When time is short, your priorities change; however, you can exercise effectively by altering one of these variables: frequency, duration, intensity and type. The first three variables will have the greatest effect on your weight, health and energy levels; but changing the type of exercise you do is good to keep challenging your muscles and to prevent boredom.WORKING OUT MORE OFTEN, FOR LONGER AND AT A HIGHER INTENSITY, WILL INCREASE THE AMOUNT OF KILOJOULES USED. THIS CAN BE THOUGHT OF AS A MATHEMATICAL EQUATION WITH WEIGHT LOSS BEING DIRECTLY PROPORTIONAL TO INCREASES IN ANY ONE OF THEM. When time is limited, increasing frequency and duration are not possible, so the most effective way to exercise is to increase the intensity of the workout. For example, if walking is your way to stay fit, you should add hills or intersperse the walking with periods of jogging to increase the intensity. Essentially, the less time you have, the harder your workout needs to be in order to be effective.TO GAUGE THE INTENSITY OF A WORKOUT, YOUR RATE OF PERCEIVED EXERTION (RPE) CAN BE MEASURED ON A SCALE OF ONE TO TEN. BARELY MOVING IS RATED AS ONE; FIVE IS MODERATE ACTIVITY AND TEN IS MAXIMUM EXERTION WHERE THE EXERCISE IS SO INTENSE YOU HAVE TO SLOW DOWN. During a speed workout, your RPE should be brought up to a five for an eight-minute warm-up. Then to be most effective, you should alternate two minutes at level nine with two minutes at level six for the next 12 minutes; the final ten minutes should be spent doing strength exercises and stretching to cool down.Image: Pressmaster/ShutterstockResources:No Time to Exercise? Why not try the Speed Workout – Dr. Joanna McMilan-Price - TheFoodCoach