Jeremy Ethier’s latest muscle-building experiment has attracted more than 1.35 million views, and it is easy to see why.
Three people trained one muscle on one side of their body every day for 30 days. They trained the corresponding muscle on the other side twice a week. MRI scans taken before and after the experiment appeared to show more growth on every daily-trained side.
For one participant, the difference was enormous. His conventionally trained chest grew by approximately 5%, while the side trained every day grew by almost 20%.
That is the source of the “almost four times faster” claim.
It is a remarkable result. It is not, however, evidence that everyone should start training the same muscle seven days a week.
What Ethier’s experiment involved
The three participants each focused on a different body part:
- Brandon trained one side of his chest every day.
- Max trained one leg every day.
- Yolinda trained one arm every day.
The opposite side was trained conventionally, twice per week.
After 30 days, the reported MRI results were:
- Yolinda: approximately 3% growth on the daily-trained arm and 1% on the other arm.
- Max: approximately 2–3% growth on the daily-trained leg and little measurable growth on the other leg.
- Brandon: almost 20% growth on the daily-trained chest and approximately 5% on the other side.
The participants rested for three days before their final scans to reduce the possibility that temporary swelling would be mistaken for muscle growth. Muscle researcher Daniel Plotkin reportedly analysed the images without being told which side had received which training.
Those precautions make the results more interesting than a simple before-and-after photograph. They do not turn the exercise into a controlled scientific trial.
There were only three participants, there was no published or peer-reviewed protocol and there was no statistical analysis showing that the results exceeded normal measurement error or individual variation.
Most importantly, the experiment did not isolate training frequency.
The daily-trained muscles did far more work
The muscles trained every day completed approximately 35 hard sets per week. The conventionally trained muscles completed about 10.
That is three and a half times as much weekly volume.
The daily group was therefore not simply doing the same training spread over more days. It was doing considerably more total work. The experiment cannot tell us whether the additional growth came from daily frequency, higher volume, the combination of the two or the way individual participants responded.
This distinction matters because published research generally finds that training frequency has much less influence on muscle growth when weekly volume is held constant.
A 2019 systematic review and meta-analysis concluded that frequency does not meaningfully affect hypertrophy when training volume is equated. Frequency is better viewed as a way to organise enough productive sets without trying to cram them into one exhausting workout.
Volume appears to matter more. An earlier dose-response meta-analysis found that greater weekly resistance-training volume tended to produce greater hypertrophy, although adding sets eventually produces diminishing returns.
A much larger 2023 network meta-analysis examined 119 studies involving 3364 people in its hypertrophy analysis. Multiple-set programs performed well, but the differences between many training prescriptions were relatively small. Higher-load, multiple-set training performed twice per week was the highest-ranked prescription for hypertrophy, not daily training.
The most defensible interpretation of Ethier’s experiment is therefore not that rest days prevent muscle growth. It is that increasing weekly volume may accelerate growth, and higher frequency can make that volume easier to distribute.
A Sensible Four-Week Specialisation Block
- Choose one muscle: Prioritise one lagging body part, not your entire body.
- Add volume gradually: Move from about 10 weekly sets to 12–16 rather than jumping to 35.
- Spread the work: Perform two to four sets across three or four weekly sessions.
- Leave something available: Finish most sets with one to three good repetitions in reserve.
- Rotate two movements: Avoid loading the same joint in exactly the same way every session.
- Watch performance: Reduce the work if repetitions fall repeatedly or joint pain develops.
After four weeks: Return to your normal volume and assess whether the extra work produced a worthwhile result.
Why soreness eventually disappeared
The first week was reportedly uncomfortable. Participants experienced soreness, fatigue and, in Max’s case, knee pain that required him to reduce the load and temporarily stop one exercise.
The soreness then began to decline. This is consistent with the repeated-bout effect: after repeated exposure to a particular physical stress, the muscles generally experience less damage and soreness from subsequent sessions.
That does not mean joints and tendons adapt at the same speed.
A muscle might feel capable of working again while an irritated shoulder, elbow or knee is quietly becoming the limiting factor. This is especially relevant for men in their 40s, 50s and 60s who may have the muscular capacity to tolerate more work but less enthusiasm from an ageing rotator cuff or elbow tendon.
Soreness is also an imperfect guide. A lack of soreness does not prove that more training is productive, while mild soreness does not automatically mean the muscle cannot be trained.
Performance and joint comfort are more useful signals. If loads or repetitions fall across several sessions, technique deteriorates or a joint begins to hurt, the additional frequency is probably exceeding your current ability to recover.
The sensible way to use the idea
The practical lesson is not to leap from 10 weekly sets to 35.
Instead, use higher frequency for a short specialisation block. Pick one muscle you want to prioritise, add a small amount of work to workouts you are already performing and monitor how you respond.
If you currently perform approximately 10 challenging sets for that muscle each week, increase gradually to around 12–16 rather than immediately tripling the workload. Spread those sets across three or four sessions so each set remains productive.
Keep most sets one to three good repetitions short of failure. Rotate between two exercises that load the muscle differently rather than hammering the same joint through the same movement every day.
Run the experiment for four weeks, then assess your performance, measurements and joint comfort.
Daily training produced striking results for three people in a carefully arranged YouTube experiment. It did not prove that seven-day training builds muscle four times faster.
What it did demonstrate is more useful: a muscle may tolerate more frequent work than many lifters assume, and spreading additional volume across the week can make a specialisation phase more manageable.
The trick is increasing the growth stimulus without allowing your joints to become the experiment’s real result.
