Background— Atrial fibrillation impairs atrial contractility, inducing atrial stunning that promotes thromboembolic stroke. Action potential (AP)-prolonging drugs are reported to normalize atrial hypocontractility caused by atrial tachycardia remodeling (ATR). Here, we addressed the role of AP duration (APD) changes in ATR-induced hypocontractility. Methods and Results— ATR (7-day tachypacing) decreased APD (perforated patch recording) by ≈50%, atrial contractility (echocardiography, cardiomyocyte video edge detection), and [Ca ²⁺ ] _i transients. ATR AP waveforms suppressed [Ca ²⁺ ] _i transients and cell shortening of control cardiomyocytes; whereas control AP waveforms improved [Ca ²⁺ ] _i transients and cell shortening in ATR cells. However, ATR cardiomyocytes clamped with the same control AP waveform had ≈60% smaller [Ca ²⁺ ] _i transients and cell shortening than control cells. We therefore sought additional mechanisms of contractile impairment. Whole-cell voltage clamp revealed reduced I _CaL ; I _CaL inhibition superimposed on ATR APs further suppressed [Ca ²⁺ ] _i transients in control cells. Confocal microscopy indicated ATR-impaired propagation of the Ca ²⁺ release signal to the cell center in association with loss of t-tubular structures. Myofilament function studies in skinned permeabilized cardiomyocytes showed altered Ca ²⁺ sensitivity and force redevelopment in ATR, possibly due to hypophosphorylation of myosin-binding protein C and myosin light-chain protein 2a (immunoblot). Hypophosphorylation was related to multiple phosphorylation system abnormalities where protein kinase A regulatory subunits were downregulated, whereas autophosphorylation and expression of Ca ²⁺ -calmodulin-dependent protein kinase IIδ and protein phosphatase 1 activity were enhanced. Recovery of [Ca ²⁺ ] _i transients and cell shortening occurred in parallel after ATR cessation. Conclusions— Shortening of APD contributes to hypocontractility induced by 1-week ATR but accounts for it only partially. Additional contractility-suppressing mechanisms include I _CaL current reduction, impaired subcellular Ca ²⁺ signal transmission, and altered myofilament function associated with abnormal myosin and myosin-associated protein phosphorylation. The complex mechanistic basis of the atrial hypocontractility associated with AF argues for upstream therapeutic targeting rather than interventions directed toward specific downstream pathophysiological derangements.