Objective:Using ultrasound imaging technology to evaluate the contraction characteristics of diaphragm in patients with chronic low back pain.Methods:Twenty nine patients with chronic low back pain and 26 healthy persons recruited from the rehabilitation department of the Third Affiliated Hospital of Sun Yat-sen University from November 2019 to April 2020 were selected and divided into the low back pain (LBP) group and the healthy control group. The thickness of the diaphragm (Tdi) of the subjects during deep breathing was evaluated by portable color Doppler ultrasound equipment under different body positions. The subjects were required to perform maximum inspiration for total lung capacity (TLC) and expiration for functional residual capacity (FRC) in the supine and standing position, respectively. The end-inspiratory diaphragm thickness (TdiTLC) and end-expiratory diaphragm thickness (TdiFRC) were recorded, and the diaphragmatic thickening fraction (DTF) was calculated. The general data of subjects with lower back pain and the correlation between Oswestry Dysfunction Index (ODI) and diaphragm function were analyzed; The diaphragm function of healthy control group and LBP group were compared; The receiver operating characteristic (ROC) curve of Tdi and DTF in the diagnosis of lower back pain were analyzed.Results:ODI lifting score was negatively correlated with standing TdiTLC ( r=-0.50, P<0.01). In intra-group comparison, the TdiTLC and TdiFRC values of healthy subjects in standing position were increased compared with those in supine position ( t=6.115, 7.314, all P<0.001); In standing position, TdiTCL and TdiFRC values in LBP group were increased compared with those in supine position ( t=2.834, 4.673, all P<0.01). In comparison between groups, TdiTLC values in supine and standing position of LBP group were significantly lower than those in healthy control group ( t=2.597, 3.338, all P<0.05); In standing position, TdiFRC of patients in LBP group was significantly lower than that of healthy control group ( t=2.098, P=0.041) and DTF value of patients in LBP group was significantly lower than that of healthy control group ( t=2.902, P=0.006). When TdiTCL≤3.3 mm in supine position was used to predict low back pain, the diagnostic sensitivity and specificity were 78.6% and 53.8%, respectively, and the area under the curve was 0.661. When TdiTCL≤4.5 mm in standing position was used to predict low back pain, the diagnostic sensitivity and specificity were 95.7% and 46.2%, respectively, and the area under the curve was 0.759. When DTF≤81.3% in standing position was used to predict low back pain, the diagnostic sensitivity and specificity were 52.2% and 84.6%, respectively, and the area under the curve was 0.698. Conclusions:It is found in our study that the diaphragm contractile function of patients with lower back pain is worse than that of normal subjects, and the difference was significant in standing position. We suggest that the right-side ultrasound image acquisition in the patient′s standing position is helpful to ensure the accuracy and objectivity of the measurement results. TdiTCL≤4.5 mm or DTF≤81.3% in standing position can be used as one of the reference indexes for the combined diagnosis of chronic low back pain.