APA Dictionary of Psychology

the inability or decreased ability to exercise to the degree considered normal for one’s age and condition because physical exertion results in difficulties such as breathlessness, pain, extreme spikes or drops in blood pressure, dizziness, muscle cramps, low heart rate, fatigue, cyanosis, and nausea or vomiting. Rather than being its own syndrome, exercise intolerance is the primary symptom of chronic diastolic heart failure as well as a symptom of a number of other disorders (e.g., cystic fibrosis, chronic fatigue syndrome, mitochondrial disease). It can be managed with a modified exercise regimen, medication, and oxygen supplementation or by breathing heliox, a mixture of helium and oxygen that passes in and out of the lungs with less resistance than standard air and thus requires less effort to breathe.