Heart patients have great trouble sticking to the exercise programmes set out by their doctors, according to a survey.
Exercise is vital after heart attack
Excercise is important during the recovery period to reduce the chance of another attack.
However, a study presented at a US conference found that virtually all patients' exercise efforts waned over a year - and 14% did not exercise at all.
Researcher Dr Shirley Moore described the findings as "a bit of a shock".
The study looked at 83 patients, all of whom had either had heart attacks, or had bypass surgery or angioplasty, a procedure to unblock clogged heart arteries.
They had been sent on cardiac rehabilitation courses where exercise programmes were set up - then given heart monitors to wear during exercise so that researchers could keep an eye on what they actually did.
Most of the patients were told to exercise every day for 35 to 45 minutes.
The idea of exercise is to strengthen the heart muscle and speed recovery.
However, the researchers found that the number of people not exercising in any particular month rose from 18% in the first month to 55% by the 12th month.
The average amount of exercise carried out per month droped from 8.7 hours in the first month to 5.3 hours a year later.
The efficiency of the workouts dropped as well - patients spent less time in the "heart rate zone" by the 12th month, although this might be a sign of increased heart fitness.
Dr Moore, from Case Western Reserve University, said: "It's critical that we work with these people to find out why there is such a drop-off.
"Either they don't understand what is considered aerobic exercise, or we've designed a programme for them that is so uncomfortable that they're not doing it."
She is now researching whether "booster" sessions could improve exercise among heart patients.
The study was presented at the annual meeting of the American Association of Cardiovascular and Pulmonary Rehabilitation in Kansas City.