The effect of pain severity on postural stability and dynamic limits of stability in chronic low back pain

Abstract

BACKGROUND:

Deficits of dynamic balance in chronic low back pain patients have been reported by different studies in terms of impaired postural control. However, they excluded the degree of pain as a determinant affecting dynamic balance.

OBJECTIVE:

To evaluate the effect of pain intensity on dynamic balance control in terms of postural stability indices (PSIs) and limits of stability (LOS) in chronic LBP patients.

METHODS:

Sixty subjects (38 men, 22 women) participated in the current study, 45 patients with chronic LBP were selected randomly and served as the chronic LBP group while 15 healthy subjects served as the asymptomatic group (AS). The chronic LBP group was further classified in terms of pain intensity into 3 subgroups; low pain (LP), moderate pain (MP) and severe pain (SP) subgroups. The Biodex Balance System was used to measure the dynamic balance control (PSIs and LOS).

RESULTS:

Statistical significant differences were found among AS group and chronic LBP subgroups in PSIs and LOS.

CONCLUSIONS:

The intensity of pain has shown to be one of the determinants affecting dynamic balance in chronic LBP patients who showed differences in the impairment of PSIs and LOS with different degrees of pain.

Keywords

Dynamic balance chronic low back pain pain

1. Introduction

Low back pain (LBP) is currently the most prevalent musculoskeletal problem in modern societies [1]. Deficits of balance in LBP patients have been reported in terms of impaired postural control [2]. However, the real source of this impairment is still unclear. Postural control is the result of an interaction among mechanical, central, and peripheral neurologic mechanisms [3]. In chronic LBP, musculoskeletal and neural deficits such as impaired proprioception and delayed response of spinal structures might result in poor postural control and impaired balance [4, 5].

Impaired postural control during quiet standing in chronic LBP patients has been reported in different researches [6, 7]. However, more dynamic situations in our daily life have not been discussed well [8]. Different studies [9, 10, 11] explained that dynamic balance in terms of postural control and dynamic limits of stability (LOS) were impaired in chronic LBP patients. Most studies excluded the degree or severity of pain as one of the determinants that could affect the dynamic balance in chronic LBP patients except for a few [8, 12].

Ruhe et al. [12] reported a relationship between the center of pressure mean velocity and self-reported pain scores which can be used clinically as an objective monitoring tool for patients under treatment or rehabilitation. Sipko and Kuczyński [8] analyzed dynamic balance in chronic LBP patients in terms of LOS. Subjects with chronic LBP showed reduced forward LOS regardless of pain level.

It is also important to note that Ruhe et al. [12] had their patients in one symptomatic group while Sipko and Kuczyński [8] divided their subjects into high and low pain sub groups only. The lack of identifying pain severity into mild, moderate and sever was one of our motives to execute this study.

In addition, their measurement was based on less surface perturbation. It is believed that using an unstable or movable balance platform could be more helpful to understand the impaired dynamic balance or postural control in relation to different degrees of pain severity of chronic LBP.

As the impact of different pain intensities on postural stability has not been clearly evaluated in chronic LBP, we hypothesize this relationship is worth investigating as it may show clinical significance for clinical application [12]. Therefore, the purpose of this study was to evaluate the effect of different pain intensities of chronic LBP patients on dynamic balance control in terms of postural stability indices (PSIs) and LOS.

2. Methods

2.1 Subjects

Sixty subjects (38 men, 22 women) participated in the current study, 45 out of them were diagnosed as chronic LBP and 15 healthy subjects served as the asymptomatic group. The chronic LBP patients were selected randomly from the Orthopedic Physical Therapy Clinic, Faculty of Physical Therapy, Cairo University. Randomizing software was used to choose the participating subjects’ numbers from the clinic’s records. Our sampling was based on the convenience sampling technique as our subjects would be furtherly assigned according to their pain intensity.

The inclusion criteria for chronic LBP patients were low back pain of nonspecific origin and chronic pain for at least 3 months. The exclusion criteria were the presence of neurological diseases, other orthopedic conditions for the spine and lower limbs, herniated disc, spondylosis, spinal canal stenosis, vestibular disturbances and surgical treatment of the spine [13]. The chronic LBP group was further classified in terms of pain intensity into 3 subgroups; low pain (LP), moderate pain (MP) and severe pain (SP) subgroups [12]. Subjects of both groups were not included in regular physical activity during the previous 6 months prior to evaluation and were informed to stop medications one weak as well after consulting their physicians. All subjects signed an informed consent prior to their participations in the study. The study procedures were approved by the Faculty of Physical Therapy Ethics Committee, Cairo University.

2.2 Pain evaluation

At the beginning, the intensity of pain in chronic LBP group was evaluated by visual analog scale (VAS) after patients were in a weight-bearing posture (walking or standing) for 5 minutes. The validity and reliability of the VAS was reported by Crossley et al. [14].

Pain intensity was rated by each subjects from 0 to 10 cm, where 0 represented ‘no pain’, and 10 represented ‘unbearable pain’. The pain grades suggested 0 to 4 mm can be considered no pain; 5 to 44 mm, mild pain (LP subgroup); 45 to 74 mm, moderate pain(MP subgroup); and 75 to 100 mm, severe pain (SP subgroup) [15]. Based on these grades the three subgroups of CLBP were created.

Table 1
Demographic data for AS group and CLBP subgroups (mean $\pm$ SD, $n=$ 15 for each)

	AS	LP	MP	SP	$P$ value
Age (yrs)	45 $\pm$ 5.9	42.57 $\pm$ 6.2	43.1 $\pm$ 5.7	46.1 $\pm$ 7.39	0.319
Height (cm)	171.75 $\pm$ 4.9	170.1 $\pm$ 5.1	170.9 $\pm$ 4.3	172.3 $\pm$ 5.1	0.587
Weight (kg)	76.75 $\pm$ 4.7	74.4 $\pm$ 4.9	76.1 $\pm$ 5.1	77.3 $\pm$ 6.5	0.483
BMI	26 $\pm$ 4.1	25.7 $\pm$ 3.9	26.1 $\pm$ 3.7	26 $\pm$ 4.5	0.994
Sex F/M	7/8	5/10	6/9	4/11	0.647

F: female; M: male; AS: asymptomatic group; LP: low pain subgroup; MP moderate pain subgroup; SP: sever pain subgroup. SD: standard deviation; BMI: body mass index.

2.3 Dynamic balance evaluation

Biodex balance system (Biodex medical systems, inc. Brookbaven technology New York) was used in this study to assess dynamic balance. The system has a movable balance platform providing up to 20 ${}^{\circ}$ of surface tilt in a 360 ${}^{\circ}$ range of motion. The platform is connected to computer software that enables the device to serve as an objective assessment of balance [16]. The evaluation of dynamic balance included both postural stability indices (PSIs) and the dynamic limits of stability (LOS). The PSIs include the anterior-posterior stability index (APSI), the mediolateral stability index (MLSI) and the overall stability index (OASI). The OASI stability score is believed to be the best indicator of the overall ability of the patient to balance the platform [17]. These measures calculate the amount of deviation from the baseline position [18]. The LOS parameters which represented the motor control skills [19] were expressed as direction control (DC) and time (T) required for completing the test. Poorer postural stability results in higher scores of PSIs and poorer dynamic LOS results in lower DC value and prolonged T, all are representing impaired dynamic balance [20, 21].

2.4 Evaluation procedures

At the beginning of the test session, each subject with chronic LBP rated his/her pain intensity using the 100 mm VAS. To evaluate the dynamic balance, two practice trials were performed prior to the test to give instructions about the test tasks and to ensure that all subjects were able to complete the test. The dynamic balance evaluation was done in two procedures:

Table 2
Postural stability indices and dynamic limits of stability (mean $\pm$ SD) between AS group and CLBP subgroups

		AS	LP	MP	SP	$P$ value
Postural stability indices	OASI	1.59 $\pm$ 0.5	1.64 $\pm$ 0.6	2.46 $\pm$ 0.42 ${}^{*}$	3.54 $\pm$ 1.2 ${}^{*}$	0.0001
	APSI	1.11 $\pm$ 0.36	1.22 $\pm$ 0.62	2.1 $\pm$ 0.51 ${}^{*}$	3.34 $\pm$ 1.2 ${}^{*}$	0.0001
	MLSI	0.91 $\pm$ 0.25	0.99 $\pm$ 0.48	1.03 $\pm$ 0.24	2.05 $\pm$ 0.61 ${}^{*}$	0.0001
Dynamic limits of stability	DC	57 $\pm$ 13.4	51 $\pm$ 12.5	30.2 $\pm$ 8.08 ${}^{*}$	22.4 $\pm$ 5.4 ${}^{*}$	0.0001
	T	76 $\pm$ 18.65	110 $\pm$ 38.1 ${}^{*}$	143 $\pm$ 29.65 ${}^{*}$	181 $\pm$ 29.1 ${}^{*}$	0.0001

AS: asymptomatic group; LP: low pain subgroup; MP moderate pain subgroup; SP: sever pain subgroup; OASI: overall stability index; APSI: anterior-posterior stability index; MLSI: mediolateral stability index; DC: direction control; T: time; SD: standard deviation; significant difference: ${}^{*}$ $p<$ 0.05.

The first procedure was a test to measure PSIs (OASI, APSI and MLSI). The eyes were opened, the test duration was 20 seconds and stability level was 3 (moderate level) [22] and constant all over the test duration. The subjects in all groups were instructed to stand barefoot and to assume proper centered position as soon as the platform was released. The feet positions were recorded by using foot angles and coordinates on the platform grid. When the task time ended the OASI, APSI and MLSI were recorded automatically. The second procedure was a test to measure LOS or motor control skills (DC and T) (75% LOS, which is the moderate skill level) [23]. Once the test started the subjects tried to move the cursor to the box which appeared on the screen with little deviation as possible. When the test was completed, the DC and T were recorded automatically. During the actual tests, every subject was instructed to assume the same foot position in both procedures. A pause of 2 minutes was taken between the two procedures to minimize errors from adaptation.

Table 3

Postural stability indices (mean $\pm$ SD) within AS asymptomatic group and CLBP subgroups

	AS	LP	MP	SP
OASI	1.59 $\pm$ 0.5	1.64 $\pm$ 0.6	2.46 $\pm$ 0.42	3.54 $\pm$ 1.2
APSI	1.11 $\pm$ 0.36	1.22 $\pm$ 0.62	2.1 $\pm$ 0.51	3.34 $\pm$ 1.2
MLSI	0.91 $\pm$ 0.25	0.99 $\pm$ 0.48	1.03 $\pm$ 0.24	2.05 $\pm$ 0.61
P value	0.0001	0.011	0.0001	0.0009
Tukey’s post hoc test
OASI/APSI (mean difference)	0.0039 ${}^{*}$ (0.48)	0.12 (0.12)	0.05 ${}^{*}$ (0.36)	0.9 (0.11)
OASI/MLSI (mean difference)	0.0001 ${}^{*}$ (0.68)	0.009 ${}^{*}$ (0.65)	0.0001 ${}^{*}$ (1.43)	0.002 ${}^{*}$ (1.4)
APSI/MLSI (mean difference)	0.3366 (0.2)	0.52 (0.23)	0.0001 ${}^{*}$ (1.07)	0.004 ${}^{*}$ (1.3)

AS: asymptomatic group; LP: low pain subgroup; MP moderate pain subgroup; SP: sever pain subgroup; OASI: overall stability index; APSI: anterior-posterior stability index; MLSI: mediolateral stability index; SD: standard deviation; significant difference: ${}^{*}$ $p<$ 0.05.

This test protocol (eye opened and moderate level of difficulty in PSI and LOS) for dynamic balance evaluation was selected for many reasons: (1) more challenging test conditions (less than stability level 3) could not be completed by chronic LBP patients while the eyes were closed; (2) chronic LBP patients deprived of visual information present increased postural instability compared to healthy subjects in quiet standing [24] and center of pressure is not significantly dependent on visual information during dynamic challenging situations [25]; (3) different pain grades may affect the results of dynamic balance evaluation [26]. Therefore, in the current study, the differentiation between intensities of pain as a topic of concern was involved while the visual feedback was being maintained.

2.5 Data analysis

Data were analyzed using a Statistical Package for Social Sciences (SPSS) version 20.0. The comparisons within and between groups were accomplished by means of analysis of variance test (ANOVA). The post-hoc Tukey’s test was applied where main interactions were found. The chi square ( $\chi^{2}$ ) statistic was used to test the effect of sex variable in all groups. The level of significance (alpha) was set at 0.05.

3. Results

ANOVA test showed no significant differences in age ( $P=$ 0.319), weight ( $P=$ 0.483), height ( $P=$ 0.587) and body mass index ( $P=$ 0.994) among AS group and CLBP subgroups. $\chi^{2}$ test showed no significant difference in sex ratio ( $P=$ 0. 647) as well (Table 1).

ANOVA test showed significant differences in OASI ( $P=$ 0.0001), APSI ( $P=$ 0.0001), MLSI ( $P=$ 0.0001), DC ( $P=$ 0.0001) and T ( $P=$ 0.0001) among AS group and CLBP subgroups. The post hoc Tukey’s test revealed that LP subgroup had no impairment in OASI, APSI, MLSI and DC and it had prolonged T. It also revealed that the MP and SP subgroups had impaired OASI, APSI and DC and only the SP subgroup had impaired MLSI. Finally, T was longest in SP subgroup (Table 2).

Results of ANOVA test comparing postural stability indices within AS group ( $P=$ 0.0001), LP subgroup ( $P=$ 0.011), MP subgroup ( $P=$ 0.0001) and SP subgroup ( $P=$ 0.0009) showed significant differences among OASI, APSI and MLSI. The post hoc Tukey’s test revealed that MLSI was smaller than either OASI or APSI in both AS group and CLBP subgroups (Table 3).

4. Discussion

The etiology of chronic LBP in most patients is not known as mentioned earlier. It may be hypothesized that a certain percentage of these patients may have suboptimal neuromuscular control, especially under dynamic conditions.

The results of this study suggest that there are differences in balance reactions for those who suffer from chronic LBP according to their pain severity. Our results were consistent with others revealing different pain grades and intensities could affect results of balance assessment suggesting the differentiation between intensities of pain as a topic of concern [12, 26].

Although others measured pain by the numeric pain rating scale [12], it is still a subjective outcome depending on the patients’ interpretation of pain as the visual analogue scale used in our study.

However, all of these studies included dynamic balance examination in quiet standing, voluntary body lean and forward body lean.

Our results concluded that the LP subgroup had no impairment in the postural stability indices (OASI, APSI and MLSI) while SP subgroup was associated with defects in postural stability indices (OASI, APSI and MLSI) and dynamic limits of stability (DC and T).

This was consistent with the findings of Lund et al. [27] who reported that chronic high intensity pain was a significant factor for differing postural performance in chronic LBP. The similar context was discovered in another study reporting expectations of low or high pain during trunk movement translate into different neuromuscular changes in trunk muscles [28].

This study also reports that T was prolonged in all chronic LBP subgroups and it was longest in the SP subgroup, which can be also explained by the changes in back muscle activation. Considering chronic LBP as the only factor related to changes in postural control, this change in normal standing posture leads to increase muscle activation of back muscles, which will result in an increased fatigue rate.

These changes in the pattern of back muscle activation have been suggested as a strategy to limit spine movements [29], In addition, muscle pain can cause marked decrease in position sense [30] possibly through increased presynaptic inhibition of muscle afferents at spinal level or by a down-regulation of cortical proprioceptive processing.

Comparing variables among AS group and chronic LBP subgroups, our results showed significant difference in the MLSI, which was impaired in SP subgroup. This could be explained as the MLSI accounted for a very small portion of OASI because of the greater gravitational moment around anteroposterior axis and increased stability around mediolateral axis in SP subgroup. This small contribution could be obvious in SP subgroup [31]. Moreover, the influence of muscle fatigue due to change in trunk position combined with pain may lead to increased instability in lower back, including subjects with chronic LBP [32].

With regard to APSI in both the MP and SP subgroups, our results were not surprising as they were consistent with others reporting an increase in body sway of chronic LBP subjects [31, 33, 34, 35]. Subjects with chronic LBP tended to keep their center of force (COF) posterior compared with healthy subjects.

Chronic LBP subjects were more likely to fulcrum about the hip and back to maintain uprightness in challenging balance tasks compared with healthy controls who maintained their fulcrum for the COF around the ankle. This posterior shift causes a relaxation of the subject’s trunk muscles, an increase in lumbar lordosis and greater compressive forces to the vertebrae and neural foramina [33].

Controversially, young persons with LBP and chro- nic LBP have adapted forward inclined posture and positioned their mean center of pressure (COP) more to the anterior due to pain and when vision was occluded. In other words, reweighting of proprioceptive input by increasing the loading of ankle extensors by leaning more forward may enhance sensory discrimination and help maintain a critical level of sensory information to adequately cope with anticipation of postural instability [34, 35].

The results from this paradigm show that chronic LBP subjects have impaired APSI. This study also observed impaired APSI in MP and SP subgroups in addition to OASI compared with AS group and LP subgroup. Arnold and Schmitz [31] reported that the OASI is very closely related to the APSI and receives a relatively small contribution from the MLSI, similarly to our findings. Because of this small contribution, it may be best to use the MLSI and APSI separately rather than the OASI.

According to our results, it is important to note that, T was the only parameter of the studied dynamic balance parameters that was impaired among all chronic LBP subgroups, it was longest in the SP subgroup and finally it was the only impaired parameters in LP subgroup. This could indicate that, whatever the pain severity was, the proprioception in individuals with chronic LBP might be disturbed and therefore the deficit in sensorimotor processing was revealed in all chronic LBP subgroups.

Numerous studies support our finding regarding prolonged T and they revealed problems with sensory information processing in patients with chronic LBP [25, 36], for example, lumbar proprioceptive deficits caused by pain produce postural deterioration [25] and the availability of sensory information could affect the dynamic limits of stability [36].

In a similar context, different researches support the notation that beside sensory information processing, the higher cognitive system plays an important role in dynamic posture control [37, 38]. The interaction between both of them is influenced by several factors, such as age, task difficulty, type of response required during the task, constraints to the postural task and individual differences [9, 40].

Based on this interaction we could claim that pain severity is one of constraints that might affect the dynamic balance performance in chronic LBP patient, therefore, T as a parameter of dynamic limits of stability was prolonged in all chronic LBP subgroups.

Recent evidence by Gao et al. [41] supports our explanation regarding prolonged T; the authors reported that subjects with chronic LBP had significantly prolonged reaction time of some trunk muscles compared with healthy controls during sudden imbalance. Moreover, it was reported that the chronic LBP group had longer reaction time than the control group [42]. Both findings could explain the impairment of time needed to complete the dynamic limits of stability test in all chronic LBP subgroups.

5. Limitations and recommendations

Although the intensity of pain has shown to be one of determinants affecting dynamic balance in chronic LBP patients in terms of postural stability indices and dynamic limits of stability, the overall number of participants was still comparably small. Similar studies with an identical experimental setup and larger sample sizes could be more appropriate.

6. Conclusion

The intensity of pain has shown to be one of determinants affecting dynamic balance in chronic LBP patients. These patients have differences in the impairment of dynamic balance in terms of postural stability indices and dynamic limits of stability. Chronic LBP patients had prolonged T regardless the pain level. Patients with high level of pain had impairment in postural stability indices (OASI, APSI and MLSI) and dynamic limits of stability (DC and T), while patients with moderate level of pain were similar to patients with high level of pain except for unimpaired MLSI. Patients with low level of pain had only prolonged T. Finally, it should be pointed out that care must be taken when evaluating CLBP patients since a wide range of balance responses exists among them.

Conflict of interest

None to report.

Footnotes

Acknowledgments

The authors would like to express their appreciation to all participants in this study for their commitment and consideration.

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The effect of pain severity on postural stability and dynamic limits of stability in chronic low back pain

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Methods

2.1 Subjects

2.2 Pain evaluation

Table 1 Demographic data for AS group and CLBP subgroups (mean ± SD, n = 15 for each)

2.4 Evaluation procedures

Table 2 Postural stability indices and dynamic limits of stability (mean ± SD) between AS group and CLBP subgroups

3. Results

4. Discussion

5. Limitations and recommendations

6. Conclusion

Conflict of interest

Footnotes

Acknowledgments

References

Table 1
Demographic data for AS group and CLBP subgroups (mean $\pm$ SD, $n=$ 15 for each)

Table 2
Postural stability indices and dynamic limits of stability (mean $\pm$ SD) between AS group and CLBP subgroups