Such exercises include hill repeats, track intervals, fartlek runs, weight training, specific running drills, and over-distance runs. Or, it might be a low intensity workout which lasts much longer than normal. For example, a 20 mile run, even at a slow speed, is a stressful workout. Most runners know this kind of stuff. What many seem to ignore is that stress accounts only for one half of the equation.
Rest is just as important as the active work.
Without rest, those hard workouts go for naught, and might even weaken us. During the work portion of exercise, body tissue is stressed by the actual contractions and movements required to perform the task. This results in microscopic tears as well as a build up of a waste product called lactic acid. The next step is for the body to heal itself. The result of this near miracle-like process is that within about 48 hours the body then finds itself at a higher level of strength than it was prior to the exercise being done.