We investigate the use of musical prosody as a coordination strategy in ensemble performance, focussing on the metrically ambiguous case of long sustained notes, where little rhythmic information can be obtained from the sparse note onsets. Using cluster analysis of the amplitude power curves of long notes in recordings of a violin-cello duo, we examine the use of varying intensity to communicate timing information over the duration of these sustained notes. The elbow method provides the optimal number of clusters, and we present the common intensity shapes employed by the violinist and by the cellist. Analysis of peaks in the intensity curves uncovers correspondences between peak positions and natural subdivisions of musical time: performers tend to use consistent curve shapes that peak at metrical subdivisions within notes, namely, at around the 0.25 and 0.75 points of the note event. We hypothesize that the 0.75 point intensity peak functions as an upbeat indicator and the 0.25 point peak serves to propel the note forward. We surmise that knowledge of the placements of these intensity peaks may be useful as auditory cues for marking musical time in held notes, and discuss how this knowledge might be exploited in anticipatory accompaniment systems and audio-score alignment.