Fig. 7. Force in the X-direction (see Fig. 5) reflecting thrust and drag, measured from the robotic pectoral fin during a single fin beat under three different imposed movement patterns (shown in different colors). Robotic models of the pectoral fin allow analysis of the effects of different movement patterns in a way not possible with studies of live animals alone. When the fin executes a cupping motion only (black trace), a force curve with two distinct peaks is produced with no drag force at the transition from outstroke (abduction) to instroke (adduction). A cupping and sweeping motion (blue trace) generates considerably higher thrust forces as well as a small drag force during the transition. Moving the fin in sweep only (red trace), produces large drag forces on the outstroke and roughly equivalent thrust on the return stroke.