The manipulative crack. Frequency analysis.

Objectives: This research was designed to analyse the frequency spectra of joint crack sounds produced during spinal manipulative therapy applied to the upper cervical spine of 50 volunteers and to determine if the spectra differed between the sexes and or for those with a history of previous neck trauma compared with those without a history of trauma.

Design: Randomised experimental study.

Setting: Macquarie University, Centre for Chiropractic, Summer Hill, New South Wales.

Subjects: Fifty asymptomatic subjects were recruited from the students and staff of the above college.

Intervention: Single, unilateral "diversified", high velocity, low amplitude, rotatory thrust technique applied to the region of the C3/4 zygapophyseal joints.

Main outcome measures: Joint crack sound wave analysis of Digital Audio Tape (DAT) recordings, taken from two skin mounted microphones positioned on either side of the cervical spine and later analysed by the use of a computer equipped with professional quality frequency spectrum analysis software.

Results: All fifty manipulations resulted in at least one audible joint crack sound and in total the fifty subjects combined produced 123 individual joint cracks. Only 9 subjects (18%) produced a single joint crack, the majority of the subjects (82%) produced either two (22 subjects) or three (10 subjects) distinct joint crack signals, while seven subjects produced four and two subjects five separate joint crack signals. Frequency analysis was performed on a total of 122 individual wave forms. Peak frequencies for all analysed crack signals ranged from 1,830 Hz to 86 Hz with an mean of 333 Hz (95% C.I., 285-380 Hz), a mode of 215 Hz and a median of 215 Hz. Statistical analysis for recorded signals revealed 95% Confidence Interval for the mean of 285-380 Hz. No statistically significant differences were found for peak frequencies between the sexes or for a previous history of trauma and no trauma and for pre-manipulative and manipulative joint cracks.