Subject
This cross-sectional observational study was conducted at the Department of Rehabilitation Medicine, Sir Run Run Shaw Hospital, School of Medicine, Zhejiang University, between May 2022 and October 2022, as part of another observational study. This study was approved by the Medical Ethics Committee of Sir Run Run Shaw Hospital, School of Medicine, Zhejiang University, under the ethics number 2022-450-01. Participants were recruited through posters and electronic advertisements. Participants who met the following inclusion criteria were considered eligible for this study: women aged between 18–60 years old with no recorded history or self-reported symptoms of LBP within the last six months. The exclusion criteria for this study were a history of surgery or injury related to the spine, pelvis, or lower limbs, scoliosis, obesity, pregnancy, or hypertension. Prior to participating in the study, all participants were fully informed of the study protocol, and informed consent was obtained.
Physical activity
PA was evaluated using the short version of the International Physical Activity Questionnaire (IPAQ). Participants were queried regarding the frequency and duration of high-intensity, moderate-intensity, and walking physical activity per day in the preceding seven days. The IPAQ data were calculated according to the IPAQ scoring protocol (accessible at http://www.ipaq.ki.se) and were categorised into three groups: low, moderate and high. The cut-off limits are outlined in Table 1.
Table 1
Physical activity category | Cut-off limits |
1 Low | - no activity is reported or - some activity is reported but not enough to meet categories 2 or 3 |
2 Moderate | − 3 or more days of vigorous activity for at least 20 min. per day or − 5 or more days of moderate intensity activity or walking for at least 30 min. per day or − 5 or more days of any combination of walking, moderate intensity or vigorous intensity activities achieving a minimum of 600 MET min/week |
3 High | − 3 or more days of vigorous activity accumulating at least 1500 MET min/week or − 7 days of any combination of walking, moderate or vigorous intensity activities achieving a minimum of 3000 MET min/week |
MET: Metabolic Equivalent |
Flexion relaxation phenomenon |
Quantification of FRP |
In this study, the Flexion-Relaxation Ratio (FRR) and the Extension-Relaxation Ratio (ERR) were used to quantify the FRP. These methods have been shown to possess good reliability and validity among the multitude of methods proposed in the literature.[22] The ratio was calculated from the electromyographic (EMG) recordings of the spinal extensors in a trunk dynamic forward flexion task. The FRR was defined as the ratio of the root mean square (RMS) of the flexion phase to the RMS of the full flexion phase during a trunk dynamic forward flexion task, while the ERR was defined as the ratio of the RMS of the extension phase to the RMS of the full flexion phase during the same task.
Forward flexion task
The participants were asked to visit the laboratory twice. They were briefed in detail on the study protocol by the researcher during the first visit to ensure that each participant fully understood the study. Then, a familiarisation procedure of the study was conducted. After the first visit, participants were asked to revisit the laboratory 24 hours later. During these 24 hours, they were told not to engage in physical work. The participants were asked to perform a trunk forward flexion task while wearing a surface EMG (sEMG) device to record sEMG signals.
The task consisted of four phases: Phase 1: the upright, relaxed standing position, with their arms by their side and their feet shoulder width apart for 4s; Phase 2: the participant performed a full forward flexion of the trunk with their knees straightened and the arms hanging naturally in front of the body, which lasted for 4s, Phase 3: keep the full flexion position for 4s. Phase 4: extended backwards to the starting position for 4 s. In addition, a metronome of 1s/vocal was taken for each participant to ensure an even, smooth rhythm of movement. A P-value less than 0.05 was considered as statistically significant.
sEMG signal
The ME 6000 sEMG system,16-channel (Mega Electronics Ltd., Kuopio, Finland), was used to record EMG signals from the erector spinae and multifidus muscles. The sampling frequency was set to 1000 Hz. The recordings were obtained from the right-side erector spinae and multifidus muscles, using Red DotTM Ag/AgCl electrodes (3M Health Care, St. Paul, MN, USA). The placement of the electrodes was in accordance with the SENIAM guidelines (accessible at http://www.seniam.org/).
Erector spinae (ES) (longissimus): the electrodes need to be placed at two finger widths lateral from the spinous process of L1.
Multifidus (MF): the electrodes need to be placed on a line from the caudal tip posterior spina iliaca superior to the interspace between L1 and L2 interspace at the level of L5 spinous process (approximately 2–3 cm from the midline) and aligned with it.
The electrode distance was set to 20mm. The electrodes and cables were fixed to the skin using hypoallergenic tape to prevent movement-related artifacts. Before attaching the electrodes, the skin was shaved and cleaned with alcohol pads.
EMG signal processing
The raw EMG signal data was exported from MEGAWIN software as ASCII files and imported into MATLAB (R2021a, manufactured by MathWorks America Co.) software for signal processing.
A fast Fourier transform was performed on the raw signal data, and the frequency spectrum was plotted for visual evaluation. To minimize potential artifacts and mains interference, the raw data was filtered using a 50 Hz notch filter and a 20–450 Hz bandpass filter.
The RMS formula was used to calculate the RMS value of the filtered EMG signal.
Statistical analysis
The data analysis was conducted utilizing the R (version 4.1.0; R Development Core Team) within the RStudio (version 2022.07.1 + 554) platform. The normality of the data was tested by utilizing a Q-Q plot and the Shapiro-Wilk normality test. The Bartlett test was used to test the homogeneity of variances. A Welch analysis of variance (ANOVA) was used to compare the FRR and ERR of the different groups. Further multiple comparisons were performed using the Games Howell Post-hoc Tests. The Kruskal-Wallis test was used to compare participants' characteristics such as age, height, weight and Body mass index (BMI).