Exercise And Heart Rate

Intensity is an essential part of the traditional cardiovascular exercise recommendations. An optimal exercise prescription for competitive training as well as for fitness improvement is a balance between the frequency, intensity and duration of exercise and an exercise mode. The latest recommendations for health-related exercise, however, are emphasizing the frequency and the total amount of exercise, e.g. adding daily steps or accumulating 1000 kcal a week (Physical Activity and Health 1996).

Maximal aerobic power

The basic variable in the intensity of the exercise is maximal aerobic power or maximal oxygen uptake (VO2max). It is the highest rate of oxygen consumption measured during maximal exercise and a "golden standard" of the cardiorespiratory fitness. Maximal aerobic power is a product of the cardiac output and arterial-venous oxygen difference. The cardiac output is a product of stroke volume and heart rate and it is constantly changing due to changes in stroke volume and heart rate. Stroke volume reaches its maximum at a relatively low intensity during exercise and the rest of the oxygen uptake increase is due to an increase in heart rate. Thus, heart rate is an important index of heart's work and an indicator of the functions of cardiovascular system during exercise. The heart rate-oxygen uptake relationship is linear within an individual at least up to so called "anaerobic threshold" (Wilmore and Costill 1994). Because heart rate is easily and noninvasively obtainable, e.g. on-line during exercise with modern telemetric heart rate monitors, it is more practical and useful for exercise intensity determination than oxygen uptake, cardiac output or stroke volume.

Resting and maximum heart rates and heart rate reserve

Heart rate at rest is determined by vagal tonus (parasympathetic nervous system). It averages between 60 and 85 bpm (beats per minute) in an adult person. Measured resting heart rate is needed for the Karvonen-formula (Karvonen 1957) for the heart rate reserve calculation (based on Karvonen 1957) and the target exercise heart rate determinations. Maximum heart rate is the highest heart rate achieved in an all-out effort. It is very individual due to heredity, fitness level and age. Endurance training has not, however, been shown to influence maximum heart rate on the population level (Wilmore et al. 1996). Maximum heart rate can be predicted using age, e.g. maximum heart rate for adults equals to 220 - age (ACSM 1990) and for 11-16-year-old children 220 bpm (Armstrong 1991). However, the standard deviation (SD) of any prediction equation published is at least 8-12 bpm (Miller et al. 1993, Whaley et al. 1992).

Exercise heart rate

Exercise heart rate is regulated by increased sympathetic activity. It varies within an individual according to heredity (size of the left ventricle in heart), fitness level, exercise mode and skill (economy of exercise). Body posture, environmental variables (temperature, humidity, altitude), state of mood and hormonal status also alter the heart rate response. Heart rate is also affected by drugs, stimulants and eating habits.

According to the goal of the exercise, however, the target heart rate and heart rate zones can be calculated as a percentage of the maximum aerobic power or heart rate. ACSM´s latest recommendation (1998) for developing and maintaining cardiorespiratory fitness in healthy adults gives 55/65%-90% of maximum heart rate (HRmax) or 40/50%-85% of oxygen uptake reserve (VO2R) as the intensity limits. Percentages of VO2max (being about 10% less than %HRmax at the same intensity) can be changed to the %HRmax with the following formula: %HRmax = (%VO2max + 28.12) / 1.28. Typically, 50-60% of the maximum heart rate represents light, 60-70% light to moderate, 70-80% moderate to heavy, 80-90% heavy and 90-100% very heavy intensity. Combining the rating of perceived exertion, e.g. Borg-scale (Borg 1982) with heart rate, makes the intensity to better meet the individual target intensity. For the most accurate exercise intensity (heart rate) determination (also for the Karvonen-formula) the measured maximum heart rate is needed.

Heart rate variability (HRV) has been shown to provide an individual method for target heart rate determination. Polar OwnZone (in Polar SmartEdge and M-series HR monitors) is based on a decrease in HRV during incremental exercise (Tulppo et al. 1996, 1998). The target heart rate determination by the OwnZone results limits corresponding to 62-84% HRmax on healthy men and women (Laukkanen et al. 1998) and 68-86% HRmax in obese adults (Byrne et al. 1999). Reproducibility of this method has been shown to be good (Kinnunen et al. 1998).

Using heart rate in exercise is difficult and confusing for many individuals, e.g. when participating aerobic classes, if they do not know their maximum heart rate. Adding beats/subtracting beats to the resting/pre-exercise heart rate helps them to better control the intensity (Laukkanen et al. 1997). This method is a new reading approach to the target heart rate charts reported also by Faigenbaum (1996).

In typical resistance training (targeting to muscle power and strength increase) heart rate does not play very important role during exercise bouts, but may be helpful in controlling the recovery time needed between the work out sessions. However, recently a heart rate guided low-resistance circuit training program has been shown to be beneficial for both aerobic and muscular fitness (Kaikkonen et al. 1999).

Other heart rate based applications

Heart rate recovery period (time) can be used to detect recovery after the exercise. The time it takes for the heart to return to its resting rate is decreased as a consequence of regular endurance training (Wilmore and Costill 1994, Gilman 1996).

Heart rate can also be used as an indicator of overstrain. Comparison between the resting heart rate and "the standing up heart rate" (body posture and venous return change) is the idea in the orthostatic test (Piha 1988). Polar Overtraining Test in Polar Precision Performance SW2.1 is the latest application in overtraining detection and is based on HRV measure during orthostatic test (Uusitalo 1998).

References

ACSM. Position stand. The Recommended Quantity and Quality of Exercise for Developing and Maintaining Cardiorespiratory and Muscular Fitness in Healthy Adults. Med Sci Sports Exerc 22:265-274, 1990.

ACSM. Position stand. The Recommended Quantity and Quality of Exercise for Developing and Maintaining Cardiorespiratory and Muscular Fitness, and Flexibility in Healthy Adults. Med Sci Sports Exerc 30:975-991, 1998.

Armstrong N. et al. The peak oxygen uptake of British children with reference to age, sex and sexual maturity. Eur J Appl Physiol 62:369-375, 1991.

Borg G.A.V. Psychophysical bases on perceived exertion. Med Sci Sports Exerc 14:377-481, 1982.

Byrne N. et al. Use of heart rate variability in prescribing exercise intensity thresholds in the obese. Int J Obes 23 (Suppl 5):567, 1999.

Faigenbaum A.D. Target Heart Rates: A New View of an Old Chart. ACSM Certified News 6(1):8-9, 1996.

Gilman M.B. The use of heart rate to monitor the intensity of endurance training. Sport Med 21(2):73-79, 1996.

Ingjer F. Factors influencing assessment of maximal heart rate. Scan J Med Sci Sports 1:134-140, 1991.

Kaikkonen H. et al. The effect of heart rate controlled low resistance circuit weight training and endurance training on maximal aerobic power in sedentary adults. Scand J Med Sci Sports, in press 1999.

Karvonen M. et al. The effect of training on heart rate. A longitudinal study. Ann Med Exp Biol Fenn 35:307-315, 1957.

Kinnunen H. et al. Reproducibility of individual training heart rate determined by Polar SmartEdge heart rate monitor. Int. Puijo Symposium, Book of abstracts, Kuopio university publications D, Medical Sciences 149:63, 1998.

Laukkanen R. et al. Is intensity control possible during aerobics classes? Med Sci Sports Exerc 29(5, Suppl): 401, 1997.

Laukkanen R. et al. Determination of heart rates for training using Polar SmartEdge heart rate monitor. Med Sci Sports Exerc 39 (Suppl 5):1430, 1998.

Maron B. J. The Athlete's Heart. Cardiol Clinics 10, May 1992.

Miller W. et al. Predicting maximal and HR-VO2 relationship for exercise prescription in obesity. Med Sci Sports Exerc 25(9):1077-1081, 1993.

Piha S.J. Cardiovascular Autonomic Function Tests. Publications of the Social Insurance Institution, Finland, ML:85, Turku, 1988.

Swain D. et al. Target heart rates for the development for cardiorespiratory fitness. Med Sci Sports Exerc 26(1):112-116, 1994.

Tulppo M. et al. Quantitative beat-to-beat analysis of heart rate dynamics during exercise. Am J Physiol 271:H244-252, 1996.

Tulppo M. et al. Vagal modulation of heart rate during exercise: effect of age and physical fitness. Am J Physiol 274:H424-429, 1998.

Uusitalo A. Ability of non-invasive and invasive methods of autonomic function measurements and stress hormones to indicate endurance training-induced stress. Acta Universitatis Tamperensis 621, Doctoral dissertation, 1998.

U.S. Department of Health and Human Services. Physical Activity and Health: A Report of the Surgeon General. Atlanta, GA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, 1996.

Whaley M. et al. Predictors of over- and underachievement of age-predicted maximal heart rate. Med Sci Sports Exerc 24(10):1173-1179, 1992.

Wilmore J.H. and Costill D.L. Physiology of Sport and Exercise. Human Kinetics, Champaign, Illinois, 1994, pp. 1-549.

Wilmore J. et al. Endurance training has a minimal effect on resting heart rate: the Heritage study. Med Sci Sports Exerc 28(7):829-835, 1996.