Previous laboratory tests have shown that joint temperatures, on exposure to low ambient temperatures, fall to a greater extent than muscle, rectal, or average skin temperatures. The fall in temperature is accompanied by an increased resistance of joints to movement, and the maximum speed with which the joint can be moved decreases. The predominant type of movement at the human knee joint and interphalangeal joints is a gliding one. The characteristics of synovial fluid explain the increased forces required to move a joint and the loss in speed of movement on exposure to cold. In vivo studies support such predictions.