How is lactic acid produced from exercise?

Hint:Our body needs carbohydrates (glucose) for producing energy. Anaerobic respiration arises when no sufficient oxygen is available for the body to convert glucose into energy.

Complete step-by-step answer:The food we eat subsequently breaks down and changes it form into a monosaccharide unit of carbohydrate which is glucose. This glucose, derived from food, supplies us energy. This happens when oxygen reacts with glucose and converts it into energy units.
When we perform rigorous exercise, continuously, our body needs oxygen to meet the energy requirements for the exercise. Sometimes, our body may not be able to provide sufficient oxygen in the body while doing intense exercises. As the level of oxygen decreases in our blood stream, the glucose in our blood breaks down through anaerobic respiration.
Due to lack of oxygen available for breakdown of glucose, the glucose in our blood forms lactate. The muscle fibers have this lactate and hydrogen ions combine to form lactic acid.
This lactic acid is in turn converted to energy, but the rate of accumulation is faster and hence the lactic acid gets accumulated in the muscles and red blood cells. This causes the muscles to sore after an intense exercise session.
Hence, during exercise, glucose is converted to lactic acid in our body due to lack of oxygen or anaerobic conditions.

Note:Lactic acid which is produced in our body after intense exercise, is harmless and does not cause any damage to the body (apart from temporary soreness), once we rest, this lactic acid level decreases and we reach into our original condition.