This paper focuses on the implementation of a continuous control of the degree of articulation (hypo/hyperarticulation) in the framework of HMM-based speech synthesis. The adaptation of a neutral speech synthesizer to generate hypo and hyperarticulated speech using a limited amount of speech data is first studied. This is done using inter-speaker voice adaptation techniques, applied here to intra-speaker voice adaptation. The implementation of a continuous control of the degree of articulation is then proposed in a second step. Finally, a subjective evaluation shows that good quality neutral/hypo/hyperarticulated speech, and also any intermediate, interpolated or extrapolated articulation degrees, can be obtained from an HMM-based speech synthesizer.