There is a lot of discussion around at the moment around whether you need to lift heavy to build bone health. 

A number of bone density programs that exist suggest that it is necessary to lift heavy & for low repetitions  (Watson et al., 2017). However, this reccommendation is based mainly from one study. In this study, participants lifted heavy weights for low reps (5 sets of 5 reps). Another group lifted light weights for higher reps.  The heavy lifting group had an improvement in bone density; the light weight group did not. 

This shows us that heavy lifting is effective, and that lifting light weights at a low effort is unlikely to be.  It does not provide information about any other scenarios (e.g. what about lifting light weights towards failure? what about lifting moderate weights for moderate repetitions?).

A number of systematic reviews indicate that as long as you are lifting weights that are heavy relative to the number of repetitions you are doing, and are progressively heavier over time, this is beneficial to bone (Souza et al., 2020), (Brooke-Wavell et al., 2022).

What does this mean in practice:

It means that you are likely to build bone if you lift a weight for 5 reps (that you are only able to lift 5-6 times in total); or a lighter weight for 10 reps (heavy enough you could only lift 10-11 times in total); or a lighter weight again for 20 reps (that you could only lift a total of 20-21 times).  That is, it doesn't matter how heavy it is, or how many reps you do, as long as you are lifting a weight that is heavy enough that you are working towards failure. 

To add to the confusion, it is probably true that for some people, lifting light weights no where near failure will be enough to maintain their bones. However, for people already doing this, or doing lots of exercise, who still have declining bone density, osteoporosis, or osteopenia, I think being aware that most likely lifting weights that are relatively heavy (for any number of reps) is likely to add some benefit. 

The other thing is practicality: for me, personally, i do not like lifting weights more than 8-10 repetitions, so I am very unlikely to choose a weight I would need to lift 15-20 times, as I find that type of lifting very tiring! Though it might SOUND easier that the weight is lighter, if the effort is matched, I really don't think it is.

Bottom line: it seems that lifting weights for low, moderate, or high repetitions can all benefit your bone health, as long as you are going towards failure (i.e. it is hard work!).

References:

1. Alnasser, S. M., Babakair, R. A., Al Mukhlid, A. F., Al hassan, S. S. S., Nuhmani, S., & Muaidi, Q. (2025). Effectiveness of Exercise Loading on Bone Mineral Density and Quality of Life Among People Diagnosed with Osteoporosis, Osteopenia, and at Risk of Osteoporosis—A Systematic Review and Meta-Analysis. Journal of Clinical Medicine, 14(12), 4109. https://doi.org/10.3390/jcm14124109 
2. Brooke-Wavell K, Skelton DA, Barker KL, Clark EM, De Biase S, Arnold S, Paskins Z, Robinson KR, Lewis RM, Tobias JH, Ward KA, Whitney J, Leyland S. Strong, steady and straight: UK consensus statement on physical activity and exercise for osteoporosis. Br J Sports Med. 2022 May 16;56(15):837–46. doi: 10.1136/bjsports-2021-104634. Epub ahead of print. PMID: 35577538; PMCID: PMC9304091.
3. Souza D, Barbalho M, Ramirez-Campillo R, Martins W, Gentil P. High and low-load resistance training produce similar effects on bone mineral density of middle-aged and older people: A systematic review with meta-analysis of randomized clinical trials. Exp Gerontol. 2020 Sep;138:110973. doi: 10.1016/j.exger.2020.110973. Epub 2020 May 23. PMID: 32454079.