Abstract: Objective: To study the changes of diaphragm function of rats when in restrictive position.Methods: Twenty SD rats were randomly divided into two groups:control group and restrictive position group.The rats in restrictive position group were hung with their upper limbs bound for 12 hours.The maximal twitch torce(Pt),maximal tetanic torce(Po),contraction time(CT),half-relaxtion time(1/2 RT),and the force frequency curve at 10,20,40,60 and 100Hz were recorded.The superoxide dismulase(SOD),malondialdehyde(MDA),succinic dehydrogenase(SDH) in the diaphragm were detected.The ultrastructures of the diaphragm were examined by electron microscope.Results: The Pt and Po in the restrictive position group were lower than those in the control group(P<0.01),but the CT and 1/2RT were longer(P<0.01);When 10,20,40,60 and 100 Hz square-wave voltage was given to stimulate the diaphragm,the tension of the diaphragm in the restrictive position group was significantly lower than that of the control group(P<0.05 to P<0.01).The SOD and SDH in the restrictive position group were significantly lower than those of control group(P<0.01),and the MDA were higher(P<0.01).The electron microscopic morphometry of the diaphragm in the restrictive position group revealed that the diaphragm myoneme was confused and broken;the mitochondria was in a state of vacuolization and expanded;Myelinfigures could be seen,and the sarcoplasmic reticulum was distended.Conclusions: The diaphragm systolic function of the rats in the restrictive position group is damaged,which may be related to the decreased activity of SOD and SDH,as well as the increase of MDA content.