To study processes related to weightlessness in ground-based cell biological research, a theoretically assumed microgravity environment is typically simulated using a clinostat – a small laboratory device that rotates cell culture vessels with the aim of averaging out the vector of gravitational forces. Here, we report that the rotational movement during fast clinorotation induces complex fluid motions in the cell culture vessel, which can trigger unintended cellular responses. Specifically, we demonstrate that suppression of myotube formation by 2D-clinorotation at 60 rpm is not an effect of the assumed microgravity but instead is a consequence of fluid motion. Therefore, cell biological results from fast clinorotation cannot be attributed to microgravity unless alternative explanations have been rigorously tested and ruled out. We consider two control experiments mandatory, i) a static, non-rotating control, and ii) a control for fluid motion. These control experiments are also highly recommended for other rotation speed settings and experimental conditions. Finally, we discuss strategies to minimize fluid motion in clinorotation experiments. •Clinostats rotate cell cultures to average-out the vector of gravitational forces.•Rotational movements induce complex fluid motions in cell culture vessels.•Suppression of myotube formation during clino-rotation is due to fluid motion.•Cell biological results from clino-rotation may not be attributed to microgravity.•Additional control experiments to determine fluid motion effects are mandatory.