The use of inertial measurement units (IMU) has increased in swimming research as it is a promising alternative to the time-consuming traditional ways of performance analysis such as the manual video-analysis. Current research mainly focuses on freestyle (front-crawl) and breaststroke swimming whereas backstroke and butterfly are underrepresented. Also, the focus is on data analysis in terms of stroke count, frequency and timing without considering the movement in relation to the measured data. This paper investigated the butterfly swimming stroke over 100 m with 10 athletes of different skill-levels (from regional to national level). Data were measured using an IMU in combination with video. Key positions of the butterfly swimming technique were analyzed and summarized across all athletes. Aim of this study was to identify the intra-cyclic characteristics of the butterfly swimming technique to find commonalities in the measured data independent of skill level or swim speed. The results may contribute to an automatic pattern recognition and detailed stroke analysis with separation into the different sub-phases (i.e. in- and upsweep, recovery). In addition, the two executed dolphin kicks per cycle can be analyzed with regard to timing and duration without using video recording.
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