Measurement of soft tissue strain in response to consecutively increased compressive and distractive loads on a friction-based test bed.

Purpose: To abstract the essential elements of chiropractic prone leg checking and subject them to controlled, experimental parametric testing.

Design: Controlled, objective, repeated-measure analysis of the dynamic response of leg positions to distractive and compressive loading conditions.

Setting: Research laboratory in a chiropractic college.

Participants: Twenty-five compression and 30 distraction subjects, most of whom were male, asymptomatic chiropractic students.

Intervention: The subjects were lowered to the prone position on a table optimized to detect dynamic leg positions, with separate sliding segments supporting each leg. A trial consisted of a 2-min control run, followed by two 2-min experimental runs in which compressive or distractive loads were applied incrementally to the table-leg segments.

Main Outcome Measure: An optoelectric system measured real-time absolute and relative leg positions.

Results: Right legs showed a greater average response than left legs under both distractive and compressive loads, and tended to respond more proportionately to incremental load increases. The average response to compression exceeded the response to distraction. Both legs showed a greater average response in the second half of the trials. Correlation of weights with responses was about four times greater in traction than compression.

Conclusion: The functional short leg is confirmed as a stable clinical reality, a multitrial mean of unloaded leg positional differences. The prone leg check may be a loading procedure, albeit unmeasured, that detects non-weight-bearing, functional asymmetry in loading responses. These probably reflect differences in left-right muscle tone, joint flexibility and tissue stiffness. The relatively nonmonotonic, nonlinear quality of left leg responses is consistent with asymmetric neurological responses.