A personal device is provided for measuring a training activity of a trainee having a body part which moves during this training activity, this movement at least partially defining said training activity. The device comprises a sensing unit adapted to repeatedly measure, during the training activity, parameters associated with the movement of the body part, and wherein the sensing unit comprising at least accelerometer means, a compass and, optimally, gyroscope means. The device further comprises means for attaching the sensing unit to the body part and a processor adapted to receive from the sensing unit the parameters and to calculate based thereon, data indicative of the training activity. In addition, there are provided a system for use by an instructor in instructing of a plurality of participants engaged in an activity, a communication system for communicating with one or more swimmers, the system comprising a transmitting module adapted to communicate in accordance with a wireless communication protocol with at least one receiving module associated with at least one swimmer, and a method for automated training one or more swimmers accordingly.