Electromechanical coupling is crucial for modeling a realistic representation of Ca+2 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, its coupling with a contraction myofilament model and the effect of mechao-calcium feedback on coupling. This novel coupled and calibrated human atrial electromechanical model was validated by reproducing the rate adaptation property of action potential, Ca+2 transient and the active force. The aim of this article is to present a new coupled model for human atrial myocyte and to analyze the mechanism behind the rate adaptation.