Improving aerobic fitness

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# 3.2 Intensity

Exercise intensity refers to the level of effort or workload at which someone should exercise to stimulate an improvement in their fitness. As mentioned in the previous section, to improve aerobic fitness the ACSM recommend moderate and/or vigorous intensity activity for most adults (Garber et al., 2011). Table 1 summarises what moderate and vigorous mean.

Exercise intensity can be measured using either heart rate or the rating of perceived exertion (RPE) method. We will look at each of these methods in turn. There are two methods of using heart rate to measure exercise intensity: the percentage of maximal heart rate (HRmax) method and the heart rate reserve (HRR), or Karvonen method.

## Table 1 Classification of exercise intensity

Intensity %HRR %HRmax Perceived exertion (rating on 6–20 RPE scale)
Moderate 40–59 64–76 Fairly light to somewhat hard (RPE 12–13)
Vigorous 60–89 77–95 Somewhat hard to very hard (RPE 14–17)
(Source: adapted from Garber et al., 2011, p. 1341)

As its name suggests, the percentage of maximal heart rate method involves prescribing exercise at a certain percentage of maximum heart rate. To find out a person’s true maximum heart rate we need to measure it in a laboratory. However, for most people this is impractical; therefore we can estimate maximum heart rate using the formula ‘220 – age’ (see Box 1).

## Box 1: Percentage heart rate method

### Step 1 – Calculate maximum heart rate (HRmax)

Estimated HRmax = 220 – age

= 220 – 30

= 190 bpm (beats per minute)

### Step 2 – Calculate exercise intensity

ACSM guidelines = 55–90% of HRmax

Lower target (55%) = 190 × 55%

= 190 × 0.55

= 104.5 bpm (we would round this up to 105 bpm)

Upper target (90%) = 190 × 90%

= 190 × 0.90

= 171 bpm

This formula gives us an idea of maximum heart rate, but we must remember that it is just an estimate and not completely accurate. Therefore using this method, according to ACSM guidelines, Mariella should exercise at a heart rate somewhere between 105 and 171 bpm. This is quite a wide range so, depending on her fitness levels, you would need to decide whether to prescribe Mariella exercise to the upper or lower end of this scale.

Please note that there are online calculators available to calculate all of this information for you. Once such calculator can be found at:

fitlinxx [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)]

The HRR method is thought to be more accurate than the percentage of maximal heart rate method because it takes the individual’s resting heart rate into account. The formula for calculating HRR can be seen in Box 2. As outlined in Table 2 it is recommended that to improve aerobic fitness, exercise intensity should be set at a moderate or vigorous level. The exact intensity set will depend on the individual's fitness level. Someone with relatively low levels of fitness who has just started an exercise programme may need to work at a lower intensity, whereas someone who has a higher level of fitness, perhaps who has been exercising for a while, may need to work at a higher intensity. This demonstrates the importance of progression in an exercise programme.

## Box 2: Heart rate reserve method

### Step 1 – Calculate maximum heart rate (HRmax)

Estimated HRmax = 220 - age

= 220 – 30

= 190 bpm (beats per minute)

### Step 2 – Measure resting heart rate (HRrest)

You would measure this either using a heart rate monitor or manually, using your fingers. Ideally it should be measured first thing in the morning. Let's imagine that Mariella's HRrest has been measured at 70 bpm.

### Step 3 – Calculate heart rate reserve (HRR)

HRR = HRmax – HRrest

= 190 – 70

= 120 bpm

### Step 4 – Calculate exercise intensity

ACSM guidelines = 40–85% HRR

Lower target (40%) = (HHR × 40%) + HRrest

= (120 × 0.40) + 70

= 48 + 70

= 118 bpm

Upper target (85%) = (HHR × 85%) + HRrest

= (120 × 0.85) + 70

= 102 + 70

= 172 bpm

Using this method, according to ACSM guidelines, Mariella should exercise somewhere between 118 and 172 bpm.

Please note that there are online calculators available to calculate all of this information for you. Once such calculator can be found at:

fitlinxx

An alternative to using heart rate methods is the RPE method of measuring exercise intensity. Essentially, the RPE method involves an individual rating how hard they feel they are working on a scale of 6–20. An RPE of approximately 12–17 is recommended to improve aerobic fitness (Garber et al., 2011). However, you should note that it is difficult to give a general recommendation for RPE, as it is by its very nature open to personal interpretation; that is, what I consider to be a 12 may be different to what you consider to be a 12.

RPE can be a useful way of measuring exercise intensity when heart rate monitoring is difficult or inappropriate. For example, some types of medication (e.g. beta blockers) given to people with hypertension (high blood pressure) lower the heart rate, and therefore heart rate measurement is not appropriate for people on this type of medication.

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