Runners focus on pace because it is theoretically accurate and easy to measure. However, from a physiological standpoint, pace does not always provide the most reliable training metric. Conditions such as humidity, heat, and high altitude all affect pace. If it’s summer, you are traveling, or you are returning from time off, it’s best to disregard pace as a primary metric and measure intensity without relying on pace.
There is no perfect calculator for adjusting pace in conditions. Some runners fare surprisingly well in heat and humidity, some require an acclimation period, while others still struggle until fall. One must look to metrics beyond pace for a more practical, adaptable solution.
Training pace (or more appropriately, pace ranges) has value. It provides an objective view of progress, especially when preparing for a goal race. It reveals areas of weakness, such as if a runner completes a 5k and 10K at the same pace. However, it is most useful in retrospect for analyzing training and progress. During a run itself, you are best suited by focusing on another primary metric: heart rate, ventilatory rate, or current race pace effort.
All of these approaches to measuring intensity require calibration. For athletes who constantly run too fast on their easy days, heart rate training may be more beneficial than the later two, especially as the runner is re-calibrating their perception of effort.
Heart rate training has its benefits and flaws. Caffeine consumption and external stress can spike your heart rate. Wrist-based optical heart rate monitors have a significant margin of error. However, heart rate provides an objective, clear metric for measuring intensity, which is often appealing to runners who prefer training by pace for similar reasons.
Heart rate aids in measuring intensity when external factors affect pace. For example, heat and humidity raise heart rate at any given pace. If you try to run at the same easy pace as during ideal temperature, you run too fast. However, if you focus on running at the same heart rate for easy runs during the summer, you maintain a similar easy effort.
The most effective method for determining HR does not rely on age-based formulas. Coach Joe Friel developed a method for determining heart rate based on a time trial. This time trial calculates lactate threshold heart rate (LTHR). The athlete runs a 30-minute time trial. The average heart rate for the final 20 minutes is LTHR (this reduces inaccuracy from runners who may start out too slow or fast). Easy effort corresponds to approximately 85% of LTHR; you can find a full list of corresponding heart rate zones in Friel’s article.
If a time trial is not practical, Phil Maffetone’s innovative 180 Formula determines maximum aerobic heart rate (which corresponds to aerobic threshold). This formula accounts for training background, health issues, and other factors beyond age.
This graph from Greg McMillan demonstrates the correlation between heart rate and ventilatory rate. Ventilatory rate gradually increases until aerobic threshold, and then sharply increases at lactate threshold, when breathing comes fast but still controlled. It continues to climb through VO2max, when breathing is heavy.
The talk test functions as a simple method for assessing ventilatory rate:
- Easy/sub-aerobic threshold: you can carry on a conversation
- Aerobic threshold/moderate effort: can speak a sentence
- Lactate threshold: can speak in short phrases (“pace feels good”)
- VO2max: can only speak a word or two at a time before gasping for air
If you learn to distinguish different ventilatory rates and are honest about your breathing on a run, ventilatory rate is just as reliable and effective as heart rate.
Current Race Pace Effort
The body does not know distance, but rather time at a particular intensity. A 17-minute 5K runner and a 35-minute 5K runner may cover the same race distance, but technically work within different intensity thresholds. The 17-min 5K runner is closer to VO2, which the 35-min 5K runner is working within the critical velocity range.
What you do not want to do is try to run your goal race pace. This approach requires running by an effort you could sustain if you were to race for a set duration of time based on your current fitness.
For example, if you are doing a lactate threshold (tempo) run, you spend 20-40 minutes running at the intensity that you could sustain for an hour-long race. If you do not feel like you could honestly sustain that pace for a full hour of race (not a full hour of the workout!).
Based on most physiological research, these race effort for common training intensities.
- Easy/sub-AeT: you should feel like you could hold the pace indefinitely
- Aerobic threshold/steady-state: 2-2.5 hour race effort
- Lactate threshold: Hour race effort*
- Critical velocity: 30-35 min race effort
- VO2max: 10-20 min race effort
* Many runners will find that threshold is a spectrum of intensities: 45-min race effort for cruise-intervals, 60-75 min effort for standard tempo runs, ~1.5 hour race effort for longer tempos.
The race effort approach works best for intermediate to experienced runners, who have a better understanding of how race-effort intensity feels.
McMillan, You (Only Faster)
Roche & Roche, The Happy Runner
Oftentimes, these metrics for measuring intensity without relying on pace work well when used together. You can use both heart rate and race effort, for example, or cross-check ventilatory rate and race effort.
How do you measure intensity on a run?