Objective: This study aimed to investigate the effects of combining progressive lumbar stabilization exercises with a self-breathing exercise program on psychosocial status, transversus abdominis muscle (TrA) thickness and contraction ratio, and pulmonary function in female university students aged 20–29 years with lumbar instability. Design: A randomized controlled trialMethods: Thirty-one female university students with lumbar instability who met the inclusion criteria participated in this randomized controlled trial. Participants were randomly assigned to the experimental (n=16) and control (n=15) groups. Both groups performed progressive lumbar stabilization exercises, while the experimental group additionally completed a self-breathing exercise program. Outcome measures included the Korean version of the Fear-Avoidance Beliefs Questionnaire, TrA thickness and contraction ratio, and pulmonary function testing. All outcomes were assessed before and after the intervention. Results: After the intervention, the experimental group showed significant improvements in TrA thickness, contraction ratio, and all pulmonary function variables (forced vital capacity [FVC], peak expiratory flow, maximal voluntary ventilation, forced expiratory volume in 1 second) (p<0.05). In the control group, only FVC showed a significant improvement (p<0.05). Between-group analysis demonstrateda significant difference in FVC (p<0.05). Conclusions: The combination of progressive lumbar stabilization exercises and a self-breathing exercise program significantly improved FVC among female university students with lumbar instability (p<0.05). These findings indicate that the combined intervention enhances pulmonary function by improving respiratory muscle coordination and chest wall expansion and may serve as foundational evidence for developing intervention programs targeting lumbar stability and pulmonary function.