Abstract: A processing device, program, method, and processing system are provided which can be more simply used by a user when estimating a condition during exercise or assisting estimation. The processing device comprises: an input/output interface configured to receive an acceleration rate detected, in a wired or wireless manner, from a sensor which is attached to or around the knee of a leg of a person and is for detecting at least the acceleration rate of the person during exercise; a memory configured to store the received acceleration rate in addition to a predetermined instruction command; and a processor configured to perform processing for estimating the condition of the knee joint of the person during exercise on the basis of the acceleration rate by executing the predetermined instruction command stored in the memory.