Determining Safe Physical Activity Levels Based on MET Score
The critical threshold for safe physical activity is 4 METs—if you can climb two flights of stairs or walk four blocks without stopping, you have adequate functional capacity to proceed with most activities safely. 1
Understanding the MET Classification System
Functional capacity is stratified into clear categories that predict cardiovascular risk 1:
- Poor (<4 METs): Increased perioperative cardiac risk and long-term cardiovascular events
- Moderate (4-6 METs): Adequate functional reserve for most daily activities
- Good (7-10 METs): Low cardiovascular risk profile
- Excellent (>10 METs): Minimal cardiovascular risk even with underlying disease
One MET equals resting metabolic rate (3.5 mL O₂/kg/min), representing baseline oxygen consumption at rest. 1
Practical Activity Assessment
Activities Indicating Poor Functional Capacity (<4 METs)
If you can only perform these activities, your functional capacity is limited 1:
- Slow ballroom dancing
- Golfing with a cart
- Playing a musical instrument while seated
- Walking at 2-3 mph on flat ground
- Light housework (dusting, washing dishes)
Activities Indicating Adequate Functional Capacity (≥4 METs)
If you can perform these activities, you have sufficient functional reserve for safe physical activity 1:
- Climbing one flight of stairs or walking up a hill
- Walking on level ground at 4 mph
- Heavy housework (scrubbing floors, moving furniture)
- Moderate cycling
- Climbing hills
- Ice skating or rollerblading
- Singles tennis
- Jogging
Clinical Risk Stratification Algorithm
Step 1: Simple Screening Questions
Ask yourself these two questions 1, 2:
- Can you walk 4 blocks without stopping?
- Can you climb 2 flights of stairs without stopping?
If you answer "no" to either question, you have poor functional capacity (<4 METs) and are at increased risk for cardiovascular complications (P<0.006 for walking blocks; P<0.01 for climbing stairs). 1, 2
Step 2: Risk Implications
If functional capacity is ≥4 METs and you are asymptomatic, you can proceed with planned activities safely, even if you have stable cardiovascular disease or risk factors. 1 The European Society of Cardiology confirms that when functional capacity is high, prognosis is excellent regardless of underlying cardiac conditions. 1
If functional capacity is <4 METs, the number of cardiac risk factors determines your safety profile 1:
- No cardiac risk factors: Proceed with activities using gradual progression
- 1-2 cardiac risk factors: Consider medical evaluation before vigorous activity
- ≥3 cardiac risk factors: Require formal cardiovascular assessment before increasing activity intensity
Structured Assessment Tools
The Duke Activity Status Index provides the most validated systematic approach to estimate functional capacity. 1, 2 This questionnaire correlates specific daily activities with MET levels and has been extensively validated in perioperative risk assessment. 3
The index assigns weighted scores to activities ranging from self-care (2.75 METs) to running a short distance (8.0 METs), providing an objective functional capacity estimate. 1
Important Clinical Caveats
Context-Specific Considerations
The 4-MET threshold has different prognostic implications depending on the clinical scenario. 1 After thoracic surgery, poor functional capacity (<4 METs) carries a dramatically increased mortality risk (relative risk 18.7; 95% CI 5.9-59), whereas for other non-cardiac contexts, the association is weaker (relative risk 0.47; 95% CI 0.09-2.5). 1
Measurement Limitations
Standard MET calculations may overestimate actual functional capacity in certain populations 4:
- Coronary artery disease patients
- Morbidly obese individuals
- Patients taking beta-blockers
The conventional assumption that 1 MET = 3.5 mL O₂/kg/min significantly overestimates resting energy expenditure in these groups, potentially leading to inappropriate activity recommendations. 4
Adiposity Confounding
Body weight-scaled MET values are confounded by adiposity, which can produce paradoxical observations. 5 Lean mass-proportional measures provide more accurate functional assessment, though standard METs remain the clinical standard. 5
Practical Application for Activity Prescription
For previously inactive adults, start with moderate-intensity activities (≥3 METs) to safely increase functional capacity and reduce future cardiac event risk. 4
If you cannot perform 4-MET activities, begin with supervised low-intensity activities and progress gradually under medical guidance. 1 The inverse relationship between functional capacity and complications means that even small improvements in exercise tolerance significantly reduce cardiovascular risk. 1
Monitor for symptoms during activity progression: dyspnea, chest discomfort, or excessive fatigue indicate you have exceeded your current functional capacity and should reduce intensity. 2