A sign language utterance can be seen as a continuous stream of motion, involving the signs themselves and inter-sign movements or transitions. Like in speech, coarticulation constitutes an important part of the language. Indeed, the signs are contextualized: their form and, most of all, the transitions will greatly depend on the surrounding signs. For that reason, the manual segmentation of sign language utterances is a difficult and imprecise task. Besides, annotators often assume that both hands are synchronous, which is not always true in practice. In this paper, we first propose a technique to automatically refine the segmentation by adjusting the manual tags isolating signs from transitions. We then study motion transitions between consecutive signs and, in particular, the duration of those transitions. We propose several computation techniques for the transition duration based on the analysis we have conducted. Finally, we use our findings in our motion synthesis platform to create new utterances in French Sign Language.