The bow is an essential element for the sound production of bowed string instruments. The playing qualities of a bow, as evaluated by players, can be divided into two categories: those related to the playability (ability to execute technical gestures) and those related to the sound (influence of the bow on the sound of the instrument). Recent work (Ablitzer?s PhD, 2011) has allowed a better understanding of the influence of manufacture parameters on the mechanical behavior of a bow. However, links with playing qualities remain difficult to establish, as the interaction of the player and the bow has not been investigated yet. The production of the ?good? vibratory motion of a bowed string (Helmholtz motion) is mainly controlled by three parameters: the bow position on the string, the bow speed and the bow force. A procedure, based on optical motion capture and piezo-sensors on the bridge of two cellos (Wofford?s PhD, 2018), has been developed in order to measure these control parameters and explore whether and how players modify them when changing bows. In addition, the dynamics of the bow (modelled as a two degrees of freedom oscillator) has been added in a bowed string model to simulate the effect of the bow mechanical parameters on the bowed string vibratory motion. It shows a clear effect on the transients, which can then be compared with measurements made with piezo-sensors on the bridge, under the strings. Results of a preliminary study with one musician, one cello and two bows will be presented.