Quantitative Analysis of Flexor Tendon Gliding Sounds of the Hand in Healthy Volunteers Using a Digital Stethoscope.

BACKGROUND: Smooth flexor tendon gliding depends on the structural harmony between the flexor tendon and its sheath. Because tendon dimensions and joint kinematics differ among digits, gliding dynamics may also differ, but in vivo assessment during active motion remains limited. The purpose of this study was to clarify whether the acoustic characteristics of flexor tendon gliding sounds at the metacarpophalangeal (MCP) joint differ among the index, middle, and ring fingers. METHODS: Gliding sounds were recorded from 40 hands of 20 healthy volunteers using a digital stethoscope placed at the palmar aspect of the MCP joint during active flexion-extension. A single examiner performed all recordings in a fixed finger order. Signals were filtered (20-500 Hz), and mean amplitude, mean power frequency (MPF), median power frequency (MDF), spectral flatness, and zero-crossing rate were extracted. A linear mixed-effects model with random intercepts for participant and for hand nested within participant was used. RESULTS: Significant digit-specific differences were found in mean amplitude, MPF, MDF, and spectral flatness (all P < .001), whereas the zero-crossing rate did not differ. The index finger showed lower amplitude and higher MPF and MDF than the other digits, and the middle finger showed the highest spectral flatness, with moderate-to-large pairwise effect sizes. CONCLUSIONS: Flexor tendon gliding produces distinct acoustic signatures that differ among digits, consistent with known anatomical and kinematic characteristics. Because tendon-pulley friction cannot be directly quantified using routine clinical imaging, these findings provide preliminary healthy-reference data for acoustic profiling of tendon disorders such as trigger finger.