21% oxygen) Heliox has a lower density than air. Breathing heliox lowers expiratory airflow resistance, decreases dynamic hyperinflation, and prolongs exercise tolerance. High intensity rehabilitative exercise training Increasing the fitness of muscles decreases the amount of lactic acid released at any given level of exercise. Since lactic acid stimulates respiration, after rehabilitative training exercising, ventilation is lower, respiration is slowed, and dynamic hyperinflation is reduced. A combination of these therapies (Combined therapies), have shown the potential to improve exercise tolerance as well. Hazards Certain conditions exist where exercise may be contraindicated or should be performed under the direction of an experienced and licensed medical professional acting within his or her scope of practice. These conditions include: Decompensated heart failure Recent myocardial infarction Hypertrophic cardiomyopathy or cardiomyopathy from recent myocarditis Active or suspected myocarditis or pericarditis Low left-ventricular ejection fraction (LVEF) Severe aortic stenosis Unstable ischemia Unstable arrythmia Irregular or resting pulse greater than 100 bpm Resting systolic blood pressure >200 mm Hg or resting diastolic blood pressure >110 mm Hg Severe pulmonary hypertension Chronic fatigue syndrome Suspected or known dissecting aortic aneurysm Recent systemic or pulmonary embolus Pneumothorax and haemoptysis Thrombophlebitis The above list does not include all potential contraindications or precautions to exercise. Although it has not been shown to promote improved muscle strength, passive range-of-motion exercise is sometimes used to prevent skin breakdown and prevent contractures in patients unable to safely self-power. See also Frailty syndrome Heat intolerance Post-exertional malaise References External links Symptoms Physical exercise Exercise physiology