If you can't find a chair, holding onto other sturdy objects of a similar height may help.
A group of muscles on the back of your lower leg that spans from your heel to your knee.
Bodyweight
This means that the exercise requires you to use your bodyweight as resistance but you also need other equipment to successfully perform the exercise.
Chair
Make sure all chairs you use are stable and can support your bodyweight without tipping over. If you are unsure, do not risk it.
Large Textbook
Anything that is 3-4 inches high and can handle you standing on it is fine.
Exercises that target the same primary muscle groups and require the same equipment.
Exercises that target the same primary muscle groups with different equipment