In normal subjects bracing impairs the function of the inspiratory muscles

Normal subjects can increase their capacity to sustain hyperpnoea by bracin g their arms on fixed objects, a procedure which is also known to reduce dy spnoea in patients with chronic obstructive pulmonary disease (COPD). In th e present study, it was tested whether bracing per se could improve the fun ction of the diaphragm. The effect of bracing on diaphragm function was studied in sia normal subje cts by recording changes in oesophageal (Delta Poes) and transdiaphragmatic (Delta Pdi) pressure during inspiratory capacity (IC) manoeuvres in the se ated and upright postures, and in the seated posture, also during bilateral phrenic nerve stimulation (BPNS) at functional residual capacity (FRC). Th e pattern of ribcage motion and deformation associated with bracing and wit h diaphragm contraction was also evaluated using inductance plethysmography and magnetometers. Bracing increased FRC by >300 mL and reduced IC by similar to 200 mL, in bo th postures. Delta Pdi during BPNS decreased on average by 15% indicating a n impaired diaphragmatic function. The ribcage was deformed with bracing an d was more distortable during BPNS. In conclusion, in normal subjects, bracing impairs the function of the insp iratory muscles and reduces ribcage stability. These negative effects canno t explain the improved capacity to sustain hyperpnoea when the arms are bra ced.