Participants
G*power 3 was used to calculate the sample size. The sample size of the 12 participants (including 20% dropout) in each group was capable of detecting a 20% difference in the single leg stance between the groups according to a similar study (19) assuming a standard deviation of 10%, a significance level of 0.05, and a power of 80%.
A total of 24 athletes aged 20–35 years old were recruited from two outpatient rehabilitation clinics. Participants were included if they had intermittent non-specific CLBP at least for 12 weeks, with VAS between 3 to 7, and training at least 3 sessions with 6 hours per week (20). Non-specific LBP is identified as LBP, which is not due to a known specific pathology (21). Participants were excluded if they had lumbar surgery experience, inflammatory spinal disease, spinal deformities, or neurologic radiating pain. Participants were also excluded from the study if they were unable to perform exercises. Participants were randomly assigned to the following two groups by block randomization method: TENS treatment or TENS plus diaphragm training (Fig. 1). Both groups underwent 12 intervention sessions over 4 weeks (3 sessions weekly).
The study procedure was clearly explained to all participants, and they signed written consent before entering into the study. The study was approved by the Research Ethics Committee at the Tehran University of Medical Sciences (IR.TUMS.FNM.REC.1398.138) and followed the principles of the Declaration of Helsinki. The trial was retrospectively registered at the Iranian Registry of Clinical Trials on www.irct.ir (Identification number: IRCT20090228001719N8).
Outcome Measurements
The following outcomes were collected by the examiner or self-report questionnaires.
Pain
Pain was assessed by the Numerical Rating Scale (NRS). The participants rated their pain on a defined 0–10 scale, where 0 is no pain, and 10 is the worst pain imaginable. NRS has different advantages, such as simplicity, reproducibility, easy comprehensibility, and sensitivity to small changes in pain (22).
Function
Core Outcome Measures Index (COMI) is a short, self-reported questionnaire for assessing important outcomes in patients with LBP. The COMI covers five different domains, with seven individual items; pain intensity (two separate items measuring back pain and leg/buttock pain), back function in everyday life (one item), symptom-specific well-being (one item), the general quality of life (one item) and disability (two separate items measuring social disability and work disability). The composite COMI score (range 0–10) is calculated using the average score of the five domains. Higher COMI scores indicate worse status. For the domain pain intensity, the data are collected using 0–10 graphic rating scales, with the higher of the two values for back pain and leg/buttock pain being used to represent the “pain” domain. Five-point scales (1–5) are used for the remaining domains, with the scores being rescaled into a 0–10 point range (score (1–5) minus 1, multiplied by 2.5). The values for the two disability items are averaged to represent the “disability” domain. The average of the five domains form the COMI is 0–10, best-worst (23).
Static Stability
Static stability was determined using the Unilateral Hip Bridge Endurance test (UHBE), which is a clinical measure of core stability (24). The UHBE test was performed with the participants lying supine with their arms across their chest, knees in flexion, and feet flat on the table. The participants performed a double-leg hip bridge, and once a neutral spine and pelvis position was achieved, the participants were instructed to extend one knee (randomly determined), so their leg was straight, and their thighs were parallel to another one. Participants were instructed to hold this position as long as possible. The test was terminated when they were no longer able to maintain a neutral pelvic position, as noted by a 10-degree change in transverse or sagittal plane alignment. Pelvic positioning in the transverse plane was monitored by a digital inclinometer attached to a mobilization belt that was tightly secured to the individual’s pelvic (Fig. 2).
Dynamic balance
Star Excursion Balance Test (SEBT) was utilized to evaluate dynamic balance (25). The SEBT was accomplished while the participant was standing on the dominant leg and asked to reach as far as they could along a grid in the anterior, posteromedial, and posterolateral directions with the tip of the great toe of the non-stance limb. Participant’s hands were held at the iliac crest during the test. Six practices followed by 2-min rest and three test trials were performed for each direction. The reaching distance of the grid was read and recorded manually, and the average of the three test trials was calculated. The order of reaching directions was randomized. The test was rejected and then repeated if a participant failed to maintain a unilateral stance, lifted or moved the standing foot from the grid, or failed to return the reaching limb to the starting position. For the assurance of accurate analysis, the data of the reaching distance was normalized by leg length to exclude the impact of the leg length. The leg length was measured from the anterior superior iliac spine most distal end to the most distal end of the lateral malleolus on each limb (26). Reaching in the anterior, posteromedial, and posterolateral directions have excellent reliability (27).
Intervention
All 24 participants completed the NRS, COMI, UHBT, and SEBT at baseline (before the intervention) and after completing the 4-week intervention. The control group received three sessions (30 min conventional TENS, impulse duration: 100 µs, frequency: 100 HZ) per week for 4 weeks (28). The experimental group received TENS plus diaphragm training for 12 sessions over a 4-week period. All participants in both group also received similar patient education information during the sessions. Breathing exercises were supervised by a physiotherapist at the beginning of each week to ensure that each breathing exercise was performed correctly. Participants were assigned two exercises a week and asked to complete each breathing exercise for 5 minutes, twice daily, for a total of 20 minutes a day at least 5 days per week. Instruction and feedback were given to participants on assessment days. When participants attended a clinic, breathing exercises were performed after electrotherapy with the therapist.
The exercises planned for each week were as follows:
First week
Supine breathing + Crocodile breathing
Second week
Supine breathing with TheraBand + Crocodile breathing with TheraBand
Third week
Seated breathing + 90/90/90 breathing
Fourth week
Seated breathing with TheraBand + 90/90/90 breathing with TheraBand
Supine Breathing
Participants were instructed to lay supine in a hook-lying position and arms in a comfortable position. They were asked to focus on breathing with their diaphragm, the breath filling into their lower abdomen and posterior chest wall. They were asked to keep their ribs depressed and keep their shoulders and neck relaxed. During resisted exercise, TheraBand was added around the thoracolumbar junction or fixing by the therapist to a distal section of the bed to enhance resistance (Fig. 3a) (19).
Crocodile breathing
Participants were instructed to lay prone with their hands in a diamond shape supporting their forehead. They were asked to try and focus on pushing their ribs out laterally and trying to breathe all the way down to the sacrum. A TheraBand was placed under the participant’s thoracolumbar junction during resistance training (Fig. 3b) (19).
Seated breathing: Participants were seated on a hard surface with their knees, hip, and ankles all at 90°. They were told to sit tall, as if a “string was pulling them up from the top of their head,” while maintaining all previously discussed breathing cues: preventing lower rib flair, breathing deeply, and relaxing their shoulders, neck, and arms (Fig. 3c) (19).
90/90/90 breathing: Participants were placed in the 90/90/90 position and were asked to hold their legs while maintaining all previously discussed breathing cues: controlling their ribs and thoracolumbar junction, breathing deeply, and relaxing their shoulders, neck, and arms (Fig. 3d) (19). During resistance training, a TheraBand or belt was placed under the participant’s thoracolumbar junction. The participant was instructed not to let the examiner pull the belt away. At home, participants were instructed to tie the TheraBand around a table or chair and leave tension in it to simulate the effect of pulling.