Background Patients who undergo open-heart surgery often experience widespread musculoskeletal and pulmonary complications. These can interfere with their functioning, resulting in soft tissue changes, worsening postural changes, and poor respiratory performance. Therefore, the prospective study aimed to compare forward head angle (FHA) and forward shoulder angle (FSA), maximal inspiratory pressure (MIP), and self-reported breathing dysfunction before and after open-heart surgery. Material/Methods In a prospective observational study of 106 patients, men and women scheduled for open-heart surgery were enrolled. Prior to surgery and before discharge from the hospital, all patients were required to assess FHA and FSA using 2-dimensional motion analysis software, MIP using a respiratory pressure meter, and breathing dysfunction using the 25-item Self Evaluation of Breathing Questionnaire (SEBQ). Paired t test was used to compare differences between before and after surgery. To evaluate associations, logistic regression analysis was performed. Results Of the 106 patients recruited, 73 completed the study. FHA (−Δ6.55±4.77, P <0.001), FSA (−Δ4.82±7.29, P <0.001), and MIP (−Δ14.07±17.02 cmH 2 O, P <0.001) values were decreased prior to discharge from the hospital compared with values before surgery. Regression analysis found that decreased MIP after median sternotomy was associated with an increase in FHA and breathing dysfunction in patients after open-heart surgery. Conclusions After open-heart surgery, increased FHA and breathing dysfunction (SEBQ) were associated with reduced MIP, which may be associated with poor respiratory performance after surgery. Therefore, postoperative physiotherapy in this patient group should aim to improve postural changes.