The purpose of this study was twofold: (a) to develop an underwater chronometer capable to provide feedback while the athlete is swimming, as well as being a control tool for the coach, and (b) to analyse its feedback effect on swim pace control compared with feedback provided by the coach and with no feedback, in 25 m and 50 m swimming pools. 30 male swimmers of national level volunteer to participate. Each swimmer swam 3 x 200 m at aerobic speed (AS) and 3 x 200 m just under the anaerobic threshold speed (AnS), each swam repetition with a different feedback condition: chronometer, coach and without feedback. Results (a) validate the chronometer system developed and (b) show that swimmers pace control is affected by the type of feedback provided, the swim speed elected and the size of the swimming pool.

• Providing concurrent feedback to swimmers improves theis swimming pace control.

• It is more important to provide feedback to control swim pace when the swimming pool is 50m long.

• Technological development as this chronometer system, could offload coach work, so coach can focus its time and attention on other performance aspects or other swimmers.

• Technological developments are more accepted by coaches when they don’t interfere on swimmers execution, that is, whet it is not necessary to implement the swimmer with cables and apparatus.