There are two kinds of physical activity which require special training. The first demands exact careful movements of the muscles. This kind of activity must be strictly controlled because even a slight movement in the wrong direction will lead a mistake. To type quickly, for example, a person needs training; the slightest movement of a finger in the wrong direction may cause a spelling mistake. A dancer who has to dance on the point of her shoes or turn around on one foot must be trained for a long period of time before she can sense her own center and balance herself. You may have seen a girl walking on a rope across an empty space, which, too, requires a lot of practice. The second kind of physical activity needs greater strength or extra effort. Most of us get tired if we try to run half a mile without stopping, but a specially-trained person can do this without much effort. Three years ago, some scientists carried out experiments, which produced meaningful and unexpected results. They wanted to find out whether a certain amount of physical exercise would injure those suffering from heart problems. They selected some male patients and trained them in continuous bicycle riding. They were surprised to find that the harmful effect of given amount of physical effort was actually less on the hearts of these trained patients than on those of the patients who were not similarly trained. This is important because it shows that regular physical exercise enables us to make better use of the oxygen we breathe in and that this training, in fact, reduces the amount of work our hearts do. Many tasks which are hard for untrained people are not hard at all for trained people. The experiments done by some scientists showed that   _  .
some male patients were asked to ride bicycles regularly in the experiments the physical exercise had more harmful effect on the hearts of the untrained patients the physical exercise was harmless to the male patients with heart trouble the physical exercise could be helpful for the patients to take in more oxygen