This study sought to determine the best method to quantify training based on heart rate data. It proposes a modification of Banister’s original performance model to improve the accuracy of predicted performance. The new formulation introduces a variable that accounts for changes in the subject’s initial performance as a result of the quantity of training. The two systems models were applied to a well-trained female monofin swimmer over a 24-week training period. Each model comprised a set of parameters unique to the individual and was estimated by fitting model-predicted performance to measured performance. We used the Alienor method associated to Optimization-Preserving Operators to identify these parameters. The quantification method based on training intensity zones gave a better estimation of predicted performance in both models. Using the new model in sports in which performance is generally predicted (running, swimming) will help us to define its real interest.