Two digital video camcorders (Panasonic NVDS33), sampling at 50 Hz with a shutter speed of 1/350 s, were used to film the participants under the water, from opposite sides of the pool Figure 1. Each camcorder was enclosed in a waterproof housing suspended underwater from a trolley that ran along the side of the pool, parallel to the participants’ swimming direction. The field of view of each camcorder was adjusted in such a way that the whole body of each participant was visible. To scale the recorded video footage and account for camcorder movement, a calibration rope, with markers every metre, was suspended horizontally in the water directly beneath the participants.