This document published by ACSM (American College of Sports Medicine) proposes a new standard to unify the inconsistent exercise intensity terminology used across public health, exercise science, and sports science. The researchers point out that ambiguous terminology reduces the effectiveness of exercise prescription, and present a five-level intensity system from 'Very Low' to 'Very High' with corresponding ratings of perceived exertion (RPE). They also recommend using physiologically grounded markers like metabolic thresholds and repetitions in reserve (RIR) instead of crude heart rate formulas or %1RM ratios that fail to account for individual differences.

1. Why Is Exercise Prescription Important -- and Complicated?

The evidence that exercise benefits health is overwhelming. Regular exercise builds fitness and endurance while promoting health. The UK Academy of Royal Medical Colleges even called exercise a potential 'Wonder Drug.' Just as dosage matters when prescribing medication, the effects of exercise depend on frequency, type, duration, and intensity.

Exercise is a potential 'Wonder Drug.'

But there is a problem. Improving outcomes and treating disease through exercise prescription requires evidence-based planning, and each field uses entirely different language.

A Historical Perspective on Exercise Prescription

Interest in exercise goes back to antiquity. Plato emphasized the importance of physical activity:

Lack of activity destroys the good condition of every human being, while movement and methodical physical exercise save it and preserve it.

-- A modern interpretation of writings by Plato (427--347 BC)

In 1975, ACSM published its first guidelines, launching a scientific approach. Yet exercise physiologists, physicians, and trainers still use inconsistent terminology, causing ongoing confusion in practice.

2. Exercise Intensity: The Core Is 'How Hard'

Among the FITT(VP) principles of exercise prescription (Frequency, Intensity, Time, Type, Volume, Progression), this document focuses most on Intensity.

Intensity is a crucial factor in determining health and fitness outcomes. Too little yields no effect; too much increases the risk of injury and adverse events.

As the graph (Fig 1) shows, higher intensity produces greater mortality reduction benefits (a), but simultaneously increases the risk of adverse events (b). Setting appropriate intensity is therefore essential -- but the problem is that each field uses different terms for 'intensity.'

There is no greater impediment to the advancement of knowledge than the ambiguity of words.

-- Thomas Reid, 1852

3. The Current Terminological Chaos

Let's look at how confusing the current terminology really is.

Cardiorespiratory Exercise

• Public health: Typically uses 'Light,' 'Moderate,' and 'Vigorous.'
• Exercise physiology/sports science: Divides by physiological response into 'Moderate,' 'Heavy,' 'Severe,' and 'Extreme.'
• Sports practice: Uses Zone 1, 2, 3, or terms like 'Tempo' and 'Easy.'

Even when the same word 'Moderate' is used, some contexts mean "able to hold a conversation," while others mean "just below the lactate threshold" -- with different standards.

Resistance Exercise

Weight training is even more confusing:

• Intensity is typically defined by percentage of 1RM (one-repetition maximum).
• But this conflates load (weight) with intensity (degree of effort).
• Even a light weight can produce very high intensity if you repeat to failure.

This table (Fig 2) shows at a glance how many different terms and standards are used across organizations and guidelines.

4. The New Standard Proposal: A 5-Level System

To resolve this confusion, the researchers propose five new standard intensity levels for universal use across all fields.

Proposed 5-Level Intensity and RPE Scale

1. Very Low - Very easy
2. Low - Easy
3. Moderate - Somewhat hard
4. High - Hard
5. Very High - Very hard

Why These Changes?

• 'High' replaces 'Vigorous': Linguistically, 'High' is a more natural step above 'Moderate,' and it aligns with the widely used term High-Intensity Interval Training (HIIT).
• 'Supramaximal' removed: Since 'Maximal' already means the highest possible intensity, a word meaning "beyond maximum" is logically contradictory.
• Avoiding 'Light' and 'Heavy': These words evoke weight (load), so terms describing degree of effort like 'Easy' and 'Hard' are recommended instead.

5. How to Set Cardiorespiratory Exercise Intensity

Many people use formulas based on percentage of maximum heart rate (%HRmax) or oxygen uptake (%VO2max), but the researchers point out that these methods fail to account for individual differences.

Recommended Method (Gold Standard)

The most accurate approach is to measure metabolic thresholds.

• First Metabolic Threshold (MT1): The point where lactate or ventilation begins to increase non-linearly. (Below this is 'Low'; above is 'Moderate.')
• Second Metabolic Threshold (MT2): The point where lactate accumulates so rapidly that equilibrium can no longer be maintained. (Above this is 'High.')

Using these thresholds allows comparable physiological stress levels regardless of individual fitness.

Methods to Avoid (Not Recommended)

• Fixed percentage formulas: "Exercise at 70% of your max heart rate" may be a comfortable aerobic workout for one person but a breathless high-intensity session for another.
• METs (Metabolic Equivalents): Fixed values that don't account for body weight or fitness level, making them unsuitable for personalized prescription.

What If You Don't Have Lab Equipment? (Practical Alternatives)

You can't measure thresholds in a lab every time. In that case, using Rating of Perceived Exertion (RPE) as a supplementary tool is recommended.

• Talk Test: If you can comfortably hold a conversation during exercise, you're typically at the 'Low' level. However, this can vary by individual, so use it only as a supplementary measure.

6. Resistance Training Intensity: It's Not All About the Weight

In weight training, people commonly prescribe "10 reps at 70% of 1RM." But this determines the load (weight), not the actual intensity (how hard it is).

The New Standard: Repetitions in Reserve (RIR)

The researchers strongly recommend using RIR (repetitions remaining until failure) instead of weight.

• RIR 0: Cannot lift even one more rep (Very High)
• RIR 2-3: 2-3 reps left in reserve (High)

As shown in the figure (Fig 5), even at the same weight (50% 1RM), stopping after only 5 reps means low intensity, but continuing to failure makes it very high intensity. In other words, how close you get to failure is the true measure of exercise intensity.

Conclusion: Speaking the Same Language for Better Health

This consensus statement proposes changing long-established terminology, so some resistance from practitioners is expected. But the researchers believe that public health, exercise science, and sports practice must share the same language for more accurate data collection and effective exercise prescription.

Key Takeaways:

1. Unify exercise intensity into Very Low, Low, Moderate, High, Very High -- five levels.
2. For cardiorespiratory exercise, use metabolic thresholds and RPE rather than simple heart rate formulas.
3. For weight training, assess how many more reps you can do (RIR) rather than weight (%1RM).

We hope these guidelines serve as an important first step toward better communication among researchers, practitioners, and the exercising public.