This paper describes results from the original randomised controlled trial (RCT) in stroke patients that showed the FES users received a reduction in quadriceps spasticity that was not seen in the control group.
The full article is published in Physiotherapy Volume 83, Issue 2, February 1997, Pages 82–89.
the_effect_of_common_peroneal_nerve_stimulation_on_quadriceps_spasticity_in_hemiplegia.pdf