It is recommended that you exercise at an intensity of 50-85% of heart rate reserve max for improvement in both health and fitness. For deconditioned individuals, you should exercise at an intensity of 40-50% of heart rate reserve max.
The total amount of work performed is one of the most significant factors in improving cardiorespiratory fitness. Improvement is similar for low-intensity, long-duration activities versus high-intensity, short-duration activities if the total energy costs of the activities are equal. However, high-intensity exercise carries a higher risk of cardiovascular complications, orthopedic injuries, and decreased adherence. Low-to moderate-intensity training programs with longer durations are recommended for most adults.
How to prescribe exercise intensity
Methods for figuring how hard your client should exercise are nearly the same as those used to assess intensity while exercising. Methods of prescribing intensity include:
1. HR methods: percentage of HR max and the Karvonen (HRR) method.
2. Rate of perceived exertion (RPE scale or Borg scale)
3. Using METS to establish the proper workload
1. Heart rate methods
Prescribing and measuring intensity by taking a pulse is very common and convenient. However this method has some limitations, including: the effect of medications (may decrease or increase the HR); the fact that both HR methods are based on the formula 220-age = max HR (accurate for only about 75% of the population); the effect of the pressor response (HR elevated due to upper body exercise, but oxygen consumption and calorie expenditure are not consistent with HR); and the initial difficulty teaching clients to take an accurate pulse.
2. Rate of perceived exertion (RPE) method
This method is valuable for assessing and prescribing intensity for several reasons, including: it may be used even if clients are taking HR altering medications; it helps clients to “listen to their bodies;” it provides an accurate gauge of approaching fatigue; and it is a widely validate, reproducible tool for monitoring intensity. RPE can be used in conjunction with the heart rate method.
3. Using METS to establish the proper workload
A MET (metabolic equivalent) is a unit of energy expenditure often used by physiologist and cardiologists. Most people , on a pound per pound basis, consume approximately the same amount of oxygen at rest (VO2 3.5 ml of O2/kg of BW/min wth BW= body weight). This resting oxygen uptake value is equivalent to one MET. In other words, having an energy expenditure of on MET means that an individual is expending only a resting level of energy (calories). If a person exercises at a five MET level of energy expenditure that mean he/she is consuming five times more oxygen and calories than at rest. (5 x 3.5 ml/kg/minute would equal 17.5 ml of O2 consumed per Kg of body weight per minute.) Therefore, 10 METS equals 10 resting , and so on.
Borg Scale Rating of Perceived Exertion
Using the original Borg scale, rate of 12-13 approximates 50% of HRR and a rate of 16 approximates 85% of HRR. Therefore, the ACSM recommends that clients exercise within an RPE range of 12-16 (meaning, to the clients, that the intensity subjectively feels somewhat hard to hard). On the revised scale, this would be between four and six. Clients can be given a range (e.g., 12-14) as part of their prescription. Most people will need instruction to help them understand how to use the RPE scale; extremely unfit or inactive clients may need two to three sessions before they use the scale properly.