Electromechanical coupling is crucial for modeling a realistic representation of Calcium transient and Ca+2 cycling. Cellular Ca+2 dynamics in atria differ fundamentally from the ventricles. A biophysically detailed electrophysiology model is hence, necessary to reproduce the experimentally observed phenomena like Ca+2 wave propagation in human atrial myocytes. In this work, we present a spatially detailed and yet computationally efficient electrophysiology model and its coupling with a contraction myofilament model. Having this fully coupled and calibrated human atrial electromechanical model we can reproduce the rate adaptation property of action potential duration, Ca+2 transient and the active force. The aim of this article is to analyse the mechanism that is behind the rate adaptation for a fully coupled human atrial electromechanical model.