All walking, running, jumping, leaping, hopping, and kicking movements will work the hip flexors in some way, whether as agonists, antagonists, or stabilizers. This is dependent on the phase of the movement. Agonists are the main muscle movers of a particular joint action. Antagonists are usually located opposite the main movers, counteracting the action. Stabilizers, also called fixators, are muscles acting as a firm base while other muscles exert force to create movement.
All skeletal muscles are movers and stabilizers – it depends on the movement and position of the body as to how the muscles are reacting at the time. Skeletal muscles are all of the above – it depends on the movement and position of the body as to what role the muscles are playing at the time.
Cardiovascular machines are great hip joint workouts in the sagittal plane, which means the joint actions of flexion and extension are being done. The treadmill is especially good; walking or running outdoors is even better (fresh air!) as long as proper footwear and running surfaces are available.