Design: hierarchical Bayesian network meta-analysis with multivariable dose-response modelling, CINeMA certainty assessment, PROSPERO registered
Population: 18,429 adults aged 40 and over across 124 randomised trials; 11,132 across 26 trials for the fracture outcome
Primary outcome: change in bone mineral density at lumbar spine, femoral neck and total hip; fracture incidence as secondary
Effect: brisk walking or jogging 0.013 g/cm² at spine (95% CrI 0.006–0.021) and 0.021 at total hip (0.005–0.038); benefit plateau around 600 MET-min/week; fractures — mixed aerobic OR 0.29 (0.11–0.78), mind-body 0.58 (0.36–0.93)

Exercise advice for bone health has been given for decades without a type or a dose attached, which is why it is so widely ignored. A hierarchical Bayesian network meta-analysis of 124 randomised trials and 18,429 adults aged 40 and over set out to supply both, comparing structured exercise modalities against each other and against no exercise for bone mineral density and for fractures.

For lumbar spine density, brisk walking or jogging (mean difference 0.013 g/cm², 95% credible interval 0.006 to 0.021) and combined aerobic-resistance exercise (0.013, 0.010 to 0.016) came out ahead. At the femoral neck it was brisk walking or jogging (0.009, 0.001 to 0.018) and mind-body exercise (0.007, 0.002 to 0.013). At the total hip only brisk walking or jogging reached benefit (0.021, 0.005 to 0.038). The dose-response curves were non-linear and inverted-U shaped, with clinically meaningful benefit reached at around 400 metabolic equivalent minutes per week for the spine and around 600 for the femoral neck and total hip — roughly two to three hours of brisk walking. In the fracture analysis (26 trials, 11,132 participants), mixed aerobic exercise gave an odds ratio of 0.29 (0.11 to 0.78) and mind-body exercise 0.58 (0.36 to 0.93); other modalities were neutral.

The inverted U is the part worth carrying into clinic. More is not better past the plateau, which means the advice to an older patient is a specific and achievable target rather than an open-ended exhortation. Two to three hours of brisk walking a week is something a 65-year-old in an Indian city can do, costs nothing, and — unlike gym-based resistance programmes — does not depend on access. Certainty was low to moderate across most estimates, so this is a prescription to give confidently and a mechanism to describe cautiously.

Prescribe a dose: about 150 minutes of brisk walking a week, built up gradually.
Add resistance work where the spine is the concern — combined aerobic-resistance performed best there.
Do not push past the plateau; the dose-response was inverted-U, so more is not better.
Offer mind-body exercise (such as tai chi or yoga) where walking is limited; it helped the femoral neck and the fracture outcome.
Record the starting activity level so the increment is measurable at review.

Why it matters

It converts 'stay active for your bones' into a modality and a number a patient can act on, and it says where the benefit stops rising.

The statistics, in plain English

The bone mineral density differences are small in absolute terms — 0.013 g/cm² at the spine is a fraction of what a bisphosphonate achieves — so this is about slowing decline, not reversing it. The fracture results are the clinically important ones and also the shakiest: an odds ratio of 0.29 for mixed aerobic exercise comes with a credible interval from 0.11 to 0.78, which is wide, and the certainty rating is low. An inverted-U dose-response means the estimated benefit falls away at the highest doses, but with fewer trials at those doses that tail is the least reliable part of the curve. Low to moderate certainty across most estimates is why the authors write 'cautious support' rather than a recommendation.