Whatever effect HVLA has on the α motor neuron pool, it has not been demonstrated by lower velocity techniques such as soft-tissue massage.
27 The higher velocity applied in HVLA may provide insight into its therapeutic mechanism, as well as emphasize the proper application of the technique. For example, evidence suggests that stabilizing the neuronal gain of the motor neuron pool via proprioceptive feedback would help alleviate aberrant and uncoordinated muscle function.
24 Furthermore, it has been theorized that such uncoordinated muscle function may lead to buckling of a vertebral unit, even without trauma.
28 This buckling is likely due to a failure in the vertebral unit's ability to oppose such buckling behavior through the proper timing in the recruitment of its attached supportive muscles.
28 If, as suggested by multiple studies,
24,27 HVLA could potentially reset this timing by stabilizing the α motor neuron pool, it would explain HVLA's long-term benefits, even in the setting of relatively short-term local effects. Beyond the hypothetical, studies have shown that dysfunctional muscle behavior is corrected after HVLA. For example, Tunnell
29 found a decrease in multifidus tonus in patients with low back pain, whereas Koppenhauer et al
30 found an increase in multifidus contraction force after HVLA.
30 Similarly, de Almeida et al
31 noted that HVLA was able to increase pelvic floor tonus in women. Herzog et al
32 found a decrease in these EMG responses after HVLA. Although the effect of HVLA on the local musculature has not been consistent, it again appears related to a normalization of previously dysfunctional behavior. These data suggest that rather than a single effect on muscle tonus, HVLA may stabilize the α motor neuron pool to correct for aberrant postural and proprioceptive behavior.