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Abstract

BACKGROUND:

Lifting heavy objects can induce postural stress and low back pain.

OBJECTIVE:

This study aimed to examine the effect of object weight expectations on trunk muscle activity and assess trunk muscle activity in people with chronic low back pain.

METHODS:

Twenty-two male college students (11 healthy and 11 participants with chronic low back pain) were recruited. The procedure was performed in three settings: lifting an expected 5-kg object, lifting an unexpected 10-kg object, and lifting an expected 10-kg object. Lifting was divided into five phases, and the muscle activity in each phase was compared between groups (chronic low back pain/control), object weight predictions, and phases.

RESULTS:

Compared to the control group, the chronic low back pain group had higher erector spinae muscle activity, regardless of weight or prediction, and significantly higher rectus femoris muscle activity in the early lifting phase of the expected 10-kg object ( $p=$ 0.043). Compared to when lifting the expected 10-kg object, erector spinae muscle activity was higher in the early lifting phase of the control group when lifting the unexpected 10-kg object ( $p=$ 0.016).

CONCLUSIONS:

Healthy individuals and individuals with chronic low back pain had different recruitment strategies for lifting objects heavier than predicted.

Keywords

Lifting trunk muscle activity unexpected weight low back pain EMG

1. Introduction

Low back pain (LBP) is highly prevalent regardless of age [1] and causes a reduction in daily activities and work ability [2]. More specifically, LBP persisting for more than 3 months requires a long recovery time and is defined as chronic LBP (CLBP) [3]. Lifting is one of the many factors that induce LBP [4], and a higher frequency of LBP has been reported in individuals who lift and carry heavy loads at work [5]. For example, frequently lifting and carrying a low-load mass with a forward-bent back doubled the risk of developing chronic LBP among healthcare workers [6].

Trunk muscle activity is essential for controlling trunk stability [7] and lifting. Moreover, lifting tasks require trunk flexion and extension, which can overload the lower back and erector spinae (ES) muscles. Accordingly, trunk muscle activity during lifting has been investigated to clarify the mechanism of lifting-induced LBP.

Lifting-induced LBP may be related to object weight prediction [8, 9], and differences in trunk muscle activity with and without weight prediction have been investigated. Studies on healthy men have shown that back muscle activity increases when lifting without weight prediction compared with when weight is predicted, leading to LBP [8, 9]. For example, lifting without predicting the weight of an object increases muscle activity in the ES [8]. Furthermore, lifting a heavier-than-expected object produces eccentric contraction of the trunk extensors early in the lifting phase [10] and an overload of the back muscles. Moreover, the activity of the back muscles is higher in the latter half of the lifting motion [9].

A comparison of normal subjects and those with a history of low back pain has shown that the EMG amplitude of the ES during lifting is higher in those with two or more episodes of LBP than in normal subjects [11], suggesting that the ES muscles may be involved in LBP during lifting. However, it is unclear whether trunk muscle activity differs between individuals with chronic low back pain (CLBP) and healthy subjects when holding a heavier-than-expected object. Understanding the differences in muscle activity between these two groups may reveal the mechanisms underlying trunk muscle dysfunction in individuals with LBP. Therefore, this study aimed to examine whether the trunk muscle activity of subjects with CLBP differs from that of healthy subjects when holding a heavier-than-expected object.

2. Methods

2.1 Participants

A total of 22 male college students (age, 20.5 $\pm$ 1.0 years; height, 172.7 $\pm$ 5.8 cm; weight, 64.3 $\pm$ 7.8 kg), including 11 participants with CLBP and 11 healthy controls, were recruited. Given that sex differences in pain thresholds and tolerance have been reported [12], only male participants were selected. Healthy control participants did not have LBP, whereas participants with CLBP had LBP for at least 3 months [13]. Pain present on the posterior trunk, between the 12 ${}^{\text{th}}$ rib and the bottom edge of the gluteal fold, was defined as LBP. For both groups, volunteers were recruited from within the author’s university and were confirmed to have met the inclusion criteria at the time of recruitment. Furthermore, the degree of current LBP was assessed using the visual analog scale (VAS) prior to lifting on the day of the experiment. The degree of maximum pain from the onset of LBP was also assessed using VAS. The following exclusion criteria were applied to both groups: history of spinal surgery, neurological disorders, and difficulty in lifting due to lower back or lower limb injuries. The inclusion and exclusion criteria were assessed by an experienced physical therapist during subject recruitment and on the day of the study. All participants were informed of the study’s aims and procedures before participation, and written informed consent for participation was obtained from all participants. The study was conducted in accordance with the Declaration of Helsinki and approved by the Ethics Review Committee of the authors’ affiliated institutions (Approval number: 18581-210218, Approval date: 2/18/2021).

2.2 Experimental task

Table 1
Experimental procedure

1.	Recognizing a 5 kg object	The participants lifted an object weighing 5 kg five times to achieve familiarization with the 5 kg weight.
2.	Lifting the expected5 kg object	The participants lifted a 5 kg object once while recognizing the weight of the object as 5 kg.
3.	Lifting the unexpected10 kg object	The participants lifted a 10 kg object once while recognizing the weight of the object as 5 kg. The object was identical in appearance to the 5 kg object.
4.	Recognizing a 10 kgobject	The participants lifted an object weighing 10 kg five times to achieve familiarization with the 10 kg weight.
5.	Lifting the expected10 kg	The participants lifted a 10 kg object once while recognizing the weight of the object as 10 kg.

Participants were asked to lift boxes of different weights (5 and 10 kg) from a platform; the weights were based on previous studies [11, 14]. Each box was made of plastic, had a black body with a green lid, and was 29.5 cm $\times$ 44 cm $\times$ 16 cm with handles on both sides. The contents of each box could not be observed by the participants during the task. The five steps of the test procedure, which are outlined in Table 1 were performed in sequence. The three measurement conditions were established as follows: lifting an expected 5-kg object, lifting an unexpected 10-kg object, and lifting an expected 10-kg object. The height of the platform was 58 cm from the floor, and the distance from the object to the participant’s toe was 20 cm. During lifting, the participants’ left and right feet were shoulder-width apart, and the elbow and knee joints were extended to minimize differences in movement between all trials. The height and distance of the platform were set to allow the participants to hold the elbow and knee joints extended during the task.

2.3 Data measurement

Muscle activity was measured using a wireless surface EMG system (DELSYS Trigno Wireless EMG System; DELSYS, Natick, MA, USA) that had a sampling frequency of 2000 Hz. The activity of the right-sided muscles was measured. The skin was rubbed with alcohol to reduce skin impedance, and the surface electrodes were positioned on the rectus abdominis (RA), internal oblique (IO), external oblique (EO), ES, gluteus maximus (GMa), rectus femoris (RF), biceps femoris (BF), and biceps brachii (BB) based on previous studies [15, 16, 17, 18]. Surface electrodes were attached parallel to the muscle fibers. A digital video camera (Exilim EX-FH20; Casio Computer Co., Ltd., Tokyo, Japan) was set up perpendicular to the plane of motion 3 m to the right of the platform, and recordings were made at a frequency of 210 Hz. To obtain kinematic data, reflective markers (QPM190, Qualisys AB, Gothenburg, Sweden) were attached to the right side of the object, right acromion, and middle fingertip. The digital video camera was synchronized with the electromyography (EMG) system.

2.4 Experimental procedure

To normalize EMG data, maximum voluntary contraction (MVC) was performed for each muscle, and each participant practiced each MVC to learn the position and measurement methods prior to the MVC test. Manual resistance was applied until the participants achieved maximum effort. Participants performed maximum isometric contractions for 3 s. The MVC for the RA was measured in a supine position using a partial sit-up with the knees flexed at 90 ${}^{\circ}$ . Manual resistance was applied to both shoulders toward trunk extension. The MVC for the IO was obtained in a supine position, with the knees flexed at 90 ${}^{\circ}$ and the trunk flexed and rotated to the right. Manual resistance was applied to the left shoulder toward trunk extension and left rotation. The MVC for the EO was measured in a supine position, with the knees flexed at 90 ${}^{\circ}$ and the trunk flexed and rotated to the left. Manual resistance was applied to the right shoulder toward trunk extension and right rotation. The MVC for the ES was acquired in a prone position, with the trunk extended and without any leg movement. Manual resistance was applied to both shoulder girdles toward trunk flexion. The MVC for the GMa was obtained in a prone position, with the right knee flexed at 90 ${}^{\circ}$ and the right hip extended. Manual resistance was applied to the right thigh toward hip flexion. The MVC for the RF was measured in a sitting position at the bedside, with the hands placed on the bed on either side of the body and the right knee extended to its maximum capacity. Manual resistance was applied to the right lower leg toward knee flexion. The MVC for the BF was acquired in a prone position, with the right knee flexed at 45 ${}^{\circ}$ and without trunk rotation. Manual resistance was applied to the right lower leg toward knee extension. The MVC for the BB was obtained in a sitting position, right shoulder and elbow in a 90 ${}^{\circ}$ flexion with the forearm supinated. Manual resistance was applied to the right forearm toward elbow extension.

Lifting practice was conducted after the MVC test (Table 1). To get familiarized with a 5 kg weight, the participants lifted a 5-kg object for 5 times. After the weight was recognized, the “lifting of the expected 5-kg object” task was performed. Subsequently, participants were told that the electrodes and markers needed to be checked for misalignment and were led behind a partition. The 5-kg object was replaced with a 10-kg one, which went unnoticed by all participants. This was termed as the “lifting the unexpected 10-kg object” task. Although the participants recognized the object as a 5 kg one, its true weight was 10 kg. Finally, to recognize a 10 kg weight, the participants lifted a 10-kg object for 5 times. Following this, the “lifting of the expected 10-kg object” task was performed.

Participants were guided throughout the lifting task using a metronome (ME110SBL; Yamaha Corporation, Shizuoka, Japan); throughout 2-s intervals, participants started lifting from a static standing position and touched the object; the object was lifted until the trunk was upright; the object was returned to the platform; and, finally, the participant returned to a static standing position.

2.5 Data analysis

Figure 1.

Phases of lifting. Lifting was divided into the following five phases: preparation, early lifting, middle lifting, late lifting, and recovery.

Data analysis software (ImageJ; National Institutes of Health, Bethesda, MD, USA) was used to divide the data into the following five phases: preparation, early lifting, middle lifting, late lifting, and recovery (Fig. 1). The interval between motion initiation and touching the object by hand was defined as the preparation phase. The interval between touching the object by hand and the initiation of the object’s marker coordinate movement was defined as the early lifting phase. The interval between the time of initial movement of the object’s marker coordinate and the point at which the object was completely lifted was defined as the middle lifting phase. The interval between the point of times when the object was completely lifted and when the object was rested on a platform was defined as the late lifting phase. The interval between the point of times when the object was rested on a platform and the end of lifting procedure was defined as the recovery phase.

The raw EMG data were bandpass-filtered between 20 and 450 Hz and full-wave-rectified using an analysis software (LabChart 8; AD Instruments, New Zealand). The root mean square (RMS) during the MVC test was calculated by identifying the compartment with the maximum amplitude in 1 s. The RMS during each phase was normalized as a percentage of the highest RMS obtained during a 1s period in the MVC test (%MVC).

2.6 Statistical analysis

Statistical analyses were performed using SPSS version 28.0 (IBM Corp.; Armonk, NY, USA). Height, weight, and age were compared between both groups. We conducted the Kolmogorov-Smirnov test to confirm the normality of the data. Nonparametric tests were used to assess age. Each muscle activity was analyzed using a three-way repeated measures analysis of variance (ANOVA) (GROUP [control/CLBP] $\times$ TRIAL [expected 5-kg object/unexpected 10-kg object/expected 10-kg object] $\times$ PHASE [preparation/early lifting/middle lifting/late lifting/recovery]). Post hoc analyses of the main effects of the TRIAL factors (above) were performed after testing for the simple-simple main effect and the simple main effect. After testing for the simple-simple main effect, post-hoc analyses were performed using Bonferroni correction, and a $p$ -value $<$ 0.0167 (0.05 divided by 3) was considered statistically significant. After testing for the simple main effect, post-hoc analyses of the main effect of factors were corrected and multiple comparisons were applied by multiplying $p$ -values by the number of comparisons. Adjusted $p$ -values $<$ 0.05 were considered statistically significant. The statistical significance of the other tests was also set at $p<$ 0.05.

3. Results

Table 2
Characteristics of the chronic low back pain group and control group

	Control group	Chronic low back pain group	$p$ -value
Age (years)	20.4 $\pm$ 1.0	20.7 $\pm$ 1.0	0.438
Height (cm)	173.3 $\pm$ 7.2	172.1 $\pm$ 4.3	0.651
Weight (kg)	64.4 $\pm$ 8.3	64.2 $\pm$ 7.7	0.971
BMI (kg/m ${}^{2})$	21.4 $\pm$ 2.3	21.7 $\pm$ 2.3	0.796

Group characteristics are expressed as the mean $\pm$ SD. Statistically significant difference: $p<$ 0.05.

There were no significant differences in age, height, weight, and body mass index between both groups (Table 2). In the CLBP group, the VAS score at the time of the experiment was 2.2 $\pm$ 1.7 cm and the maximum pain since onset was 5.7 $\pm$ 1.9 cm.

Table 3

Erector Spinae and Rectus Femoris muscle activity in each group with and without weight prediction

		Control group		Chronic low back pain group
		Expected	Unexpected	Expected	Unexpected
		10 kg	10 kg	10 kg	10 kg
Erector spinae	Preparation	9.5 $\pm$ 4.9	9.5 $\pm$ 4.4 ${}^{\text{d}}$	13.0 $\pm$ 3.7	13.6 $\pm$ 4.0 ${}^{\text{d}}$
(%MVC)	Early lifting	16.6 $\pm$ 7.9 ${}^{\text{a,h}}$	18.6 $\pm$ 7.4 ${}^{\text{h}}$	25.5 $\pm$ 4.5 ${}^{\text{a}}$	23.8 $\pm$ 5.9
	Middle lifting	26.5 $\pm$ 9.2 ${}^{\text{b}}$	28.0 $\pm$ 10.4 ${}^{\text{e}}$	36.9 $\pm$ 9.0 ${}^{\text{b}}$	39.6 $\pm$ 12.1 ${}^{\text{e}}$
	Late lifting	21.2 $\pm$ 7.2 ${}^{\text{c}}$	20.4 $\pm$ 6.7 ${}^{\text{f}}$	28.3 $\pm$ 6.7 ${}^{\text{c}}$	28.9 $\pm$ 7.7 ${}^{\text{f}}$
	Recovery	13.3 $\pm$ 6.2	12.5 $\pm$ 5.2	16.4 $\pm$ 4.0	16.7 $\pm$ 5.4
Rectus femoris	Preparation	1.9 $\pm$ 0.9	2.0 $\pm$ 1.5	3.6 $\pm$ 3.0	2.0 $\pm$ 0.9
(%MVC)	Early lifting	1.8 $\pm$ 0.9 ${}^{\text{g}}$	1.7 $\pm$ 1.0	3.2 $\pm$ 2.1 ${}^{\text{g,i}}$	1.9 $\pm$ 0.7 ${}^{\text{i}}$
	Middle lifting	2.1 $\pm$ 1.0	2.1 $\pm$ 1.2	3.4 $\pm$ 2.1	2.5 $\pm$ 1.1
	Late lifting	2.0 $\pm$ 0.9	1.9 $\pm$ 1.1	3.9 $\pm$ 3.1	2.6 $\pm$ 1.0
	Recovery	2.7 $\pm$ 1.5	2.6 $\pm$ 2.0	3.9 $\pm$ 2.6	4.1 $\pm$ 2.3

Muscle activity data are expressed as the mean $\pm$ SD. Unit of measurement: % MVC. ${}^{\text{a-g}}$ Significant difference in both group (Control group vs. Chronic low back pain group) ( ${}^{\text{a}}$ – expected 10 kg in the early lifting phase, $p=$ 0.004) ( ${}^{\text{b}}$ – expected 10 kg in the middle lifting phase, $p=$ 0.014), ( ${}^{\text{c}}$ – expected 10 kg in the late lifting phase, $p=$ 0.026), ( ${}^{\text{d}}$ – unexpected 10 kg in the preparetion phase, $p=$ 0.033), ( ${}^{\text{e}}$ – unexpected 10 kg in the middle lifting phase, $p=$ 0.025), ( ${}^{\text{f}}$ – unexpected 10 kg in the late lifting phase, $p=$ 0.011), ( ${}^{\text{g}}$ – expected 10 kg in the early lifting phase, $p=$ 0.043). ${}^{\text{h-i}}$ Significant difference in both trial (Expected 10 kg vs. Unexpected 10 kg) ( ${}^{\text{h}}$ – Control group in the early lifting phase, $p=$ 0.016), ( ${}^{\text{i}}$ – CLBP group in the early lifting phase, $p=$ 0.001).

Figure 2.

Comparison of muscle activity with and without low back pain. Mean and standard deviation for the electromyographic activity of the erector spinae (a, expected 5 kg; b, unexpected 10 kg; c, expected 10 kg) and rectus femoris (d, expected 10 kg). ${}^{**}p<$ 0.01 and ${}^{*}p<$ 0.05.

Figure 3.

Comparison of muscle activity with and without weight prediction. Mean and standard deviation for the electromyographic activity of the (a) erector spinae, (b) rectus femoris, (c) biceps femoris, and (d) biceps brachii muscles. ${}^{*}p<$ 0.0167.

EMG data are expressed as %MVC. Three-way repeated measure ANOVA revealed a significant three-way interaction for the ES muscle ( $F_{(4.549,90.977)}=$ 3.022; $p=$ 0.017; partial $\eta^{2}=$ 0.131). ES muscle activity was significantly higher in the CLBP group than in the control group in the early lifting phase of the expected 5-kg object ( $p=$ 0.013); in the preparation ( $p=$ 0.033), middle lifting ( $p=$ 0.025), and late lifting phases ( $p=$ 0.011) of the unexpected 10-kg object; and in the early lifting ( $p=$ 0.004), middle lifting ( $p=$ 0.014) and late lifting phases ( $p=$ 0.026) of the expected 10-kg object (Table 3; Fig. 2). Post-hoc analyses showed that ES muscle activity was higher when lifting the unexpected 10-kg object than the expected 10-kg object in the early lifting phase of the control group ( $p=0.016)$ (Table 3, Fig. 3).

Three-way repeated measure ANOVA revealed a significant three-way interaction for the RF muscle ( $F_{(3.541,70.811)}=$ 2.701; $p=$ 0.043; partial $\eta^{2}=$ 0.119). In the early lifting phase of the expected 10-kg object, RF muscle activity was significantly higher in the CLBP group than in the control group ( $p=$ 0.043) (Table 3, Fig. 2). Post-hoc analyses showed that RF muscle activity was higher when lifting the expected 10-kg object than the unexpected 10-kg object in the early lifting phase of the CLBP group ( $p=$ 0.001) (Table 3, Fig. 3).

Three-way repeated measure ANOVA showed no three-way interaction but a significant two-way interaction of TRIAL $\times$ PHASE for the BF and BB muscles ( $F_{(3.541,70.811)}=$ 2.701; $p=$ 0.043; partial $\eta^{2}=$ 0.119). Multiple comparisons following the simple main effect test showed that BF and BB muscle activity was higher when lifting the unexpected 10-kg object than the expected 10-kg object in the middle lifting phase (BF $p=$ 0.003; and BB $p<$ 0.001) (Table 3, Fig. 3).

Three-way repeated measure ANOVA showed no three-way interaction but a significant two-way interaction of TRIAL $\times$ PHASE for IO and GMa (IO: $F_{(2.310,46.195)}=$ 6.566; $p=$ 0.002; partial $\eta^{2}=$ 0.247; and GMa: $F_{(3.903,78.058)}=$ 13.023; $p<$ 0.001; partial $\eta^{2}=$ 0.394). Multiple comparisons following the simple main effect test showed no significant difference in IO and GMa muscle activity between the expected and unexpected 10-kg object lifting tasks. Three-way repeated measure ANOVA showed no three- or two-way interaction for the RA and EO muscles. No main effects of GROUP or TRIAL were observed.

4. Discussion

This study investigated muscle activity during 3 different lifting tests. The control and CLBP groups differed in their recruitment patterns when lifting objects heavier than expected.

The ES muscle activity was higher in the CLBP group than in the control group during lifting, regardless of the weight or prediction. It has been reported that repetitive lifting induces measurable fatigue in the ES muscles [19]. Thus, it can be hypothesized that muscle fatigue and overloading may occur during repeated lifting.

RF muscle activity was also higher in the CLBP group than in the control group during the early lifting phase (i.e., the interval between the participant’s hand touching the object and the object beginning to move) of the expected 10-kg object lifting task. There was no difference in ES and RF muscle activity between both groups during the early lifting phase of the unexpected 10-kg object lifting task. These findings indicate that, when lifting an object of predictable weight, the CLBP group prepared for the lifting task by increasing ES and RF muscle activity before the object moved. However, these strategies were not applied when lifting an object heavier than expected. A previous study reported that vastus lateralis muscle activity was higher in healthy individuals than in patients with CLBP when lifting an object from the ground with both hands [14]. However, RF muscle activity was higher in the CLBP group than in the control group in the present study. Perhaps, the differences in quadricep muscle activation between both studies is owing to differences in height when lifting from the floor or from a platform [14]. In the current study, participants were instructed to maintain knee extension during lifting. Compared to the control group, the CLBP group demonstrated increased RF activity and knee extension immobilization, indicating that compensation for the trunk muscle dysfunction may have occurred.

This study focused on differences in muscle activity with and without prediction. In the control group, ES muscle activity during the early lifting phase was higher when lifting the unexpected 10-kg object than when lifting the expected 10-kg object. There was no difference in muscle activity between groups, but BB and BF activity were higher when lifting the unexpected 10-kg object than when lifting the expected 10-kg object. Thus, when lifting an object that was heavier than expected, BB and BF activity increased regardless of the group, whereas, in the control group, ES activity increased before the object was moved. In previous studies using healthy participants, ES muscle activity was higher when the weight of an object was unexpected than when it was expected [8, 20]. Similarly, when participants could not predict the object’s weight, ES activity increased only in the control group in the present study. Muscle activity was also previously measured in healthy participants during unilateral upper limb raising, and ipsilateral ES and BF muscle activation was found to precede the deltoid muscle before the upper limb moved [21]. Accordingly, ES and BF may function as postural control muscles. Moreover, delayed ES feedforward has been reported in patients with lumbar spine instability [22]. Given the results of this study, healthy participants could use strategies to recruit the ES muscle earlier and control posture when lifting objects that are heavier than expected. In contrast, it was suggested that the ES muscle does not control posture before object movement in the CLBP group.

Overall, this study showed that the CLBP group had higher ES muscle activity than the control group, regardless of an object’s weight or predicted weight. Individuals with CLBP were unable to recruit their trunk muscles appropriately when lifting objects that were heavier than predicted, suggesting trunk muscle dysfunction. This may lead to worsening or prolongation of LBP, but it was not investigated in this study and requires further research.

This study had several limitations. First, the sample size was small. In this study, it was difficult to recruit large number of individuals with CLBP. Further studies with more participants will be needed in the future. Each participant’s motion during lifting was not analyzed. Although the participant’s movements were standardized in this study, differences in motion caused by the presence or absence of predictions may have occurred. In future studies, it would be worth considering the participant’s motion, as well as trunk muscle EMG. Moreover, although this study was conducted with specific weights (5 and 10 kg), the type of object weight is not specified in actual situations. Therefore, it is necessary to increase the variation in object weights in further studies.

In conclusion, the CLBP group had significantly higher ES muscle activity during lifting than the control group. When lifting an object that was heavier than expected, BB and BF activity increased regardless of LBP. Moreover, ES activity also increased before the object was moved, but only in the control group. These findings suggest that healthy individuals and individuals with CLBP had different muscle recruitments for lifting objects heavier than predicted.

Author contributions

CONCEPTION: C.S. and M.E.

PERFORMANCE OF WORK: C.S., H.H., K.S.

INTERPRETATION OR ANALYSIS OF DATA: C.S., H.H. and M.E.

PREPARATION OF THE MANUSCRIPT: C.S., H.H., R.H., H.Y., K.S., T.T. and M.E.

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: C.S., H.H., R.H., H.Y., K.S., T.T. and M.E.

SUPERVISION: C.S. and M.E.

Ethical considerations

All participants were informed of the study’s aims and procedures before participation, and written informed consent for participation was obtained from all participants. The study was conducted in accordance with the Declaration of Helsinki and approved by the Ethics Review Committee of the authors’ affiliated institutions (Approval number: 18581-210218, Approval date: 2/18/2021).

Funding

This study was supported by a Grant-in-Aid program from Niigata University of Health and Welfare.

Footnotes

Acknowledgments

Not applicable.

Conflict of interest

The authors declare that they have no competing interests.

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Comparison of trunk muscle activity while lifting objects of expected and unexpected weight with and without low back pain

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Methods

2.1 Participants

2.2 Experimental task

Table 1 Experimental procedure

2.4 Experimental procedure

2.5 Data analysis

3. Results

Table 2 Characteristics of the chronic low back pain group and control group

Author contributions

Ethical considerations

Funding

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Experimental procedure

Table 2
Characteristics of the chronic low back pain group and control group