The physiological rationale for using heart rate

In healthy individuals and cardiac patients, the common aim of using heart rate is to act as a marker of the physiological strain of the exercising skeletal muscles. Specific to the cardiac patient, the role of heart rate (in conjunction with systolic blood pressure) also acts as a key indicator of myocardial strain (Froelicher and Myers, 2000).

As a marker of the body's general physiological strain during aerobic exercise, heart rate is usually described as a percentage of maximal heart rate

(%oHRmax). The %HRmax has for many years provided a practical substitute marker of the percentage of maximal aerobic power (%VO2max). This is based on the assumption that HRmax and VO2max coincide (Astrand and Rhyming, 1954; Astrand and Christensen, 1964). For an individual, heart rate for a given %VO2max does not change, regardless of training status, fitness level or age (Skinner, et al., 2003). The use of %HRmax allows for the relative comparison of exercise intensity of people of differing ages. Correspondingly, the use of %VO2max allows for the relative comparison of individuals of different levels of maximal aerobic power (aerobic fitness). In recognising the heterogeneity of cardiac populations, relative to both age and fitness (Lavie and Milani, 2000), the use of these two relative measures (%HRmax and %VO2max) allows for the same exercise prescription principles to be applied to all patients.

The %VO2max, linked to the level of muscle-produced lactate in the blood, is a good indicator of the balance between aerobic and anaerobic metabolic needs of the exercising skeletal muscle (Christensen, 1931; Robinson, 1938; Astrand and Rhyming, 1954; Astrand, et al., 2003). It is such a balance that forms the basis of the intensity targets that are so widely recommended in the recognised guidelines mentioned earlier and to be discussed later in this chapter. The significance of this contribution between aerobic and anaerobic processes (Krogh and Lindhard, 1920) underpins the physiological effectiveness of the exercise session, both in determining the correct intensity and in achieving the required duration of the aerobic exercise session.

A pivotal study by Karvonen, et al. (1957) enabled a more accurate coupling between heart rate and %VO2max. The key aspect was the use of the difference in heart rate between rest and maximal exertion, known as the heart rate reserve. Over a period of training, resting heart rate decreases, whereas maximal heart rate remains much the same; thus, the gap between rest and maximal heart rate increases. This formula accounts for the widening heart rate reserve over the weeks and months of exercise training. The result of this approach is that a given percentage of maximal heart rate reserve (%HRRmax) theoretically represents the same %VO2max, although a given %HRmax does not represent the same %VO2max, except near maximal aerobic exercise intensities, as illustrated in Figure 3.1.

More recent recommendations by the ACSM (1998) have referred to a VO2 reserve method, where there is a matching of heart rate reserve and VO2 reserve (the difference between resting VO2 and VO2max). This method has further increased the accuracy of the link between heart rate and oxygen uptake in representing the work intensity of the exercising skeletal muscle (Swain and Leutholz, 1997). With any of the formulas, accuracy is dependent on the validity of the measured or estimated values. In this case both maximal heart rate and resting heart rate validity need to be considered. Measuring a true resting heart rate requires the patients to monitor themselves at home when they wake first thing in the morning.

Oxygen Heart Rate Rest

%VO2max

<20

20-

-39

40-

-59

60-

-84

>85

100

%HRRmax

<20

20

-39

40-

-59

60-

-84

>85

100

%HRmax

<35

35

-54

55-

-69

70-

-89

>90

100

RPE

<10

10

-11

12-

-13

14

-16

17-19

19-20

Figure 3.1. Summary of the relationship between the percentages of maximal aerobic power (%VO2max), maximal heart rate reserve (%HRRmax), maximal heart rate (%HRmax) and Borg's rating of perceived exertion (RPE).

(Adapted from Pollock, et al., 1978; ACSM, 1994,1998, 2000; Noble and Robertson, 1996.)

Figure 3.1. Summary of the relationship between the percentages of maximal aerobic power (%VO2max), maximal heart rate reserve (%HRRmax), maximal heart rate (%HRmax) and Borg's rating of perceived exertion (RPE).

(Adapted from Pollock, et al., 1978; ACSM, 1994,1998, 2000; Noble and Robertson, 1996.)

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  • martin
    What is the physiological rationale of measuring blood pressure?
    8 months ago

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