Techniques based on motion capture can be useful in analyzing a wide range of musical styles and practices: in this case, Transylvanian village music. We focused on a repertoire known as ‘Gypsy songs of sorrow’, played by professional Gypsy musicians during parties and celebrations of their own community. Two parameters were the object of study: rhythmic duration, and synchronization between musicians (a violinist and a viola player). Results show that rhythm is a local variant of aksak and is based on two duration units (S=short, L=long) which respect the formula 2: 3 < S:L < 3: 4. Performances are characterized by large variations of the S: L ratio from period to period, which have an expressive function. Tracking the bow's movements with motion capture techniques allowed to show that these variations are related to a swinging interpretation, which also involves a voluntary asynchrony between the two musicians.