Coaching Field Notes

Load Isn't Fatigue

By Kyle Ruth 8 min read
Kyle Ruth seated between events at the CrossFit Games, checking his watch
Photo: @Ashley.Marie.Visuals

On this week's Ruth Performance Collective call, we started a new four-week theme: fatigue. The major question for the first call was how do we track training load and readiness for an athlete in a mixed modal sport? The problem is that most of the popular systems for monitoring fatigue just don't really have the flexibility to track well for mixed modal sport.

For decades, strength and conditioning coaches have been tracking volume-load:

Volume Load = Sets x Reps x Load

This is a fantastic measurement for monitoring strength training load and nearly all of the strength systems we use today were influenced by it. However, measuring volume-load only tracks one contributor to fatigue: external load, and it doesn't capture endurance training stress at all.

Endurance researchers in the early 1990’s began creating their own separate models for tracking endurance training load, starting with Training Impulse (TRIMP), then in the early 2000’s Training Stress Score (TSS). Both systems use training duration and intensity as their base, but the measurement of intensity differs slightly. TRIMP uses the athlete's heart rate reserve during the session multiplied by duration to give you a picture of internal load. TSS uses intensity relative to the athlete’s functional threshold power (FTP) multiplied by duration as the measurement of external load.

Each of these systems does a great job of measuring what it is intended to measure, but what happens when an athlete is training for a mixed modal sport that combines strength training, endurance training, and metcons?

External load vs internal load

External load is what we as the coach actually wrote down in the athlete’s training program: sets, reps, kilograms, pounds, meters, intensity. Internal load is what it actually costs the athlete: what heart rate they hit at that intensity, and what their session RPE was when they completed the training.

As coaches, we often assume that an easy session is easy, a moderate session is moderate, and an intense session is intense. It’s not uncommon for coaches to lay out a training week with the intention to alternate between high training stress and low training stress sessions.

The Plan vs What the Athlete FeltAn illustrative week. Bars show the planned session RPE: high (8) on Monday, Wednesday and Friday, low (3) on Tuesday, Thursday and Saturday, rest on Sunday. A line shows the RPE the athlete reported: 8, 7, 9, 4, 8, 3. Tuesday's easy zone-2 run was planned at RPE 3 and reported at RPE 7.The Plan vs What the Athlete FeltAn alternating high / low weekPlanned (bars)Reported session RPE0510RPEMONTUEWEDTHUFRISATRestSUNEasy zone-2 runPlanned RPE 3, reported RPE 7Illustrative example. Session RPE: 0 to 10, rated after the session.

However, just because that's what the prescription instructs the athlete to do doesn't mean that's how the athlete actually absorbed that training load. As coaches, we’ve all seen this: we prescribe an “easy zone-2 run”, and the athlete reports legs feeling heavy and high RPE throughout the entire session. The external load (60min @ zone-2 intensity) and internal load (RPE 7 x 60min) don’t match.

What wearables are actually measuring

Garmin and Whoop both lean heavily on heart rate to estimate training load. Garmin’s training load is built on EPOC (excess post-exercise oxygen consumption), which is an estimate of how much energy it takes your body to recover after a session. Whoop’s Strain is built on cardiovascular strain from heart rate. They’re different models, but heart rate is still the main input. On the call I lumped these together as variations of TRIMP because they’re measurements of internal load that are mostly based on heart rate.

From my perspective wearables provide a better measurement of training stress than using exclusively external training load because they tell us how the athlete actually responded to the session. However, one internal response doesn’t tell us the whole cost of the training, especially if training wasn’t limited by heart rate.

Heart rate can absolutely get high in a strength session. The problem is that it doesn’t consistently reflect how demanding the work is on the muscles doing it. An athlete might do an entire metcon where they’re failing handstand push-ups and heavy cleans and their heart rate’s 155 BPM. What’s stopping them in the moment is local muscular fatigue, and that’s the part the score doesn’t fully capture.

I’m sure all of you coach clients with a Garmin or a Whoop who’ve told you “my Whoop says it was a low intensity session” after they went to maxes and were exhausted afterward. A low cardiovascular load doesn’t mean the session was easy.

That’s why wearables are so good with endurance and so bad at tracking mixed modal training: they capture the cardiovascular cost really well and underrepresent heavy lifting and local muscular fatigue. We need something that lets the athlete tell us how hard the entire session felt.

What Each Load Tool SeesFour ways to count training load compared on a heavy strength day and an endurance day. Volume load sees strength but misses endurance. Training stress score (intensity relative to FTP times duration) and TRIMP (heart rate reserve times duration) see endurance but miss a heavy strength day. Session RPE (RPE times duration) sees both.What Each Load Tool SeesSame week, four ways to count itHEAVY LIFTDAYENDURANCEDAYVolume loadVolume load = sets x reps x loadSees itMisses itTraining stress scoreTSS = intensity relative to FTP x durationMisses itSees itTraining impulseTRIMP = heart rate reserve x durationBarely movesSees itSession RPEsRPE = RPE x durationSees itSees itHeavy lift day: RPE 8 x 70 min = 560 sRPE

Session RPE x duration

The solution is to use something like Session RPE (sRPE) to monitor training stress across modalities.

Session RPE = RPE x Session Duration (minutes)

The athlete rates the entire session on a 0 to 10 scale (or 0-5) within about 30 minutes of finishing; if it goes much longer, their ability to accurately recount the session drops a little bit. Then you literally multiply minutes by RPE.

In the example week I built for the call, a hybrid athlete had a huge heavy lifting session on Thursday: a volume load of 12,825 pounds. Their endurance training load was zero, and their heart rate load barely moved because it was true low-rep strength work. But it was a 70-minute session and they rated it an 8, so session RPE scored it 560, the biggest day of their week. It captures strength training, it captures metcons, and it captures endurance training.

One Week, Four Load ScoresAn example training week scored four ways, each row scaled to its own biggest day. Thursday is a 70-minute heavy lifting session. Volume load makes it the biggest day of the week at 12,825 pounds. Training stress score gives it 0. Training impulse gives it 48, one of the lowest days. Session RPE gives it 560, the biggest day of the week.One Week, Four Load ScoresThursday = 70 min of heavy lifting at RPE 8Volume loadsets x reps x load12,825 lbTraining stress scoreintensity vs FTP x duration0Training impulseHR reserve x duration48Session RPERPE x duration560MONTUEWEDTHUFRISATSUNExample week from the call. Each row is scaled to its own biggest day.

The research backs this up in our sport. Tibana and colleagues (2018) had athletes do Fran and Fight Gone Bad and found session RPE tracked heart rate load closely in both. Falk Neto and colleagues (2020) had athletes do the same functional fitness workout twice, once all-out max effort and once at RPE 6. Heart rate load found no significant difference between the two sessions, while session RPE load was more than double for the all-out max effort.

Heart rate load is still useful for steady aerobic work, so if your athletes already wear one, keep it as a second measurement tool (I still use my Garmin and track HR for session intensity). Session RPE is the one number that works for every session they do.

From daily scores to readiness

One session score tells you about one day, but fatigue doesn’t automatically reset overnight. It stacks up across the week, so readiness comes from comparing this week to what the athlete has been doing. As a coaching community, we talk about progressive overload and smart jumps all the time, but we don’t really have a good way to measure them unless we’re measuring training load. That takes two numbers.

Acute workload is this week’s total. Add up session RPE x duration for every session: five 60-minute sessions at RPE 5 is 5 x 300 = 1,500. Chronic workload is the average weekly total from the previous four weeks (the current week isn’t counted). It tells you how much training load the athlete is prepared for, because it’s what their body has been absorbing for the last month.

Divide acute by chronic and you get one number that tells you how this week compares to what the athlete is prepared for. Picture two athletes with the same last four weeks: 1,400, 1,600, 1,500 and 1,500, a chronic workload of 1,500.

The first athlete has a 1,650 week. That’s a ratio of 1.1: a little more than they’re used to, which is what a smart jump looks like. A ratio of 1.0 means this week matched their usual week, and a ratio of 2.0 means their internal training load doubled.

The second athlete is the one you all know. They’ve been training around 1,500 a week, they decide “I’ve got to be ready for a comp,” and this week comes in at 2,725, a ratio of 1.8, almost double what they’re prepared for. Then they’re exhausted and tweaked their bicep and “can’t figure out what happened.” We do know what happened: their training volume went up too fast.

The same thing happens when an athlete who’s been inconsistent all of a sudden commits, or when an athlete gets cleared after an injury and jumps straight back into full training. That’s where the overuse injuries and the tweaks show up. Spikes relative to chronic load are associated with injury (Gabbett 2016). I’m not claiming that regulating jumps in internal training load prevents all injuries, but as a coach it shows you the spike, and you make an educated call rather than coaching on “vibes.”

The flip side matters just as much, because high load on its own isn’t the problem. In the same research, athletes who were used to high training loads had fewer injuries than athletes on low loads. If you gradually build that second athlete up to 4,000 a week, keeping every jump small along the way, they’re less likely to get injured at 4,000 than they were jumping from 1,500 to 2,725. That’s why it’s worth tracking: it tells you how fast you can push the load up for that athlete.

This Week vs the Last FourTwo example athletes with the same last four weeks of training load: 1,400, 1,600, 1,500 and 1,500, a chronic workload of 1,500. In the first, this week is 1,650, a ratio of 1.1, a smart jump. In the second, this week is 2,725, a ratio of 1.8, a spike.This Week vs the Last FourAcute workload ÷ chronic workloadChronic = average of the last 4 weeks (1,500)A smart jumpWK 1WK 2WK 3WK 41,650THIS1,650 ÷ 1,500 = 1.1A spikeWK 1WK 2WK 3WK 42,725THIS2,725 ÷ 1,500 = 1.8Session RPE x duration, totaled per week. Example numbers.

Start this week

If you coach inside CoachRX, you can literally go into your settings and toggle session RPE on; when people finish a session, it pops up. If you coach with spreadsheets, building this in is as simple as a box for RPE and a box for session duration. In other software, add two exercises at the end of every session: RPE and duration.

Then every week, total it, compare it to the last four, and look at the days where what you planned and what the athlete reported don’t match. That easy zone-2 run that came back at RPE 7 tells you which athletes need a follow up conversation and maybe a reactive deload. Turn it on for one athlete today, and in four weeks you’ll have your first chronic load to compare against.

On the call, we also went one layer deeper: looking at the Ruth Ratio for tracking individual movement volume load against its chronic volume. This tells you which body segments are taking the biggest training load and guides how much volume you can prescribe per movement while minimizing injury risk. That part stays inside the Collective.

- Kyle