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The effect of standing interventions on acute low-back postures and muscle activation patterns
Applied Ergonomics 58 (2017) 281e286
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Applied Ergonomics journal homepage: www.elsevier.com/locate/apergo
The effect of standing interventions on acute low-back postures and muscle activation patterns Kayla M. Fewster, Kaitlin M. Gallagher 1, Jack P. Callaghan* Department of Kinesiology, University of Waterloo, Waterloo, ON, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 November 2015 Received in revised form 29 June 2016 Accepted 5 July 2016
Occupations requiring prolonged periods of constrained standing are associated with the development of low back pain (LBP). Many workplaces use improvised standing aids aimed to reduce LBP. Unfortunately, there is little scientific evidence to support the use of such standing interventions in effectively reducing LBP. To assess some commonly implemented standing interventions, thirty-one participants stood in four different standing positions (Level Ground (control), Sloped, Elevated, and Staggered) for 5 min each. The use of an elevated surface changed the lumbar spine posture of participants such that participants stood in a more flexed lumbar spine posture. This change in lumbar spine posture may be an indication that the elevated standing aid intervention can positively impact lumbar spine posture in standing pain developers and potentially reduce LBP. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Low back pain Standing aid Standing intervention Prolonged standing
1. Introduction Occupations requiring prolonged static standing are associated with low back pain (LBP) development (Andersen et al., 2007; Tissot et al., 2009). Approximately 50% of individuals (Marshall et al., 2011; Nelson-Wong et al., 2008; Nelson-Wong and Callaghan, 2010b; Gallagher et al., 2014) are susceptible to acute LBP development during prolonged standing (pain developers, PDs). Those identified as PDs have a 3 times higher likelihood of seeking clinical care for LBP within 2 years (Nelson-Wong and Callaghan, 2014). Many workplaces have begun using improvised standing positions aimed to reduce LBP and standing aids are commonly recommended by various occupational health and safety associations (Canadian Center of Occupational Health and Safety (CCHOS), 2008, Occupational Safety & Health Administration (OSHA), 2012). Unfortunately, there is little scientific evidence to support the use of such standing interventions in effectively reducing LBP during prolonged standing. Therefore, the purpose of this work was to investigate the short-term differences in lumbar spine posture and muscle activation patterns between PDs and non-pain developers (NPDs) utilizing a variety of commonly implemented standing
* Corresponding author. Faculty of Applied Health Sciences, Department of Kinesiology, University of Waterloo, Waterloo, N2L 3G1 Ontario, Canada. E-mail address: [email protected] (J.P. Callaghan). 1 Present address: Health, Human Performance and Recreation, University of Arkansas, Fayetteville, AR, USA. http://dx.doi.org/10.1016/j.apergo.2016.07.002 0003-6870/© 2016 Elsevier Ltd. All rights reserved.
interventions. PDs have been shown to stand with greater lumbar lordosis (extension) than NPDs during a prolonged standing task (Sorensen et al., 2015). A positive relationship was displayed between lordosis angle and maximum reported low back pain scores on a visual analog scale. A radiographic study that imaged the sagittal lumbar spine found that PDs stood closer to the maximum extension angle of their lower lumbar arc (L3-S1) than NPDs (Gallagher et al., 2014). Seated breaks between prolonged bouts of standing have also been shown to help temporarily alleviate LBP development, a movement that also induces lumbar spine flexion movement (Gallagher et al., 2014). Based on these postural characteristics of PDs when compared to NPDs, inducing flexion in the lumbar spine via an altered standing position could provide a means to mitigate LBP development when standing. Motor control patterns also differ between PDs and NPDs during prolonged periods of level ground standing. PDs demonstrate increased bilateral gluteus medius muscle co-activation, while NPDs tend to exhibit reciprocal firing of these muscles (NelsonWong et al., 2008). In two follow up studies, gluteus medius cocontraction indexes (Nelson-Wong and Callaghan, 2010b) were higher, and cross-correlations values (Marshall et al., 2011) were higher and positive in PDs compared to NPDs, indicating gluteus medius co-activation. This co-activation response is hypothesized to be a potential pre-disposing factor or symptom of pain development since it is evident at the start of a standing task prior to pain development (Nelson-Wong and Callaghan, 2010b).
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Alternate standing positions have been recommended in attempt to reduce LBP and help workers tolerate prolonged bouts of standing. To date, recommended standing interventions have attempted to incorporate posture and movement into the design of the intervention (Table 1). The majority of standing investigations have focused on postural changes associated with standing interventions and little work has looked at the influence of standing interventions on muscle co-activation patterns. Muscle activation patterns are a distinguishing feature between PDs and NPDs (Nelson-Wong et al., 2008), and therefore it is important to consider when evaluating the successfulness of a standing intervention. By examining the influence of alternative standing positions on key variables that differentiate PDs from NPDs, we can gain insight into what standing interventions could be helpful at reducing or preventing LBP. A change in muscle co-activation in PDs, resulting in a decrease in muscle co-activation, may be an indication that an alternative standing position could be effective at reducing LBP across PDs. The purpose of this study was to evaluate alternate standing positions based on how they impact lumbar spine posture and muscular activity. We hypothesized that the use of standing interventions, sloped surface, elevated single foot rest, and staggered stance will move participants into a more flexed lumbar posture compared to level ground standing with a subsequent increase in muscle activation. Results from this study may explain if recommended standing interventions can positively change motor control strategies and lumbar spine posture previously linked to low back pain development during prolonged standing. In this study, lumbar spine angle and hip and trunk muscle activations were measured while participants stood on level ground and the three alternate standing positions.
2. Methods 2.1. Participants Twenty-three participants (11 male, 12 female) between the ages of 18e35 participated in this study. Exclusion criteria included
any previous history of low back pain that was significant enough to seek medical intervention or that resulted in greater than three days off work or school, previous lumbar or hip surgery, employment in a task that required prolonged static standing during the past 12 months, and the inability to stand for at least two hours. All participants had previously participated in a prior prolonged standing simulation that categorized participants as either PDs (11 participants) or NPDs (12 participants) based on self-reported visual analog scores (VAS). A participant was considered a PD if they reported any change in VAS score greater than 10 mm from baseline during the prolonged standing simulation (Gallagher et al., 2014; Marshall et al., 2011; Nelson-Wong et al., 2010). Ethics approval for research involving Human Subjects was obtained from the Office for Research Ethics at the University of Waterloo.
2.2. Instrumentation Four pairs of disposable surface EMG electrodes (Blue Sensor, Ambu A/S, Denmark) were placed on the lumbar erector spinae (LES, above and below the third lumbar spinous process) and gluteus medius muscles (GM, approximately 50% of the distance between the iliac crest and greater trochanter). The GM muscle was chosen because GM activation patterns are a distinguishing feature between PDs and NPDs during prolonged standing. PDs tend to display gluteus medius co-activation, meaning that both muscles are activated together. NPDs tend to display gluteus medius reciprocal firing, meaning that one muscle is being activated while the contralateral muscle is not (Nelson-Wong et al., 2008). As a result the goal of monitoring GM activations was to examine the influence of different standing interventions on activation patterns across PDs and NPDs. A reference electrode was placed over the spinous process of the seventh cervical vertebrae. EMG signals were differentially amplified using a common mode rejection ratio of 115 dB (at 60 Hz; input impedance of 1010 U), analog band-pass filtered from 10 to 500 Hz and gained by a factor of 500e5000 (AMT- 8, Bortec, Calgary AB, Canada). The specific gain used was tailored to each individual muscle using sub-maximal test contractions and real-time visual feedback to best fill the input range of
Table 1 A summary table of commonly implemented standing interventions and previous studies’ general findings. Study
Evaluated standing intervention
Assessment
Findings of standing intervention
Dolan et al., 1988
One leg elevated onto a 20 cm platform
Compared low back muscle activity when level standing to standing with one leg elevated on a platform.
Gallagher and Callaghan (2016)
Declining sloped surface of 16
Compared the posture differences between level standing and standing on a declining sloped surface during prolonged standing.
Nelson-Wong and Callaghan (2010a)
Self selected alternation between standing on a 16 incline and decline surface
Compared differences between using a sloped surface during prolonged standing and level ground prolonged standing.
Gallagher et al. (2013)
Short-term differences between standing on incline and decline sloped surfaces
Gallagher (2014)
Posture differences between standing on a declining sloped surface and standing with one foot elevated on a platform.
To examine the short and long term responses to standing when using a sloped surface on pelvis, lumbar, and trunk angles. Radiographic assessment of sagittal lumbopelvic postures between PDs and NPDs when standing on level ground, standing with one leg elevated and standing on a sloped surface
Standing with one leg elevated and resulted in increased lumbar flexion and increased low back muscle activity when compared to level standing. Sloped standing reduced LBP scores by 59.4% for PD when compared to level ground. All participants showed hip joint flexion, trunk- tothigh angle flexion, and posterior translation of the trunk center of gravity towards the ankle joint when standing on the sloped surface compared to level ground. Using a sloped surface reduced LBP scores by 59.4% for PD when compared to level standing and decreased gluteus medius co-activation levels. NPDs showed increased bilateral gluteus medius co-activation of these muscles. Using a sloped surface increased trunk flexion and posterior rotation of the pelvis. The elevated surface was most effective at causing lumbosacral lordosis flexion, and the declined sloped surface was more effective at inducing L1/L2 intervertebral joint flexion.
K.M. Fewster et al. / Applied Ergonomics 58 (2017) 281e286
the A/D converter (Winter 2009). The gained signals then remained fixed for the remainder of the collection and the signals were then sampled at 2048 Hz using a 16-bit A/D conversion card. Maximum Voluntary Contractions (MVCs) were collected from each muscle for normalization purposes. MVCs for the LES were obtained as the participant lay prone on a table with their torso hanging off the edge of the table at the level of the anterior superior iliac spine. Participants crossed their arms over their chest, bent their torso towards the ground as a starting position and then extended their trunk to resistance applied by the experimenter. GM MVC’s were obtained through resisted hip abduction in the side lying position. Ten second rest trials were taken in both the prone and supine resting positions. An Optotrak Certus motion capture system (Northern Digital Inc. Waterloo, ON, sampling at 32 Hz) was used to track movement of infrared markers during the data collection. Two rigid bodies were used to track movement of the spine, one rigid body containing 4 infrared markers was placed at the level of L1/L2 (upper lumbar spine), and one rigid body containing 5 infrared markers was placed over the sacrum. Four different standing positions were assessed in this study (Fig. 1): 1. Standing with both feet side by side on level ground (Level) 2. Standing with one foot raised on a platform (Elevated). The height of the platform was adjusted such that the participant’s thigh to trunk angle was 135 . This posture is described as physiological normal for the lumbar spine compared to erect standing where some curvature of the lumbar spine is still maintained (Keegan, 1953). 3. Standing on a sloped surface (Sloped), with a 16-degree decline (toes facing down). 4. Staggered Standing (Staggered). The foot that was placed on the elevated surface was the same one that was the front foot during the staggered standing trials. The participant moved the front foot forward such that the arch of that foot was in line with the toes of the other foot. It is important to note that the raised leg in the elevated standing condition was always the same leg that was placed forward in the staggered stance. From this point forward the raised/ forward leg will be referred to as the “leg up” and the other leg will be referred to as the “leg down”.
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2.3. Experimental procedures and protocol A maximum lumbar extension trial was recorded for a reference posture of lumbar spine angle. During this trial the participant was told to keep their knees locked and bend backwards about their lumbar spine without shifting their hips forward. The participant then stood in the four standing conditions for 5 min each. A short duration of 5 min was chosen in attempt to gauge the postural response of the standing interventions in the absence of time varying and pain factors. The first condition was always Level standing. The remaining three standing conditions were then randomized for each participant. The participant stood for 5-min and then sat for 1-min between the trials, in an attempt to minimize the development of low back pain. During each condition the participant worked at a computer desk performing a standardized typing task. The desk was adjusted to 5e6 cm below their wrist when their elbows were placed at 90 (Kroemer and Grandjean, 1997). For the sloped surface, the height of the table was higher to accommodate standing on the surface. 2.4. Data processing and analysis 2.4.1. Kinematic data Kinematic data was imported into Visual 3D (C-Motion, Kingston, ON) to calculate lumbar spine joint angles. Each marker’s coordinate data were filtered using a 6 Hz s order dual pass Butterworth filter. Mean lumbar angles were determined using a Flexion/Extension e Lateral Bend-Axial Rotation sequence. The average lumbar spine angle was computed across each 5 min trial for all standing conditions and expressed with respect to maximum lumbar extension. 2.4.2. Electromyography data All EMG signals were highpass filtered using a 30 Hz, second order Butterworth filter, to remove any ECG contamination (Drake and Callaghan, 2006). A notch (bandstop) filter was applied with filter cutoff frequencies from 59 to 61 Hz, to remove any 60 Hz electrical contamination (Mello et al., 2007). EMG signals were then full wave rectified and dual passed filtered through a fourth order Butterworth filter with a cutoff frequency of 6 Hz (Nelson-Wong et al., 2008). The resulting linear enveloped signals were then normalized to MVC. Resting activation was then subtracted off the normalized EMG signal. The processed EMG signals were down sampled to 32 Hz prior to further data analysis as a data reduction
Fig. 1. The four standing conditions tested. From left to right: level, elevated, sloped and staggered.
K.M. Fewster et al. / Applied Ergonomics 58 (2017) 281e286
measure. Cross-correlation analyses were used to quantify the common signal between right and left GM and LES signals. Cross-correlation is an analysis technique used to quantify the similarity in shape and the phase delay between two time-varying waveforms (NelsonWong et al., 2009). A highly positive correlation indicates that the two signals are acting together in phase, whereas a highly negative correlation indicates that one signal is at maximum and the other is at a minimum (out of phase). Practically, this technique is often used to access the degree of co-activation between two muscles (in the case of this study between the right and left pairs of the LES and GM). Positive cross-correlation values indicate muscles are being activated together (co-activating), while negative cross-correlation values indicate that one muscle is being activated while the other is not, implying reciprocal firing of the muscles (Nelson-Wong et al., 2009). Five 1-min blocks of the bilateral gluteus medius EMG from each standing trial were entered into a custom Matlab program (Version 8.5 Mathworks, Inc., Natick, MA, USA) to compute the cross-correlation coefficients, Rxy, with the following equation:
Rxy ðtÞ ¼
1 T
ZT 0
xðtÞyðt þ tÞdt Rxx ð0ÞRyy ð0Þ
Where Rxy(t) is the normalized cross-correlation of two signals, x(t) and y(t) at a phase shift t with a potential range of values between 1 and þ1 and T is the length of the record. The maximum crosscorrelation of each 1-min block was computed across a phase shift of ± 500 ms, then the average Rxy was calculated over each 5-min trial to provide an overall pattern of co-activation during each standing condition (Nelson-Wong et al., 2009). This analysis provided one average Rxy value for the right and left GM and LES for each standing condition. Average EMG signals were used to obtain an estimate of muscle activation levels during the standing conditions. Average EMG for the right and left LES, and GM were obtained by taking an average of the normalized EMG signals over each 5-min trial, for each standing condition.
2.5. Statistical analysis No significant gender effects were observed and therefore the data were collapsed across gender. A two-way mixed general linear model assessed the influence of Pain Status and Standing Position on lumbar spine posture and Cross-correlation values. A three-way mixed general linear model assessed the influence of Pain Status, Leg Up vs. Leg Down, and Standing Position on average EMG activation levels (SPSS v20, IBM Corporation, Somers, NY, USA). Any significant effects or interaction effects were further evaluated using the Tukey post hoc test. An alpha level of 0.05 was set for significance.
3. Results 3.1. Kinematic differences A main effect of standing condition for lumbar spine angle was observed (p ¼ 0.008). When using the elevated standing aid, participants stood with greater lumbar spine flexion (23.07 ± 9.43 degrees of flexion) compared to level Ground (20.32 ± 8.36 ) and Sloped (20.20 ± 8.88 ) (Fig. 2). There were no significant differences between the Staggered condition and any of the other standing conditions (Fig. 2).
25.00
Lumbar Spine Angle With Respect to Maximum Extenstion (Degrees)
284
A
A,B
B
B
20.00
15.00
10.00
5.00
0.00 Level
Elevated
Staggered
Sloped
Standing Position
Fig. 2. Mean (standard error) lumbar spine angles across standing conditions (degrees) with respect to maximum extension (note: maximum extension ¼ 0 ) for each of the investigated standing conditions (i.e. a greater lumbar spine angle indicates more flexion). Means indicated with different letters are significantly different.
3.2. Muscle activation differences 3.2.1. Cross-correlation differences A significant interaction effect of Pain Status x Standing Position was found for GM cross-correlation. This was largely driven by a difference that was exhibited between PD and NPD, GM crosscorrelation during the Level ground condition (PD Rxy ¼ 0.14 ± 0.12; NPD Rxy ¼ 0.0055 ± 0.015, p ¼ 0.026) and a different directional response in the Elevated standing condition (Fig. 3). No significant differences were observed in crosscorrelation values for any of the three standing interventions investigated across PDs and NPDs (Fig. 3). This was as a result of an increase in cross-correlation values for NPD while using each of the standing interventions (Fig. 3). No significant differences were observed in cross-correlation values for the LES. 3.2.2. Average EMG activation Average activation levels for the right and left muscles were reported as leg up and leg down as per the leg that was lifted up/ placed forward during the elevated/staggered standing trials and the leg that was down on the floor/placed back. No significant differences were observed for the GM and LES activation levels across leg, pain group or standing conditions. The EMG values were low on average, while not statistically different, PDs had the tendency to have slightly higher average GM activation levels (Table 2). 4. Discussion Consistent with our hypothesis, lumbar spine posture was altered with the use of a standing intervention. Contrary to our hypothesis, there were no accompanying changes in muscle activation patterns across standing interventions. The Elevated standing aid resulted in greater lumbar spine flexion when compared to the Level and Sloped conditions. Across all standing conditions PDs exhibited a positive GM cross-correlation value, indicating that the GM muscles were co-activated. This study has shown that PDs display significant differences in muscle activation patterns compared to NPDs during level standing. PDs did demonstrate a slight reduction in the co-activation of the GM when using standing interventions (Fig. 3); however, these positions did cause an increase in GM muscle activation patterns in NPDs similar to the magnitudes found for PDs. The increase in lumbar flexion with the
K.M. Fewster et al. / Applied Ergonomics 58 (2017) 281e286
285
0.2
Right-Left Average GM Cross-Correlation Values
PD
0.15
NPD
B"
0.1
0.05
A"
0 Level
-0.05
Elevated
Staggered
Sloped
Standing Condition
Fig. 3. The average Right-Left GM cross-correlation values for each standing condition. Standard Error bars are displayed. Means indicated with different letters are significantly different.
Table 2 A summary table of the average GM activation for PDs and NPDs across standing positions.
Level Elevated Staggered Sloped
PD NPD PD NPD PD NPD PD NPD
use of the Elevated standing aid may be a possible mechanism for decreasing pain in PDs during prolonged standing given the reduction in co-activation and a change in posture to less extended positions e two factors previously linked to increased pain reporting (Nelson-Wong et al., 2008; Sorensen et al., 2015). The use of an elevated surface changed the lumbar spine posture of participants such that participants stood in a more flexed lumbar spine posture, which compares well with previous work (Dolan et al., 1988; Gallagher, 2014). Pain developers have been shown to stand with greater lumbar lordosis (extension) than NPDs during a prolonged standing task (Sorensen et al., 2015). This change in lumbar spine posture, which is the same postural difference that is observed between PDs and NPDs, may be an indication that the elevated standing intervention can positively change lumbar spine posture in PDs and potentially reduce LBP. A follow-up study of prolonged standing while using an elevated standing aid is required to determine if it would be a successful intervention for reducing prolonged standing induced LBP. The sloped surface and staggered stance were unsuccessful at changing lumbar spine posture when compared to flat standing. The lack of change in lumbar spine posture with the use of a sloped surface was surprising. This result is in contrast to previous findings
Average activation (Percent MVC)
Standard deviation
1.05 0.75 1.09 0.93 1.19 0.64 1.28 0.83
0.74 0.59 0.64 0.58 0.88 0.33 1.2 0.62
(Gallagher et al., 2013; Nelson-Wong and Callaghan, 2010a), who found an increase in lumbar flexion angle when using a sloped surface and a 59.4% reduction in LBP (Nelson-Wong and Callaghan, 2010a). Gallagher (2014), also found no difference in lumbar spine angle between standing on level ground versus a decline slope when using internal x-ray measurements. Combined with the results of the current study, it seems that the sloped surface may not alter lumbar spine angle. The use of a sloped surface may induce other posture alterations such as hip posture changes, this may explain the previous success in LBP reduction with this standing intervention (Nelson-Wong and Callaghan, 2010a). In addition, Nelson-Wong and Callaghan 2010a, investigated posture differences with the use of the sloped surface over a much longer period of time when compared to this study. This may also contribute to the differences in posture seen between this study and the previous study. In the case of the staggered stance, this alternate standing position was hypothesized to help individuals tolerate prolonged standing by allowing them to sway their trunk forward to rest their body weight on the ball of the forward foot (Zacharkow, 1988). By alternating the weight bearing foot, and shifting the body weight back and forth, this standing intervention has the potential to reduce LBP development during prolonged standing by inducing
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small movements early on when standing. A gross measure such as average lumbar flexion angle may not be sensitive enough to demonstrate the small flexion-extension motions this intervention may induce. Moving forward it may be beneficial for future work to investigate the staggered stance intervention with smaller lumbar movements throughout a longer period of standing. Despite the change in lumbar spine flexion angle for the elevated standing aid, no changes in GM and LES muscle activation between PDs and NPDs across each of the standing interventions were noted in this study. As found previously (Nelson-Wong et al., 2008), during the Level standing condition NPDs had significantly lower GM cross-correlation values when compared to PDs. This indicates that PD’s displayed co-activation of the bilateral GM muscles, whereas NPDs did not. When using each of the three alternate standing positions no significant differences between PDs and NPDs were observed. Although not statistically significant, using the standing interventions increased GM muscle coactivation patterns for NPDs only. This is a potential concern because the NPD group exhibited a pattern when using the standing interventions that could be indicative of identifying highrisk individuals for developing LBP in level standing. A limitation to this study is that participants were exposed to the level standing condition prior to any of the standing interventions; therefore there is the potential for order effects. We chose this method so that we were able to obtain a true baseline for level ground standing. Second, each participant was only exposed to each standing condition for 5 min. This may not have been sufficient time to distinguish muscle activation patterns. This time frame was chosen to mitigate LBP development so it did not influence the evaluation of basic changes of lumbar spine posture and muscle activation.
5. Conclusions The elevated standing aid intervention introduced changes in standing style that resulted in increased lumbar spine flexion when compared to level standing. The sloped surface and staggered stance were unsuccessful at changing lumbar spine posture when compared to level standing. This change in lumbar spine posture, which is the same posture difference seen between PDs and NPDs, may be an indication that the elevated standing aid intervention can positively change lumbar spine posture in PD and potentially reduce LBP. The interventions show promise for PDs but raise concern that they could introduce unwanted changes in NPDs so broad based applications for all workers should be undertaken with caution. Future work should assess longer durations when using standing interventions and assess hip and thigh postural alterations.
References Andersen, J.H., Haahr, J.P., Frost, P., 2007. Risk factors for more severe regional musculoskeletal symptoms: a two-year prospective study of a general working population. Arthritis Rheum. 56, 1355e1364. Canadian Center of Occupational Health and Safety (CCHOS), 2008. Working in A standing position, 2012. Dolan, P., Adams, M.A., Hutton, W.C., 1988. Commonly adopted postures and their effect on the lumbar spine. Spine 13, 197e201. Drake, J.D., Callaghan, J.P., 2006. Elimination of electrocardiogram contamination from electromyogram signals: an evaluation of currently used removal techniques. J. Electromyogr. Kinesiol. Off. J. Int. Soc. Electrophysiol. Kinesiol. 16, 175e187. Gallagher, K.M., Callaghan, J.P., 2016. Standing on a declining surface reduces transient prolonged standing induced low back pain development. Appl. Ergon 56, 76e83. Gallagher, K.M., Campbell, T., Callaghan, J.P., 2014. The influence of a seated break on prolonged standing induced low back pain development. Ergonomics 57 (4), 555e562. Gallagher, K.M., 2014. The Relationships of Prolonged Standing Induced Low Back Pain Development with Lumbopelvic Posture and Movement Patterns. University of Waterloo (Doctorate Thesis). Gallagher, K.M., Wong, A., Callaghan, J.P., 2013. Possible mechanisms for the reduction of low back pain associated with standing on a sloped surface. Gait Posture 27, 313e318. Keegan, J.J., 1953. Alterations of the lumbar curve related to posture and seating. J. Bone Jt. Surg. Am. Vol. 35-A, 589e603. Kroemer, K.H.E., Grandjean, E., 1997. Fitting the Task to the Human: a Textbook of Occupational Ergonomics. CRC Press. Marshall, P.W.M., Patel, H., Callaghan, J.P., 2011. Gluteus medius strength, endurance, and co- activation in the development of low back pain during prolonged standing. Hum. Mov. Sci. 30, 63e73. Mello, R.G., Oliveira, L.F., Nadal, J., 2007. Digital Butterworth filter for subtracting noise from low magnitude surface electromyogram. Comput. Methods Programs Biomed. 87, 28e35. Nelson-Wong, E., Callaghan, J.P., 2014. Transient low back pain development during standing predicts future clinical low back pain in previously asymptomatic individuals. Spine 39, 379e383. Nelson-Wong, E., Howarth, S.J., Callaghan, J.P., 2010. Acute biomechanical responses to a prolonged standing exposure in a simulated occupational setting. Ergonomics 53, 1117e1128. Nelson-Wong, E., Callaghan, J.P., 2010a. The impact of a sloped surface on low back pain during prolonged standing work: a biomechanical analysis. Appl. Ergon. 41, 787e795. Nelson-Wong, E., Callaghan, J.P., 2010b. Is muscle co-activation a predisposing factor for low back pain development during standing? A multifactorial approach for early identification of at-risk individuals. J. Electromyogr. Kinesiol. 20, 256e263. Nelson-Wong, E., Howarth, S., Winter, D.A., Callaghan, J.P., 2009. Application of autocorrelation and cross-correlation analyses in human movement and rehabilitation research. J. Orthop. Sports Phys. Ther. 39 (4), 287e295. Nelson-Wong, E., Gregory, D.E., Winter, D.A., Callaghan, J.P., 2008. Gluteus medius muscle activation patterns as a predictor of low back pain during standing. Clin. Biomech. 23, 545e553. Occupational Safety & Health Administration (OSHA), 2012. Youth Worker Safety in Restaurants (ETool) - Prolonged Standing, 2012. Sorensen, C.J., Norton, B.J., Callaghan, J.P., Hwang, C., Van Dillen, L.R., 2015. Is lumbar lordosis related to low back pain development during prolonged standing? Man. Ther. 1, 1e5. Tissot, F., Messing, K., Stock, S., 2009. Studying the relationship between low back pain and working postures among those who stand and those who sit most of the working day. Ergonomics 52, 1402e1418. Winter, D.A., 2009. Biomechanics and Motor Control of Human Movement, fourth ed. John Wiley & Sons Inc., Hoboken, New Jersey (Chapter 15). Zacharkow, D., 1988. Posture: Sitting, Standing, Chair Design, and Exercise. Thomas, Springfield, Ill, USA.
Contents lists available at ScienceDirect
Applied Ergonomics journal homepage: www.elsevier.com/locate/apergo
The effect of standing interventions on acute low-back postures and muscle activation patterns Kayla M. Fewster, Kaitlin M. Gallagher 1, Jack P. Callaghan* Department of Kinesiology, University of Waterloo, Waterloo, ON, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 November 2015 Received in revised form 29 June 2016 Accepted 5 July 2016
Occupations requiring prolonged periods of constrained standing are associated with the development of low back pain (LBP). Many workplaces use improvised standing aids aimed to reduce LBP. Unfortunately, there is little scientific evidence to support the use of such standing interventions in effectively reducing LBP. To assess some commonly implemented standing interventions, thirty-one participants stood in four different standing positions (Level Ground (control), Sloped, Elevated, and Staggered) for 5 min each. The use of an elevated surface changed the lumbar spine posture of participants such that participants stood in a more flexed lumbar spine posture. This change in lumbar spine posture may be an indication that the elevated standing aid intervention can positively impact lumbar spine posture in standing pain developers and potentially reduce LBP. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Low back pain Standing aid Standing intervention Prolonged standing
1. Introduction Occupations requiring prolonged static standing are associated with low back pain (LBP) development (Andersen et al., 2007; Tissot et al., 2009). Approximately 50% of individuals (Marshall et al., 2011; Nelson-Wong et al., 2008; Nelson-Wong and Callaghan, 2010b; Gallagher et al., 2014) are susceptible to acute LBP development during prolonged standing (pain developers, PDs). Those identified as PDs have a 3 times higher likelihood of seeking clinical care for LBP within 2 years (Nelson-Wong and Callaghan, 2014). Many workplaces have begun using improvised standing positions aimed to reduce LBP and standing aids are commonly recommended by various occupational health and safety associations (Canadian Center of Occupational Health and Safety (CCHOS), 2008, Occupational Safety & Health Administration (OSHA), 2012). Unfortunately, there is little scientific evidence to support the use of such standing interventions in effectively reducing LBP during prolonged standing. Therefore, the purpose of this work was to investigate the short-term differences in lumbar spine posture and muscle activation patterns between PDs and non-pain developers (NPDs) utilizing a variety of commonly implemented standing
* Corresponding author. Faculty of Applied Health Sciences, Department of Kinesiology, University of Waterloo, Waterloo, N2L 3G1 Ontario, Canada. E-mail address: [email protected] (J.P. Callaghan). 1 Present address: Health, Human Performance and Recreation, University of Arkansas, Fayetteville, AR, USA. http://dx.doi.org/10.1016/j.apergo.2016.07.002 0003-6870/© 2016 Elsevier Ltd. All rights reserved.
interventions. PDs have been shown to stand with greater lumbar lordosis (extension) than NPDs during a prolonged standing task (Sorensen et al., 2015). A positive relationship was displayed between lordosis angle and maximum reported low back pain scores on a visual analog scale. A radiographic study that imaged the sagittal lumbar spine found that PDs stood closer to the maximum extension angle of their lower lumbar arc (L3-S1) than NPDs (Gallagher et al., 2014). Seated breaks between prolonged bouts of standing have also been shown to help temporarily alleviate LBP development, a movement that also induces lumbar spine flexion movement (Gallagher et al., 2014). Based on these postural characteristics of PDs when compared to NPDs, inducing flexion in the lumbar spine via an altered standing position could provide a means to mitigate LBP development when standing. Motor control patterns also differ between PDs and NPDs during prolonged periods of level ground standing. PDs demonstrate increased bilateral gluteus medius muscle co-activation, while NPDs tend to exhibit reciprocal firing of these muscles (NelsonWong et al., 2008). In two follow up studies, gluteus medius cocontraction indexes (Nelson-Wong and Callaghan, 2010b) were higher, and cross-correlations values (Marshall et al., 2011) were higher and positive in PDs compared to NPDs, indicating gluteus medius co-activation. This co-activation response is hypothesized to be a potential pre-disposing factor or symptom of pain development since it is evident at the start of a standing task prior to pain development (Nelson-Wong and Callaghan, 2010b).
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Alternate standing positions have been recommended in attempt to reduce LBP and help workers tolerate prolonged bouts of standing. To date, recommended standing interventions have attempted to incorporate posture and movement into the design of the intervention (Table 1). The majority of standing investigations have focused on postural changes associated with standing interventions and little work has looked at the influence of standing interventions on muscle co-activation patterns. Muscle activation patterns are a distinguishing feature between PDs and NPDs (Nelson-Wong et al., 2008), and therefore it is important to consider when evaluating the successfulness of a standing intervention. By examining the influence of alternative standing positions on key variables that differentiate PDs from NPDs, we can gain insight into what standing interventions could be helpful at reducing or preventing LBP. A change in muscle co-activation in PDs, resulting in a decrease in muscle co-activation, may be an indication that an alternative standing position could be effective at reducing LBP across PDs. The purpose of this study was to evaluate alternate standing positions based on how they impact lumbar spine posture and muscular activity. We hypothesized that the use of standing interventions, sloped surface, elevated single foot rest, and staggered stance will move participants into a more flexed lumbar posture compared to level ground standing with a subsequent increase in muscle activation. Results from this study may explain if recommended standing interventions can positively change motor control strategies and lumbar spine posture previously linked to low back pain development during prolonged standing. In this study, lumbar spine angle and hip and trunk muscle activations were measured while participants stood on level ground and the three alternate standing positions.
2. Methods 2.1. Participants Twenty-three participants (11 male, 12 female) between the ages of 18e35 participated in this study. Exclusion criteria included
any previous history of low back pain that was significant enough to seek medical intervention or that resulted in greater than three days off work or school, previous lumbar or hip surgery, employment in a task that required prolonged static standing during the past 12 months, and the inability to stand for at least two hours. All participants had previously participated in a prior prolonged standing simulation that categorized participants as either PDs (11 participants) or NPDs (12 participants) based on self-reported visual analog scores (VAS). A participant was considered a PD if they reported any change in VAS score greater than 10 mm from baseline during the prolonged standing simulation (Gallagher et al., 2014; Marshall et al., 2011; Nelson-Wong et al., 2010). Ethics approval for research involving Human Subjects was obtained from the Office for Research Ethics at the University of Waterloo.
2.2. Instrumentation Four pairs of disposable surface EMG electrodes (Blue Sensor, Ambu A/S, Denmark) were placed on the lumbar erector spinae (LES, above and below the third lumbar spinous process) and gluteus medius muscles (GM, approximately 50% of the distance between the iliac crest and greater trochanter). The GM muscle was chosen because GM activation patterns are a distinguishing feature between PDs and NPDs during prolonged standing. PDs tend to display gluteus medius co-activation, meaning that both muscles are activated together. NPDs tend to display gluteus medius reciprocal firing, meaning that one muscle is being activated while the contralateral muscle is not (Nelson-Wong et al., 2008). As a result the goal of monitoring GM activations was to examine the influence of different standing interventions on activation patterns across PDs and NPDs. A reference electrode was placed over the spinous process of the seventh cervical vertebrae. EMG signals were differentially amplified using a common mode rejection ratio of 115 dB (at 60 Hz; input impedance of 1010 U), analog band-pass filtered from 10 to 500 Hz and gained by a factor of 500e5000 (AMT- 8, Bortec, Calgary AB, Canada). The specific gain used was tailored to each individual muscle using sub-maximal test contractions and real-time visual feedback to best fill the input range of
Table 1 A summary table of commonly implemented standing interventions and previous studies’ general findings. Study
Evaluated standing intervention
Assessment
Findings of standing intervention
Dolan et al., 1988
One leg elevated onto a 20 cm platform
Compared low back muscle activity when level standing to standing with one leg elevated on a platform.
Gallagher and Callaghan (2016)
Declining sloped surface of 16
Compared the posture differences between level standing and standing on a declining sloped surface during prolonged standing.
Nelson-Wong and Callaghan (2010a)
Self selected alternation between standing on a 16 incline and decline surface
Compared differences between using a sloped surface during prolonged standing and level ground prolonged standing.
Gallagher et al. (2013)
Short-term differences between standing on incline and decline sloped surfaces
Gallagher (2014)
Posture differences between standing on a declining sloped surface and standing with one foot elevated on a platform.
To examine the short and long term responses to standing when using a sloped surface on pelvis, lumbar, and trunk angles. Radiographic assessment of sagittal lumbopelvic postures between PDs and NPDs when standing on level ground, standing with one leg elevated and standing on a sloped surface
Standing with one leg elevated and resulted in increased lumbar flexion and increased low back muscle activity when compared to level standing. Sloped standing reduced LBP scores by 59.4% for PD when compared to level ground. All participants showed hip joint flexion, trunk- tothigh angle flexion, and posterior translation of the trunk center of gravity towards the ankle joint when standing on the sloped surface compared to level ground. Using a sloped surface reduced LBP scores by 59.4% for PD when compared to level standing and decreased gluteus medius co-activation levels. NPDs showed increased bilateral gluteus medius co-activation of these muscles. Using a sloped surface increased trunk flexion and posterior rotation of the pelvis. The elevated surface was most effective at causing lumbosacral lordosis flexion, and the declined sloped surface was more effective at inducing L1/L2 intervertebral joint flexion.
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the A/D converter (Winter 2009). The gained signals then remained fixed for the remainder of the collection and the signals were then sampled at 2048 Hz using a 16-bit A/D conversion card. Maximum Voluntary Contractions (MVCs) were collected from each muscle for normalization purposes. MVCs for the LES were obtained as the participant lay prone on a table with their torso hanging off the edge of the table at the level of the anterior superior iliac spine. Participants crossed their arms over their chest, bent their torso towards the ground as a starting position and then extended their trunk to resistance applied by the experimenter. GM MVC’s were obtained through resisted hip abduction in the side lying position. Ten second rest trials were taken in both the prone and supine resting positions. An Optotrak Certus motion capture system (Northern Digital Inc. Waterloo, ON, sampling at 32 Hz) was used to track movement of infrared markers during the data collection. Two rigid bodies were used to track movement of the spine, one rigid body containing 4 infrared markers was placed at the level of L1/L2 (upper lumbar spine), and one rigid body containing 5 infrared markers was placed over the sacrum. Four different standing positions were assessed in this study (Fig. 1): 1. Standing with both feet side by side on level ground (Level) 2. Standing with one foot raised on a platform (Elevated). The height of the platform was adjusted such that the participant’s thigh to trunk angle was 135 . This posture is described as physiological normal for the lumbar spine compared to erect standing where some curvature of the lumbar spine is still maintained (Keegan, 1953). 3. Standing on a sloped surface (Sloped), with a 16-degree decline (toes facing down). 4. Staggered Standing (Staggered). The foot that was placed on the elevated surface was the same one that was the front foot during the staggered standing trials. The participant moved the front foot forward such that the arch of that foot was in line with the toes of the other foot. It is important to note that the raised leg in the elevated standing condition was always the same leg that was placed forward in the staggered stance. From this point forward the raised/ forward leg will be referred to as the “leg up” and the other leg will be referred to as the “leg down”.
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2.3. Experimental procedures and protocol A maximum lumbar extension trial was recorded for a reference posture of lumbar spine angle. During this trial the participant was told to keep their knees locked and bend backwards about their lumbar spine without shifting their hips forward. The participant then stood in the four standing conditions for 5 min each. A short duration of 5 min was chosen in attempt to gauge the postural response of the standing interventions in the absence of time varying and pain factors. The first condition was always Level standing. The remaining three standing conditions were then randomized for each participant. The participant stood for 5-min and then sat for 1-min between the trials, in an attempt to minimize the development of low back pain. During each condition the participant worked at a computer desk performing a standardized typing task. The desk was adjusted to 5e6 cm below their wrist when their elbows were placed at 90 (Kroemer and Grandjean, 1997). For the sloped surface, the height of the table was higher to accommodate standing on the surface. 2.4. Data processing and analysis 2.4.1. Kinematic data Kinematic data was imported into Visual 3D (C-Motion, Kingston, ON) to calculate lumbar spine joint angles. Each marker’s coordinate data were filtered using a 6 Hz s order dual pass Butterworth filter. Mean lumbar angles were determined using a Flexion/Extension e Lateral Bend-Axial Rotation sequence. The average lumbar spine angle was computed across each 5 min trial for all standing conditions and expressed with respect to maximum lumbar extension. 2.4.2. Electromyography data All EMG signals were highpass filtered using a 30 Hz, second order Butterworth filter, to remove any ECG contamination (Drake and Callaghan, 2006). A notch (bandstop) filter was applied with filter cutoff frequencies from 59 to 61 Hz, to remove any 60 Hz electrical contamination (Mello et al., 2007). EMG signals were then full wave rectified and dual passed filtered through a fourth order Butterworth filter with a cutoff frequency of 6 Hz (Nelson-Wong et al., 2008). The resulting linear enveloped signals were then normalized to MVC. Resting activation was then subtracted off the normalized EMG signal. The processed EMG signals were down sampled to 32 Hz prior to further data analysis as a data reduction
Fig. 1. The four standing conditions tested. From left to right: level, elevated, sloped and staggered.
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measure. Cross-correlation analyses were used to quantify the common signal between right and left GM and LES signals. Cross-correlation is an analysis technique used to quantify the similarity in shape and the phase delay between two time-varying waveforms (NelsonWong et al., 2009). A highly positive correlation indicates that the two signals are acting together in phase, whereas a highly negative correlation indicates that one signal is at maximum and the other is at a minimum (out of phase). Practically, this technique is often used to access the degree of co-activation between two muscles (in the case of this study between the right and left pairs of the LES and GM). Positive cross-correlation values indicate muscles are being activated together (co-activating), while negative cross-correlation values indicate that one muscle is being activated while the other is not, implying reciprocal firing of the muscles (Nelson-Wong et al., 2009). Five 1-min blocks of the bilateral gluteus medius EMG from each standing trial were entered into a custom Matlab program (Version 8.5 Mathworks, Inc., Natick, MA, USA) to compute the cross-correlation coefficients, Rxy, with the following equation:
Rxy ðtÞ ¼
1 T
ZT 0
xðtÞyðt þ tÞdt Rxx ð0ÞRyy ð0Þ
Where Rxy(t) is the normalized cross-correlation of two signals, x(t) and y(t) at a phase shift t with a potential range of values between 1 and þ1 and T is the length of the record. The maximum crosscorrelation of each 1-min block was computed across a phase shift of ± 500 ms, then the average Rxy was calculated over each 5-min trial to provide an overall pattern of co-activation during each standing condition (Nelson-Wong et al., 2009). This analysis provided one average Rxy value for the right and left GM and LES for each standing condition. Average EMG signals were used to obtain an estimate of muscle activation levels during the standing conditions. Average EMG for the right and left LES, and GM were obtained by taking an average of the normalized EMG signals over each 5-min trial, for each standing condition.
2.5. Statistical analysis No significant gender effects were observed and therefore the data were collapsed across gender. A two-way mixed general linear model assessed the influence of Pain Status and Standing Position on lumbar spine posture and Cross-correlation values. A three-way mixed general linear model assessed the influence of Pain Status, Leg Up vs. Leg Down, and Standing Position on average EMG activation levels (SPSS v20, IBM Corporation, Somers, NY, USA). Any significant effects or interaction effects were further evaluated using the Tukey post hoc test. An alpha level of 0.05 was set for significance.
3. Results 3.1. Kinematic differences A main effect of standing condition for lumbar spine angle was observed (p ¼ 0.008). When using the elevated standing aid, participants stood with greater lumbar spine flexion (23.07 ± 9.43 degrees of flexion) compared to level Ground (20.32 ± 8.36 ) and Sloped (20.20 ± 8.88 ) (Fig. 2). There were no significant differences between the Staggered condition and any of the other standing conditions (Fig. 2).
25.00
Lumbar Spine Angle With Respect to Maximum Extenstion (Degrees)
284
A
A,B
B
B
20.00
15.00
10.00
5.00
0.00 Level
Elevated
Staggered
Sloped
Standing Position
Fig. 2. Mean (standard error) lumbar spine angles across standing conditions (degrees) with respect to maximum extension (note: maximum extension ¼ 0 ) for each of the investigated standing conditions (i.e. a greater lumbar spine angle indicates more flexion). Means indicated with different letters are significantly different.
3.2. Muscle activation differences 3.2.1. Cross-correlation differences A significant interaction effect of Pain Status x Standing Position was found for GM cross-correlation. This was largely driven by a difference that was exhibited between PD and NPD, GM crosscorrelation during the Level ground condition (PD Rxy ¼ 0.14 ± 0.12; NPD Rxy ¼ 0.0055 ± 0.015, p ¼ 0.026) and a different directional response in the Elevated standing condition (Fig. 3). No significant differences were observed in crosscorrelation values for any of the three standing interventions investigated across PDs and NPDs (Fig. 3). This was as a result of an increase in cross-correlation values for NPD while using each of the standing interventions (Fig. 3). No significant differences were observed in cross-correlation values for the LES. 3.2.2. Average EMG activation Average activation levels for the right and left muscles were reported as leg up and leg down as per the leg that was lifted up/ placed forward during the elevated/staggered standing trials and the leg that was down on the floor/placed back. No significant differences were observed for the GM and LES activation levels across leg, pain group or standing conditions. The EMG values were low on average, while not statistically different, PDs had the tendency to have slightly higher average GM activation levels (Table 2). 4. Discussion Consistent with our hypothesis, lumbar spine posture was altered with the use of a standing intervention. Contrary to our hypothesis, there were no accompanying changes in muscle activation patterns across standing interventions. The Elevated standing aid resulted in greater lumbar spine flexion when compared to the Level and Sloped conditions. Across all standing conditions PDs exhibited a positive GM cross-correlation value, indicating that the GM muscles were co-activated. This study has shown that PDs display significant differences in muscle activation patterns compared to NPDs during level standing. PDs did demonstrate a slight reduction in the co-activation of the GM when using standing interventions (Fig. 3); however, these positions did cause an increase in GM muscle activation patterns in NPDs similar to the magnitudes found for PDs. The increase in lumbar flexion with the
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285
0.2
Right-Left Average GM Cross-Correlation Values
PD
0.15
NPD
B"
0.1
0.05
A"
0 Level
-0.05
Elevated
Staggered
Sloped
Standing Condition
Fig. 3. The average Right-Left GM cross-correlation values for each standing condition. Standard Error bars are displayed. Means indicated with different letters are significantly different.
Table 2 A summary table of the average GM activation for PDs and NPDs across standing positions.
Level Elevated Staggered Sloped
PD NPD PD NPD PD NPD PD NPD
use of the Elevated standing aid may be a possible mechanism for decreasing pain in PDs during prolonged standing given the reduction in co-activation and a change in posture to less extended positions e two factors previously linked to increased pain reporting (Nelson-Wong et al., 2008; Sorensen et al., 2015). The use of an elevated surface changed the lumbar spine posture of participants such that participants stood in a more flexed lumbar spine posture, which compares well with previous work (Dolan et al., 1988; Gallagher, 2014). Pain developers have been shown to stand with greater lumbar lordosis (extension) than NPDs during a prolonged standing task (Sorensen et al., 2015). This change in lumbar spine posture, which is the same postural difference that is observed between PDs and NPDs, may be an indication that the elevated standing intervention can positively change lumbar spine posture in PDs and potentially reduce LBP. A follow-up study of prolonged standing while using an elevated standing aid is required to determine if it would be a successful intervention for reducing prolonged standing induced LBP. The sloped surface and staggered stance were unsuccessful at changing lumbar spine posture when compared to flat standing. The lack of change in lumbar spine posture with the use of a sloped surface was surprising. This result is in contrast to previous findings
Average activation (Percent MVC)
Standard deviation
1.05 0.75 1.09 0.93 1.19 0.64 1.28 0.83
0.74 0.59 0.64 0.58 0.88 0.33 1.2 0.62
(Gallagher et al., 2013; Nelson-Wong and Callaghan, 2010a), who found an increase in lumbar flexion angle when using a sloped surface and a 59.4% reduction in LBP (Nelson-Wong and Callaghan, 2010a). Gallagher (2014), also found no difference in lumbar spine angle between standing on level ground versus a decline slope when using internal x-ray measurements. Combined with the results of the current study, it seems that the sloped surface may not alter lumbar spine angle. The use of a sloped surface may induce other posture alterations such as hip posture changes, this may explain the previous success in LBP reduction with this standing intervention (Nelson-Wong and Callaghan, 2010a). In addition, Nelson-Wong and Callaghan 2010a, investigated posture differences with the use of the sloped surface over a much longer period of time when compared to this study. This may also contribute to the differences in posture seen between this study and the previous study. In the case of the staggered stance, this alternate standing position was hypothesized to help individuals tolerate prolonged standing by allowing them to sway their trunk forward to rest their body weight on the ball of the forward foot (Zacharkow, 1988). By alternating the weight bearing foot, and shifting the body weight back and forth, this standing intervention has the potential to reduce LBP development during prolonged standing by inducing
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small movements early on when standing. A gross measure such as average lumbar flexion angle may not be sensitive enough to demonstrate the small flexion-extension motions this intervention may induce. Moving forward it may be beneficial for future work to investigate the staggered stance intervention with smaller lumbar movements throughout a longer period of standing. Despite the change in lumbar spine flexion angle for the elevated standing aid, no changes in GM and LES muscle activation between PDs and NPDs across each of the standing interventions were noted in this study. As found previously (Nelson-Wong et al., 2008), during the Level standing condition NPDs had significantly lower GM cross-correlation values when compared to PDs. This indicates that PD’s displayed co-activation of the bilateral GM muscles, whereas NPDs did not. When using each of the three alternate standing positions no significant differences between PDs and NPDs were observed. Although not statistically significant, using the standing interventions increased GM muscle coactivation patterns for NPDs only. This is a potential concern because the NPD group exhibited a pattern when using the standing interventions that could be indicative of identifying highrisk individuals for developing LBP in level standing. A limitation to this study is that participants were exposed to the level standing condition prior to any of the standing interventions; therefore there is the potential for order effects. We chose this method so that we were able to obtain a true baseline for level ground standing. Second, each participant was only exposed to each standing condition for 5 min. This may not have been sufficient time to distinguish muscle activation patterns. This time frame was chosen to mitigate LBP development so it did not influence the evaluation of basic changes of lumbar spine posture and muscle activation.
5. Conclusions The elevated standing aid intervention introduced changes in standing style that resulted in increased lumbar spine flexion when compared to level standing. The sloped surface and staggered stance were unsuccessful at changing lumbar spine posture when compared to level standing. This change in lumbar spine posture, which is the same posture difference seen between PDs and NPDs, may be an indication that the elevated standing aid intervention can positively change lumbar spine posture in PD and potentially reduce LBP. The interventions show promise for PDs but raise concern that they could introduce unwanted changes in NPDs so broad based applications for all workers should be undertaken with caution. Future work should assess longer durations when using standing interventions and assess hip and thigh postural alterations.
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