When we deal with post rehab clients with pain, we sometimes forget about the physiology of pain. Our perception of pain is cause by a stimulus that triggers the sensation of pain. In most situations, the pain stimulus must be present for some period of time to trigger the pain response. In assessing the client’s pain we must determine the “tissue tolerance level” or the level of stimulus that must be present to trigger the pain response. The classic example is the client who complains of pain in the lower extremity after standing for a period of time. If this pain stimulus is consistent during the time period, then this is the tissue tolerance level for client. We may find increasing the strength of the surrounding muscle and the stability of the joint structures may, with time, increase the amount of time the stimulus must be present before the client reports pain. This is a shift in the tissue tolerance level. Now the client is able to function for a longer period before reporting pain. Continued exercise may further enhance the tissue tolerance levels.