An alternating pressure pad (1) comprises inflatable cells (9,10) supplied with air by supply lines (7, 8) from a compressor (2) via supply line (3). A pressure controller controls the output of the compressor. Also connected to supply line (3) is an angle detection valve (40) restricting flow of air to atmosphere with increase in its angular position and a pressure transducer (17). Any increase in pressure due to an increase in angle of the detection valve (19) results in back pressure in the detection line. The pressure controller (2) uses the pressure transducer (17) to detect this change in pressure and then acts to adjust compressor output to the cells (9, 10) accordingly. The angle detection valve (40) is positioned under the head section of the pad to move with the head rest section.