Strength Trainer and how WHOOP measures muscular load in lifting

Podcast episode originally published on April 27, 2023

Strength training data is hard to measure when heart rate alone misses much of the work. In Episode 219 of the WHOOP Podcast, Chief Technology Officer at WHOOP Jaime Waydo and host Will Ahmed explain how Strength Trainer uses exercise logging plus sensor data to quantify muscular load, combine it with cardiovascular load, and reflect that work in Strain, Sleep Need, and Recovery. This article answers seven practical questions: what problem Strength Trainer solves, how the load model works, why the PUSH Technologies acquisition mattered, how the feature was tested, what you see in the WHOOP app, how your scores change, and where the roadmap goes next.

Note: This article covers WHOOP 4.0. For the latest hardware, see current WHOOP hardware.

For a full walkthrough from Waydo and Ahmed, watch Episode 219 of the WHOOP Podcast on YouTube.

What problem does Strength Trainer solve for people who lift weights?

Strength Trainer solves an undercounting problem. Traditional heart-rate driven tracking can capture cardiovascular effort well, but many lifting sessions create a large amount of stress in muscles, joints, bones, and connective tissue without keeping heart rate elevated the entire time.

That was the starting point for Waydo and Ahmed. They described a common frustration for people who lift: a session can feel brutally hard, yet the data you see afterward can look modest because the signal came mostly from heart rate. Strength Trainer was built so the work from barbell lifts, dumbbells, kettlebells, machine work, and bodyweight movements can show up in the training picture more clearly.

In practical terms, that means WHOOP is trying to capture two different kinds of demand in the same workout. Cardiovascular load reflects how hard your heart and circulation worked. Muscular load reflects the stress placed on the musculoskeletal system during resistance exercise. If you do squats, presses, pull-ups, sled pushes, or a bodyweight circuit, that second category matters a lot.

Waydo framed the feature around that missing credit. In the surrounding discussion, she made clear that the goal was not a separate lifting log sitting off to the side. The goal was to connect lifting stress to the same outcomes people already watch on WHOOP, especially Strain, Sleep Need, and next-day Recovery.

As Waydo put it:

"Strength Trainer really allows you to track your weightlifting workouts on WHOOP and get credit for the load that is happening to your muscles, your joints, your skeleton, and really think about how that ties into the overall picture of your strain and then what that does to your sleep and your recovery."

What you should take away

• Strength Trainer was built for workouts where heart rate alone misses part of the training load
• Muscular load is meant to reflect stress on muscles, joints, bones, and related tissues during resistance exercise
• The feature ties lifting stress back to the same WHOOP outcomes people already use, including Strain, Sleep Need, and Recovery

How does WHOOP calculate muscular load during a strength workout?

WHOOP calculates muscular load by combining volume and intensity. The model uses the exercise you logged, the weight involved, your body weight when it applies, and movement data from the sensor to estimate how much load your musculoskeletal system handled.

Waydo broke the logic into two parts. First, WHOOP looks at how much volume you lifted. That includes external weight, but it can also include your body weight for movements such as pull-ups, push-ups, planks, or other bodyweight exercises. Second, WHOOP looks at how hard the movement was performed. In the episode, she pointed to movement speed, the size of the motion, and the effort required to move the load.

That approach lines up with what WHOOP later described in the research and development behind Strength Trainer. The internal research team said the metric had to do two things: use the inertial sensors already in the band, specifically the accelerometer and gyroscope, and stay grounded in biomechanics. The result was a model that goes beyond simple bar weight. A squat and a curl can use the same external weight and still place very different demands on the body because they involve different movement patterns, different effective mass, and different muscle groups.

Ahmed gave a simple example in the episode. If you are doing a bicep curl, WHOOP can track the motion pattern, combine it with the weight in your hand, and use the speed of that repetition to estimate strain on the body. A full-body squat produces a different result because more tissue is involved and more total mass is moving.

Waydo summarized the calculation this way:

"It's looking at both how much volume did you lift. So what's the weight. And it's not just the weight of the weights that you were lifting. It's also your body weight if you're in a bodyweight exercise. So that's how much volume you're lifting. And then it's how hard are you working in that exercise. And so how fast did you move it. How big of a motion did you go through."

What you should take away

• WHOOP estimates muscular load from both volume and intensity, rather than from weight alone
• Body weight can count toward load when the movement makes body weight part of the work
• Accelerometer and gyroscope data help WHOOP distinguish between different exercises, speeds, and movement sizes

For Waydo's full explanation of how weight, body weight, movement speed, and motion size feed muscular load, watch the full episode on YouTube.

How did WHOOP add Strength Trainer to existing WHOOP 4.0 hardware?

WHOOP added Strength Trainer through software and algorithm work, not a new band. Ahmed said the feature grew out of the acquisition of PUSH Technologies and the work of bringing PUSH motion and velocity expertise onto existing WHOOP 4.0 hardware.

That matters because a new capability often brings an assumption that new hardware will be required. Ahmed said the team wanted the opposite outcome. Once WHOOP acquired PUSH Technologies, the task became taking the outside algorithms and fitting them onto the accelerometers and gyroscopes already inside WHOOP 4.0. In other words, the sensor people were already wearing became the platform for the new lifting feature.

Waydo added an important technical point. Company integration is hard, especially when hardware, software, algorithms, and product teams all need to work together. She said the PUSH Technologies team became part of the broader engineering effort rather than staying isolated as a bolt-on product. That made Strength Trainer feel like part of the WHOOP app instead of a separate tool.

Ahmed described the handoff from acquisition to feature this way:

"A lot of it was taking their hardware and the algorithms that sat on top of their hardware and finding ways to integrate that directly into your existing WHOOP 4.0 sensor."

The result was an over-the-air update path. Members could download the latest WHOOP app, receive updated software, and access the feature without changing bands. That is a useful case study in how WHOOP can add new measurement layers when the signal is already available in the sensor and the limiting factor is model development.

What you should take away

• Strength Trainer was built to work on existing WHOOP 4.0 hardware through software and algorithm updates
• PUSH Technologies supplied motion and velocity expertise that helped WHOOP expand into muscular load
• The feature depends on accelerometer and gyroscope signals that were already present in the band

How was Strength Trainer tested and validated before launch?

Strength Trainer went through a long testing cycle before launch. Waydo said every exercise had to be built, tested, and validated, and the process involved more than a year of work in WHOOP Labs with both internal testers and external participants.

The scale of that work matters because Strength Trainer is not a single movement detector. In the episode, Ahmed said the app included more than 500 exercises at launch, and Waydo explained that every one of those exercises required repeated data collection. The team brought in people with different lifting backgrounds and different movement styles, then had them repeat the same exercises over and over so the algorithms could learn what variation looked like in the real world.

WHOOP later gave more detail in the same research and development behind Strength Trainer article. According to that piece, testing included thousands of hours in WHOOP Labs, a mix of men and women, a range of skill levels, and movement patterns that covered major barbell lifts, accessory work, and bodyweight training. WHOOP said the initial validations alone included more than 10,000 repetitions.

Waydo also described diversity inside the validation process. Some participants were new to lifting, some were experienced, some were younger, and some were older. That matters for a product that has to recognize the same exercise when it is performed with different tempos, range of motion, and levels of technical polish.

In Waydo's words:

"Every single one of those has to be built, has to be tested, has to be validated that the algorithms are working correctly for that exercise."

What you should take away

• Strength Trainer was tested in WHOOP Labs for more than a year before launch
• The launch library included more than 500 exercises, and each one required repeated validation work
• WHOOP said internal validation collected more than 10,000 repetitions across major movement patterns

If you want the full discussion of how WHOOP Labs handled testing, exercise diversity, and repeated movement data, watch the full episode on YouTube.

What happens in the WHOOP app during a Strength Trainer workout?

In the WHOOP app, Strength Trainer gives you two entry points. You can start with a prebuilt workout, or you can build your own routine from the exercise library and follow it live as you train.

Waydo said the prebuilt side includes workouts from named athletes such as Michael Phelps, Patrick Mahomes, and Sloane Stephens, along with a broader library labeled by difficulty level. That gives people a quick starting point when they want to train but do not want to build a session from scratch. The app also shows required equipment before you start, which is especially useful when you are in a hotel gym or a small apartment setup.

The custom side is where the feature becomes a training log. Ahmed said you can choose from roughly 500 exercises, set reps and weights, arrange lifts in order, and create supersets. During the workout, the app guides you set by set, tracks active and rest time, and lets you adjust weight or reps as needed. Afterward, the workout is saved with the details attached.

That saved history is one of the most useful parts of the feature. Ahmed said he had spent years relying on memory or notes from a trainer. With Strength Trainer, weights, reps, sets, and timing stay inside the WHOOP app, which makes it easier to repeat the same session later or compare one version of a lift to another.

Ahmed described the in-workout flow this way:

"You'll be able to put in some 500 different exercises that you might do. You'll input the number of reps, the amount of weight. You'll do the workout in real time. It'll guide you through the workout as you're doing it."

If you want help thinking about programming once the tracking piece is in place, Episode 217 on building a strength training program with Dr. Andy Galpin is a useful next read.

What you should take away

• Strength Trainer lets you choose between prebuilt workouts and fully custom routines
• The workout flow includes exercise selection, reps, weight, timers, rest periods, and saved history
• Saved workouts make it easier to repeat sessions, compare lifts over time, and keep a clean record inside the WHOOP app

How does Strength Trainer change Strain, Sleep Need, and Recovery?

Strength Trainer changes Strain by adding muscular load to the score. Sessions that once looked lighter because they relied mostly on heart-rate data can now produce a higher Activity Strain and a higher Day Strain when the lifting load was large.

Ahmed and Waydo both described this as the main payoff of the feature. After the workout ends, WHOOP shows more than a single score. You can see how much of the workout load came from the cardiovascular side and how much came from the muscular side. Ahmed said that heavy lifts with lower heart rates often show a larger muscular share, while more continuous efforts can tilt the other way.

The post-workout details also add metrics that matter to lifters. Ahmed highlighted total tonnage, while both speakers discussed intensity as a metric tied to movement pace, effort, and how much of the body is involved in a given exercise. Waydo said the app can also show the heart-rate graph alongside individual lifts or supersets so you can see which parts of the session spiked heart rate and which parts allowed it to settle.

The next-day implications are part of the value. Ahmed said Sleep Need can rise after a lifting session because WHOOP now credits the extra work done by the musculoskeletal system. He also said the next Recovery reading will be interpreted in light of the full workload from the prior day, rather than a partial workload driven mainly by heart rate.

Ahmed put that directly:

"Your sleep need will be a little higher, so you'll notice that in the WHOOP app. WHOOP might be a little more forgiving on why your recovery is lower the next day, because it'll now absorb, of course, the muscular strain that you've put on your body."

For a deeper look at how lifting stress and rest fit together, Episode 187 on strength training, conditioning, and rest adds useful context.

What you should take away

• Muscular load can raise Activity Strain and Day Strain when a workout stressed the body more than heart rate alone suggested
• Post-workout details can show the split between cardiovascular load and muscular load
• Heavy lifting can increase Sleep Need and change how the next Recovery result is interpreted

For Ahmed and Waydo's explanation of how muscular load changes Strain, Sleep Need, and next-day Recovery, watch the full episode on YouTube.

What is next for Strength Trainer on WHOOP?

The launch described in Episode 219 was the base layer for a larger roadmap. Waydo said the next steps included trend tracking, equipment-aware exercise search, better workout comparisons over time, and tighter sharing loops between coaches, trainers, and athletes.

That roadmap makes sense because Strength Trainer already captures structured workout data. Once sets, reps, weight, timing, and movement signals are stored in one place, the next useful question is how those sessions compare across weeks and months. Waydo specifically pointed to trends as an early next step, including the ability to compare today's session with a similar workout from a month earlier.

Ahmed added another practical direction: making it easier to credit a workout after the fact. The launch version focused on a live training flow, where you start the workout and move through sets in real time. He said WHOOP wanted to reach a point where you could add a session later, identify the time window, and still get credit for the work performed.

Search and sharing were two more clear roadmap items. Waydo described a future state where you could search the exercise library by the equipment available in front of you, and Ahmed described coach-to-athlete or friend-to-friend workflows where a workout could be sent through the app and then returned with results attached.

Waydo summed up one near-term piece of that roadmap like this:

"One of the next things that's coming is trends. Obviously, you want to know how this workout compares to one that was similar that you did a month ago."

The original Strength Trainer launch announcement shows what shipped first, but the podcast makes clear that the longer plan was always broader than version one.

What you should take away

• The first release of Strength Trainer was designed as a starting point, not a finished endpoint
• Trend views, equipment-based search, and coach-to-athlete workout sharing were already on the roadmap
• After-the-fact workout logging was another planned way to make muscular load easier to capture

The bottom line

• Strength Trainer was built to measure the part of lifting stress that heart rate alone can miss
• WHOOP calculates muscular load from both volume and intensity, using logged exercises plus motion data from the accelerometer and gyroscope
• Bodyweight movements can count toward muscular load because WHOOP includes body weight when the exercise makes it part of the work
• Strength Trainer was designed to run on WHOOP 4.0 through software and algorithm updates rather than requiring new hardware
• WHOOP said internal testing covered more than 500 exercises and more than 10,000 repetitions during early validation work
• The WHOOP app lets you choose prebuilt workouts, build custom routines, track sets and supersets live, and save a full archive of reps and weights
• Muscular load can raise Activity Strain, increase Sleep Need, and change how the next Recovery result is interpreted after a lifting session
• Trend tracking, equipment-aware search, workout sharing, and after-the-fact logging were all part of the future roadmap described in the episode

Frequently asked questions about things discussed in this episode

How does WHOOP measure muscular load in Strength Trainer?

WHOOP measures muscular load by combining the exercise you log with weight, body weight when it applies, and movement data from the accelerometer and gyroscope to estimate how much work your musculoskeletal system handled.

What does WHOOP do for bodyweight exercises in Strength Trainer?

WHOOP can include body weight in the load calculation when the movement makes body weight part of the work, such as in pull-ups, push-ups, planks, and similar exercises.

How does WHOOP combine muscular load and cardiovascular load?

WHOOP combines muscular load and cardiovascular load inside Strain so a lifting workout can reflect both the heart-rate response and the stress placed on muscles, joints, and bones.

What does WHOOP show after a Strength Trainer workout?

WHOOP shows workout details that can include Activity Strain, the split between muscular and cardiovascular load, heart-rate response by exercise, total tonnage, and the saved record of reps, sets, and weight.

What does WHOOP do for Sleep Need after heavy lifting?

WHOOP can raise Sleep Need after a demanding Strength Trainer workout because the app now accounts for muscular work that older heart-rate only views could miss.

What does WHOOP require to use Strength Trainer?

WHOOP requires the latest WHOOP app and a supported band so the feature can use the updated software and the motion sensors already built into the hardware.

What does WHOOP do if you want to repeat a workout later?

WHOOP saves Strength Trainer sessions in the workout history so you can reopen an earlier routine, review the previous weights and reps, and run that session again.

For people who spend real time under the bar, Strength Trainer gives WHOOP a way to connect the reps you logged with the Strain, Sleep Need, and Recovery that follow.