Georgia State University explains that the metabolic pathway that is common to both cellular respiration and fermentation is anaerobic respiration. Anaerobic respiration takes place when there is no oxygen present in the cells.
Georgia State University explains that in humans, this can occur when muscles are under pressure and are lacking oxygen. If the body could not utilize anaerobic respiration, the cells would die when oxygen supply is limited.
During fermentation, carbohydrates are converted into acids or alcohol. About.com describes the mechanism as being similar to the production of glucose in humans during anaerobic respiration. However, despite fermentation also taking place in the absence of oxygen, it is bacteria that carry out the process.