During whole-body exercise, fatigue is difficult to quantify; however, changes to mechanical, physiological and psychological systems during exercise are associated with the development of fatigue. To quantify fatigue, one must therefore assess changes occurring in these variables. The purpose of this study was to demonstrate a method to assign weightings to selected variables and to combine them into a single value quantifying changes occurring during exercise. Twelve female recreational runners performed one hour of treadmill running, during which heart rate, respiration rate, stride frequency and six selected psychological variables were collected at defined intervals throughout the run. Data were normalised and a principle component analysis was performed. The resulting first eigenvector was termed the "contribution vector" and indicated the weighting of each variable towards the global exercise-induced changes in the body. The projection of data onto the contribution vector resulted in a value described as the "fatigue index". An assessment of the generalisation of the method to new data was performed using a leave-one-out cross-validation procedure and indicated that the index is accurate to within 3.01% of the maximum index value measured. The method developed here has the advantage over current methods due to its multifactorial, causal and customisable nature.
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