"Extensor muscles" refer to a group of muscles in our body that help us extend or straighten our joints. They work opposite to flexor muscles, which help in bending or flexing our joints. Full definition
But the muscle it specifically targets is the «gluteus maximus» or the «glutes» a bunch of muscles that act as the main extensor muscle of the hip. (fitsaurus.com)
As subjects ran on a treadmill wearing the exosuit, the actuation unit pulled on the wires, which acted as a second pair of hip extensor muscles applying force to the legs with each stride. (sciencedaily.com)
While you do get some activation of the hip extensor muscle groups (hamstrings and glutes) the quads are taking the majority of the load. (fitnesscrest.com)