How ArchiFit works: the next-set rules and the 8-week analysis

What a suggestion reads

A suggestion is one line above the weight and reps fields, such as "Next set: 67.5 kg × 6, heavier than the last set". In the app it is labeled "ArchiFit AI suggestion". The fields never fill from it: they start at the first set of your last workout of that exercise, and you type what you actually lift.

Words used below. The top weight of a workout is its heaviest weight. The sets at the top are the sets done at that weight. The top and bottom of the range are 8 and 6 for a 6–8 plan. Rest times, InBody numbers and the time of day are not read. For body-weight exercises only reps count.

Rule 1. One step heavier

When a rule goes up, the new weight is the old weight plus 5% of it, rounded to the exercise's weight step, and never less than one step. In numbers: next = weight + max(step, 5% of weight rounded to the step).

The American College of Sports Medicine recommends a 2–10% load increase, smaller for small-muscle exercises, once a lifter can do one or two reps over the target on two consecutive sessions (ACSM, 2009). ArchiFit's step sits inside that range. When it goes up is set by rules 2 to 4, and those triggers are ArchiFit's own.

Rule 2. The next set within today's workout

After each set, ArchiFit looks only at the set you just did.

• Past the top of the range: one step heavier (rule 1), for the bottom of the range. 65 kg × 9 on a 6–8 plan gives 67.5 kg × 6. On an lb setting, 145 lb × 9 gives 150 lb × 6.
• Inside the range or under it: the same weight, with the reps kept inside the range. 65 kg × 7 gives 65 kg × 7, and 65 kg × 5 gives 65 kg × 6. Within a workout the weight is not lowered when reps drop from fatigue.
• No plan: the set you just did, again.
• Body-weight set: the same reps.
• A lighter day from rule 4: every set that day keeps the lighter weight and its reps.

Rule 3. The first set of the day

Before the first set, ArchiFit reads your last workout of that exercise and, for some directions, the one before it.

For a body-weight exercise, the first set is your best reps from last time plus one, with no plan or with build muscle, and your best reps for the other directions.

Rule 4. The way out of a stall (build muscle)

A lift counts as stuck when the last three workouts have the same top weight and the first set at that weight did not gain reps from the first of the three to the last. On a build-muscle plan, the next three workouts then go:

1. 90% of the stuck weight, for the top of the range plus 2 reps.
2. 95% of the stuck weight, for the top of the range.
3. The stuck weight again, for the bottom of the range.

The lighter weights are rounded to the exercise's step and always stay below the stuck weight. If you train at other weights in between, rules 2 and 3 take over again.

Rule 5. When the weekly plan sets a weight

The weekly report ends with a plan for your next workouts. When that plan moved a body part's weight up a step or down (see the 8-week analysis), the day's first set for that body part uses the plan's weight and reps instead of rule 3. When the plan only carried last time's weight over, rule 3 stands.

kg and lb

Weights are stored in kg. On an lb setting they show to the nearest 0.5 lb (60 kg is 132.5 lb), and suggested weights land on the lb step: a 2.5 kg step is 5 lb, so 65 kg is suggested as 145 lb and 67.5 kg as 150 lb. The lb steps are 2.5, 5, 10 and 20 lb, matching 1, 2.5, 5 and 10 kg. Phones whose region is the United States start in lb; all others start in kg.

How the plan is picked

A plan has a direction, days per week, a split and a rep range. ArchiFit recommends each one with the rules below, and you can change every choice. The recommendations come from these rules alone, without the AI.

Direction from a body composition reading

With height, sex and body fat percentage from an InBody sheet (or typed in), and skeletal muscle mass for the second step:

1. Body fat at or above 25% for men or 30% for women: lose body fat.
2. Skeletal muscle above the top of its standard band, with body fat at or below the top of its band: keep current body.
3. Anything else: build muscle.

The standard bands follow the bars on an InBody sheet, by height and sex. Body fat: 10–20% for men and 18–28% for women. Skeletal muscle: 90–110% of a standard worked out from height and sex. The standard weight is 22 × height² for men and 21 × height² for women (height in metres). The standard lean mass is 85% (men) or 77% (women) of that, and the standard skeletal muscle is 56% (men) or 54.8% (women) of the lean mass. No formula for that last figure is published, so ArchiFit fitted it to sample sheets, and it matches them within about 2%. Age is not used. If a number the rule needs is missing, no direction is recommended.

The 25% and 30% lines match the 2009 cut-offs of the American Society of Bariatric Physicians, as reported by Shah and Braverman (2012). The same paper notes that the World Health Organization's 1995 definition used 25% for men and 35% for women.

Split, rep range and weekly sets

• Split: up to 4 days a week, a 2-day split, so at 4 days each body part comes round about twice a week. At 5 or more days, a 3-day split. The plan goes in order, not by weekday.
• Split choices: 2-day upper and lower, 2-day push and pull, 3-day chest and triceps / back and biceps / legs and shoulders, 3-day push, pull and legs, or your own split of 2 to 6 days.
• Rep range: 6–8 reps is recommended for every direction, losing fat included, so the weight is kept while cutting. 12–15 reps is there to pick.
• Weekly sets for chest, back and legs: 12 to build muscle, 10 to lose body fat, 4 to keep the current body. Smaller body parts get half, and a single day holds 2–6 sets for a big body part and 2–4 for a smaller one.

The 8-week analysis

The analysis is the last page of the weekly report. Your phone finds every signal below from your sets. With your consent, an AI model then writes a few paragraphs about them. Each signal is reported for one body part at a time.

What it looks at

The 8 weeks ending with the report's week, with sets grouped by finer body part (such as upper chest or lats) and by day. A day's order is where that body part came in the day. A day whose top weight is more than 30% above or below that body part's usual top weight is treated as other equipment and left out of comparisons.

Stall

The same top weight on at least 3 recent days across at least 3 weeks, with the last of them in the report's week or the week before. ArchiFit compares those days with up to 3 days before them: place in the day, number of sets, first-set reps and days between sessions. The longest stall is reported.

Reps rising at one weight

On at least 3 days at one top weight, the reps of the same set never fell and rose by 2 or more. The page shows an estimated one-rep max from that set, first day to last, using the Epley equation, weight × (1 + reps/30), the same as weight × (1 + 0.0333 × reps) (as stated in Macarilla et al., 2022), for sets of 12 reps or fewer. If every set at that weight reached the top of the plan's range (8 reps without a plan) on the last day, the next weight is one step heavier.

Pattern

At the top weight used most often, with at least 6 days at it, ArchiFit compares the reps of the same set on two kinds of day. One comparison is days when the body part came first against days it came later. The other is days after a break of 4 days or more against 3 days or fewer. A pattern is reported when each kind has at least 2 days and the averages differ by 1.5 reps or more.

Next weight against the plan's reps

For each body part's last day in the past 14 days, with a plan: if every set at the top weight reached the top of the range, one step heavier. If the best set fell short of the bottom of the range, a lighter weight where the bottom of the range should land by Epley, rounded down to the step, below the top weight and not under 80% of it. For example, 100 kg for at best 4 reps on a 6–8 plan gives 92.5 kg. Up to six body parts are listed.

Up since the start, and gains

For a body part first logged in the last 28 days, with at least 2 days, the analysis reports when the last day's Epley estimate is more than 5% above the first day's. For the others, it compares the top weight of the last 4 weeks with the 4 weeks before.

What the optional AI does

Only after you agree, the phone sends the numbers behind these signals to ArchiFit's server: up to 12 body parts and 24 days each, with month and day but no year, no name and no ID, plus your plan's choices. An AI model from Anthropic writes short paragraphs about each signal. For a stall or a pattern it may also pick one change: move that body part first in the day, or set its rest to 90, 120, 150 or 180 seconds.

Before a paragraph is shown, the app checks it. Every number in it must appear in what was sent. It may not tell you what to do, and it may not use words such as failure, mistake, injury or pain. It must also fit a length limit. Without the AI, the rules make the changes themselves:

• A stalled body part that had moved later in the day goes first again.
• A body part that does better when it comes first goes first.
• Rising reps that filled every set go one step heavier.
• A next weight from the plan's reps becomes the weight.

These changes fill the plan for the next workouts (rule 5). If the server does not answer, the page still shows the phone's numbers, without paragraphs. If you decline, nothing is sent.

What leaves the phone, and when

InBody numbers, names, years, cardio records, photos and videos are never sent. The privacy page links to the full policy.

ArchiFit's suggestions are general fitness information, not medical advice. InBody is a trademark of InBody Co., Ltd.; ArchiFit is not affiliated with InBody.