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Joel G. Pickar, D.C., Ph.D., Professor, Palmer Center for Chiropractic Research, Palmer College of Chiropractic, Davenport, Iowa

Manual therapies, like mobilization and manipulation, by their very nature impose mechanical inputs upon the body. A substantial body of literature identifies primary afferents capable of responding to mechanical inputs. Sensory input arrives at the spinal cord along Group I (A), II (Aß), III (A), and IV (C-fiber), afferents from superficial and deep tissues. Muscle stretch is an adequate stimulus for the receptive endings of Group Ia and II muscle spindle afferents. It can also stimulate Group Ib Golgi tendon organs afferents although muscle contraction appears more effective. Subpopulations of Aa skin afferents and Group II, III and IV skin and deep tissue afferents respond to innocuous and/or noxious mechanical input. Some are sensitized to mechanical input by the presence of chemical mediators in the extracellular space. In an animal model, grade III mobilization of the normal knee joint reduces secondary hyperalgesia produced at the more distal ankle joint. Large diameter Group I muscle afferents as well as Group II and III muscle and joint afferents could contribute to the antihyperalgesia via central serotonergic or adrenergic receptors, but not opioidergic or GABAergic receptors. In humans, stimulation of muscle spindle afferents using vibration reduces pain thresholds in unexercised leg muscle, but in contrast, exacerbates the delayed onset muscle soreness of eccentrically exercised leg muscles. Spinal manipulation has been applied to the lumbar spine in an animal model. Impulse loading of the paraspinal tissues activates both muscle spindle and Golgi tendon organ afferents. The duration of the impulse load strongly affects the instantaneous discharge frequency of Group Ia and Group II muscle spindle afferents. The magnitude of their discharge increases non-linearly as the impulse duration becomes similar to that used clinically. Further work is necessary to determine whether the force-time profile of a particular manual therapy and the condition of the tissues to which the therapy is delivered has selective effects on primary afferents or the central neurons on which they synapse.

Recommendations: 1) Determine the discharge characteristics (i.e. the pattern or frequency of action potentials) of primary sensory neurons in response to the force-time profiles used in manual therapies. 2) Determine how the patterns of activity from neurons innervating superficial versus deep tissues affect the signaling properties of neurons in the central nervous system. 3) Determine if manual therapies produce long-lasting changes in tissue biomechanics which would presumably produce long-lasting changes in mechanosensory input.

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