Introduction: Breast cancer is a major cause of morbidity and mortality, leading to functional deficiencies in ventilation, muscle performance, balance and posture.Objective: To describe the effects of the muscle energy technique (MET) for quadratus lumborum on maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP) in patients with stage I and II breast cancer.Materials and methods: Quasi-experimental study with a pretest-posttest design conducted in 10 patients treated at a highly specialized healthcare center in Bogotá. Physical therapy assessment and three physiotherapy sessions with the MET to measure MIP and MEP were carried out before and after the intervention using a respiratory pressure meter.Results: The average MIP was 41% of the reference value at the beginning of the intervention, which increased to 69% at the end of the sessions. On the other hand, the initial average MEP was 33%, while the post-intervention average MEP was 51%. The average change rate for MIP was 68% and 57% for MEP. The Wilcoxon signed-rank test was performed, achieving a statistically significant difference (z=-2.807, p=0.005).Conclusion: Applying the MET on the quadratus lumborum muscle improves its performance and increases respiratory muscle strength.