This article introduces an integrated and biologically-inspired theory of decision-making, motor preparation, and motor execution. The theory is formalized as an extension of the diffusion model, in which diffusive accumulated evidence from the decision-making process is continuously conveyed to motor preparation brain areas, where it is filtered out through a second accumulation processing stage. The resulting motor preparation variable is then transmitted to the response-relevant muscles when it exceeds a threshold level of activation, corresponding to the beginning of motor execution. The transmission continues until a threshold amount of force has been produced by the muscles to issue the response. We tested this dual-stage dual-threshold diffusion model by continuously probing the electrical activity of response-relevant muscles through electromyography (EMG) in four choice tasks that span a variety of domains in cognitive sciences, namely motion perception, numerical cognition, recognition memory, and lexical knowledge. The model provided a good quantitative account of behavioral and EMG data, and systematically outperformed previous models. This work represents an advance in the integration of processes involved in simple decisions, and sheds new light into the interplay between decision and motor systems.