Affecting up to 2 in 3 of the adult population during the course of a year, back pain is a very common disorder [1,2]. Back pain is a leading reason for doctor consultation, hospitalisation and other health and social care service utilisation [3], and is considered as the most costly musculoskeletal disease in industrialized countries [4]. Over the last few decades, research has focused on understanding possible mechanisms responsible for low back dysfunction (LBD). The presence of spinal proprioceptive deficit in LBD population is one of the current debates that could explain the patho-physiology of back pain [5-7]. Proprioception is a special term often used to describe the complex interaction between afferent and efferent input to control body movement and position [8]. Proprioception is thought to have a key role in maintaining normal spinal movement and stability [5,9]. Many authors have demonstrated that sensory-motor deficits are present in LBD patients; these deficits can affect segmental spinal stability and eventually lead to articular damage and subsequent chronic pain [10,11]. Sensori-motor dysfunction associated with LBD may include disturbances in a wide range of control mechanisms; the patho-physiological mechanisms and characteristics of these disturbances in different spinal lesions, however, require further investigation [12].

Understanding the role of such mechanisms in normal persons and in subjects with LBD could be crucial for improving back pain management, particularly those aspects related to reaching sound diagnosis, providing plausible explanation of the problem, providing relevant information and prescribing effective rehabilitation approaches [13]; yet, research is needed to investigate such an issue [14]. Although attention was drawn to include proprioceptive re-training exercises in current back rehabilitation programs [15]; very little research, however, exists to support this [8]. Impaired proprioception may be a major risk factor for recurrent injuries after the integrity of the muscles and ligaments has been restored [16]. Proprioceptive deficits are thought to be both a consequence and a cause of injury [17]. Proprioception and muscle control training could be the main key elements for rehabilitation of patients with LBD; still, little research has attempted to quantify spinal proprioception and detect proprioceptive deficits in patients with LBD [18,19].

LBD is a frequent condition with a wide range of clinical picture and causes. Twelve years ago, it was suggested that a gap in the literature existed concerning the detection of the degree to which proprioceptive deficits occur with different spinal lesions [9]; the situation has not significantly changed and research is still needed to discern such as aspect. The purpose of this study was to determine whether repositioning accuracy as a measure of proprioception differs in subjects with and without LBD, and to investigate the difference in the degree of repositioning accuracy in relation to the cause of the LBD, whether mechanical or discogenic.

The main aim of the present study was to compare the lumbar repositioning accuracy, as a measure of proprioception, in two different LBD populations compared to control subjects. The findings of the current study suggest that proprioception is affected in subjects with back dysfunction and that the proprioceptive deficit might constitute a part of the global dysfunction experienced by patients. The results of the study showed significant differences in the lumbar repositioning accuracy between the control and the two LBD groups. Lumbar repositioning AE were significantly greater in the LBD groups compared to the control group. The healthy subjects repositioned their back more accurately to the target position. However, no differences were found between the two LBD groups, showing that the proprioceptive deficits occur with the same degree regardless to the cause of the LBD whether mechanical or discogenic.

The difference in the lumbar repositioning accuracy between the control and the MLBD groups can be explained according to the essential basis of proprioception explained by Parkhurst and Burnett [17], where they stated that mechanoreceptors operate so that increased stretch or tension produces an increase in afferent signals, while impulses decrease with shortening. Accordingly, both increased and decreased muscle stretch may cause mechano-receptive dysfunction. In addition, the traditional view that joint receptors play the major role in controlling proprioception has been challenged in favour of the suggestion that muscle receptors may play an essential, perhaps primary, role [26]. Accordingly, it is assumed that muscle fatigue or shortening associated with LBD will have a drastic effect on the normal function of the muscle spindle, an important component for ensuring the correct positioning of the lumbosacral spine. Other explanation that may account for the results is that pain inhibition of local muscles such as lumbar multifidus may result in alterations in the normal muscle recruitment pattern resulting in the repositioning deficits [24].

Impaired postural control and lumbar proprioceptive deficits observed in patients with LBD were suggested to be a possible consequence of a feedback error resulting from sensory loss, information processing deficit, or both [21]. This may explain the differences in lumbar repositioning accuracy between the control and the DLBD groups. The non-significant difference between the two LBD groups suggest that the impaired position sense in both LBD groups can possibly be attributed to the fact that receptors important for proprioception are affected with dysfunction in the lumbar spine, and these proprioceptive deficits are not compensated by proprioceptive mechanisms outside the lumbar spine [9]. The instability concept in low back syndrome can provide another explanation for the proprioceptive deficits that occur with the same degree in the two LBD groups. The stability system of the spine consists of three components; the passive component (spinal column), the active component (spinal muscles) and the nervous component (neural control unit). Any overloading or disrupted function of any of these three components, as a result of mechanical or discogenic causes may lead to proprioceptive deficits and increased risk of injury [12].

The significant difference in the lumbar repositioning accuracy, which had been reported in this study, are in line with the earlier observations of Gill and Callaghan [9], who demonstrated that differences in proprioception do exist between individuals with LBP and those free from back pain. The findings were also in agreement with Taimela et al. [26], who noticed the presence of delayed lumbar muscle responses to sudden loads and impaired ability to sense a change in lumbar position as a result of paraspinal muscle fatigue. The results of this study is also supported by the earlier study conducted by Brumagne et al. [18], who suggested that patients with LBD may have proprioceptive disturbances possibly due to altered paraspinal muscle spindle afferents and impaired central processing of this sensory input. The findings were also supported by the study conducted by O'Sullivan et al. [24], who found that the inability of subjects with lumbar segmental instability to reposition the lumbar spine accurately into a neutral spinal posture is due to deficiency in lumbar proprioceptive awareness.

The findings of this study, however, differ from the earlier findings of Lam et al. [27] and Lee et al. [7], who failed to find significant difference in active lumbar repositioning accuracy between the study groups. This might be attributed to the inclusion of a heterogeneous (non-specific) LBD sample, which limited the ability to detect a difference between the study populations. Lee et al. [7] suggested, however, that impairments in proprioception do exist in individuals with back dysfunction, but it is better detected when assessed with a motion perception threshold measure.

The study done by Newcomer et al. [8] also failed to find difference between individuals with LBP and control subjects for the same reason, i.e., inclusion of a heterogeneous symptomatic study population, which may have limited their findings. In addition, the technique of measurement was not accurate enough. Testing the subjects in standing without immobilization of the legs provided extra-proprioceptive afferent input from the lower limbs and vestibular system. Newcomer et al. [28] repeated the same study, but with partial immobilization of the lower extremities and pelvis and they were able to detect a difference in the repositioning error that was not detected in the first study. The current study adopted the use of seated-compressed position, with adequate immobilization of the pelvis and the lower extremities, over the standing position because of its functional relevance and to minimize additional proprioceptive inputs from other distant receptors that have been noted in the standing position [24]. Also, in this position, the feet were held with no contact with the floor to minimize the proprioceptive input from the lower limbs. The current study included two particular subgroups of LBD patients, with distinctive inclusion/exclusion criteria and strict screening procedure, in order to recruit homogenous groups and to overcome limitations reported in previous studies.

The study has, however, been limited by several factors. First, the Biodex system 3 allows measuring the lumbar repositioning accuracy for movements in the sagittal plane only, i.e., lumbar flexion and extension, which limited the ability to measure the lumbar repositioning accuracy in side binding and rotation movements. In addition, the Biodex system 3 cannot measure fractions of degrees, which might have affected the accuracy of measurement. The main limitation is that the Biodex system 3 provided a considerable amount of sensory input due to large contact area with the body leading to increased cutaneous feedback.

It might be argued that the significant difference in the lumbar repositioning accuracy between the control and the two LBD groups can be attributed to the fact that spinal pain may result in coordination dysfunction during dynamic tasks, with alteration in the normal agonist-antagonist activity, particularly, as there is strong evidence to suggest that disturbances in neuromuscular control and motor performance may result directly from a reaction to the presence of pain [24]. However, during the current testing procedure, no subject reported severe pain in the back at the time of testing that might have interfered with the measurement. Furthermore, the results of the study showed weak correlation between pain and AE in the DLBD group (r = 0.225) while there was no correlation between pain and AE in the MLBD group (r = -0.043), which could eliminate any direct influence of pain on the study.

The study made significant contribution to the body of knowledge as it provided further evidence that proprioceptive dysfunction do exist in individuals with LBD. Moreover, the study is the first to compare two subgroups of LBD and investigate the difference in proprioceptive deficits between them. The study suggests that patients with LBD may have altered spinal proprioceptive function and disrupted joint position sense in the lumbar region compared to healthy individuals. The findings of the current study support the importance of incorporating a screening test for monitoring proprioceptive deficits in individuals with back dysfunction.

Differences in proprioception do exist between individuals with back dysfunction and normal subjects. Patients with back dysfunction had a less refined position sense than healthy individuals. The proprioceptive deficits do exist regardless of the cause of the back dysfunction, i.e., mechanical or discogenic, and may represent an important aspect of the patho-phsyiology of such a condition. Further research is needed to determine the different spinal lesions associated with a reduction in proprioception, and to determine whether such deficits can be corrected with specific exercise interventions and the role of proprioceptive retraining in the rehabilitation of back dysfunction.