The present invention relates to the field of medical technology and in particular to determining a respiratory activity in patients with chronic obstructive pulmonary disease (COPD) based on surface electromyography measurements taken from the intercostal muscles on the chest of a subject. A processing apparatus for processing an electromyography signal indicative of an activity of a target muscle in a human or animal body that relates to the measurement of respiratory effort amid an activity of at least one further muscle is presented. The processing apparatus comprises a first interface for receiving a first electromyography signal obtained from the target muscle and the further muscle at a first location on the body; a second interface for receiving a second electromyography signal obtained from the further muscle at a second location on the body; and an analysis unit for determining a similarity signal based on the first electromyography signal and the second electromyography signal, wherein the analysis unit is further configured to determine a respiratory phase as an inspiratory phase if the similarity signal obtained from the first and the second electromyography signal is below a first predetermined threshold and/or determine a respiratory phase as an expiratory phase if the similarity signal obtained from the first and the second electromyography signal exceeds a second predetermined threshold.