The effective forces transmitted by high-speed, low-amplitude thoracic manipulation

Twenty asymptomatic volunteers each received three spinal manipulative treatments to the thoracic spine. The treatments consisted of a straight posterior-to-anterior high-speed, low-amplitude thrust to the transverse process of T3-T10 using a reinforced hypothenar contact. All treatments were given by a full-time practicing clinician with 3 years of experience.

The primary objective of this study was to quantify local measures of loading applied by the clinician on the volunteers during spinal manipulative treatments and to compare these local measures of loading with previously described global measures.

The sparse information on the mechanics of spinal manipulative treatments deals exclusively with global force or pressure measurements. On the basis of these global data, incorrect conclusions may be drawn about the beneficial effects of spinal manipulative therapy, the loading of internal structures, and the risks associated with these treatments.

Twenty asymptomatic subjects each received three posterior-to-anterior, high-speed, low-amplitude spinal manipulative treatments to the transverse process of the thoracic spine. Total force, local force, contact area, peak pressure, and average pressure at the contact interface between clinician and subject were measured continuously by use of a thin, flexible pressure pad. Local and global measures of loading were compared and analyzed by use of nonparametric statistics (alpha = 0.01).

The average peak total force was 238.2 N. The average peak local force over a target area of 25 mm2 was 5 N, indicating that global measures of loading vastly overestimate the local effective forces at the target site. The peak pressure point moved, on average, 9.8 mm during the course of the manipulation.

To the authors' best knowledge, this is the first study to quantify local, effective measures of loading and compare them with the global measures typically used. The conclusions are limited because the study used a single clinician. The effective loading of specific target sites is much smaller than the global measures might suggest. This result occurs because as the forces during spinal manipulative treatment increase, so does the contact area; therefore, much of the total treatment force is taken up by non-target-specific tissues. Because of the vast discrepancy between the global and local measures of loading, it is suggested that risk-benefit assessments of high-speed, low-amplitude spinal manipulative treatments should be made, including local measures of loading. Finally, because theoretical approaches and the inverse dynamics approach can provide only global measures of loading, the results of such studies should be interpreted with caution.