Meet Forge: the thinking behind your next workout
Over a year of research, design and listening to lifters. Here’s how Forge turns your goals, training history and real effort into a workout that fits you.
Forge is the workout algorithm inside JustGains. It helps answer two everyday questions: what should I train today, and what should I aim for on my next set? It brings together your goals, experience, equipment, available time and completed training history to make those decisions easier.
A year of work behind a simple suggestion
We’ve spent over a year designing Forge: studying training research, examining how other workout apps approach programming and progression, and listening to what lifters say when those tools fall short. That included reading complaints and discussions on Reddit, where the details of an ordinary gym session often reveal problems a feature list misses.
Weight jumps that feel impractical. Repetitive workouts. Equipment that doesn’t match the gym. Those frustrations helped shape our priorities: useful variety, respect for your setup, and suggestions you can understand and adjust. Our aim was to build the best science-based algorithm we could, with as much attention to the experience of using it as to the calculations underneath.
The work goes beyond choosing exercises. We’ve built rules for exercise selection, training volume, session timing, history matching, effort feedback and practical weight increments. Then we test awkward cases: a first workout, incomplete history, a long break, an unexpectedly hard set, or a weight you change yourself.
One system, from the plan to the next set
When you ask Forge to build a workout, it chooses exercises and a set structure around your settings, recent training and muscle usage. Auto Progression then works inside a started workout, suggesting weights and reps as you log. You can also use it in a workout you built yourself.
Start with you
Your goals, experience, equipment, time and history.
Build the workout
Forge chooses exercises and a set structure around that information.
Log what happened
The weight, reps and effort you actually completed.
Adapt the next target
Auto Progression updates unfinished sets. Your log informs future workouts.
These are different jobs. Building a session determines how much work is in the plan. Auto Progression helps you carry out that plan; it does not add sets or rewrite your completed results.
Your history gives Forge a starting point
Auto Progression looks at completed working sets of the same exercise. Each session contributes its strongest working-set evidence, so lighter, fatigued sets later in a workout do not make the next session start weaker. Weight, reps and any effort you reported help estimate a suitable next target. A planned set does not count as something you achieved, and doing more sets does not earn an automatic strength increase.
Recent sessions carry more influence than older ones. A session from six weeks ago has half the weight of equivalent new evidence; at twelve weeks, a quarter. Forge considers up to 24 eligible sessions from the last six months, depending on the history available on your device.
View chart data
| Weeks since a completed workout | 6 weeks: 50% influence · 12 weeks: 25% |
|---|---|
| 0 | 100.00 |
| 2 | 79.37 |
| 4 | 63.00 |
| 6 | 50.00 |
| 8 | 39.69 |
| 10 | 31.50 |
| 12 | 25.00 |
| 14 | 19.84 |
| 16 | 15.75 |
| 18 | 12.50 |
| 20 | 9.92 |
| 22 | 7.87 |
| 24 | 6.25 |
That fading line describes how much we rely on an old log, not how much strength you have lost. After a longer break, Forge also starts more cautiously. With no useful history, Auto Progression keeps your existing targets until a completed working set gives it something to work with.
Today gets a say, too
Ten reps with two more possible tells a different story from ten reps with nothing left. Choose whichever scale is easier for you: reps in reserve (RIR) counts clean reps left; RPE rates effort out of 10. 2 RIR = RPE 8, 1 RIR = RPE 9, and 0 RIR = RPE 10. The simulator and workout loggers use the same conversion. Switching scales keeps the meaning of your entries intact.
The RIR-based effort scale has been studied as a practical way to guide training load. It is useful feedback, even though people’s estimates are imperfect. Research on the effort scale and RIR accuracy.
As you complete sets, Forge adjusts the next unfinished target in that exercise. If a familiar weight turns out much harder than expected, the next suggestion can ease off immediately. It also accounts for the work you have already done, so a lighter later set can be a sensible adjustment.
Example: three recent sessions of 80 kg × 10 with two reps left. Target: 8–12 reps with two left, using 2.5 kg steps.
- Starting suggestion from history
- 80 kg × 10 reps
- Next suggestion after 80 kg × 8 with no reps left
- 65 kg × 10 reps
Bar length shows suggested load from zero. Calculated by Forge’s progression engine. Change the inputs in the simulator →
Leave effort blank if you’re unsure. Forge can repeat demonstrated work and account for sets already done, without inventing spare reps or rewarding missing feedback with a strength increase. Very easy effort reports are also treated conservatively. Your effort target is never silently recorded as how the set actually felt.
Progress does not mean maxing out every set
Our default is to aim for about two reps in reserve unless your workout specifies another effort target. In one eight-week study of trained lifters, stopping with one or two reps left produced similar quadriceps growth to reaching failure, with less acute fatigue. Refalo and colleagues, 2024.
View chart data
| Completed sets of 10 reps | No reps left (RPE 10) | Two reps left (RPE 8) |
|---|---|---|
| 0 | 0.00 | 0.00 |
| 1 | 9.48 | 4.93 |
| 2 | 14.18 | 7.38 |
| 3 | 17.96 | 9.34 |
| 4 | 21.23 | 11.04 |
| 5 | 24.18 | 12.57 |
The graph shows Forge’s fatigue estimate, not measurements from that study. Research informs our approach; the exact curves and adjustments are our design choices. The complete algorithm has not been clinically validated, and it cannot measure pain, technique or how ready you feel.
Progress can mean another rep at the same weight before a heavier weight becomes practical. Forge stays inside your workout’s rep range and rounds suggested loads down to the equipment step you choose. Recent performance with reported effort can allow a small increase, but reopening a workout never counts as new progress.
Turn it on, stay in control
- Open a started workout’s options menu (⋮). Under Set suggestions, switch Last Sets to Auto Progression. Choose a default for new workouts in Workout Settings. These preferences are saved for your account on this device; a choice made for an individual workout takes priority.
- Choose your load step in kg or lb. Use the change in the weight you log: total weight for a total-load exercise, or per-side weight for an exercise logged per side.
- Choose RIR or RPE for effort. Tap a set number to report how it felt: two more clean reps is 2 RIR / RPE 8. In the mobile app you can also hold the completion circle, slide to an effort value, and release.
- Review the suggestion and train. Enter a different weight or rep count whenever you need to. Completing a set accepts the numbers shown; reporting effort helps refine the next target.
Switching back to Last Sets leaves your entries and completed sets intact. Auto Progression currently adapts ordinary weighted rep sets, including failure sets. Warmups, drop sets, myoreps, timed work and assisted or bodyweight exercises keep their existing prescriptions.
See the thinking for yourself
Open the Forge simulator, choose an example, and change one thing. Make the last session older. Leave effort blank. Log a tougher set. Watch the next suggestion change using the same engine as your workouts, without changing your real history.
Run progression checks lets you try the built-in behavior tests with one click. Behind those checks, our automated tests cover manual overrides, completed sets, duplicate history, equipment steps and many combinations of training inputs. We want Forge’s decisions to be understandable, testable and useful when you’re standing in front of the rack.
FAQ
Keep learning
RPE vs RIR
RPE and RIR describe the same thing from opposite sides: effort against reps left.
What is progressive overload?
Progressive overload means asking a little more of your body over time.
Sets, reps, and training volume
Training volume is how much work you actually do, usually counted in hard sets.
What is an estimated 1RM?
An estimated 1RM predicts your max from a completed set without requiring a true max attempt.