You can substitute the item you are standing on for anything as long as it is sturdy, can support your weight, and will not move when you press on it.
A group of muscles on the back of your lower leg that spans from your heel to your knee.
Smith Machine
This machine can be used to reduce the amount of stabilizer muscles needed to perform an exercise.
Steps
If unavailable this can be substituted for a household item such as a large textbook, provided that it is sturdy and can support your bodyweight without sliding on the floor.
Exercises that target the same primary muscle groups and require the same equipment.
Exercises that target the same primary muscle groups with different equipment