A brick workout refers to the stacking of two disciplines during the same workout, one after the other with minimal to no interruption in between. As you switch modes of exercise, your body needs to effectively and efficiently prepare for the next demand while recovering from the previous exercise demand. Your heart rate increases significantly as your body tries to shift the blood flow from the muscles of the first exercise to the demands of the muscles of the next. Brick workouts help your body handle the aerobic, anaerobic, and muscular demands of a triathlon event.
For the triathlete, the most obvious reason to brick train is that this routine follows the specificity principle of training. You do not compete in each phase of the race in isolation. The transition is almost a fourth discipline. You need to train accordingly and practice these transitions from one exercise to another within your training regimen. For the non-triathlete, this style of training is a great way to increase your calorie burn. You can burn close to fifteen calories per minute based on a vigorous effort on both the cycling and the running ends of the workout.