Lactic acid (or lactate) is an essential molecule found within the muscle cells in order to aid recovery.
Within the last ten or so years, science has determined that lactic acid is actually not responsible for the burning sensation within the muscle after anaerobic training. These findings were contrary to previous beliefs. In actual fact, lactic acid is what aids in reducing that burning sensation within muscle cells (or muscle fibres). The burning sensation is scientifically known as acidosis (since the burn is due to an acidic environment forming).
So how does it work? The chemistry is fairly complex and tedious, so let's just stick to the basics:
1. Your muscles require ATP (adenosine tri-phosphate) which is a primary source of energy found within the body. Without ATP, your muscles would not work at all!
2. When you initiate a contraction of a muscle cell (or muscle fibre), the energy for this is derived from splitting ATP into ADP (adenosine di-phosphate) plus Pi (inorganic phosphate) through hyrolisation. This process results in the accumulation of H+ ions.
3. The accumulation of H+ ions creates an acidic environment which is responsible for the "burn" sensation experienced particularly with anaerobic exercise. Eventually the H+ ions will accumulate to the point where the environment is so acidic, the muscle will fail to work. This can be one reason why you may experience muscular failure during resistance training. This is a safety mechanism the body employs to prevent overloading the muscles to the point of injury.
4. A molecule called pyruvate acts as a "buffer" that comes and cleans up H+ ions within the muscle fibre. Pyruvate binds with 2 of the H+ ions to form lactate. This process neutralises the environment and allows muscle function to return to normal.
As a result, lactate (or lactic acid) is not directly responsible for acidosis.
You can improve the efficiency of this process by training at a high intensity for short periods of time (either steady-state or interval training). The more efficient your muscles are at dispelling acidic build-up, the more efficient they will work when placed under overload from exercise.
Lactic acid is also not responsible for DOMS (delayed onset muscle soreness). DOMS occurs due to microscopic muscle damage as a result of exercise - not from the build-up of the lactic acid that helps to reduce the acidity of muscle cells.