Exercise intensity is a foundational element in any effective fitness program, determining the type of physical adaptation the body undergoes. The level of effort dictates the results, whether the goal is building strength, improving endurance, or enhancing overall health. Intensity can be measured in two distinct ways: absolute and relative. Absolute intensity is a universal measure, while relative intensity is a personalized metric that accounts for individual differences in fitness, fatigue, and physical capacity. Understanding and utilizing relative intensity transforms a generalized exercise plan into a highly individualized training prescription.
Defining Relative Intensity
Relative intensity refers to the amount of effort exerted during activity compared directly to an individual’s own maximum capacity. This perspective acknowledges that the same activity can feel vastly different depending on the person performing it. For example, a six-mile-per-hour run represents the same absolute speed for everyone, but for a trained marathon runner, this is a light warm-up, while for a novice, it is a near-maximal effort.
This personalized approach makes relative intensity a far more meaningful measure for training than absolute metrics like distance, weight lifted, or speed. Relative intensity is expressed as a percentage of a person’s individual maximum heart rate, maximum oxygen consumption (\(\text{VO}_2\text{max}\)), or perceived exertion.
An individual’s relative intensity is not static and must be adjusted daily for factors like stress, sleep quality, and recovery status. On a day following poor sleep, an exercise that might normally be 70% of a person’s maximum capacity could feel like 85%, and the training intensity must be lowered to accommodate that fatigue.
Measuring Effort with the RPE Scale
The most accessible and practical method for measuring relative intensity is the subjective Rating of Perceived Exertion (RPE) scale. This scale asks exercisers to rate how hard they feel they are working, factoring in all sensations, including muscle fatigue, heart rate, and breathing rate. While the original Borg scale used a 6–20 range, the simplified 1–10 scale is now commonly used for its straightforward application.
An RPE of 1 represents no effort at all, such as sitting down, while an RPE of 10 signifies an all-out, maximal effort that cannot be sustained for any longer. A moderate-intensity workout typically falls around RPE 5, where the individual is slightly breathless but can still speak in short phrases or sentences. When the effort increases to RPE 7, conversation becomes difficult, and the exerciser can only manage a few words at a time.
For resistance training, RPE is closely linked to the concept of Repetitions in Reserve (RIR), which indicates how many more repetitions a person could complete before reaching muscular failure. An RPE 8, for instance, means the exerciser finished the set feeling they had exactly two repetitions left in the tank.
Advanced Physiological Measurement Techniques
Objective measurement of relative intensity, particularly for cardiovascular exercise, is often determined by calculating a percentage of the heart rate reserve. The Heart Rate Reserve (HRR) method, also known as the Karvonen formula, provides a highly accurate way to set personalized target heart rate zones. This method accounts for both the maximum heart rate (MHR) and the resting heart rate (RHR), offering a more precise measure than simply using a percentage of MHR alone.
The calculation first finds the HRR by subtracting the RHR from the estimated MHR, which is often approximated as 220 minus the person’s age. The target heart rate is then calculated by multiplying the desired intensity percentage by the HRR and subsequently adding the RHR back to that value. The moderate intensity range typically corresponds to 50–70% of the HRR, while vigorous activity is generally set higher, between 70–85% of the HRR.
For resistance training, an objective relative intensity can be calculated using the One Repetition Maximum (1RM) test. The 1RM is the maximum amount of weight an individual can lift for one single repetition with proper form. Training intensity is then expressed as a percentage of this maximal lift, such as performing a set at 85% of 1RM.
Structuring Training Based on Relative Intensity
The practical application of relative intensity is to align the effort level with specific physiological goals, as different intensities produce distinct training adaptations. Low relative intensity, typically below 60% of HRR or RPE 3, is utilized for active recovery and establishing a foundational aerobic endurance base. This level of effort allows for extended duration without excessive fatigue.
Moderate intensity, often between RPE 5 and RPE 7, is effective for improving aerobic capacity and is frequently used in long-duration endurance work. Pushing the relative intensity higher, into the RPE 8 to RPE 10 range or above 85% of 1RM, stimulates adaptations associated with strength, power, and maximal oxygen uptake (\(\text{VO}_2\text{max}\)) improvement. For instance, a program targeting muscle hypertrophy requires a high relative intensity set, often measured at RPE 8 or 9, to ensure sufficient muscle fiber recruitment.