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What kind of game is it?
Muscle Diary is a simulation game where you plan a pixel-art character's training week, food, and sleep, then see how strong they can get in 12 weeks (84 days). First you set your character's sex, height, weight, age, body fat, lifting experience, and job, plus how many reps you can do at a given weight on the bench press, squat, and deadlift. Then you choose a goal.
For each day of the week, you pick a muscle group (Chest, Legs, Back, Full body, or Rest) and an intensity (Light, Normal, or To failure). Press "Start training," and the days advance automatically, one at a time, as your lean mass, body fat, and one-rep max (1RM) on all three lifts change. After 12 weeks, you'll see your character at the start and after 12 weeks side by side with your results, which you can share on X or through your phone's share menu.
The English version uses pounds (lb) and feet and inches, and personal records count in 5 lb steps.
Features
Research-based formulas behind the gains
Your one-rep max (1RM) is estimated from the weight and reps you enter using the Epley formula. Muscle growth starts from a monthly limit for your experience level (1%, 0.75%, or 0.5% of body weight per month), then factors in that day's stimulus, protein, sleep, how much you eat, and how much room you have left before your natural limit (based on the fat-free mass index, or FFMI). Calories burned are calculated the same way as in Diet Diary, with the Mifflin-St Jeor equation and METs, and any calories not used to build muscle go to body fat.
For example, if someone brand new to lifting trains each muscle group twice a week, eats a little extra, and changes up their plan every 4 weeks, they gain about 4 lb (1.8 kg) of muscle on average in 12 weeks, and their bench press 1RM goes up about 20%. The formulas and sources are explained in detail on the game's "How the Math Works" page.
Rest is part of training
Each muscle group you train builds up fatigue (0–100), which recovers with sleep each night. Seven hours of sleep recovers 30 a night, but five hours only 18. Train a muscle that's still tired and you get less stimulus (half at fatigue 50). Train "To failure" and push that muscle's fatigue over 80, and there's an 8% chance of injury, which keeps you from training it for 7 days. You get to experience a basic rule of lifting: building in rest days gets you further than going hard every day.
Food and protein
For how much you eat, choose Cut (−500 kcal), Maintain, or Bulk (+300 kcal). Cutting burns body fat but makes it harder to build muscle. Protein helps up to 0.73 g per lb of body weight, but adding a shake to reach 1 g per lb doesn't make muscle grow any faster. The right plan depends on whether you want to lose fat or build muscle.
Days that don't go to plan
- Keep the same plan for 4 weeks and you hit a plateau: stimulus drops to 0.8 times. Change your plan and it goes back to normal
- When motivation drops, you skip the gym (a 5% chance below 60 and 40% below 30, even higher on weekends and rainy days)
- A night out with friends adds 1,200 kcal and cuts that night's sleep by 1 hour (more often on Fridays)
- On weekday evenings the gym is sometimes packed, and you can't get a good workout in
- Catch a cold and you can't train for 3 days
- Set a new personal record and your motivation goes up
An example week
Here's one possible week, starting with the default character: a man in his 30s, about 5'9" and 200 lb with 28% body fat, brand new to lifting, with an on-the-go job, who can bench 105 lb for 5 reps (a 1RM of about 122.5 lb). He uses the default plan (Mon: Chest, Tue: Legs, Thu: Back, Sat: Full body, other days Rest; Normal intensity, Maintain, Enough protein, and 7 hours of sleep).
| Day | What happens |
|---|---|
| Monday | Chest day. At first your body is still learning how to produce force, so the bench press climbs quickly |
| Tuesday | Leg day. Fatigue in the chest recovers with sleep |
| Wednesday | Rest day. Motivation recovers a little |
| Thursday | Back day. The deadlift goes up |
| Friday | A planned rest day turns into a night out with friends: extra calories and a late night |
| Weekend | Full body on Saturday, rest on Sunday. The bench press hits a new personal record at the next 5 lb step, and motivation goes up |
Events happen at random, so the same plan gives different results each time you play.
Tips
- Aim for a "weekly stimulus" of about 6 under the plan for efficient gains (training chest, legs, and back twice a week each at Normal comes out to 6)
- Training the same muscle group "To failure" two days in a row makes injury more likely. Put a rest day or a different muscle group in between
- Get at least 7 hours of sleep. With 5 hours, fatigue is slow to clear and muscle grows only 60% as fast
- If you hit a plateau, try changing which muscle groups you train on which days
For full instructions, see "How to Play" on the game's site.
FAQ
- Does it cost anything?
- No, it's free. You don't need an account or an app.
- Can I play if I don't know how much I can lift?
- Yes. An estimate based on your weight and lifting experience is already filled in, so you can start right away.
- Are the weight and lifts I enter sent anywhere?
- No. Your inputs and game progress are stored only in your browser.
- Can I stop and continue later?
- Yes. Your progress is saved automatically, and you can press "Continue" next time to pick up where you left off.
- Will I get as strong as the game says?
- The numbers in the game are estimates based on research and widely used guidelines. Real results vary with your body and lifestyle. This game is not medical advice or individual training guidance, so talk to a doctor, certified trainer, or other professional about real training and nutrition.