Upper-Body Pitch Control Differentiates Sprint Butterfly Performance in Youth Swimmers: An IMU-Based Analysis

Efficient segmental pitch control is critical for butterfly swimming propulsion and alignment, yet its role in youth performance remains unclear. This study quantified head, shoulder, and hip pitch kinematics using wearable inertial measurement units (IMUs) in 41 competitive swimmers (9-11 years). Participants performed two maximal 25-m butterfly trials and were classified into faster and slower groups. Pitch angle, velocity, frequency, time, and pitch deviation index were extracted. Between-group differences were assessed using independent t-tests, and associations with performance were examined using Pearson correlations. Faster swimmers exhibited smaller head pitch angles during the Breath phase (p < 0.001, d = -2.01), along with greater shoulder pitch velocities and frequencies (all p < 0.05, d = 0.67-1.07). They also demonstrated shorter pitch times and lower pitch deviation indices (all p < 0.05, d = 0.66-1.92), indicating more efficient and stable movement patterns. In contrast, hip kinematics showed fewer and less consistent differences between groups. Several head and shoulder variables during the Breath phase were moderately correlated with sprint time (r = 0.32-0.43, p < 0.05). These findings suggest that sprint butterfly performance in youth swimmers is primarily associated with more controlled and stable upper-body pitch motion, particularly during breathing.