It is important for instructors to understand the issues associated with the use of target heart rate for assessing the intensity of their clients’ workouts. The first issue is the often-overlooked fact that we are estimating a person’s maximal heart rate (usually with some age-based formula), and maximal heart rate estimates typically have considerable standard variations.
Consequently, we often have people exercising considerably above or below the percentage range designated by a given formula. (For example, a 40-year-old is instructed to exercise between a heart rate of 108 and 144 bpm (representing 60 percent and 80 percent of 220 minus age), when in reality this 40-year-old’s actual maximal heart rate is 165 instead of an age-predicted 180). Neither of these situations is desirable, especially when individuals with elevated risk factors are exercising at higher percentages of their actual maximal heart rate. Of course, the opposite situation is not optimal in that the person will likely be under-training and, therefore, fall short of his/her fitness goals.
A second important issue relates to the considerable variability of relative intensity tolerance — anaerobic threshold — among individuals. A combination of genetics and training allow people to tolerate various percentages of their maximal aerobic work capacity. For example, a genetically gifted, highly trained endurance athlete may be able to engage in a prolonged workout at approximately 85 to 90 percent of his/her V02max, whereas an unconditioned individual may struggle to maintain a steady workout at only 50 to 60 percent V02max. Consequently, when you consider the potential of these first two factors in advising a client to exercise between a textbook-standard percentage heart rate range (say 60 to 90 percent predicted maximal heart rate), the unfortunate result could be someone significantly overexerting or under-training.
The third issue concerns the actual mechanics of accurately measuring one’s pulse — a skill more difficult than often portrayed by fitness instructors. Each detail of pulse monitoring — locating one’s pulse, not pressing too hard (especially on the carotid), correct timing, and measuring during (not after) cessation of the activity — contributes to the ultimate accuracy and usefulness of the resultant count.
It is certainly not uncommon for someone to under- or overcount their pulse by one or more beats. This error is then magnified by multiplying the count to arrive at a minute rate (e.g., undercount by one in 10 seconds equals an under count of six beats per minute).
The fourth issue centers around the normal day-to-day variability of heart rate response to a given external work demand (for example, running at an eight-minute mile pace). An individual may erroneously perceive an activity to be equivalent to previous demands that in reality require a greater or lesser physiological and cardiovascular response. Internal factors such as muscular soreness, glycogen stores, degree of rest/fatigue and fluid levels, combined with external (environmental) influences such as thermal stress (wet bulb temperature), wind resistance and terrain, individually and collectively influence actual exercise heart rate response during any given exercise session.