behavioral changes have been reported as alternation between stoop and squat postures(31–34) and between trunk and hip kinematics. (35,36) Behavioral adaptations during a repetitive asymmetric lifting task have been seen as an increase in the amount of forward bending of the spine when initiating lifts and an increase in the lateral bending velocity, (37) both of which are behaviors identified by the Lumbar Motion Monitor model to increase LBD risk. (23) Recently, near-infrared spectroscopy (NIRS) has been used to quantify changes in the muscle physiology. (38,39) There is direct evidence to suggest the link between tissue oxygenation levels (as measured by NIRS) and localized muscle fatigue. (40) Specifically, studies have shown high correlations between changes in the median frequency response of electromyographic signals and physiological measures obtained from the NIRS system. (40,41) Decrease in tissue oxygenation levels during seated WBV exposure has been demonstrated by Maikala and Bhambhani. (42,43) In our prior work, we have shown changes in erector spinae oxygenation levels with the same type of repetitive asymmetric lifting task that was used for this study. (37) The aim of this research was to determine if the behavioral and physiological changes associated with repetitive asymmetric lifting tasks are more pronounced when the lifting is preceded by WBV exposure. Relative to lifting tasks that are not preceded by WBV, repetitive asymmetric lifting activity preceded by WBV exposure is hypothesized to (1) accelerate the decrease in tissue oxygenation in the back muscles; (2) lead to a more rapid increase in sagittal plane range of motion while performing a controlled spine flexion-extension motion protocol; (3) result in more frequent and pronounced compensatory behaviors during a defined lifting task as measured by increases in spine kinematic measures; and (4) increase biomechanical loading of the spine. METHODS Experimental Design This research is part of a larger study investigating the effects of vibration exposure and lifting task precision demands on biomechanical changes experienced during a repetitive lifting task. Since our initial analyses indicated that there were no significant interactions between lifting precision demands and vibration exposure, this article is focused on analyzing the effects of repetitive lifting under high task precision demands that is preceded by a simulated driving task which was performed with and without WBV. The repeated measures design counterbalanced the sequences of WBV exposure, and was scheduled at least a week apart. For each of the experimental conditions, participants lifted a box (15% of their maximum
capacity) repetitively over a 60-minute period using a stoop posture. The dependent variables were selected to assess changes in the physiological and behavioral responses. During the 60minute simulated driving task and the 60-minute repetitive asymmetric lifting task, changes in muscle physiology were assessed using hemoglobin levels derived from the NIRS signals. Subjective work load was evaluated using a self-report measure (Borg CR-10 scale).(45) Behavioral and biomechanical changes during the repetitive lifting task were assessed using measures of lift duration, three-dimensional spine kinematics between (T1 and S1), and three-dimensional spine movements. In addition, three-dimensional peak motions were evaluated during a flexion-extension motion assessment protocol (FEMAP) to evaluate changes in motor performance during the simulated driving and repetitive lifting tasks. Sample Seventeen healthy volunteers, 13 males and 4 females between the ages of 18 and 32 (mean = 21.3 years, s.d. = 4.1 years) participated in the study. Mean height and weight of the participants were 1.76 m (s.d. = 0.07 m) and 79.7 kg (s.d. = 11.2 kg). Participants had no prior history of musculoskeletal disorders of the back, neck, shoulder, arms, or legs within the past six months. All participants were recruited from a university student population and had no experience in manual material handling jobs. All participants signed an institutional review board (IRB) approved consent document prior to participating. Apparatus A custom-built WBV platform was designed and developed for this study (Figure 1a). The seated participants were vibrated at a frequency of 5 Hz (sinusoidal vibrations) and vertical acceleration levels of 0.1 g as they performed a simulated driving task. (44) The seat did not have a back support. A passive structure was constructed from Creform materials to create a circular conveyor system. Lifts originated from a height of 0.25 m above the floor and 0.43 m in front of the participants, and terminated 0.86 m above floor level, 0.43 m from the spine to the participant’s left side. The origin and destination conveyors provided 90 degrees of asymmetry to the participants left side (Figure 1b). The destination conveyor had 0.006 m side-to-side space between the box and the guide rails, and a vertical clearance of 0.02 m. The wooden box (0.4 × 0.3 × 0.25 m) lifted by the participants had handles and was filled with reams of paper to the desired weight. Self-report measures (Borg CR-10 scale)(45) were obtained to quantify changes in the overall subjective work load during the seating and the repetitive lifting task. NIRS was used to evaluate changes in muscle physiology during the 60 minutes of simulated driving and the 60 minutes of lifting activity. The NIRS system provides a direct measurement of oxygen delivered and utilized at the muscle site. Sensors from the two-channel INVOS 4100 Cerebral Oximeter (Somanetics Corporation, Troy, MI) were attached over the erector spinae
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The steering wheel and pedal assembly was adjusted to each participants’ desired driving position.
FIGURE 1. Experimental setup during the simulated driving task (a) and the lifting task (b).
muscles at the L3 level. Changes in total, oxygenated, and deoxygenated hemoglobin levels were continuously monitored at 85 Hz. Previously, studies have demonstrated oxygenated hemoglobin levels to be more sensitive in detecting physiological changes during repetitive work(37,40); thus, this is the NIRS measure used in this article. Three-dimensional spine motions were captured (at 120 Hz) using a magnetic motion capture system (The Motion Monitor, Chicago, IL) to assess spine kinematic changes associated with the repetitive asymmetric lifting task and performance changes during the FEMAP performed periodically throughout the data collection process. Bertec force plates (Bertec Corp, Columbus, OH) were used to measure ground reaction forces that provided input into a three-dimensional dynamic linked-segment model within the Motion Monitor System that computes the three-dimensional movements acting at L5/S1. Two Inscale (Terra Haute, IN) force scales, positioned beneath the conveyors, were used to identify the timing of the initial lift and the box placement in the data stream collected during the lifting task. Maximum lifting strength, used to scale the box weight for each individual participant’s capability, was measured using a dynamometer. During 60 minutes of the simulated driving task, participants interacted with the Scania Truck Driving Simulator (SCS Software, Prague, Czech Republic). The driving controls included a steering wheel with integrated pedals mounted in front of the vibrating platform (Racin’ Pro, Subsonic). 174
Procedures At the beginning of each participant’s first experimental session, lifting strength was assessed using a maximum isometric assessment in which the participant attempted to pull upwards on a handle located at the same height and distance from the ankles as the handles on the box during initial part of the lifting task. The box weight which was adjusted to 15% of each individual subject’s maximum lifting strength (indicated by the larger of the two exertions). On average, the box weighed 11.06 kg (s.d. = 3.53 kg). Eleven motion capture system sensors were attached to the participants at the following locations: on the back of the head, at the top of the thoracic spine (T1), over the top of the sacrum (S1), and bilaterally between the elbow and shoulder joint, between hip and knee joint, between knee and ankle joint, and on the wrist using velcro straps and tape. The location of the upper and lower arm and legs sensors was approximately halfway between the adjacent joints; a calibration procedure was used to measure the distance of the sensor from each adjacent joint. Additionally, the two NIRS sensors were placed bilaterally on the erector spinae muscles at the L3 level. Prior to beginning each session, the participant’s maximum range of motion (ROM) in the sagittal plane was measured. With eyes closed, participants were asked to bend forward maximally to obtain each individual’s maximum trunk flexion capacity. 60-minute Simulated Driving Period Prior to the lifting task in each session, the participants performed the simulated driving task during which they were seated on the vibration platform for 60 minutes without back support. Prior to one driving session, the platform was turned on, therein exposing the participants to vibration; in the other session, the participants sat on the same platform for the same amount of time, but it did not vibrate. The participants were instructed that they were not judged on their driving ability. Before beginning the 60-minute driving period, and every 20 minutes thereafter during the 60 minutes of simulated driving, the participants performed the Flexion-Extension Motion Assessment Protocol (FEMAP) described below. The Lifting Task Using the handles, participants repetitively lifted the box from a conveyor in front of them (0.25 m above the floor) to the conveyor located to their left side (0.86 m above floor) without moving their feet and while using primarily a stoop posture. The task was paced so that the participants performed 10 lifts/min for 10 minutes. An audio signal provided every 6 sec indicated when each lift should be initiated. At the end of each 10-minute lifting period, the FEMAP (described below) was performed and a Borg scale rating was obtained, after which another 10-minutes of lifting was initiated. This process of lifting for 10 minutes followed by the FEMAP
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continued until 60 minutes of lifting was completed or until the participant indicated he or she was fatigued and no longer able to continue. Where participants terminated their lifting before the end of the 60-minute period, a final FEMAP was performed and a Borg rating was obtained. Participants were given an opportunity to practice the lifting task before starting the first 10-minute block of lifts. Flexion-Extension Motion Assessment Protocol (FEMAP) Given that Parnianpour and colleagues (1988) reported out-of-plane motions became more prevalent with localized back muscle fatigue, a protocol was developed to periodically evaluate trunk kinematic performance at selected points during the subject’s participation in the experiment. This assessment took approximately 30 seconds and was conducted at the beginning of the 60-minute driving task and every 20 minutes thereafter. Once the lifting task was initiated, the FEMAP was conducted in between each 10 minutes of the lifting activity. (37) In this procedure, the participant with eyes closed repeatedly flexed forward to a target position that was defined as two-thirds of their maximum trunk flexion capacity as fast as they could and returned to an upright standing posture. These motions were repeated 10 times, during which spine kinematic data were collected. Spine position information was provided via auditory feedback; an auditory tone signaled when participants reached the target position and when they returned back to their upright standing posture. Prior to the first experimental session, participants were allowed to practice the flexion-extension task until they felt comfortable with this procedure. Data Analysis Self-report scores obtained from Borg scale ratings were used to identify changes in the overall subjective work load during the 60 minutes of simulated driving and the 60 minutes of repetitive lifting activity. For the muscle physiology measure, the average over each 20-minute period of the driving task and each 10-minute period of the lifting task was calculated. While the simulated driving data were normalized to baseline values, (37) the lifting data were normalized to the initial 10 minutes of the lifting activity. To assess behavioral and biomechanical changes during the prolonged lifting task, the peak three-dimensional spine kinematics and movements were obtained for each lift. The timing of the individual lifts was calculated based on the force scale data. Means of the duration data and peak kinematic and movement measures for each 10 minutes of lifting activity were computed to identify parameters indicative of behavioral and biomechanical adaptations during lifting. For each flexion-extension movement task during the FEMAP, three-dimensional peak displacements and movement velocities were obtained. The resulting data from the NIRS and motion monitor system measures were analyzed using a within subjects repeated measures ANOVA procedure using IBM SPSS, version 19
FIGURE 2. Borg rating - The Borg rating measure during the lifting task as a function of vibration and time. Vertical bars indicate standard error of the mean.
(IBM Corporation, Armonk, NY). Due to the difference in the nature of the task between seating and lifting, the Borg, NIRS, and the FEMAP measures were separately analyzed for the 60 minutes of simulated driving and the 60 minutes of repetitive lifting. For the participants who were not able to complete 60 minutes of repetitive lifting (N = 3, one session each), we used a conservative imputation approach of carrying their last measured value forward so they could be included in the ANOVA procedures. A paired t-test comparison was conducted when significant interactions were obtained between vibration exposure and time. In all statistical tests, a p-value 0.05). Muscle Physiology Measures Oxygenated hemoglobin levels were used to quantify changes in the erector spinae muscle physiology. Oxygenated hemoglobin levels from the left and right erector spinae muscles were highly correlated (r > .92) in both the vibration and no-vibration conditions. Considering the high correlations between the left and right erector spinae muscles, further results focus on the changes oxygenated hemoglobin levels measured on the right side, which was contralateral to the load placement. Figure 3 shows the percentage change of oxygenated hemoglobin derived from the NIRS system decreased significantly over time (p < 0.001) during the lifting task. More specifically, the oxygenated hemoglobin levels decreased significantly for the initial 30 minutes of the repetitive lifting task after which no significant change was observed for this
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Changes in three-dimensional spine movements during the repetitive asymmetric lifting task are shown in Figure 5. Overall, the forward bending movement on the spine (Figure 5a) was unaffected by the repetitive lifting activity over time (p > 0.05). While the twisting movement on the spine (Figure 5b) increased significantly over time (p = 0.012), the lateral bending movement on the spine (Figure 5c) decreased significantly over time (p < 0.001). Exposure to WBV showed no effect on the movement measures. The three-dimensional movement velocities of the spine increased significantly (p < 0.01) over time (Figure 6). In addition to the time-dependent effects, twisting velocities were larger when the lifting followed WBV (62.0 vs 56.9 degrees/ sec, p = 0.025).
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FIGURE 3. NIRS - Oxygenated hemoglobin levels obtained from the NIRS system during the lifting task as a function of vibration and time. Vertical bars indicate standard error of the mean.
physiological measure. WBV exposure prior to the lifting task did not affect these changes in oxygenated hemoglobin levels (p > 0.05). Behavioral Changes with Repetitive Lifting The overall lift duration during the repetitive asymmetric lifting task decreased significantly over time (p < 0.001). This trend was linear (r2 = 0.72) with an average decrease of 0.02 seconds with every 10 minutes of repetitive lifting activity. Exposure to WBV did not change the overall lift duration. A summary of the ANOVA results based on the analysis of the data obtained during the lifting task are shown in Table I. Figure 4a shows the amount of forward bending increased significantly over time (p = 0.016). A post hoc analysis showed that the magnitude of the forward bending during the last 10 minutes of repetitive lifting task was significantly greater than that observed during the initial 10 minutes. WBV exposure showed no effect on this measure. The amount of spine twisting during the repetitive lifting task (Figure 4b) was significantly greater after exposure to whole body vibration (28.8o vs 25.4o, p = 0.046). However, there were no temporal trends in the overall twisting motions across the 60 minutes of lifting. Lateral bending motion of the spine (Figure 4c) was not affected by exposure to WBV (p > 0.05) and did not significantly change over the 60 minutes of lifting.
Flexion-Extension Motion Assessment Protocol (FEMAP) FEMAP During the Driving Task None of the three-dimensional spine kinematics variables showed any change due to the vibration exposure. However, the 60 minutes of simulated driving did significantly affect the peak sagittal plane range of motion (p = 0.047), but not the twisting or lateral bending ROM. (p > 0.05). Peak forward bending, extension, and lateral bending velocities of the spine assessed during the FEMAP also increased significantly across the simulated driving period (p < 0.05). Likewise, there was a similar trend for the twisting velocity (p = 0.061). FEMAP results obtained During the Lifting Task WBV exposure prior to lifting did not affect the spine ROM in the sagittal, frontal, or transverse planes, and showed no significant interactions with time (Table II). Across the 10-minute blocks of lifting, the peak sagittal plane range of motion increased significantly over time (p = 0.003). However, the peak deviations in the axial and coronal planes were not affected by the repetitive lifting activity. The FEMAP spine movement velocities increased significantly across the samples obtained between each 10-minute block of lifting (p < 0.05). For the lateral bending velocity, there was a significant interaction between the vibration exposure and the amount of time spent lifting (Table II). Post hoc analysis showed that this measure increased after 20 minutes of repetitive lifting when lifting was preceded by quiet seating,
TABLE I. The p-Values From the ANOVA Results for Peak Values Averaged Across the 100 Lifts Within Each 10-min Block of Lifting for the Three-Dimensional Spine Kinematic and Movement Measures Independent Variables Vibration Exposure (VE) Time (T) VE x T
Flexion
Twist
Lateral Bend
0.335 0.016 0.895
0.046 0.572 0.575
0.582 0.551 0.625
Forward Bending Moment
Twisting Moment
Lateral movement
Extension Velocity
Twisting Velocity
Lateral Bending Velocity
0.509 0.204 0.299
0.420 0.012 0.604
0.523 < 0.001 0.472
0.440 0.005 0.508
0.025 < 0.001 0.587
0.392 0.001 0.833
Notes: All p-values < 0.05 are indicated by the bold font.
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FIGURE 4. Lifting - The amount of (a) forward bending, (b) twisting, and (c) lateral bending as a function of vibration and time. Vertical bars indicate standard error of the mean.
whereas, only marginal increase (p = 0.057) was observed when lifting was preceded by WBV exposure (Figure 7d). For the twisting velocity, there was a marginally significant interaction (p = 0.069) between vibration exposure and time spent lifting (Figure 7c). No significant interaction between vibration exposure and time spent lifting was observed for the flexion and extension movement velocities of the spine (Figures 7a and 7b). DISCUSSION
T
he purpose of this study was to understand the sequential effects of seated WBV exposure on repetitive asymmetric lifting activity. Overall, exposure to WBV showed no significant effect on the ratings of perceived exertion and muscle physiology measures during the 60 minutes of repetitive asymmetric lifting. As for behavioral measures, repetitive asymmetric lifting activity preceded by WBV exposure resulted in
FIGURE 5. Lifting - (a)forward bending movement, (b) twisting movement, and (c) lateral bending movement as a function of vibration and time. Vertical bars indicate standard error of the mean.
significantly larger twisting motions and twisting velocities of the spine. Changes in the Overall Subjective Workload Our results showed that the overall subjective work load increased significantly over time during the 60 minutes of repetitive asymmetric lifting. These results are similar to other studies in the literature that have shown increases in Borg ratings of perceived exertion during repetitive lifting tasks. (34,45–47) Increases in perceived physical effort over time have been shown previously to be associated with the development of muscle fatigue. (48–51) However, exposure to WBV showed no effect on the overall subjective work load, indicating that this subjective measure of fatigue is largely affected by the prolonged seating and lifting activity independent of the vibration exposure. Changes in Muscle Physiology Measures Tissue oxygenation measures obtained from the NIRS system were used to quantify changes in the erector spinae
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activity is indicative of fatigue development in the erector spinae muscles. Our hypothesis that the exposure to WBV would lead to a larger drop in the tissue oxygenation measures was not supported. As such, our results are in disagreement with the prior studies that have shown erector spinae fatigue with seated WBV exposure. (15,16) In these prior studies, participants were holding an extra weight and were seated with trunk flexed forward. (15,16) These variations would further engage the back extensor muscles and potentially enhance fatigue development. Studies that have looked at a more realistic scenario have found similar back muscle fatigue for WBV exposure and quiet sitting without WBV exposure as seen in the current study. (52,53) It is possible that exposure to WBV increased the amount of co-contraction from the trunk musculature, thereby fatiguing the abdominal muscles; however, we did not measure abdominal muscle physiology.
FIGURE 6. Lifting - (a) Extension, (b) twisting, and (c) lateral bending velocities of the spine as a function of vibration and time. Vertical bars indicate standard error of the mean.
muscle physiology. During the repetitive lifting activity, the oxygenated hemoglobin levels decreased significantly over time. Previously, studies have shown development of muscle fatigue, as measured with EMG spectral analysis, is associated with decrease in oxygenated hemoglobin levels.(40,41) Thus, the decrease in oxygenated hemoglobin levels during the simulated driving and the repetitive asymmetric lifting
Behavioral Changes after Exposure to WBV The amount of twisting and the twisting velocity of the spine during the repetitive asymmetric lifting tasks were significantly higher when participants were exposed to WBV. Exposure to WBV has been known to affect neuromuscular control of the spine. Specifically, studies have reported increased latency in the feedback response after exposure to seated WBV. (13,14,54) Thus, the larger twisting motions and movement velocity after exposure to WBV could be a coping strategy to compensate for the poor feedback response in controlling precise motions during box placement. Twisting motions increase facet contact pressures and tensile stresses on selected annulus disc fibers, (55) thereby increasing the risk of injury. In their lumbar motion monitor model, Marras et al. (23) identified twisting velocity as one of the key factors used to determine LBD risk. Both the amount of twisting and the velocity of twisting have been reported to increase coactivity from the trunk musculature, (36,56) and increased co-contraction has been demonstrated to increase LBD risk. (57) Behavioral Changes Associated with Time The behavioral changes reported here are similar to the results obtained in our prior work. (37) In this study, twisting movements typically peaked while picking the boxes in a flexed posture. One of the tissues under strain in the stooped posture from which the lifts originated is the annulus fibrosis,
TABLE II. P-Values From the ANOVA Results for the Three-Dimensional Spine Kinematics Measures During the FEMAP Collected in Between Lifting Activity Independent Variables Time (T) VE x T
ROM (Sagittal)
ROM (axial)
ROM (coronal)
Forward Bending Velocity
Extension Velocity
Twisting Velocity
Lateral Bending Velocity
0.944 0.003 0.932
0.840 0.323 0.580
0.442 0.752 0.306
0.673 < 0.001 0.439
0.882 0.001 0.969
0.239 0.024 0.069
0.368 0.033 0.008
p < 0.05 are indicated by bold values.
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FEMAP
T
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he flexion-extension motion assessment protocol was developed to test whether prolonged repetitive asymmetric lifting that was preceded by whole body vibration exposure leads to deterioration in motor performance when performing a simple flexion-extension task. Our hypothesis was partially supported in that participants showed an increase in their sagittal plane range of motion during the simulated driving and the lifting tasks. Larger deviations of the spine in the sagittal plane suggest creep in the passive structures of the spine (59) or a change in the neuromuscular response. However, the lack of differences between the vibration conditions suggest that it is the lifting task rather than the vibration exposure that is contributing to this increased motion in the sagittal plane. There were some limitations concerning this study. First, we should recognize that all our participants were novice manual handlers from a university population, and previous studies have shown differences between experienced and inexperienced lifters. (60–,62) One should be cautious in generalizing the results obtained from this study to experienced lifters. Second, the participants were not provided a backrest during the seated driving task. The addition of a backrest would substantially change the transmission of vibrations to the human body. (63) Third, the workplace layout was fixed in our study. Due to this, the relative work height and reach distance to the box would have varied with the anthropometric diversity in the sample. Fourth, the participants were restricted from lifting with their legs. This was done to limit the movement degrees of freedom to the spine and upper extremities. Clearly, the ability to increase the use of lower extremities in lifting could significantly alter the results found in this study. CONCLUSION
W
ith a repetitive asymmetric lifting activity, behavioral changes that increase the risk of back injury included more forward bending, larger twisting movements, and larger three-dimensional movement velocities. This risk was further elevated when lifting activity was preceded by WBV exposure as there was more spine twisting and larger twisting velocities observed with exposure to WBV.
FIGURE 7. (a) Forward bending, (b) extension, (c) twisting, and (d) lateral bending velocities of the spine sampled during the FEMAP which was run every 10 minutes during the lifting task. Vertical bars indicate standard error of the mean.
which is not well suited to resist large twisting movements. Thus, the increase in twisting movements along with simultaneous forward bending motions would increase the risk of low back injury. (58) Further, three of the five factors that changed over time with the repetitive lifting activity (amount of forward bending, twisting velocity, and lateral bending velocity) have also been shown to increase the risk of back injury. (23)
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