Swim-bike training doesn't provide the mechanical loading needed to build or maintain bone density, but structured heavy resistance and impact training measurably does — and the loading needs to happen at near-daily frequency to work
Interpretation
Olmedillas et al.'s review of 31 studies found road cycling is not osteogenic and adult cyclists frequently show lower lumbar-spine bone density than other athletes; Gómez-Bruton et al.'s review of 64 studies found competitive swimmers' bone density is typically similar to sedentary controls — the two disciplines that dominate triathlon training volume are both bone-neutral at best. Bailey and Brooke-Wavell's dose-response RCT found unilateral hopping only produced a significant bone-density gain at daily frequency (7 days/week); 2 or 4 days/week showed no significant change. This directly explains why occasional strength sessions alone may underdeliver on bone benefit without enough frequency.
Practical implication
Recommend dedicated impact/resistance loading for triathletes concerned about bone health — swim-bike volume alone won't provide it — and set expectations around frequency: the dose-response evidence suggests near-daily loading, not just 1-2 sessions a week, is what actually moved bone density in the RCT evidence.
Sources
3 sources, independently verified.
Road cycling is not an osteogenic (bone-building) activity, and adult road cyclists frequently show lower bone mineral density than other athletes, particularly at the lumbar spine; mountain biking or mixed-sport participation appeared to partially offset this. The core evidence that swim-bike training alone doesn't provide the loading needed to build or maintain bone.
Competitive swimmers' bone density is typically similar to sedentary controls and lower than athletes in high-impact sports, with higher bone turnover markers. The review's own framing — swimming 'does not seem to negatively affect bone mass' but isn't among the best sports for building it — extends the cycling finding above: the two swim-bike disciplines that dominate triathlon training volume are both bone-neutral at best.
Testing 0, 2, 4, or 7 days per week of unilateral hopping (50 multidirectional hops, generating 2.5-2.8 bodyweights of peak ground-reaction force): femoral-neck bone density changed by -0.3%, 0.0%, +0.9% and +1.8% respectively — only the daily condition produced a significant gain (95% CI 0.8-2.8%). Direct dose-response evidence that impact loading for bone needs to be close to daily, not just regular, to produce a measurable effect.