The limits to exercise performance and the future of fatigue research

It is immensely difficult to provide a detailed historical account of the development of fatigue as a concept in the exercise sciences. However, the study of fatigue stretches back centuries to scientists such as Galvani who provided the ideas and tools to undertake experiments related to the electrical impulses needed to animate skeletal muscles [1]. The definitive work of Mosso in the 18th Century stands as a landmark in the study of fatigue [2]. In his book La fatica (fatigue) [2] he concluded that there were two phenomena which categorized fatigue; “The first is the diminution of the muscular force. The second is fatigue as a sensation. That is to say, we have a physical fact which can be measured and compared, and a psychic fact which eludes measurement” [p.154]. Notably, early textbooks such as Physiology of muscular exercise by Bainbridge in 1931 [3] pointed out that the limit of exercise “has often been ascribed to the capacity of the heart alone, but the facts as a whole indicate that the sum of the changes taking place throughout the body brings about the final cessation of effort” [p.176]. It is an interesting fact that research into fatigue is highly complex and consensus about the aetiology of this human condition still evades us. Not surprisingly, even after centuries of research in this area, fatigue is still very much a part of medical and social discourse. There may be several reasons for this, not least of which could be the loss or change in the meaning of the term fatigue. The Oxford Dictionary [4] defines fatigue as “extreme tiredness after exertion; reduction in efficiency of a muscle, organ etc. after prolonged activity”. Compare this definition to that of exhaustion which is often used interchangeably by exercise physiologists and which is defined as “a total loss of strength; to consume or use up the whole of” [4]; clearly, these are substantially different meanings. In addition to these general meanings, there is wide variation in definitions of fatigue in the exercise sciences which include statements such as “the failure to maintain the required or expected force” [5], or “a loss of maximal force generating capacity” [6] or “a reversible state of force depression, including a lower rate of rise of force and a slower relaxation” [7]. There are many more statements or definitions like these which attempt to capture the specific observation that there has been a decline in the ability to produce skeletal muscle tension of a given magnitude in order to quantify the amount of fatigue that has developed.