Advances in sensing technology make the task of quantifying expressive human body movement more feasible than ever before. Success will enable breakthroughs in Humancomputer interaction (HCI) and control paradigms. In most areas, however, expressivity remains vague and difficult to define. We examine the movements of instrumental conductors at an elementary level to define particular qualities of a beat. In our test case, we focus on the difference between sostenuto and staccato articulation styles as a base for expressive qualities. We show that it is possible to define generic low-level movement features, we call movement primitives, to quantify the qualitative aspects of these two different articulation styles across a range of different tempi. Our movement primitives include the mean and variance of the magnitude of velocity and acceleration, and measures of spatial curvature. Each of these is measured from the ictus of one beat through the ictus of the next beat in a standard 4/4 beat pattern. The discriminative power of these features is demonstrated by two-tail t-tests and verified through Naïve Bayes classification experiments. The results demonstrate that our use of movement primitives effectively describes characteristics of expression revealed in each beat of two different articulation styles.