Study design and settings
This study was part of a parallel group, randomized-controlled trial that run from July 2016 through March 2017 (registration no. IRCT201507258035n2 in the Iranian Registry of Clinical Trials) [21]. Eligible subjects were recruited from the physiotherapy clinic of Kermanshah University of Medical Science, Kermanshah, Iran. To conduct the research, we received permission from the Ethic Committee of Kermanshah University of Medical Science (code number: kums.rec.1395.169).
Participants
In the first session, the study was explained to all participants; they read and filled out the informed consent form about the study purpose and procedure. Three consecutive or 5 intermittent absences in attending sessions, caused them to be excluded. Using the Pocock formula with considering of 80% power and 95% confidence level, the sample size was computed. Regarding the results of a previous clinical trial and based on the change in the mean score of hamstring flexibility (Mean1 = 5.7, Mean2 = 3, SD1 = 3.33, SD2 = 2.9) [22], 15 patients were estimated per group.
After receiving all the permissions and approvals, then, forty-five eligible subjects were randomly assigned to three groups of static stretching (n = 15), strengthening exercise in muscle lengthened position (n = 15) and control (n = 15), using a block randomization procedure of size 2 (Fig. 1). In this clinical trial, random allocation to each group was performed by a statistician. This person who defined allocation sequences for the study, those who assigned patients to each group and the evaluator were blinded to the patients’ allocation sequence and the allocation sequence remain concealed until patients were enrolled and assigned to interventions.
Study eligibility criteria included having non-specific LBP for more than 3 months, pain intensity from 3 to 6 according to the visual analogue scale (VAS), obvious hamstring muscle shortness in SLR test, and age between 18 and 60 years. To perform the SLR test, the participants were positioned in supine with fewer clothes and without a pillow under their head, their hips medially rotated and adducted, and their knees extended. The tester lifted the patient’s legs forcing the posterior ankle upward while keeping their knees in a fully extended position. The tester continued to lift the participant’s legs by flexing at the hips until they complained of pain or tightness in their backs or back of their legs [23]. Having any pathology or anomaly in lower limbs such as neuropathic pain, malignancy, inflammatory diseases, and severe osteoporosis, arthritis and/or bone diseases were the study exclusion criteria. The patients and researcher were not aware of the existence of different intervention groups.
Intervention
Common conventional physiotherapy program for all three groups included 15 min of heat therapy (hot pack), 15 min application of transcutaneous electrical nerve stimulation (High frequency TENS) to the area of low back and performing the commonly used exercises for back pain. While the control group received only routine above care, subjects in the exercise groups received additional interventions.
In the first group, passive static stretching (SS) of the hamstring was performed using a spring for three sets of 2 min with 2 min rest in between while the subjects were in supine position and knee was fully extended. In the second intervention group or strengthening in lengthened position (SLP) group, in seated posture on a chair, while the thighs of subjects were supported on a surface and their hip joints were kept in 120 degrees of flexion (knee joint in full extension); subjects extended their hip against a spring and contracted hamstring muscles. The other hip and knee joints were kept flexed in 90 degrees. In the control group, nothing other than the common interventions (heat therapy and TENS) were done for the participants. The interventions were applied during 12 sessions, three times per week.
Outcome measure
Assessment of balance
YBT is one of the most important tests for the evaluation of dynamic balance. This is a functional test, which requires strength, flexibility, and neuromuscular control, and stability, range of movement, balance, and proprioception. Because of its speed, efficiency, portability, consistency, and objectivity, it is a popular functional test [11].
The YBT needs lower limb strength and range of motion [12]. The advantages of this test are its standard protocol and high inter-rater (0.99–1.00) and intra-rater (0.85–0.91) reliabilities [13]. The reliability of YBT is reported to be for the anterior 0.99; posteromedial 1.0; posterolateral 0.99; and composite 0.97 [16].
For balance assessment, Y-Balance test was performed in three directions (anterior (ANT), posteromedial (PM) and posterolateral (PL)) for all participants at the baseline and after 12 sessions of intervention.
In this test, the participants tried to reach the anterior, posteromedial, and posterolateral reach directions with one foot as far as they were able while they were standing on the other foot. If their reaching foot kicked the ground or the stance foot failed to maintain a unilateral stance on the ground, the test was recognized as invalid. The maximum reach distance of three trials in each reach direction was recorded by investigator and the mean value in each reach direction was chosen for data analysis. The test was done for both limbs. The reach distance data were normalized to lower limb length. For lower limb length, while lying in the supine position, the distance between anterosuperior iliac spine (ASIS) and the center of the ipsilateral medial malleolus of tibia was measured.
Predictor variables
To determine the important and significant variables, all variables, including age, sex, height, weight, leg dominance (right or left), reach direction, phase (before and after intervention), type of intervention (three study groups), pain and disability entered the model and the significant variables were entered the final model. Visual Analogue Scale (VAS) (0–100; 0 – no pain; 100 – worst pain you can imagine) and Persian translated version of the Oswestry disability questionnaire (0 = no disability, 100 = totally disabled) were used to assessment of pain intensity and functional status, respectively [24].
Statistical analyses
Quantitative and qualitative data were expressed as mean (SD) and frequency (percent), respectively. The possible differences in demographic and clinical features between groups were evaluated using the analysis of variance (ANOVA) and chi -square test. The normality of data distribution was confirmed (Kolmogorov-Smirnov test).
The Generalized Estimation Equations (GEE) method was used to analyze variation in outcomes across the intervention groups and predictor variables. GEE are methods of parameter estimation for correlated data. When data is recorded with a measurement unit in a time sequence, these repeated measurements are correlated. If these conditions are not considered, the standard errors of the parameter estimates will be large and the significantly results will be not reliable.
So, considering the structure of correlations between observations, GEE was run to investigate the adjusting effect of variables on Y-Balance test reach directions (dependent variables). By the way, unlike general linear models that applied to the independent measures for each subject, this model is able to identify patterns for subjects with correlated measures, (12 measurement of Y-Balance test).
In the first step, a backward strategy to select significant variables was applied at P-value< 0.05, then the significant variables were entered in the final model.
Analyze of the experimental data was started through the use of SPSS ver.22 (SPSS Inc., Chicago, IL, USA) software and two-sided P-values < 0.05 were considered as level of significant.