Falling is one of the most frequent incidents in daily and occupational activities leading to high number of injuries. There are several factors contributing to fall, such as holding a bag, external load, fatigue [1,2] and age related changes [3]. Falls lead to serious injuries and can cause fear of falling perception [4]. These incidences are highly costly in terms of losing physical abilities for doing routine and occupational activities as well as medical care [5] and as a consequence, inability to do routine work.

The assessment of postural sway during quiet standing on the force plate system is commonly used for quantitatively assessment of the static balance. This assessment consists of measurement of center of pressure (COP) position of ground reaction force during stance maintenance. The postural stability maintenance is important in avoiding falling and therefore injury avoidance. One of the daily activities challenging the postural system is load carriage mostly in a backpack form [5]. Previous researches have shown that such load carriage can affect static postural control, balance and gait pattern [5-7]. However, the previous studies conducted on external loads have been focused on the impacts of assymetric-distributed external loads on the balance control just in healthy subjects.

Postural control in the subjects with chronic low back pain (CLBP) during quiet standing was assessed in several studies. Findings of these studies showed a poor postural control in LBP subjects, compared with healthy subjects [8-10]. Altered postural control because of pain or injury may lead to reduced body control resulting in a further injury risk.

However, majority of the previous conducted studies examining the external load impacts on the balance control, have been focused on the asymmetrically distributed loads [11]. Zultowski et al. indicated that asymmetrical loading on the body increases postural sway in a medial-lateral direction [11].

To date, however few studies have addressed postural control strategies in a symmetrical-distributed external load, none of them was conducted on the LBP populations. Therefore, the behavior of the CLBP subjects exposed to an it has not yet been comprehended that how LBP subjects behave in standing condition with applying an external load. In other words, are there any differences in the effects of external load distribution on postural control strategy in healthy and CLBP subjects?

Therefore, the present study was aimed to analyze the control of posture in the subjects with CLBP during quiet standing on the force plate system in comparison with matched healthy adults in tow test conditions of with and without symmetrical external load.

Participants

This was a descriptive and cross-sectional study conducted on the 44 subjects, of which twenty three were adult subjects with chronic LBP and the others (n=21) were healthy individuals as the controls. The inclusion criteria were: (1) ability to complete the quiet standing task for at least 90 seconds, (2) ability to follow instructions, (3) no visual or hearing impairment, (4) no neuro-muscular or orthopedic dysfunction as well as taking any medications affecting the subject’s balance. LBP group were included if they had episodic LBP for 12 months or more, and excluded if they had serious spinal pathology, nerve root pain, previous history of spinal surgery, structural deformity of the spine, uncorrected vision, recent pregnancy, vestibular or respiratory disorder, diabetes and recent lower limb pathology or if they were using any medicine that could affect balance. They had no history of pain extending the gluteal fold. Subjects were tested when they were pain free (i.e. pain score lower than 2 on Visual Analogue Scale). Although LBP patients were assessed in their relatively painfree episode, some residual motor control changes may be perpetuated even when they are in remission, leading to the ongoing cycle of painmotor control dysfunction-pain [12]. Therefore, to identify the nature of motor control changes or their measurement properties, the effect of pain must be considered in the design.

Most CLBP subjects did not have a more specific diagnosis than Non specific LBP by physician.

The healthy participants were matched by age, height, weight and body mass index (mean age 27.6 ± 5.2 years, mean weight 71.09 ± 10.1 kg, mean height 174.1 ± 7.4 cm, mean body mass index 23.4 ± 3.4 kg/ m²) against the LBP participants (mean year 30.2 ± 6.3 year, mean weight 73.39 ± 8.2 kg, mean height 179 ± 8.5 cm, mean body mass index 23.1 ± 3.7 kg/m²). Also they were matched by physical fitness based on self reported checklist. They were not professional athletics and usually did two or three sessions per week low level activity.

Experimental procedures

Prior to the measurements, the purpose and procedure of the study were explained in detail to all subjects and written informed consent was obtained from all of them. All of the experimental procedures of the present study was approved by the ethics committee of University of Social Welfare and Rehabilitation Science.

In this study the combined factor of two load conditions in two groups was selected. The experiment consisted of two trials of quiet standing for 60 s with two levels of external loads including no-load and 12 kg load that was uniformly distributed in the front and back of the custom made waistcoat.

All trials were performed on a force plate (Z812A model, Kistler Instruments AG, Switzerland) in a quiet laboratory setting (the Ergonomics Laboratory of USWRS). For each condition the subject was asked to stand on the force plate with his/her feet approximately at pelvis width, to look straight ahead and keep the arms at his/her sides in a comfortable position. They were instructed to stand as still as possible for 60 seconds. The sampling frequency was set at 100 Hz for all trials. To avoid transient oscillations, the first five-second section of each recording series was omitted. The remaining period (55 seconds) was used for the computation of center of posture (COP) parameters. The force plate output was filtered with a low-pass Butterworth filter (cut-off frequency of 10 Hz). For this study the following parameters were used: standard deviation (SD) of amplitude in anterior-posterior (A-P) and medial lateral (M-L) directions (SDx and SDy), SD velocity in A-P and M-L directions (SDVx and SDVy), mean total velocity (Vm), area [13] and phase plane parameters [14].

(SAP, SML, SR ) were determined. The formula used to calculate each parameter is presented in table 1.

Parameter	Formula
SD of amplitude (mm)
AP
ML
SD of velocity (mm/s)
AP	, where
ML	, where
Phase plane portrait (Arbitrary Unit)
SAP
SML
SR
Mean total velocity (mm/s)
Area (mm2)	,where

Table 1: Formulae used to calculate COP parameters.

Statistical analysis

A two-way analysis of variance (ANOVA) was conducted to assess the influencing effects of two different load conditions (Noload and 12 kg load) on the participants’ scores on the COP measures. The statistical significance level was set as p=0.05 for all analyses in this study. Statistical analyses of the data were conducted using the statistical package of SPSS (version 16).

The descriptive summaries of the COP-based measures for the load and no-load conditions and in both groups are presented in table 2. There was homogeneity of variance between groups as assessed by Levene’s test for equality of error variances. There was no significant interaction between the groups and load conditions. Results showed that the load conditions have significant effects on the determined parameters as SDx [F(1, 84)=5.74, p=0.01], SDY [F(1, 84)=5.74, p=0.01] , AP S [F(1, 84)=3.9, p=0.04], R S [F(1, 84)=3.9, p=0.04] , ML S [F(1, 84)=3.9, p=0.04]. The groups have not significant effects on each COP parameters (Table 3).

Table 2: Descriptive data for COP measures in two load conditions in healthy and low back pain subjects.

Table 3: Summary of Two way analysis of variance for six measures of postural performance: F ratios and P values by variable.

The main finding of the present study is a semi linear effect of the external load on postural control, as indirectly assessed by COP, in both A-P and M-L directions. The assessments of the sway parameters showed a significant increase in the six out of nine measures of the postural sway [standard deviation of the amplitude of COP in A-P and M-L directions as well as in phase plane parameters (S_AP,S_ML,S_R )] by applying the load mass.

Previous studies have shown that physiological and mechanical factors can influence the postural control in quiet standing. . In this study mechanical factor was actually manipulated by adding external load. From a mechanical perspective, upright stance is modeled as an inverted pendulum and the added load would make such a system less stable. Indeed, in line with the predictions of the inverted pendulum model [15], shifting the location of the body’s center of mass to its natural position resulted in an increase in both the phase plane and SD of amplitude parameters.

Previous studies have reported this adverse effect on human postural control during quiet standing. Ledin and Odkvist reported greater postural sway as a result of mass placing on the chest and back [16]. Heller et al. showed that COP motion was greater in the backpack condition compared with the unloaded condition [17]. In the most previous studies, asymmetric loading such as backpack, adding weight on the shoulder or on the back were used. This applied load type induces asymmetric load distribution on the body resulting in a decreased level of balance and finally falling. In this study, we attempted to assess the effects of symmetrical–distributed external load positioning on the balance. Under the applied situations in this study, symmetric loading increased the sway measures.

In the present study, physiological mechanism could not contribute to the increased level of postural sway. It seems that the evaluated task was not enough demanding, which influence physiological measures such as heart rate and respiration changes.

Numerous researches have shown poor postural control in LBP subjects, as compared with healthy subjects [8,9,18,19]. We expected that LBP subjects behave differently under the loaded condition. Taken together with the potential adverse effects of applying external load, it seems that such a load may not be a sufficient challenge to affect the postural control in the LBP subjects. These small ratio differences suggest that the presence of an external load imposed a similar level of postural demand on both groups.

The present study has some limitations: First, learning effects may have existed. However, before data collection, practice was provided and the order of the conditions was randomly selected, so these effects are unlikely to have influenced the findings. Second, the sample size of this study was likely insufficient to detect significant differences. And finally, the last but not the least limit of this study was that the LBP subjects were assessed in relatively pain free period. Different levels of pain in an individual may influence the postural control strategies adopted by the subject [20].

Body sway, as quantified by traditional COP measures, increased with applying external load on the body. No interaction between group and external load was found on balance. It appears that this may be due to low load. Higher external load or asymmetrically distributed load may be shows difference in better way.