Sacroiliac Joint (SI Joint) Manipulation Decreases H-Reflex

What Murphy, Dawson & Slack (1995) Found

Can adjusting the sacroiliac (SI) joint influence spinal cord reflex activity?

In a 1995 study published in Electromyography and Clinical Neurophysiology, Murphy, Dawson, and Slack investigated whether sacroiliac joint manipulation alters the H-reflex, an objective neurophysiological measure of spinal motor neuron excitability.

Their findings suggest that manual therapy at the SI joint may produce measurable changes in spinal reflex function.

The H-reflex (Hoffmann reflex) is an electrically evoked analog of the stretch reflex. It is commonly used to assess:

• Alpha motor neuron excitability

• Spinal cord reflex pathway integrity

• Changes in neuromuscular control

Alterations in H-reflex amplitude can reflect changes in central nervous system modulation at the spinal level (Knikou, 2008).

The Study

Brian A. Murphy and colleagues examined the effect of sacroiliac joint manipulation on H-reflex amplitude. After performing SI joint manipulation, they measured reflex responses using electromyography (EMG).

The key finding:

Sacroiliac joint manipulation resulted in a significant decrease in H-reflex amplitude.

(Murphy et al., 1995)

A reduction in H-reflex amplitude suggests decreased motor neuron pool excitability — potentially reflecting inhibitory modulation within the spinal cord.

Why This Matters

The sacroiliac joint is richly innervated with nociceptive and mechanoreceptive afferents. Prior research has shown that SI joint stimulation can activate trunk and gluteal musculature reflexively (Indahl et al., 1999).

If manipulation alters afferent input from the SI joint, this could:

• Modify spinal cord excitability

• Influence muscle tone

• Affect movement patterns

• Alter pain perception

Manual therapy is increasingly understood to have neurophysiological effects beyond simple mechanical repositioning (Bialosky et al., 2009).

Clinical Implications

A decrease in H-reflex amplitude following SI joint manipulation may indicate:

• Short-term modulation of spinal reflex circuits

• Reduced hyperexcitability in the motor neuron pool

• Potential normalization of neuromuscular control

This supports the view that spinal manipulation may exert effects through the nervous system — not solely through structural alignment.

However, it is important to note:

• The study measured immediate neurophysiological changes.

• It did not directly measure long-term clinical outcomes.

• Reflex modulation does not automatically translate to symptom improvement.

A Balanced Perspective

Research on spinal manipulation and reflex modulation has produced varying results depending on region treated, subject population, and methodology (Knikou, 2008).

Nevertheless, Murphy et al. (1995) provided early experimental evidence that sacroiliac joint manipulation can measurably influence spinal reflex activity.

This contributes to a growing understanding that:

The effects of manipulation are neurophysiological as well as biomechanical.

Conclusion

Murphy, Dawson, and Slack (1995) demonstrated that sacroiliac joint manipulation decreases H-reflex amplitude, suggesting modulation of spinal motor neuron excitability.

For clinicians, this study reinforces an important concept:

Manual therapy may influence the nervous system in measurable ways — offering insight into mechanisms beyond joint mechanics alone.

References

Bialosky JE, Bishop MD, Price DD, Robinson ME, George SZ. The mechanisms of manual therapy in the treatment of musculoskeletal pain: A comprehensive model. Man Ther. 2009;14(5):531–538.

Indahl A, Kaigle A, Reikeras O, Holm SH. Sacroiliac joint involvement in activation of the porcine spinal and gluteal musculature. J Spinal Disord. 1999;12:325–330.

Knikou M. The H-reflex as a probe: Pathways and pitfalls. J Neurosci Methods. 2008;171(1):1–12.

Murphy BA, Dawson NJ, Slack JR. Sacroiliac joint manipulation decreases the H-reflex. Electromyogr Clin Neurophysiol. 1995;35:87–94.

Sacroiliac joint manipulation decreases the H-reflex, Murphy, B.A., Dawson, N.J., Slack, J.R.

Electromyogr Clin Neurophysiol , 1995;35:87-94