[Research Rundown] Overtraining in Runners: 3 Metrics That Reveal If You’re Doing Too Much

Ever had one of those runs where your legs feel like concrete and your motivation is M.I.A.? As runners, we juggle work, life, and miles – so it’s no surprise that fatigue can sneak up on us. The big question is, how do you know when you’re genuinely overdoing it versus just having an off day? A new study offers some answers. It turns out both your heart and your feelings can tell you a lot about your fatigue. And balancing the two might be the key to smarter training and avoiding burnout.

Today, we’re looking at a fresh research study titled “Monitoring fatigue state with heart rate‐based and subjective methods during intensified training in recreational runners.” Consider this your research rundown on how to use both heart rate data and your own perception to gauge fatigue. Let’s dive in!

What’s the Difference Between Being Tired and Overtraining in Runners?

This study is basically the sports science version of “How much can we push before it’s too much?” Researchers took 24 recreational runners (people like us, not pros) through a training rollercoaster. First, everyone did a 3-week normal training period (baseline). Then came a 2-week overload where training volume and intensity were cranked up (~80% more intense than usual!). Finally, a 1-week recovery period let them rest up. Before and after each phase, the runners ran a 3000m time trial to see how their performance changed.

FIGURE 1 Study design. T0–T3 refers to test days, when the 3000 m running test and reactivity jump test were performed. C1–C6 refers to control running tests which consisted of a submaximal running test, countermovement jump test, and 6x3‐min high‐intensity interval training session. During the baseline period, training load of the 3rd week was slightly decreased (−20%) to ensure sufficient recovery before the overload period. DOI: 10.1002/ejsc.12115

Why all this? The goal was to see how different fatigue monitoring methods respond when training stress goes through the roof.

The researchers tracked a mix of heart rate-based measures (like resting heart rate and heart rate variability recorded at night) and subjective measures (like how sore or tired runners felt, mood, and “readiness to train” ratings). They wanted to know which signals (heart vs. subjective) are most sensitive to heavy training and whether they can actually tell who’s on track and who’s overcooked.

In fact, the study found that combining night heart rate + readiness to train + the HR/effort index correctly identified who was overreaching with over 90% accuracy.

In real life, this matters because as a self-coached runner you don’t have a team of sports scientists watching your every step. You need simple ways to monitor yourself. This study tested some of those ways head-to-head in a pretty intense training scenario.

Overreaching vs. Responding: How Runners React Differently to Hard Training

After the overload training block, the runners split into two clear groups based on how they reacted:

• Responders (RESP): About half the runners thrived on the harder training. These folks got faster – improving their 3k time trial by ~2.5%. They handled the extra load and came out feeling fitter.

  
  

• Overreached (OR): The other group hit a wall. Their 3k performance didn’t improve or even got slightly worse (~0.6% slower) despite all that hard work. They showed signs of excessive fatigue, called overreaching (and while overreaching isn't as bad as overtraining in runners, it's still fatigue that lasts days or weeks and tanks your performance while providing no fitness improvement once you've recovered.)

Fatigue is individual: Two runners can do the same training; one blossoms and another burns out. It doesn’t mean one is “weaker” – it’s just how our bodies respond.

So why did the same training plan make some runners fitter and others exhausted? Individual differences in recovery and stress, for one. But importantly, the study looked at how to detect those differences before a race or time trial makes it obvious. This is where heart rate and subjective measures come in.

Heart Rate vs. How You Feel: The Best Ways to Track Running Fatigue

The researchers tracked a bunch of indicators daily to see which ones flared up with fatigue:

• Heart-Rate Based Metrics: They checked things like nocturnal heart rate (your sleeping or resting heart rate at night) and heart rate variability (HRV). HRV is a fancy way to measure the tiny time differences between heartbeats – it’s often lower when you’re stressed or fatigued and higher when you’re recovered. They also looked at an HR-running power index, which essentially relates heart rate to running effort – think of it as how efficient or strained your heart is for a given run.

• Subjective Metrics: Each morning the runners answered a quick questionnaire about how they felt. Key things were readiness to train (basically “how good to go do I feel?”), soreness/fatigue in the legs, sleep quality, overall stress, and mood. This is similar to what many of us might log in a training diary or a recovery app – e.g., rating how tired or fresh you feel on a scale.

FIGURE 5 Subjective recovery in responders (RESP) and individuals with suspected overreaching (OR). Dashed circle indicates significant between‐group difference in the change from BL. DOI: 10.1002/ejsc.12115

So which metrics raised the alarm first? Here’s what they found:

• The subjective markers (how runners felt) were the quickest to respond when training load shot up. Especially readiness to train and leg soreness changed a lot during the overload, showing significant fatigue. In fact, those who ended up overreached reported a bigger drop in readiness to train and bigger spike in soreness than the responders did. These simple self-checks were super sensitive to the training stress – your body’s basically yelling “I’m tired!” through feelings of heavy legs and low motivation.

  
  

• The heart-rate measures were a bit more nuanced. Interestingly, just looking at resting heart rate or exercise heart rate alone didn’t consistently change for everyone during the two-week overload. In other words, simply seeing a higher heart rate on your easy run isn’t a foolproof sign of trouble – some runners’ heart rates stayed normal even as training ramped up. However, when they dug deeper, night-time heart rate and HRV told a story. By the second week of intense training, the overreached group showed a higher nocturnal resting HR (up 3%) and lower HRV (-0.7%), whereas the responders had a lower nightly HR (~-2.8%) and slightly higher HRV (+2.1%), indicating better recovery. These differences were statistically significant, meaning the fatigued runners’ hearts were beating a bit faster at rest and with less variability – classic signs of stress.

  
  

Some differences popped up early. Changes in the HR-running power index (how much harder the heart was working for the same output) started to separate the groups after just 6 days of overload, and changes in nocturnal HR and HRV followed within about 9–10 days. Meanwhile, the subjective feeling of “I’m not ready to train” diverged around day 11. So, objective heart data might give a slight head-start in warning you of accumulating fatigue, but the subjective markers don't lag far behind.

No single measure was perfect on its own, but together they paint a clear picture. In fact, the study found that combining night heart rate + readiness to train + the HR/effort index correctly identified who was overreaching with over 90% accuracy.

How to Balance Training Load and Recovery to Prevent Burnout

If you’re training for a race or chasing a personal best, you know there’s a fine line between productive training and overdoing it. This research matters because it shows simple ways to keep tabs on that line. Here’s what it means in plain terms:

• Fatigue is individual: Two runners can do the same training; one blossoms and another burns out. It doesn’t mean one is “weaker” – it’s just how our bodies respond. The study confirmed that tolerance to a given training load varies a lot in recreational runners. For self-coached athletes, this is a reminder to run your own race (as it were) when it comes to training volume. What works for your buddy might be too much for you, or vice versa.

  
  

• Your feelings are valid data: Sometimes we’re told to “ignore the pain” or “push through fatigue.” But in reality, those subjective feelings (tiredness, poor mood, heavy legs) can be early warning signs of overtraining. Research backs this up – a 2016 systematic review found that subjective measures (like mood, fatigue, soreness ratings) are often more sensitive and consistent than heart rate or other objective measures in tracking training stress. In other words, how you feel is real – trust it. If you wake up dreading your run and your legs feel like lead, your body might be saying it’s time for a break.

  
  

• Heart data adds context: While feelings matter, combining them with some heart rate data can give a fuller picture. Think of heart rate and HRV as a peek “under the hood” at your body’s engine. An elevated resting heart rate or dropping HRV trend over a few days can signal mounting fatigue or stress on your systems. However – and this is important – heart-rate metrics have natural day-to-day ups and downs. The study from Bosquet et al. (2008) noted that changes in HR/HRV with overtraining are usually small, often similar in size to normal daily fluctuations. So you shouldn’t freak out over one bad reading; instead, look for trends over several days. And always interpret those trends alongside how you feel. As one review concluded, heart data only becomes meaningful when you compare it with other signs and symptoms of fatigue.– basically, use your heart and your head.

  
  

• Performance isn’t the only marker: One surprising point the researchers made is that performance itself (like your pace or time trial results) might lag behind these fatigue signals, especially for recreational runners. You might still be hitting decent times due to fitness gains masking fatigue. So don’t wait until you bomb a race to realize you’re burnt out. Pay attention to the subtle cues beforehand.

Smart Fatigue Monitoring for Self-Coached Runners: What to Do Now

So, how can you use these findings to train smarter and stay healthy? Here are some practical tips to implement:

1. Keep a Training Log with RPE and Mood: Every day, jot down a quick rating of how hard your workout felt (RPE – Rating of Perceived Exertion) and how you’re feeling overall (energized, flat, sore, etc.). This study used a 1–5 scale for things like readiness to train and fatigue, which you can easily adapt. Over time, you’ll spot patterns. For example, if your easy runs suddenly feel hard (high RPE) and you’re unusually irritable or exhausted, it’s a cue to cut yourself some slack. In fact, monitoring your own “wellness” scores can catch overload early – subjective self-report measures often flag fatigue quicker than many high-tech metrics.

   
   

2. Measure Resting Heart Rate or HRV (But Keep It Simple): Consider tracking your resting heart rate or heart rate variability each morning. There are smartphone apps and wearables that can do this in a couple of minutes while you’re still in bed. Look for trends, not single days. A normal fluctuation is fine, but if you see, say, a consistent 5+ beat increase in your morning pulse for several days or your HRV is dropping below your typical range, it might mean accumulating fatigue or stress. One case study with a recreational runner confirmed that using HRV alongside perceived effort is practical and insightful – together they reflected training adaptations and flagged excessive training load. The key is consistency: measure in the same conditions (morning, before caffeine!) and track over weeks.

   
   

3. Watch the Heart-Effort Relationship: Pay attention to how your heart rate at a given pace compares to normal. If you’re having to run slower or your heart is pounding harder than usual to hit your normal pace, that’s a sign of fatigue. Sports scientists call this an uncoupling of internal vs. external loadgssiweb.org. For example, if you usually run 8:00 minutes/mile at 150 bpm but today that pace has you at 160 bpm and feeling awful, your body might be under-recovered. The study’s HR-running power index is essentially that concept – when it climbed, it signaled a runner was on the way to overreaching. On the flip side, if you see your heart rate lower for a given effort over time and you feel good, that’s a positive adaptation.

   
   

4. Don’t Ignore Soreness and Sleep: The runners who got overreached reported notably sore legs and feeling less ready to train. Sure, some muscle soreness is normal after hard workouts, but if it’s not subsiding or it’s paired with poor sleep and mood, listen up. Build in rest days and lighter sessions when needed. It’s far better to take one easy week than be forced to take a month off because of burnout or injury. Remember, you get fitter not just by training hard, but by recovering well.

   
   

5. Use the “Two-Thirds” Rule: This is a nifty idea inspired by the study’s finding that combining measures gives the best prediction. If two out of three of your monitoring markers are showing fatigue (for instance: your mood is low, your morning HR is elevated, but your workout paces are still okay), consider that a red flag. Don’t wait for all systems to fail. Two out of three “bad” scores = time to chill, adjust your training, get some extra sleep, or prioritize recovery. Conversely, if two out of three are in the green, you’re probably fine to proceed.

   
   

What the Science Says About Heart Rate, HRV, and Overtraining Risk

The findings of this study line up with a growing body of research encouraging a blend of objective and subjective monitoring:

• A 2016 review in Sports Medicine also found that changes in heart rate measures alone are often too small to clearly signal overtraining – they easily get lost in the noise of daily variation. The authors recommended using heart rate or HRV alongside how the athlete feels to truly make sense of it. In short: your watch can tell you something’s up, but your own perception confirms what it is.

• Other studies have consistently shown self-reported wellness questionnaires (mood, stress, fatigue ratings) tend to react quickly to training load changes and can predict performance dips or illness risk. These simple check-ins often outperform high-tech metrics in sensitivity. This is great news for self-coached runners – it means a low-cost, low-tech tool (your own brain and a notebook) is actually one of the best monitoring devices you have!

• Research on heart rate variability in endurance athletes finds it useful, but it’s most powerful when combined with training logs. For example, a 2021 case report on a recreational runner found that tracking HRV trends together with training volume and perceived effort helped identify fatigue and optimize performance. Her HRV dropped sharply right before she got sick, serving as an early warning. The science is increasingly supportive of a holistic approach: mix subjective and objective info for a 360° view of your fatigue and recovery status.

Finally, sports scientists like to talk about “internal load” vs “external load.” External is the work you do (miles, pace, etc.), and internal is how your body responds (heart rate, RPE, etc.). When these two fall out of sync – for instance, you keep piling on miles (external) but your heart rate, sleep, and mood (internal) are trending in the wrong direction – it’s a sign you might be heading toward trouble. The consensus across studies is that a bit of smart monitoring can catch that mismatch early.

 The Takeaway: Train Smarter, Recover Better, and Avoid Overtraining

Listen to both your heart and your head. For the self-coached runner, this research is an empowering reminder that you don’t need an entire lab to monitor your training. A heart rate monitor (or even just a finger on your pulse) and honest self-reflection can work wonders. Pay attention to the signals your body gives – whether it’s an elevated heartbeat, dead-tired legs, or just a gut feeling that you’re dragging – and adjust accordingly.

Remember, the goal of monitoring isn’t to make you paranoid about every data point, but to help you train smarter and happier. Think of it as being your own coach: some days you crack the whip, other days you play the compassionate coach who says “go easy, you need a rest.” The science says that approach leads to better results and fewer breakdowns.

So the next time you’re debating whether to push through a hard workout or to back off, check in with your heart rate and your personal “how-do-I-feel-o-meter.” If both are screaming fatigue, don’t be a hero – be smart and recover. Your future self (and your race times) will thank you. As this study showed, the best results come when we train hard and recover hard – using a mix of objective and subjective cues to find that sweet spot.

Happy (and healthy) running, everyone!

Track Smarter with COROS: The Right Tools Make a Difference

When it comes to monitoring your fatigue and recovery, having a solid wearable makes life a whole lot easier. I’ve been using COROS for years, and I’m excited to officially partner with them, because their watches do exactly what we runners need them to do: track the data that actually matters without drowning you in fluff.

If you're looking to keep tabs on how your body is handling training, here’s how a COROS watch can help:

✅ Resting Heart Rate – Track your baseline and spot trends over time

✅ HRV (Heart Rate Variability) – COROS EvoLab includes HRV tracking to help you monitor recovery

✅ Sleep Tracking – Poor sleep is often a sign you’re not recovering well; COROS tracks sleep quality and duration

✅ Effort Scores & Fatigue Trends – See how hard your workouts hit you and whether your recovery is keeping up

✅ Training Load & Readiness – EvoLab shows whether your current training is building fitness or tipping into overload

✅ Run Data You’ll Actually Use – Pace, HR, elevation, cadence, and more, all cleanly displayed and easy to interpret

💥 Bonus for You: Use this link and promo code RUNEXP at checkout to get a free extra watch band with your purchase. 🎉

Whether you’re new to heart rate monitoring or ready to get more dialed in, a COROS watch can help you connect the dots between how you feel and what your training is doing under the hood!

Sources

 The insights and data in this post are based on the following sources:

1. Bosquet L, Merkari S, Arvisais D, Aubert AE. Is heart rate a convenient tool to monitor over-reaching? A systematic review of the literature [published correction appears in Br J Sports Med. 2008 Dec;42(12):1016]. Br J Sports Med. 2008;42(9):709-714. doi:10.1136/bjsm.2007.042200

   
   

2. Boullosa D, Medeiros AR, Flatt AA, Esco MR, Nakamura FY, Foster C. Relationships between Workload, Heart Rate Variability, and Performance in a Recreational Endurance Runner. J Funct Morphol Kinesiol. 2021;6(1):30. Published 2021 Mar 22. doi:10.3390/jfmk6010030

   
   

3. Nuuttila OP, Uusitalo A, Kokkonen VP, Weerarathna N, Kyröläinen H. Monitoring fatigue state with heart rate-based and subjective methods during intensified training in recreational runners. Eur J Sport Sci. 2024;24(7):857-869. doi:10.1002/ejsc.12115

   
   

4. Saw AE, Main LC, Gastin PB. Monitoring the athlete training response: subjective self-reported measures trump commonly used objective measures: a systematic review. Br J Sports Med. 2016;50(5):281-291. doi:10.1136/bjsports-2015-094758

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