From the Wikipedia article about Martin-Baker ejection seats:
In 1944, James Martin was asked by the Ministry of Aircraft Production to develop methods for fighter pilots to escape their aircraft. Martin decided that the best method involved ejection of the seat with the occupant sitting in it, aided by an explosive charge. After ejection, the pilot would separate from the seat and open his parachute by pulling a ripcord in the usual way.
At that time there was little information on how much upward thrust the human body could withstand. Data relating to “g” forces in catapult launching of aircraft involved horizontal thrust and was therefore inapplicable to the new problem. Tests would have to be conducted to find out how much upward “g” force a person could tolerate. These were done by shooting a seat up a near-vertical path, loading the seat to represent the weight of the occupant, and measuring the accelerations involved.
A 5-metre (16 foot) test rig was built in the form of a tripod, one of the legs being in the form of guide rails. The seat was propelled up the guide rails by a gun, consisting of two telescopic tubes energised by an explosive cartridge. The guide rails were provided with ratchet stops every 75mm (3 inches), so that the seat was automatically arrested at the top of its travel.
Studies were conducted to find the limits of upward acceleration that the human body could stand. The first dummy shot with the seat loaded to 200lb was made on 20 January 1945, and four days later one of the company’s experimental fitters, Bernard Lynch, undertook the first “live” ride, being shot up the rig to a height of 4 feet 8inches. In three further tests, the power of the cartridge was progressively increased until a height of 10 feet was reached, at which stage Lynch reported the onset of considerable physical discomfort. The first seat was successfully live-tested by Lynch on 24 July 1946, who ejected from a Gloster Meteor travelling at 320 miles per hour (510 km/h) IAS at 8,000 feet (2,400 m) over Chalgrove Airfield in Oxfordshire.
There’s some footage included in a recent Smithsonian Channel video showing Bernard Lynch and the testing involved from the 1940s:
Bernard Lynch was an engineer fitter at British aviation firm Martin-Baker. But his main claim to fame was as the fearless test subject for their signature invention: the ejection seat.