Low-frequency H-reflex depression in trained human soleus after spinal cord injury

► Retrograde signaling during muscle activity may alter spinal reflex activity in SCI. ► Electrical stimulation of paralyzed soleus improved torque and fatigue resistance. ► After 2 years of training, H-reflex depression was not restored in trained limbs. ► Subjects who began training early showed more normalization of H-reflex depression. ► Early muscle activity may help foster retrograde changes in spinal reflex pathways.