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.
The amount of air taken in (inspiration) or breathed out (expiration) during normal breathing is called tidal air. Reserve air is the air that can be forced out of the lungs after normal expiration. The amount of air that the lungs can bring in during forced inspiration is called complemental air. Vital capacity is the volume of air exhaled during forced breathing and is the sum of tidal air, reserve air, and complemental air. The lungs are never completely empty of air.