Impact forces have been associated with the development of musculoskeletal injuries. However, results of epidemiologic studies that assess the association between impact loading and the development of acute or chronic injuries do not support this notion. There is agreement that excessive impact force may produce damage to the human musculoskeletal system and that there is a window of loading in which biologic tissue reacts positively to the applied impact load. However, it seems that the impact forces and stresses acting on cartilage, bones, ligaments, and tendons during running are typically within an acceptable range. Running on soft or hard surface materials creates different feelings of comfort. A muscle-tuning model suggests that muscles in general attempt to avoid vibrations using a tuning strategy to establish a critically damped mechanical system. Thus, the different feeling may manifest itself in the form of changed comfort or performance, a concept that is certainly not in agreement with the previous paradigm of impact forces and cushioning. (C) Lippincott-Raven Publishers.