Exercise prescription--North American experience.

The principles of exercise prescription are reviewed with respect to North American experience. The required regimen must be safe, therapeutically effective, and ensure a high rate of compliance. Precautions to increase the safety of exercise are discussed. Cardiac emergencies are sufficiently rare events (less than 1 in 200,000 hours even in post-coronary classes) that the need for immediate medical supervision of a well-designed programme can be questioned. The prime determinant of the response to training is the intensity of effort relative to the individual's initial fitness. Post-coronary patients often have a great potential for training due to their previous inactivity, but this will not be realised if the prescribed exercise is of insufficient intensity. Exercise programmes are plagued by a high 'drop-out' rate; 50% of normal middle-aged volunteers are lost in 6 months, and even with post-coronary programmes losses can be 60--70% over 4 years. Simple suggestions are made for improving compliance with the required exercise prescription.