In experienced athletes, "hitting the wall" is conventionally believed to be due to the body's glycogen stores being depleted, with "second wind" occurring when fatty acids become the predominant source of energy.
[5][7] Some scientists believe the second wind to be a result of the body finding the proper balance of oxygen to counteract the buildup of lactic acid in the muscles.
[9] The phenomenon has come to be used as a metaphor for continuing on with renewed energy past the point thought to be one's prime, whether in other sports, careers, or life in general.
Anaerobic metabolism to some degree then takes place in the muscle and this less ideal energy production produces lactic acid as a waste metabolite.
This is the case even without exercise in people with respiratory disease, challenged circulation of blood to parts of the body or any other situation when oxygen cannot be supplied to the tissues involved.
Some people's bodies may take more time than others to be able to balance the amount of oxygen they need to counteract the lactic acid.
This theory of the second wind posits that, by pushing past the point of pain and exhaustion, runners may give their systems enough time to warm up and begin to use the oxygen to its fullest potential.
A big push[clarification needed] in Ironman Triathlon ten years ago introduced the idea of heart rate training and "tricking" one's body into staying in an aerobic metabolic state for longer periods of time.
[17] Exercise intolerance such as muscle fatigue and pain, an inappropriate rapid heart rate in response to exercise (tachycardia), heavy (hyperpnea) and rapid breathing (tachypnea) are experienced until sufficient energy is produced via oxidative phosphorylation, primarily from free fatty acids.
Oxidative phosphorylation by free fatty acids is not achievable with isometric and other anaerobic activity (such as lifting weights), as contracted muscles restrict blood flow (leaving oxygen and blood borne fuels unable to be delivered to muscle cells adequately for oxidative phosphorylation).
[17][18] The second wind phenomenon in GSD-V individuals can be demonstrated by measuring heart rate during a 12 Minute Walk Test.
[22][31] For the regularly active patients, it took more strenuous exercise (very brisk walking/jogging or bicycling) for them to experience both the typical symptoms and relief thereof, along with the sign of an increased heart rate dropping, demonstrating second wind.
[22][31][19] In young children (<10 years old) with McArdle disease (GSD-V), it may be more difficult to detect the second wind phenomenon.