For bone, the evidence points heavy: in the LIFTMOR trial the working load was 80–85% of a one-rep maximum or above, for 5 sets of 5, under close supervision. But that is a destination, not a starting line. Most people should begin with 8–12 reps at a moderate effort — Too Fit to Fracture puts general strength work at 5–8 on a 0–10 scale, and advises starting at the lower end if you are sedentary, new to strength training or at high fracture risk — spend the first few weeks on technique, and move toward heavier work only once your form holds and your clinician has cleared it.
What does “80–85% of your one-rep max” actually mean?
Your one-rep max (1RM) is the heaviest load you can lift for a single good repetition — one more, or one with a rounded back, and it doesn't count. 80–85% of that is a weight you can move for roughly five hard reps and no more. So “5 sets of 5 at 80–85%” isn't five comfortable sets: it's five sets that each finish with little left over, with two to three minutes of rest between them. That is the dose behind the LIFTMOR protocol and behind HiRIT in general.
Why does heavy-and-few beat light-and-many for bone?
Muscle grows across a wide range of loads. Bone is fussier. It adapts to strain that is meaningfully larger than what it already experiences every day, applied briefly rather than endlessly repeated. Twenty easy reps deliver a lot of repetitions and very little novel strain — which is also why walking alone rarely builds bone. Five hard reps deliver the opposite. High load, low volume, full recovery between sets: that is the shape of a set that signals bone to adapt.
How do you judge load without testing a true one-rep max?
You almost never need to test one, and for most people with low bone density you shouldn't — a single maximal effort on a lift you are still learning is exactly when technique fails. Bone Fit™ offers a gentler substitute: use a 10-rep max. The heaviest weight you can lift ten times before failure is about 75% of a 1RM. If you can do twenty reps before failure, you are working below 60% — under the minimum overload the Bone Fit™ manual describes as necessary for muscle adaptation in untrained people. The other practical tool is RPE, or rate of perceived exertion: a number for how hard the set actually felt. Bone Builder prescribes effort as an RPE number on a 0–10 scale, where 10 is a set you could not have added a single rep to. Bone Fit™ frames the same idea as reps before form deteriorates — about eight reps at 80% of your max, about fifteen at 60%.
| What you're training | Reps per set | Effort (RPE 0–10 unless noted) | Where it fits |
|---|---|---|---|
| Learning a new movement | 8–12 | 4–5 | Technique work at the start of any lift |
| General strength and function | 8–12 | 5–6 | Foundation, and early Build weeks — Too Fit to Fracture's stated range is 5–8 |
| Getting measurably stronger | 6–8 | 7 | Build, roughly weeks 5–8 |
| Heavier progressive loading | 5–6 | 7–8 | Build, roughly weeks 9–12 |
| Bone-targeted heavy loading (HiRIT) | 5 reps × 5 sets | 80–85% 1RM | Peak only — screened, coached and supervised. Bone Builder does not put you here with an osteoporosis diagnosis or a high-risk profile |
| Back-extensor endurance | 3–5 holds of 3–5 seconds | Low by design | Daily after a vertebral fracture (UK consensus, 2022) |
When should you add weight, and how much?
Not before the pattern is solid, because load applied to a poor pattern is the actual risk. LIFTMOR itself opened with a lighter on-ramp so participants could master technique before the load went up. The Bone Fit™ manual makes a related point about free weights: they are preferable to machines only when the exercise has been taught with proper form and the person has enough strength to control the whole movement without compensating. In practice that means Hip Hinge Practice and Hip Hinge with Bar before a Barbell Deadlift, Chair Squat or Bodyweight Squat before a Barbell Back Squat, and often the Trap-Bar Deadlift as a friendlier first heavy hinge — the neutral grip makes it easier to keep the chest up and the spine neutral. Once that is in place:
- Progression has to actually happen — the Bone Fit™ manual is blunt that it must occur. A weight that never changes eventually stops producing adaptation.
- Reassess your 10-rep max regularly. Bone Fit™ suggests that once you can manage 12–15 reps at a given load, increase it by 2–10%.
- Add weight only when every rep of every set looked like the first. If the last rep rounds or grinds, today's weight is the right weight — and neutral spine is the line that matters.
- Train the same muscle group 2–3 times a week, with 24–48 hours of rest in between. LIFTMOR used just two 30-minute sessions a week.
- Expect months, not weeks. LIFTMOR measured its bone-density changes over eight months, because bone remodels slowly.
Does supervision really matter at these loads?
Yes — and this is the part most articles skip. LIFTMOR's 101 postmenopausal participants (mean age 65) trained in small supervised groups, no more than eight people per instructor. Under those conditions the record was strong: 92% adherence and a single minor adverse event across the whole trial, with lumbar-spine bone density rising 2.9% while the usual-care control group lost 1.2%. The researchers are explicit that they do not recommend replicating the protocol unsupervised, and the Bone Fit™ manual's own summary of the LIFTMOR work notes that the program requires a high level of supervision. Technique under a near-maximal load is the one variable you cannot reliably check on yourself.
What if your bone density is very low, or you've had a fracture?
Then the honest answer to “how much weight” is: less, later, and with your physical therapist involved. A history of vertebral or hip fracture, a T-score of −3.0 or lower, or pain right now all place you on the Foundation track and away from loaded forward-bending and impact work — see exercise after a vertebral fracture. Too Fit to Fracture likewise advises starting at a lower intensity if you are sedentary, new to strength training or at high fracture risk. And for people who have had a vertebral fracture, the UK consensus statement summarised in the Bone Fit™ manual recommends daily back-extensor work at low intensity for endurance — exercises repeated 3–5 times and held for 3–5 seconds — rather than loaded heavy.