It has been shown that a training effect is obtained as soon as exercise takes the heart rate above 70% of the maximum rate. This figure is reached by untrained people more quickly than trained people.
Once there is an indication that the heart rate has risen, it is already above this minimum and the risk of overexertion and a negative experience increases. It is therefore clear that levels of exertion do not have to be excessively high in order to create a training effect.
A level of 70% of the maximum heart rate is achieved during moderate effort; this can be maintained for longer periods with little or no negative impact. A handy rule of thumb is that you should be able to carry on talking without getting out of breath during activity.