Keep your leg at a 90-degree angle and your knee and foot at the same height. Avoid rotating your spine at any point during the exercise. Pause at the top for two seconds before lowering your knee to the floor. Repeat 15-25 times before performing the exercises with the other leg.
Inhaled air is about 20-percent oxygen. Exhaled air is about 15-percent oxygen. Therefore, about 5-percent of breathed air is consumed in each breath. That air is converted to carbon dioxide. So, as far as how much air is actually used, human beings take in about 550 liters of pure oxygen per day. A person who is exercising uses a lot more oxygen than that.
How much does air weigh? It might not seem like it, but air has weight. Anything with mass has weight, and we know air has mass because (for example) we can feel it when the wind blows. The total weight of the atmosphere exerts a pressure of about 14.7 pounds per square inch at sea level. You don't notice this weight, however, because you are used to it. If you live in Denver, Colorado, which is at an elevation of about 5,000 feet, then about 85% of the atmosphere is above you, resulting in an air pressure of about 12.5 pounds per square inch.