This research aimed to design a training program using synchronous training and to determine its effect on certain physical abilities and the performance level of junior swimmers in the 400-meter individual medley. The experimental method was used, as it was suitable for the nature of this research. This involved a two-group experimental design, one experimental and one control, using pre- and post-test measurements. The sample was randomly selected from the research population, consisting of 20 swimmers, who were randomly and equally divided into two groups (one experimental and one control), each consisting of 10 swimmers. In addition, 5 swimmers from the original population but outside the research sample were included for a pilot study. Among the most important results was the superiority of the experimental group (synchronous training) over the control group in post-test measurements, demonstrating improvement in certain physical abilities (transitional speed, speed endurance, strength endurance, general endurance, and flexibility) and the performance level of junior swimmers in the 400-meter individual medley. Among the most important recommendations are: 1- Implementing the proposed synchronized training program for junior 400-meter individual medley swimmers. 2- Raising awareness among coaches and swimmers about the importance of synchronized training and providing the necessary training equipment to implement this type of training.