Full length articleEffects of unstable shoes on trunk muscle activity in patients with chronic low back pain
Introduction
Unstable shoes were developed for the general population with the aim of allowing wearers to benefit from the proprioceptive stimuli of training on uneven ground while performing the activities of normal daily living. As a result, many studies have focused on the effects of unstable shoes on the kinematics and electromyography of lower limb muscles in a standing posture and gait [[1], [2], [3]]. However, studies related to the effects of unstable shoes on spine kinematics and trunk muscle activity are limited. In one of these studies, Buchecker et al. assessed the spinal alignment, concurrent angular velocity, and EMG activity of trunk muscles during bipedal stance in healthy adults [4]. They concluded that wearing unstable shoes provoked more motion at the thoracolumbar and lumbopelvic levels and increased lumbar erector spinae (ES) activity in a bipedal stance when compared to standard (control) footwear.
More recently, Lisón et al. reported that unstable shoes increase trunk muscle activity of the ES and rectus abdominis (RA) and affect lumbar lordosis during gait compared to control flat shoes in a sample of young healthy participants [5]. Thus, these authors suggest that the use of unstable shoes may have potential implications in promoting spine stability, particularly in strengthening trunk muscles in the healthy population or perhaps even in low back pain (LBP) treatment. Accordingly, previous longitudinal studies have shown the effectiveness of unstable shoes in reducing pain [6,7] and disability [8,9] in different populations of chronic LBP patients. While recent studies on unstable shoes have provided some encouraging findings regarding the potential health benefits of these shoes, the overall body of published work is relatively small and the methodologies and focuses of these studies are diverse. Hence, the basic mechanism(s) by which unstable shoes influence gait pattern in either healthy volunteers or those with disabilities remain unclear. In particular, the precise factors determining the effectiveness of unstable shoes in LBP patients are still unknown. However, changes in trunk muscle activity may represent one possible mechanism underlying the therapeutic effect of unstable shoes on back pain [5,[9], [10], [11]].
Previous studies on unstable shoes in healthy adults have shown a clear increase in center of pressure (CoP) displacements in the anterior-posterior direction in the bipedal stance compared with that of traditional flat-sole shoes [2,12]. Effects in the medio-lateral direction have also been noted in healthy adults [2,3], and as a result, higher EMG lower-limb and trunk-muscle activity (involved in maintaining joint movement and positional control) has also been reported [[2], [3], [4], [5]]. In addition, there are also differences in postural control during standing in chronic LBP patients compared to healthy individuals [13]. For instance, during a more challenging standing condition (with visual occlusion), people with chronic LBP demonstrate increased CoP displacement and velocity which is thought to result from their impaired ability to maintain postural stability [14]. Other work has modelled mechanisms by which altered motor control strategies in this region serve as a potential cause and/or effect of LBP [15,16]; this work describes three inter-coordinated subsystems that are collectively responsible for adapting to the spinal stability requirements during various postures and movements (passive, active, and neural subsystems), and points out that dysfunctional neuromuscular-control strategies could result in clinical instability. Indeed, it is well established that chronic LBP patients demonstrate a variety of apparently dysfunctional neuromuscular-control strategies [17].
With all the above in view—and assuming that lumbopelvic region muscle-activity coordination is important for generating mechanical spinal stability—we hypothesize that the stimulus provided by introducing an element of imbalance (using unstable footwear), in a population that could require enhanced spinal stabilization (chronic LPB patients), might change trunk muscle activation amplitudes during gait. Therefore, the purpose of this work was to compare the EMG activity levels of trunk muscles (ES, RA, obliquus internus [OI], and obliquus externus [OE]) during gait in a sample of chronic LBP patients when wearing unstable shoes compared to wearing conventional (control) flat shoes.
Section snippets
Participants
Forty-three patients aged 18 to 65 years with a diagnosis of nonspecific chronic LBP lasting at least 3 months were recruited from the Orthopedic Surgery Service at the Hospital to participate in this cross-sectional study. The exclusion criteria were: a Roland Morris Disability Questionnaire (RMDQ) score < 4, obesity (BMI ≥ 30 kg/m [2]), diagnosis of a spinal tumor or infection, spinal fracture, lumbar radiculopathy, systemic (autoimmune, infectious, vascular, endocrine, metabolic, or
Results
A total of 43 nonspecific chronic LBP patients were screened in this study. Eight patients were excluded because they did not meet the inclusion criteria, and 35 patients were finally enrolled (18 women, 17 men; 51.1 ± 12.4 y [22 patients ≥ 50 y, 13 < 50 y]; 26 ± 3.8 kg/m [2]; 9.3 ± 5.2 RMDQ score). There were no missing data nor any statistically significant gender or age-related differences for any of the studied variables, and so all the data were pooled for the subsequent analyses. The
Discussion
To the best of our knowledge, this is the first study to investigate the immediate effects of wearing unstable shoes on trunk muscle activity during gait in chronic LBP patients. Our results show that the unstable shoes produced significantly higher ES, RA, and OI%EMG muscle activity levels compared to conventional flat shoes, with a large effect size for the ES. These results in LBP patients are concordant with our previous study which analyzed trunk muscle activity during gait in 48 healthy
Conflict of interest statement
The authors have no conflicts of interest to declare and have no financial or other interests in the product studied (MBT shoes).
Acknowledgements
The authors would like to acknowledge the support (language help) from the University CEU Cardenal Herrera (INDI 16/35), and the Instituto de Salud Carlos III, Spain, (PI12/02710).
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