The effects of familiarization with loading,weight and size of loading on neuromuscular responses during sudden upper limb loading in chronic low back pain patients

Abstract

BACKGROUND:

Changes in the motor control of the spine were found in patients with chronic low back pain (CLBP). Sudden loading of the spine is supposed to be the cause of about 12% of lower back injuries. However, some aspects of this problem, such as alterations in the sensory-motor control of the spine, remain questionable.

OBJECTIVE:

To investigate the effects of familiarization with loading, weight and size of loading on neuromuscular responses during sudden upper limb loading in CLBP patients.

METHODS:

In this quasi-experimental study surface electromyography of the erector spinae (ES) and transverses abdominis/internal oblique (TrA/IO) and external oblique (EOA) muscles were recorded in 7 men and 13 women with CLBP and 20 asymptomatic subjects (10 men and 10 women) aged 18–45 years from the general community familiarization. Moreover, investigating control of the posture measurements of the center of pressure (COP) and vertical ground reaction force (GRF) or Fz were recorded using a force plate. Data were analyzed using paired $t$ -test and independent $t$ -test with the significance level of 0.05.

RESULTS:

Data analyses were performed using SPSS version 18. Some electromyography and force plate variables were significantly different for different conditions in each group and between the asymptomatic and low back pain groups ( $p\leqslant$ 0.05).

CONCLUSION:

Several motor control changes were observed in the CLBP patients. These patients showed decreased trunk muscle activity as well as too early and too delayed responses compared to asymptomatic subjects.

Keywords

Chronic low back pain surface electromyography posture

1. Introduction

Low back pain (LBP) is a common problem [1] with point prevalence and life time prevalence of 15%–30% and 50%–85%, respectively. About 10% of those experiencing LBP develop chronic pain that may result in early retirement and heavy health burden [2]. Changes in trunk muscle activation have been observed in chronic low back pain (CLBP) patients following limb movement or loading using surface electromyography [3]. Inefficiency of the erector spinae (ES) and abdominal muscle responses may increase the flexion moment of the lumbar spine and damage the lumbar structures [4]. Trunk stability resulting from sufficient muscle activity is necessary for both the prevention and treatment of low back problems [5]. In addition, adequate control of the posture is a major requirement for performing daily activities [6] that may be disturbed during unstable conditions [7]. Increased electrical muscle activity before loading [8], limited effect of prediction in CLBP patients [9], and similar spinal muscle responses between healthy controls and CLBP patients [10] have been reported.

Sudden loading of the spine mechanisms occur during slips, falls, and hits as well as on addition of extra load to an object held in one’s hands. Moreover, moving the contents of a container unexpectedly can suddenly load the spine [11, 12, 13, 14]. These conditions are believed to cause about 12% of all low back problems. In order to maintain postural balance and produce efficient muscular activity in these conditions, the neuromotor system must be activated properly [14]. Following unexpected loading, the central nervous system (CNS) attempts to compensate for deviations between the desired and actual kinematics, potentially increasing the excessive spinal loads and spinal damage [12, 13, 14]. Previous studies have indicated that contraction of the trunk muscles surrounding the spine provides stiffness to the spine and increases stability [15]. Impaired motor control of the spine may result in LBP chronicity [16]. It is believed that to detect alterations in the neuromotor control of the spine rather than direct pain effects, pain intensity score should be $\leqslant$ 3 on the visual analog scale (VAS $\leqslant$ 3) [17].

The first aim of the present study was to determine whether motor and postural responses after familiarization with loading would differ during sudden upper limb loading (SULL) in asymptomatic and CLBP subjects. The second aim was to determine whether motor and postural parameters changed following increased loading weight in asymptomatic and CLBP subjects. The third aim was to determine if any differences existed between CLBP and asymptomatic subjects in terms of the motor and postural control based on the loading size.

2. Methods

2.1 Study population

We enrolled 7 men and 13 women with CLBP as well as 20 asymptomatic subjects (10 men and 10 women) aged 18–45 years, who were recruited through leaflet advertising [18]. Because more women agreed to participate in the study sooner, both genders were not equally distributed. The two groups were matched by age, height, and weight. Any pain, tenderness, or discomfort between the edge of the twelfth ribs and the gluteal folds for three or more months [19, 20, 21], VAS $\leqslant$ 3 [21], and the absence of any lumbar radicular signs were the inclusion criteria for the LBP patients [22]. In addition, the absence of any respiratory, neurological, or orthopedic conditions was also an inclusion criterion for the asymptomatic subjects [23]. Participants were excluded if they were unwilling to continue the test and if the pain exacerbated during the test (Appendix).

Figure 1.

The test methodology.

2.2 Test methodology

This quasi-experimental study was performed as described in Fig. 1. The correct posture was defined as standing over the force plate with the knees completely extended and elbows flexed at 90 ${}^{\circ}$ with a basket in the hands, the ears covered, and the eyes either closed or open. Three boxes were used in this research as loads (1 – a small box weighing 1% of the subject’s body mass, 2 – a small box weighing 2% of the subject’s body mass, 3 – a large box weighing 2% of the subject’s body mass). An electromagnetic system was used to fix these boxes approximately at eye level, directly above the basket. In each condition, any one of the above boxes was released at a random time into the basket without warning [23]. All trials were done with the eyes open. The impact time was indicated with a marker switch in the basket. Figure 1 illustrates the study procedure. The Tehran University of Medical Sciences Ethics Board approved this study (Ethical code: 130/3428; date: 18/3/2013), and it was performed according to the principles of the Declaration of Helsinki.

2.3 Electromyography and force plate recordings

Surface Electromyography (EMG) was conducted using the ME 6000 (Mega Electronics Ltd. Kuopio, Finland). EMG of the muscles on the right and the left sides was recorded after the subjects’ skin was prepared by shaving, rubbing, and cleaning with alcohol pads. The middle points of each pair of the surface EMG electrodes (Ag-AgCl discs) were then placed as follows: transversusabdominus/internal oblique (TrA/IO) approximately 2 cm medial and inferior to the anterior superior iliac spine [24], the external oblique (EOA) 10 cm lateral to the umbilicus with a 45 ${}^{\circ}$ vertical orientation [25], and the ES muscle at the L3–L4 level approximately 4 cm lateral from the midline. The longitudinally oriented electrodes along the fibers of the muscle were positioned with a center-to-center electrode distance of 2.5 cm [26]. Band-pass filtering and sampling of the EMG data were performed between 20 Hz and 500 [27, 28] using Qualisys Track Manager software version 2.7 (QTM, Gothenburg, Sweden). In addition, data were exported to MATLAB software version 7 (MathWorks, USA) for analyses. A time window of 200 ms before and 250 ms after perturbation was considered for the analysis [28]. Normalization of the root-mean-square (RMS) of the EMG signal was performed according to the method used by Kanekar et al. [29] and Silfies et al. [30]. Consequently, using the MATLAB software, the integral of the EMG signal (IEMG) representing muscle activity and the muscle onset latencies were calculated. Muscle onset was defined as the point when the RMS signal crossed the threshold of the mean plus three standard deviations of the pre-perturbation baseline signals. Crossing the threshold for at least 20 ms was considered as muscle onset to avoid misinterpreting an EMG spike activity as muscle onset during signal processing [28].

A force plate device to assess control of the posture (Kistler Company, Switzerland) was used to measure the Fz (vertical component of ground reaction force), peak latency (time to peak of the muscle electrical activity), the total center of pressure (COP) excursion, and COP excursion along the Y axis (YCOP). The data from the EMG systems, marker switch, and the force plate were collected synchronously [31].

2.4 Statistical analyses

Data analyses were performed using SPSS software version 18 (SPSS Inc. Chicago, USA). The normality of the data was examined using the Kolmogorov-Smirnov test. Paired $t$ -tests were used to calculate the within-group differences following studied conditions. Independent $t$ -tests were used to calculate the between-group differences in the studied conditions. For the force and COP data, the mean value of the three measurements and for the EMG data, the mean value of three measurements for each side was calculated. Left and right sides values were pooled for subsequent analyses [10]. $P$ value $<$ 0.05 was considered significant.

3. Results

3.1 Descriptive statistics and analysis of the differences between CLBP and asymptomatic groups in the studied conditions

There were no significant between-group differences in the demographic characteristics of the subjects. In this study, surface electromyography and force data of 46 subjects were recorded. There was noise in the force plate signals of six subjects; therefore, the data of these six subjects were excluded. Table 1 illustrates the statistics and between-group analyses of the studied variables for any five conditions. Furthermore, Table 2 lists the $P$ values calculated using the independent $t$ -test to compare the differences between the asymptomatic and CLBP groups. As shown in Table 1, some studied variables significantly deferred between the groups ( $p\leqslant$ 0.05).

Table 1
The statistics of the EMG variables for each condition (between-group analyses)

Item	Muscle	Condition	Asymptomatic		LBP		$P$ values
			(mean $\pm$ SD)		(mean $\pm$ SD)
Onset latency	TrA/IO	LL	20	$\pm$ 57	55	$\pm$ 219	0.077
		RLL	$-$ 35	$\pm$ 189	49	$\pm$ 236	0.046
		RHL	6	$\pm$ 83	$-$ 16	$\pm$ 108	0.454
		HLL	$-$ 12	$\pm$ 71	$-$ 19	$\pm$ 92	0.788
	EOA	LL	36	$\pm$ 77	36	$\pm$ 118	0.995
		RLL	$-$ 77	$\pm$ 471	30	$\pm$ 125	0.279
		RHL	45	$\pm$ 109	18	$\pm$ 87	0.401
		HLL	31	$\pm$ 70	$-$ 32	$\pm$ 207	0.767
	ES	LL	$-$ 9	$\pm$ 100	$-$ 36	$\pm$ 334	0.828
		RLL	$-$ 84	$\pm$ 166	$-$ 35	$\pm$ 1	0.130
		RHL	$-$ 52	$\pm$ 68	$-$ 83	$\pm$ 132	0.359
		HLL	$-$ 53	$\pm$ 70	$-$ 127	$\pm$ 242	0.443
Peak latency (ms)	TrA/IO	LL	102	$\pm$ 44	88	$\pm$ 61	0.416
		RLL	100	$\pm$ 58	103	$\pm$ 57	0.872
		RHL	113	$\pm$ 51	89	$\pm$ 42	0.103
		HLL	87	$\pm$ 36	87	$\pm$ 41	0.986
	EOA	LL	99	$\pm$ 56	79	$\pm$ 46	0.170
		RLL	91	$\pm$ 45	68	$\pm$ 40	0.093
		RHL	97	$\pm$ 59	96	$\pm$ 50	0.926
		HLL	106	$\pm$ 41	103	$\pm$ 54	0.853
	ES	LL	92	$\pm$ 45	61	$\pm$ 39	0.026
		RLL	78	$\pm$ 54	55	$\pm$ 38	0.093
		RHL	94	$\pm$ 43	86	$\pm$ 40	0.542
		HLL	97	$\pm$ 45	103	$\pm$ 39	0.659
IEMG	TrA/IO	LL	0.95	$\pm$ 0.28	0.78	$\pm$ 0.22	0.044
		RLL	0.94	$\pm$ 0.3	0.74	$\pm$ 0.32	0.050
		RHL	1.08	$\pm$ 0.52	0.86	$\pm$ 0.43	0.161
		HLL	1.13	$\pm$ 1.13	0.85	$\pm$ 0.40	0.090
	EOA	LL	1.01	$\pm$ 0.34	0.93	$\pm$ 0.32	0.453
		RLL	0.93	$\pm$ 0.29	0.81	$\pm$ 0.29	0.218
		RHL	1.11	$\pm$ 0.72	0.88	$\pm$ 0.33	0.208
		HLL	1.20	$\pm$ 0.80	0.92	$\pm$ 0.33	0.157
	ES	LL	1.37	$\pm$ 0.33	1.18	$\pm$ 0.24	0.045
		RLL	2.26	$\pm$ 4.57	1.04	$\pm$ 0.28	–
		RHL	1.53	$\pm$ 0.62	1.24	$\pm$ 0.26	0.057
		HLL	2.32	$\pm$ 3.83	1.24	$\pm$ 0.22	–
Fz peak latency (ms)		LL	106	$\pm$ 16	106	$\pm$ 23	0.992
		RLL	109	$\pm$ 62	69	$\pm$ 19	0.641
		RHL	104	$\pm$ 26	107	$\pm$ 19	0.611
		HLL	109	$\pm$ 44	109	$\pm$ 23	0.995
COP excursion (mm)		LL	190	$\pm$ 41	190	$\pm$ 48	0.886
		RLL	185	$\pm$ 40	187	$\pm$ 41	0.873
		RHL	192	$\pm$ 36	190	$\pm$ 41	0.874
		HLL	188	$\pm$ 35	189	$\pm$ 46	0.921
YCOP excursion (mm)		LL	143	$\pm$ 33	142	$\pm$ 36	0.732
		RLL	143	$\pm$ 25	141	$\pm$ 30	0.832
		RHL	146	$\pm$ 28	145	$\pm$ 29	0.869
		HLL	148	$\pm$ 28	143	$\pm$ 34	0.603

Table 2

$P$ values calculated using a paired $t$ -test to compare parameters within groups

Item	Muscle	Condition	Asymptomatic	LBP	Group $P$ values
Onset latency	TrA/IO	RLL, LL	0.613	0.147	0.249
		RHL, RLL	0.463	0.053	0.151
		HLL, RHL	0.560	0.939	0.785
	EOA	RLL, LL	0.460	0.309	0.696
		RHL, RLL	0.397	0.727	0.029
		HLL, RHL	0.190	0.734	0.216
	ES	RLL, LL	0.992	0.334	0.474
		RHL, RLL	0.955	0.038	0.126
		HLL, RHL	1	0.468	0.773
Peak latency (ms)	TrA/IO	RLL, LL	0.836	0.264	0.361
		RHL, RLL	0.406	0.568	0.843
		HLL, RHL	0.614	0.895	0.273
	EOA	RLL, LL	0.665	0.629	0.953
		RHL, RLL	0.932	0.0.33	0.212
		HLL, RHL	0.940	0.644	0.957
	ES	RLL, LL	0.411	0.6	0.757
		RHL, RLL	0.156	0.007	0.230
		HLL, RHL	0.001	0.109	0.279
IEMG	TrA/IO	RLL, LL	0.855	0.397	0.595
		RHL, RLL	0.168	0.011	0.331
		HLL, RHL	0.171	0.740	0.199
	EOA	RLL, LL	0.123	0.019	0.592
		RHL, RLL	0.191	0.036	0.073
		HLL, RHL	0.150	0.338	0.259
	ES	RLL, LL	0.073	0.010	0.702
		RHL, RLL	0.010	0.001	0.447
		HLL, RHL	0.037	0.852	0.719
Fz peak latency (ms)		RLL, LL	0.194	0.131	0.077
		RHL, RLL	0.109	0.069	0.439
		HLL, RHL	0.471	0.772	0.775
COP excursion (mm)		RLL, LL	0.062	0.654	0.399
		RHL, RLL	0.074	0.602	0.477
		HLL, RHL	0.292	0.889	0.466
YCOP excursion (mm)		RLL, LL	0.867	0.769	0.741
		RHL, RLL	0.288	0.389	0.951
		HLL, RHL	0.561	0.641	0.453

3.2 Within- and between-group analyses of the response changes in the studied conditions

Table 2 provides the $P$ values calculated using paired $t$ -test to compare the dependent variables for the studied conditions in each group. In addition, this table lists the $P$ values calculated using the independent $t$ -test to investigate the differences between the CLBP and asymptomatic subjects in terms of the motor and postural strategies in the studied conditions ( $p\leqslant$ 0.05).

4. Discussion

This study investigated the neuromuscular control and possible changes in information processing in the chronic low back pain patients using investigated by surface electromyography and force plate devices. With minimal or no pain in the CLBP patients in the present study at the time of testing (VAS $\leqslant$ 3), any changes in the neuromuscular responses were mainly considered long-term alterations in the sensory-motor control [31].

4.1 The effects of loading familiarization on neuromuscular responses during SULL in CLBP patients

In the LBP patients, less electrical activity in the TrA/IO and ES muscles in the LL condition and less electrical activity and longer onset latency in the TrA/IO muscle in the RLL were observed. As shown in Table 2, in the LBP subjects, less electrical activity of the ES and EOA muscles were observed in the RLL condition than that in the LL condition. This decrease in the muscle activity may be interpreted as less stiffening or preparatory muscle activity in the trunk muscles or a compensatory (inhibition) mechanism by the CNS to minimize the stress on the lumbar tissues in these patients [32, 33]. These alterations in the trunk muscle activity may cause lower back injuries due to micro trauma. The decrease in muscle activity in the study is consistent with the reports of Sihvonen et al. who showed decreased electrical muscle activity during functional movements in LBP patients [34]. In contrast, this finding contradicts the findings of D’hooge et al. who showed increased electrical activity of the ES muscles with and without load [35].

Moreover, the greater TrA/IO latency in the patients in RLL condition suggests that learning mechanisms may be disturbed in the patients and that they cannot regulate their responses following familiarization of loading.

Alterations in motor control are sometimes interpreted in the context of the pain-spasm-pain model, according to which, pain results in increased muscle activity, resulting in increased pain. In contrast, as per the pain adaptation model, pain decreases muscle activity during muscle activation as an agonist and increases muscle activity during muscle activation as an antagonist [32]. During flexor type SULL on the spine, only the decreased electrical activity of the TrA/IO in the LBP group is consistent with the pain adaptation model [32].

Moreover, in LBP subjects, less IEMG of the EO muscle was observed in the RHL condition than in the unexpected heavy loading (UHL) condition. Longer and shorter peak latencies of the TrA/IO and ES muscles, respectively, were observed in the RHL condition than in the UHL condition. Moreover, compared to the UHL condition, the RHL condition produced a higher increase in the latency of the EO muscle in the LBP subjects than in the asymptomatic subjects.

As the above findings show, before familiarization to heavy loading, the EO muscle showed greater electrical activity in the LBP patients. This may be interpreted as an overestimation in these patients. In addition, in these patients, familiarization shortened the peak latency of the ES muscle and lengthened the peak latency of the TrA/IO muscle. This may be interpreted as a CNS strategy to counteract the flexor moment produced by this type of loading. A higher increase in the latency of the EO muscle in the LBP subjects than in the asymptomatic subjects following familiarization may be interpreted as a wrong modulation of the motor responses in the patients.

4.2 The effects of loading weight on neuromuscular responses during SULL in CLBP patients

In the LBP subjects, shorter latency of the TrA/IO muscle was observed in the RHL condition compared to that in the RLL condition. Crisis in providing stability in the lumbar spine during heavier loading may be a reason for the shorter latency of the TrA/IO muscle.

In addition, compared to the RLL condition, the RHL condition produced a greater increase in the latency of the EO muscle in asymptomatic subjects than in LBP subjects. This may be indicative of the normal response of the neuromuscular system in increasing the latency of the EO muscle following heavier flexor type loading to postpone the more flexor moment. These findings are consistent with those reported by Elizabeth et al., according to which, there were changes in the muscle activity following movement demand changes. Shorter latency of the TrA/IO muscle in the present study may be interpreted as attempt of the CNS to accelerate co contraction of the trunk muscles and provide stiffness and spinal stability. The activities of the transversus abdominis and internal oblique muscles significantly affected IAP and lumbar stability [32].

4.3 The effects of loading size on the neuromuscular responses during SULL in CLBP patients

Moreover, in the asymptomatic subjects, more IEMG and longer peak latency of the ES muscle were found in the HLL condition than in the RHL condition. The findings of the present study are consistent with those of Alearts et al. who reported that corticomotor excitability in the primary motor cortex (M1) is modulated according to the force requirements of the observed grasping and lifting actions [33]. Force requirements of the observed action during movement observation, is a modulating factor in corticomotor excitability of the observer’s primary motor cortex (M1). Lifting heavy objects induces higher M1-excitability than lifting light objects. These findings may support the motor simulation hypothesis, explaining the activation of the motor systems to predict or infer the weight of the lifted object [34].

When lifting objects of identical mass but different sizes, individuals perceived the smaller objects as being heavier than the larger ones (the ‘size-weight’ illusion, SWI). Differences in the mismatch between the actual sensory information and the expected sensory information would induce differences in the apparent weight of each stimulus. It has been demonstrated that individuals are unable to fully scale the load force to the actual mass of the SWI-inducing cubes. Establishing perceptual judgments or interacting with the world highlights are two effective factors for information processing strategies [35].

It is believed that cues from the observed kinematic profile are more effective than the object cues on the encoding of force in the observer’s motor system. The sensory and motor systems are interrelated. The parietal node, namely the inferior parietal lobule (IPL), is assumed to provide a ‘goal-description’ of the observed action, while the frontal part, namely the inferior frontal gyrus (IFG), is suggested to represent the kinematic features of the observed actions. In fact, during action execution, the IFG provides a significant input to M1 and a similar ‘parallel’ IFG-M1 circuit is hypothesized to be recruited [34].

5. Conclusion

As described, several motor control changes were observed in CLBP patients. These patients showed decreased trunk muscle activity along with too early and too late responses compared to asymptomatic subjects. Moreover, an alteration in the neuromuscular responses following loading familiarization, weight and size were observed in these patients. The underlying mechanisms for some of the observed motor control changes in the CLBP patients are debatable. These may be related to the fact that “human motor control system is a very complicated multi-disciplinary system that has a wide variety of motor responses”. Moreover, the results of this study can be used to prevent chronic low back pain, as well as to enrich knowledge and the methods of exercise therapy in patients with chronic low back pain.

5.1 Study limitation

We noted that there are several potential limitations of this study. Firstly, the experiment was performed on participants with nonspecific low back pain and cannot be generalized for other groups of low back pain patients. Secondly, we did not measure muscle strength, proprioception or flexibility being the possible factors related to the observed differences between asymptomatic and symptomatic participants. Thirdly, this study was conducted among a special age range. The physician and physical therapist should be carefully considered before applying the results to other groups of patients.

Footnotes

Acknowledgments

This study was performed at the Gait and Motion Analysis Laboratory, Department of Physiotherapy, School of Rehabilitation Sciences, Iran University of Medical Sciences. This research did not receive any specific grant from any funding agencies.

Conflict of interest

The authors report no conflict of interest.

Abbreviations

Appendix

The study propulation.

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The effects of familiarization with loading,weight and size of loading on neuromuscular responses during sudden upper limb loading in chronic low back pain patients

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSION:

Keywords

1. Introduction

2. Methods

2.1 Study population

2.3 Electromyography and force plate recordings

2.4 Statistical analyses

3. Results

3.1 Descriptive statistics and analysis of the differences between CLBP and asymptomatic groups in the studied conditions

Table 1 The statistics of the EMG variables for each condition (between-group analyses)

4. Discussion

4.1 The effects of loading familiarization on neuromuscular responses during SULL in CLBP patients

4.2 The effects of loading weight on neuromuscular responses during SULL in CLBP patients

4.3 The effects of loading size on the neuromuscular responses during SULL in CLBP patients

5. Conclusion

5.1 Study limitation

Footnotes

Acknowledgments

Conflict of interest

Abbreviations

Appendix

References

Table 1
The statistics of the EMG variables for each condition (between-group analyses)