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.
Oxygen makes up 21% of the air around us. In exhaled air, this reduces to 15% in exhaled air. Each breath exchanges a volume of 500ml of air between our lungs and the surroundings, known as the tidal volume. So, a reduction of 6% of oxygen means that we consume 30ml of oxygen per breath. Normal breathing rate is at 12breath/min, corresponding to an oxygen consumption of 360ml/min.