During heart or lung surgery, the flow and gas exchange of blood are controlled extracorporeally by perfusionists with the help of a heart-lung machine (HLM). Perfusionists work predominantly in a sitting position during an surgical operation, whereby they have to adapt to the working environment provided by the HLM. Studies on the musculoskeletal stress of perfusionists are still insufficient. In order to investigate posture, movement analyses are carried out, and different joint angles of perfusionists are recorded throughout the operation. The recorded joint angles were evaluated according to the specifications of the German Social Accident Insurance (DGUV) and categorized according to the degree of strain. Five surgeons/perfusionists were followed in two clinics, and their posture was recorded while working on the HLM. During all phases of the operation, high loads were measured in the areas of trunk inclination, back curvature, head inclination and shoulder joints. The measurements show that the test subjects adopted a forced posture in the moderate or stressful joint angle range for an average of 50% of the entire duration of the operation. The measurement results confirm the assumption that perfusionists are sometimes exposed to considerable physical strain when working on the heart-lung machine.