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The main role of a swimming coach is to provide frequent feedback and efficient planning to swimmers during training sessions. Quantitative and objective performance evaluation can better assist the coach. Inertial measurement units (IMUs) are widely used for motion analysis in sports because they can detect a variety of performance-related metrics. In this study, we propose a new performance evaluation feedback using goal metrics extracted from IMU and investigate its effect on the swimmer's weekly progress. The measurement was conducted once per week with 15 competitive swimmers for 10 consecutive weeks using an IMU worn on the sacrum. Each swimmer was asked to swim five one-way laps at maximum velocity in front crawl, and the coach recorded the lap times with a stopwatch, which served as the main representative of swimming performance. The swimmers were divided into two groups, an experimental and a control group, and the coach received phase-based feedback only for the experimental group. The feedback quantified the swimmer’s performance in each swimming phase (wall push-off, glide, strokes preparation and swim) and the whole lap in every swimming lap of the test. He then used the feedback to adjust the training for each individual and focus on the weaknesses identified. The results showed that the experimental group had significantly lower lap time (p-value < 0.05) and higher performance regularity than the control group from the sixth training session. This study showed that phase-based assessment feedback can help swimming coaches to create a more efficient training plan through detailed quantitative assessment.