Fitness & Sportmedizin

Studies which deal with the change of endurance performance in leisure and health sports, mostly apply the measurements of the mean and dispersion for group comparisons. Also, they interpret significant changes in the performance of an average athlete statistically and practically. However, the average athlete exists only in theory, but not in reality. In accordance with this, individual, often deviating, changes in power and endurance cannot be assessed in a differentiated way, which would be necessary to design an adequate training schedule for each individual athlete. It is also impossible to distinguish accurately between performancechangeenhancement, biological variability of performance and measurement errors for the individual athlete. Looking at clinical and training-relevant criteria, the challenge which is often neglected methodologically is the measurement and evaluation of individual adaption reactions. Comparing mean value changes and a measure for the individual change of a single athlete, the Reliable Change Index (RCI) shows that this may cause training-relevant differences. These differences can be accompanied either by an improvement of individual performance or, in a negative case, by a degradation of individual performance, even if the average performance of the group changes just in one direction. The consideration of the individual case is a condition sine qua non condition for effective individual design of training in health- and leisure-oriented endurance training, as well as in other training contexts. KEY WORDS: Reliability, Measurement Accuracy, Intraindividual  Training Effects, Change Indices, Enduranc