Target Heart Rate

Question:

One of my clients recently told me that even when training at her max target heart rate (THR), she does not break into a sweat. Her VO2 max is not that good either (below average). She is 49 years old and is not on any meds. When she was in college, her gym instructor told her she should not train over 145-155 bpm. I have tried a couple of other formulas to determine her THR. The digital one at work says her THR could be as high as 181 bpm. She said that she feels comfortable training at that rate. Do you think it is safe idea to let her train at 181 bpm?

Answer:

The prescription of exercise intensity has always been an approximate endeavor because adjustments of intensity often rely on an individual’s perception of effort. Measurements that accurately reflect the intensity of exercise in terms of metabolic demand, including oxygen consumption (VO2) and blood lactate, are limited to a laboratory setting. By contrast, the heartbeat (the split-second sequence of contractions of the heart’s four chambers) is the most easily measured physiological indicator of exercise intensity and thus offers us a very reliable and objective variable with which to work. Research has shown that the heart rate (HR) observed at slightly below the ventilatory threshold (the exercise intensity above which the volume of carbon dioxide expired is greater than the volume of oxygen inspired; akin to the lactate threshold) is a better indicator of the exercise intensity that can be sustained for prolonged periods than other physiological measures, such as blood lactate concentration, work output and ventilation. This is good news for the fitness professional since determining your clients’ HRs is obviously much easier than determining their blood lactate concentrations or VO2.

If your client is 49 years old, the most common prediction equation (220 minus age) would suggest her target HR for sustained aerobic exercise is 120 to 137 bpm (70 to 80% of her age-predicted maximum HR of 171). If also accounting for her resting HR, her target HR is 138 to 149 bpm (assuming a resting HR of 60 bpm). However, since the standard deviation associated with the estimate of max HR is ±11 bpm, a prescription of exercise intensity based on an estimated max HR is difficult at best. All people of the same age do not have the same max HR. Sixty-eight percent of the population will have a max HR within one standard deviation of the population’s average, and 95 percent will fall within two standard deviations of the average. Thus, while the equation predicts that your 49 year old client has a max HR of 171 bpm, 68 percent of all 49 year olds will have an actual max HR between 160 and 182 bpm, and 95 percent will fall between 149 and 193 bpm. This rather large margin of error can lead to prescribing an exercise intensity that is either too low or too high to achieve the optimal benefit. The equation tends to overestimate max HR in highly fit people and to underestimate max HR in unfit people.

It is not even clear from where the most often used max HR formula of 220 minus age originates, as there is no published research for the formula. Its origin has been attributed to Fox et al, who estimated a “line of best fit” on a series of raw and mean data compiled from a review of research on physical activity and heart disease. There are at least 43 formulas in the literature to predict max HR, none of which provides acceptable accuracy of max HR prediction.

A more accurate way of determining the max HR would be to measure your client’s HR while she performs a graded-intensity exercise test, with the last stage eliciting an all-out effort, at which point she would reach her actual max HR. Because of the inherent risk in performing a maximum cardiovascular test (especially with older adults), you should use caution in deciding whether to use a maximum test to determine the max HR.

Once you know your client’s actual max HR, knowing exactly what target HR to prescribe is also complicated, since there is great variability among individuals regarding how long a given percentage of max HR can be sustained. My suggestion is to first find out what your client’s actual max HR is and then have her exercise at 70 to 80 percent of that HR for her aerobic exercise sessions of 30 to 60 minutes (using higher percentages for interval training). If that intensity still feels easy for her, then bump up the intensity a bit and use the subjective measure of rate of perceived exertion (RPE) in combination with the objective measure of HR. Since you mentioned she feels comfortable at a HR of 181 bpm, she may simply have a very high max HR for her age.

References:

1. Boulay, M.R., Simoneau, J-A., Lortie, G., and Bouchard, C. (1997). Monitoring high-intensity endurance exercise with heart rate and thresholds. Medicine and Science in Sports and Exercise. 29(1):125-132.
2. Fox, S.M., Naughton, J.P., and Haskell, W.L. (1971). Physical activity and the prevention of coronary heart disease. Annals of Clinical Research. 3(6):404-432.
3. Heyward, V.H. Advanced Fitness Assessment & Exercise Prescription. Champaign, IL: Human Kinetics, 1998.
4. Howley, E.T., Bassett, D.R., and Welch, H.G. (1995). Criteria for maximal oxygen uptake: Review and commentary. Medicine and Science in Sports and Exercise. 27(9):1292-1301.
5. Howley, E.T. and Franks, B.D. (1992). Health Fitness Instructor’s Handbook. Champaign, IL: Human Kinetics.
6. Robergs, R.A. and Landwehr, R. (2002). The surprising history of the “HRmax=220–age” equation. Journal of Exercise Physiology Online. 5(2).
7. Wilmore, J.H. and Costill, D.L. (1988). Training for Sport and Activity: The Physiological Basis of the Conditioning Process. Champaign, IL: Human Kinetics.