Exercise physiologist Martti Karvonen is remembered today as a founding father of cardiovascular disease epidemiology and prevention in Finland. He was born in 1918 and received his medical degree from the University of Helsinki in 1945 and his PhD degree from the University of Cambridge in 1950. He then became the director of the Department of Physiology at the Institute of Occupational Health in Helsinki, and eventually went on to become the director general of that institute.

Karvonen served as chief physician for both the Finnish air force (1956 to 1966) and the Finnish army (1974 to 1978). Even after his retirement he remained active in the medical community and continued to act as a consultant for the WHO Division of Cardiovascular Diseases.[1]

Karvonen’s upbringing in North Karelia, a rugged part of eastern Finland, defined his approach to cardiovascular medicine. It became apparent to him that many men in the area were dying of heart disease after returning from service in the Second World War. He noticed that the local diet was rich in calories and fat, and observed that serum cholesterol levels in men from eastern Finland were higher than those of men in western Finland. Karvonen became interested in understanding the reason for cultural differences in cardiovascular event rates and the role of diet in the development of atherosclerosis.[1]   

Karvonen’s pioneering work in cardiovascular epidemiology began in the mid-1950s when he brought his observations about Finnish males to the attention of Ancel Keys, and the two scientists undertook the first epidemiological studies of risk factors on coronary heart disease in eastern and western Finland. Their collaboration eventually led to the inclusion of Finland in the hallmark 1958 Seven Countries study that established a relationship between lifestyle and the prevalence of atherosclerosis in various parts of the world. 

In 1972 Karvonen’s advice resulted in the creation of the North Karelia Project, a community-based program to address the high burden of coronary heart disease affecting the area. This program was credited with triggering major lifestyle changes across Finland, which in turn led to reductions in mean serum cholesterol and blood pressure levels, lower smoking rates, and, most important, a reduction in cardiovascular disease mortality rates and stroke-related risk factors.[2] In 1979 Karvonen’s 12-year Finnish Mental Hospital study proved that a diet low in cholesterol was associated with reductions in coronary heart disease, and the field of preventive cardiology was born.[3]

The Karvonen formula and exercise intensity
While Karvonen played a major role in establishing a link between lifestyle factors and cardiovascular events, he is also known for developing a formula to determine a target heart rate (HR) for aerobic activity.

In the Karvonen formula, peak HR is the maximum heart rate achieved during exercise stress testing and K is a coefficient: target HR = resting HR + (peak HR – resting HR) x K. In a study published in 1957, Karvonen examined the effect of different training intensities on resting, working, and maximum heart rates.[4] He found that training at an intense level will cause a decrease in working heart rate, which is a direct indication of increased peak oxygen consumption and cardiorespiratory fitness. He identified the threshold at which improvements are seen as 60% of the heart rate reserve (HRR), which is expressed as the difference between the peak heart rate and resting heart rate. Although the benefits of exercise were well documented, this finding was monumental in defining the parameters for exercise intensity that produced tangible results. 

We know now that the intensity range to improve and maintain cardiorespiratory fitness is broad and depends on a number of factors, including age, underlying health, individual physical activity habits, and baseline functional status. Davis and Convertino found the Karvonen formula to be a reasonably accurate method for estimating exercise intensity.[5] Despite occasionally overestimating or underestimating exercise intensity in certain patients, the formula provided general rule-of-thumb training heart rates, with light-intensity exercise defined as activity using 30% to 40% of the heart rate reserve, moderate-intensity activity as using 40% to 60% of HRR, and vigorous-intensity activity as using 60% to 90% of HRR. 

There are several methods for determining exercise intensity that differ from the Karvonen formula. When cardiopulmonary exercise testing is available, a percentage of either the oxygen uptake reserve or the maximal oxygen uptake can be calculated. Measures of absolute intensity include metabolic equivalent tasks (METs), absolute oxygen uptake, and caloric expenditure. Unfortunately, these methods do not account for individual differences and commonly misclassify exercise intensity. Several subjective tools of determining exercise intensity have also been developed and can be used as adjuncts to the objective methods. These include the Borg Rating of Perceived Exertion (RPE) scale, the talk test, and the OMNI scale. 

Exercise prescription
Although the benefits of regular physical activity are well known, physicians are often uncertain how to provide appropriate recommendations when patients ask for advice. Ideally, physicians should provide patients with an exercise prescription based on the FITT-VP6 principle: 
Frequency: How often to exercise.
Intensity: How hard to exercise, as determined by methods described above.
Time: Duration of each exercise session.
Type: Kind of exercise.
Volume: Product of frequency, intensity, and time to give an overall estimate of energy expended when following the exercise prescription.
Progression: Rate of progression for frequency, intensity, and time.

Exercise prescriptions should include both aerobic activity and resistance training. Aerobic activity involves the repetitive, rhythmic motion of large muscle groups, and can be seen in activities such as running and cycling. For those unaccustomed to regular exercise, aerobic activity should begin at a lower intensity and shorter duration, with gradual progression over time until target volume is achieved. In general, exercise time should be increased before intensity is increased. An example of a reasonable progression would be an increase of 5 to 10 minutes per session every 1 to 2 weeks.[6] 

The benefits of aerobic activity can also be obtained from interval training, which involves high-intensity exercise interspersed with light-intensity activity or rest. Aerobic interval training can result in a larger increase in peak oxygen consumption than continuous training,[7] and may be most useful for those at the extremes of exercise capacity.

For adults, the American Heart Association (AHA) currently recommends 30 minutes of moderate-intensity aerobic activity at least 5 days a week for a total of 150 minutes, or 25 minutes of vigorous-intensity aerobic activity at least 3 days a week for a total of 75 minutes, or a combination of moderate-intensity and vigorous-intensity aerobic activity and moderate- to high-intensity muscle strengthening activity at least 2 days a week for additional health benefits. If the goal is to lower blood pressure or cholesterol levels, an average of 40 minutes of moderate- to vigorous-intensity aerobic activity 3 to 4 days a week is recommended.[8]

With the recent increase in childhood obesity, physical activity is becoming increasingly important for children. Currently, the AHA recommends that all children older than 2 years participate in at least 60 minutes a day of enjoyable, moderate-intensity physical activities that are developmentally appropriate and varied. If scheduling a full 60-minute break for daily activity is impossible, children should have at least two 30-minute periods or four 15-minute periods to engage in vigorous-intensity physical activities appropriate for their age, gender, and stage of physical and emotional development.[9] 

It is known that the increased muscular strength resulting from resistance training is associated with decreased risk of all-cause mortality.[10] Other benefits of this type of exercise include reductions in functional limitations, improvements in self-efficacy and quality of life, and increases in bone mass and strength. The current recommendation is to train each major muscle group against resistance 2 to 3 days a week, with two to four sets during each session and eight to twelve repetitions per set.[6] The intensity of resistance should be approximately 60% to 80% of the one-repetition maximum for each individual, which can be achieved using resistance bands, free weights, machines with stacked weights, or pneumatic resistance.

It is also important to note that physical inactivity is associated with inferior health outcomes. Therefore, in addition to providing exercise recommendations, physicians should advise patients to reduce time spent being physically inactive. Lastly, special attention should be paid to patients with underlying cardiac conditions who are ready to start an exercise program. These individuals would benefit most from enrolling in a cardiac rehabilitation program with pre-participation cardiac assessment and development of a specialized exercise routine. 

Conclusions
Before writing a prescription for an antihypertensive or antihyperglycemic agent, physicians might consider writing a prescription for exercise. The benefits of physical exercise on overall health have been known for centuries, with exercise prescriptions dating as far back as 600 BC.[11] 

Finland’s Martti Karvonen played a pivotal role in the science of exercise prescription by helping to establish the link between lifestyle and risk factor development, and by determining the role of exercise intensity in improving cardiorespiratory fitness. The formula he developed remains one of the most widely used methods for determining a target heart rate for aerobic activity. 

Physicians are often approached by patients for advice on exercise and are in an ideal position to promote this invaluable behavior by providing safe, useful, and objective exercise recommendations. 

Competing interests
None declared.

This article has been peer reviewed.

Dr Ignaszewski is a PGY-3 resident in internal medicine at SUNY Upstate Medical University in Syracuse, New York. Dr Lau is a cardiologist at Lions Gate Hospital. Ms Wong is a medical student at the University of British Columbia. Dr Isserow is a cardiologist at the University of British Columbia.