Estimating Aerobic Exercise Intensity and Volume

Worked Example: Calculating Calories Burned

Suppose "Joe" weighs $150 \text{ lbs}$ ($68.2 \text{ kg}$) and jogs at $8 \text{ mph}$, which corresponds to $11.8 \text{ METs}$. How many calories does he burn in $30 \text{ minutes}$?

1. Convert METs to relative $\text{VO}_2$: $11.8 \times 3.5 = 41.3 \text{ mL} \cdot \text{kg}^{-1} \cdot \text{min}^{-1}$
2. Convert to absolute $\text{VO}_2$ (L/min): $41.3 \times 68.2 = 2817 \text{ mL} \cdot \text{min}^{-1}$ $2817 / 1000 = 2.817 \text{ L} \cdot \text{min}^{-1}$
3. Calculate kcal per minute: $2.817 \times 5 = 14.1 \text{ kcal} \cdot \text{min}^{-1}$
4. Calculate total for 30 minutes: $14.1 \times 30 = 423 \text{ kcal}$

Subjective Methods for Estimating Intensity

In real-world settings, clinicians often use subjective scales to monitor intensity. These are highly effective for clients to self-regulate during exercise.

• The Talk Test:
  • Moderate Intensity: The individual can talk but cannot sing.
  • Vigorous Intensity: The individual can only say a few words before pausing for breath.
• Borg’s RPE Scale (6–20): This scale is designed to correlate with heart rate. A simple way to use it is to multiply the rating by 10 (e.g., a rating of 11 $\approx$ 110 bpm).
• Borg’s CR-10 Scale: A 0–10 scale where moderate intensity is roughly 5–6 and vigorous intensity is 7–8.

Exercise Volume and the FITT Principle

The total volume of an exercise program is the product of Frequency, Intensity, and Time (FITT). Volume is a critical metric because there is a clear dose-response relationship between exercise volume and health outcomes.

Clinical Recommendations

To reduce the risk of cardiovascular disease (CVD) and premature mortality, a total energy expenditure of $500\text{--}1,000 \text{ MET-min} \cdot \text{wk}^{-1}$ is recommended. This can be achieved through several time-based targets:

Pro-Tip: $2 \text{ minutes}$ of moderate-intensity exercise is roughly equivalent to $1 \text{ minute}$ of vigorous-intensity exercise.

Progression: "Start Low and Go Slow"

When beginning a new program, the primary goal is adherence and safety. The "start low and go slow" principle is essential to prevent injury and burnout.

For most adults, a safe way to advance is to increase the duration (time) of each session by $5\text{--}10 \text{ minutes}$ every $1\text{--}2 \text{ weeks}$ during the initial $4\text{--}6 \text{ weeks}$ of training. Once a baseline of regular activity is established (usually after one month), the other components of the FITT principle (frequency and intensity) can be gradually adjusted upward.

By combining objective metabolic calculations with subjective feedback and a structured progression, exercise professionals can design programs that effectively improve cardiorespiratory fitness while minimizing risk.