Functional balance is an essential element in one's gait and daily life activities involving sight, hearing, vestibular apparatus, proprioceptive sense, position sense, muscular force, and cognition [1]. Postural control is achieved by the integration of the information regarding body movement sensed through the somatosensory system in the central nervous system and the appropriate reaction of the musculoskeletal system [2]. Proprioception is sensed from mechanoreceptors of muscles, ligaments, and joints and retains stability and bearings of the body during its static and dynamic movements [3,4].

Mok et al. [5] has found that an efficiency of postural recovery after an unexpected perturbation becomes lowered in people with chronic back pain and it is associated with poor use of spinal motion as a component of the postural strategy. Nies and Sinnott [6] found that low back pain patients had greater postural sway and were less likely to keep their balance on one foot with their eyes closed, compared to healthy people. Moreover, Mientjes and Frank [7] reported that a significant increase in medial/lateral instability in chronic low back pain patients was found during performance of tasks involving the removal of vision, especially in combination with increased task complexity. It was hypothesized that the increased body sway in patients with low back pain results from, among other things, injury and/or damage to proprioceptive tissues in the lumbar spine. Also, abnormal electromyography (EMG) activities may be attributable to the impairment in postural control [8,9].

However, previous studies about postural control were limited to the chronic back pain group, and had little evidence related with acute back pain. In several studies involving patients with acute back pain, delayed action time and reduced muscular activities of the trunk muscle were reported during their postural perturbation [10-12]. On the other hand, in patients with chronic back pain, changes of muscular activities were found not only in the trunk, but also in the lower extremities [13,14]. Thus, the postural control strategy in patients with acute back pain might be different from that in patients with a chronic back pain condition.

Based on this background, the aim of this study was to evaluate the changes of static and dynamic postural stability in patients with acute back pain. We also attempted to find differences in the instability pattern and the center of gravity (COG) distribution, depending upon the location of pain.

This study investigated a postural control pattern in a healthy population with eyes open and with eyes closed in the static state and the dynamic condition, with the influence of experimental acute back pain during postural control. The results revealed 1) a significant deviation in the right quadrant and the posterior quadrant for both the static and the dynamic postural control test in regards to both the healthy condition and the acute low back pain groups; 2) OAI, API, and MLI were more significantly increased while eyes were closed than while eyes were opened during both the static postural control test and the dynamic postural control test in both the healthy condition and the acute low back pain; 3) OAI and API were both significantly increased during the static postural control test with no significant change during the dynamic postural control test after the pain induction; and 4) a greater posterior deviation of COG after induction of right back pain during the static postural control test.

While someone is standing quietly, his body makes continuous movements even with his feet fixed on the ground. Such movements are small and reflexive processes to maintain his postural equilibrium. Since a human being has a high center point of mass and a small support base, he has difficulty maintaining a vertical posture. However, the multi-joints structure of the body allows a man to keep his balance in a variety of body configurations, even while he is in motion. It is considered that one's ability to rapidly adjust the movement timing of a muscle in response to any unexpected postural perturbation is very important for maintaining posture and balance.

Alterations in somatosensation from the limbs or the trunk, however, have been shown to modulate automatic postural responses [16]. Postural control involves the coexistence of stability and mobility. Hip and spine articulations are thought to contribute to the maintenance of postural stability [17,18]. Activity began with the ankle joint muscle and then radiated in sequence to the thigh muscle and then the trunk muscle on the same dorsal or ventral side of the body [19].

Recent studies have estimated that 14% of the United States population or more suffer from pain related to joints and the musculoskeletal system. Muscular injuries are a common cause of disability in the population, and muscle problems are the most common cause of low back pain [20]. There were a lot of reports about postural instability and impaired balance control in patients with low back pain [5,7,9,21]. They reported that patients with low back pain had MLI in most cases and API in some cases while eyes were closed. These results were more significant in the aspect regarding increased task complexity. Some authors asserted that increased MLI resulted from decreased proprioception and decreased spinal movement while one's posture was maintained.

Individuals with chronic low back pain demonstrated delayed and reduced EMG activity in the trunk muscle in response to postural perturbations [19,22]. In other studies, patients with low back pain showed decreased gluteus medius activity, which could be associated with reduced hip stability [13]. Jacobs et al. [14] reported that subjects with chronic low back pain revealed less early-phase activity at the internal oblique muscle and the gastrocnemius muscle and less late-phase activity at the erector spinae muscle, the rectus abdominis muscle, and the tibialis anterior muscle. Pandy et al. [23] reported that one's mediolateral balance during walking was maintained by the hip abductor, the vastus muscle groups, the soleus muscle, and the gastrocnemius muscle. Thus, abnormal muscular activity not only in the trunk muscle but also in muscles of the lower extremities in patients with low back pain was one of the reasons for their MLI.

Regarding the acute back pain, Nelson-Wong et al. [24] reported that prolonged standing, which induces back pain has some negative impact on the balance response. This study showed increased static postural instability, particularly in the anterior/posterior direction after the induction of experimental back pain. Thus, the postural stability strategy to maintain balance may be different between acute back pain and chronic back pain patients.

In patients with experimental acute back pain, Boudreau et al. [12] investigated the muscular activity in the erector spinae muscle and the external oblique muscle. This study showed a delay in the onset time and a decrease in the amplitude of EMG activity for both muscles in response to whole-body perturbation after the induction of pain. Hodges et al. [11] reported a constant reduction of the external oblique abdominal muscle activity after the experimental pain induction, compared with the erector spinae muscle. Thus, imbalanced and abnormal muscular activities of the flexor muscle and the extensor spinal muscle might be the reason for the API while pelvic and hip muscular functions were maintained in patients with acute back pain.

della Volpe et al. [25] reported that subjects with chronic low back pain showed an increased sway during the dynamic postural control test, and Radebold et al. [9] reported that patients with chronic low back pain demonstrated poorer balance performance at the higher level of task difficulty, which revealed similar results compared to our study. This difference might result from the changes in muscular activity in the chronic back pain group. Krause et al. [26] performed five types of weight-bearing tests and reported that the activity of the hip abductor increased greatly while standing with one foot on the irregular surface, compared to standing with two feet on the flat surface. This result is supported by Clark et al. [27], who reported that in the dynamic torso extension exercise, the activity of the hip flexor and biceps femoris increased more greatly than that of the paraspinal muscle when the exercise intensity increased. It means that the lower extremity muscle played an important role in dynamic postural control. Interestingly, our study results showed that there was no significant change in instability during the dynamic postural control test after the induction of pain. We can assume that both trunk muscles and lower extremity muscles are used to maintain balance in the dynamic condition while mainly trunk muscles are used in the static condition, so there was little effect of injection in the paraspinal muscle during the dynamic postural control test. Another hypothesis is that although the changes of the instability index were not statistically significant, those also tended to increase after the induction of pain. Therefore, if the sample size had been much larger, it could have shown a significant change.

In our study, both the right deviation and the posterior deviation in the static postural control test were significant in the normal healthy population as well as in the acute back pain condition. The increase in the posterior deviation is thought to be related to the increase in API. In contrast to the study conducted by Lim et al. [28], where they evaluated the balance of chronic backache patients using the Balance Master system (NeuroCom, Clackamas, OR, USA) our study showed a trend of the patients deviating toward the painless side; the right back pain group and the left back pain group showed left deviation and right deviation, respectively. These findings are thought to be a habituated avoidance response for reducing pain caused by the paralumbar and pelvic muscular spasm while the patient is standing. Their results are different from those of our study, and the reason for the differences is unclear. Thus, further study is needed.

As discussed above, further investigation is required to find out whether muscular activities of the lower extremities would be altered, and whether the COG pattern differs from that in patients with chronic back pain. Also, further investigation is required on whether right handed subjects utilize the right spinal muscle more frequently while maintaining their balance.

As a result of measuring postural stability changes in subjects with experimental low back pain, the overall instability was increased regardless of the visual information and location of pain in the static postural control test. Particularly, the increase in API had a significant role. In the dynamic evaluation, the postural instability was not changed regardless of the visual information and the pain location as the pelvic and lower extremity muscles may be not affected.