Thresholds, Heart Rate, and the Keys To Training Longevity
By Coach Johnny B (Ms, CFL3, LMT)
Six years ago, I started training for triathlons. I was also training CrossFit six days a week (which are high-intensity interval workouts with heavy ass weights). When I started training for triathlons, my belief, based on the work of people like TriDot and CrossFit, was that I needed to train harder (more threshold and VO2 max) to get faster. It worked for exactly one year. The following year, I broke my midfoot, got a Lisfranc injury, tore my UCL (elbow), and got a year-long case of plantar fasciitis, which all added up to me having to delay doing an Ironman I was registered for for a year.
Why?
Because every session was at or above lactate threshold, and all the metabolites (chemical byproducts from the training) never got cleared and started eating away at my tissues like battery acid, which eventually led to my body breaking down.
I am not alone in this doom spiral line of thinking: the #1 reason we see injuries at Atlanta Sports Recovery is because everyday athletes (aka people who do not get paid to train/workout/play a sport), go too HARD too often, with little to no respect for how adaptation occurs: recovering and absorbing the training.
This week, I wanted to share a little bit about the importance of training zones and thresholds for cardiovascular exercise (such as running, cycling, CrossFit, HIIT, and even combat sports like Jiu-jitsu) in the hope that this education can help a few people reading this possibly prevent the terrible breakdown I experienced.
The Importance Of Thresholds
The first thing that is essential to know is what thresholds are. Thresholds as physiological tipping points — markers that tell your body, “Hey, this effort level changes the metabolic game and now we have to start using different fuel and different chemical reactions are happening.”
There are two critical thresholds here:
- Aerobic Threshold (AeT): Where fat burning is maximized and effort still feels easy.
Lactate Threshold (LAT / FTP): The point where lactate begins to accumulate rapidly. Go beyond this too often, and fatigue and breakdown set in fast (especially after 35 years of age)
Heart Rate Zones and How They Correlate To Thresholds
Everyone’s two significant thresholds occur at different points during exercise. A common correlate for seeing where they are is heart rate. The rule of thumb is that your Aerobic Threshold is the top of your heart rate zone 2 (on a five heart rate zone chart) and that your Lactate Threshold is the top end of your heart rate zone 4. (See chart below for visual.)
The reality is that these correlates aren’t perfect. The best way to understand your thresholds is from a VO2 Max test or lactate threshold test (both of which we do at ASR), but they are pretty reliable assuming you have an accurate max heart rate (220-your age is bulls shit BTW, you need to do a lactate heart rate threshold test).
If heart rate isn’t your jam, paces or a % of your power (wattage) can also work as a correlate. A common way to test your lactate threshold is to have you run a 5k for time, and based on that, you can create rough “pace zones” or “power zones” that correlate to the physiology happening inside your body.
Why these thresholds and zones matter:
Training near your aerobic threshold builds endurance (zone 2 training) without being too expensive on your body. Training just below or at lactate threshold (tempo and threshold work) improves speed and stamina without tipping into burnout, when done only a few intentional sessions per week. Excessive training above threshold (VO2 Max work) will cause a breakdown that you can’t recover from. That being said, it is essential to train your VO2 max at least twice a month.
How To Monitor Recovery & Make Decisions About What Zone To Train
Heart Rate Variability (HRV) is the beat-to-beat variation between heartbeats. Higher HRV = better parasympathetic (rest-and-digest) activity. Lower HRV = heightened sympathetic (fight-or-flight) stress. NOTE: HRV is incredibly personal; comparing your HRV to anyone else’s makes you a dumb dumb, so don’t do it.
Here is how to use it with your zones:
Low HRV for 2+ days? You might be under-recovered or overreaching. Opt for changing training to lower intensity work.
Consistently rising HRV over the course of a week? You’re adapting well. TIME TO GET AFTER IT.
Use trends, not daily fluctuations. Everyone has off days.
Resting Heart Rate (RHR): The Canary in the Coal Mine
Your resting heart rate (RHR), especially when measured first thing in the morning, is a super-simple, but very accurate window into how taxed your body is.
Elevated RHR (5–10 bpm above normal average resting) = accumulated fatigue, poor sleep, illness, or under-recovery. Time to opt for lower intensity training.
Lower-than-normal RHR = high parasympathetic tone, strong recovery, and your body’s aerobic systems are improving. Time to get after it!
Pairing HRV + RHR is a powerful recovery monitoring tool
Low HRV + high RHR = 🚨 Red flag: scale back
High HRV + normal RHR = ✅ Green light: go hard
Final Thoughts: Train Intelligently, Recover Aggressively, Perform Better
Use thresholds and zones to structure intensity intelligently so you don’t crush yourself.
Use HRV and RHR to decide when to push or pull back and which zone to train in. Remember it’s trend lines, not one bad recovery night. If you have green/yellow lights on your Whoop for 6 days and then have one red, don’t stress. However, if your Whoop or Garmin says ‘unbalanced’ for the last 7 days, you need to listen.
Use discipline to let easy days be easy — and hard days be hard.
If you aren’t tracking HRV and Resting Heart Rate, buy a Garmin and start doing it now. (opinion: The Whoop is trash compared to Garmin. I’ve used both extensively.)
The result of these habits?
Less fatigue. More consistency. Better performance/fitness over time and fewer injuries.
Want help figuring out your zones?
We offer VO₂ Max and Lactate Threshold Testing at Atlanta Sports Recovery. It’s the best way to personalize your training, avoid burnout, and keep showing up strong — season after season.
