Can Ib axons be selectively activated by electrical stimuli in human subjects?

A tendon-vibration technique, used to raise the electrical threshold of muscle spindle Ia afferent fibers above that of Golgi tendon organ Ib afferent fibers in animals, was tested on human subjects. After prolonged tendon vibration, electrical stimulation of the posterior tibial nerve was ineffective or markedly less effective in eliciting Hoffmann (H-) reflexes in the soleus muscle at previbration threshold intensities. With stimulus intensity held constant at values between 1.0 to 1.4 × threshold, postvibration H-reflex myoelectric amplitudes returned to previbration values usually within 60 min. However, at higher electrical stimulus intensities (1.8 × threshold), postvibration H-reflex amplitudes were produced at or near previbration values irrespective of postvibration recovery time; in contrast, initial postvibration tendon tap reflexes were potentiated. Findings suggest that it is indeed possible to selectively activate Ib afferent fibers in humans by electrical stimuli.