H-reflex

The H-reflex is a reflectory reaction of muscles after electrical stimulation of sensory fibers (Ia afferents stemming from muscle spindles) in their innervating nerves (for example, those located behind the knee).

H-reflex is analogous to the mechanically induced spinal stretch reflex (for example, knee jerk reflex) because both cases muscle-spindle innervating fibers are activated.

H-reflex is used to assess fitness of astronauts. H-reflex was the first medical experiment completed on the International Space Station.

History
It was first described by Paul Hoffmann (hence the name) in 1910.