How do bones actually get stronger?
- Danny George
- 6 hours ago
- 7 min read
"I don't want to lift anything heavy, I'll hurt myself." "I should probably build up some muscle first before I try real weights." If you've had either of those thoughts, you're not alone. It's one of the most common, and most understandable fears among older adults starting a strength program. It's also, according to the research, largely backwards.
To understand why, it helps to understand what's actually happening inside your bones when you exercise, and why the intensity you're avoiding may be exactly what your bones are waiting for.
Bone Is Alive, and It's Listening
Bone isn't a fixed, inert structure like a support beam in a house. It's living tissue, constantly being broken down and rebuilt in a process called remodeling. Cells called osteoclasts clear away old bone tissue, while cells called osteoblasts build new bone in its place. Which process wins out - building or breaking down - depends heavily on the signals bone is receiving from daily life.
The main signal that tells bone to build is mechanical: the physical strain placed on it during movement, especially load-bearing and resistance exercise. Deep inside bone tissue, cells called osteocytes act as the sensors in this system. They detect mechanical strain and translate it into biochemical signals - a process researchers call mechanotransduction, a process that ultimately tells osteoblasts to lay down new bone. This is the modern, cellular explanation for something researchers have understood in principle for over a century: bone gets stronger specifically in response to the forces placed on it.
Why Intensity Matters More As We Age for Stronger Bones
Here's the part that surprises most people: aging doesn't just cause bone loss, it also makes bone less responsive to the loading signals that build it back. A 2019 review in Current Osteoporosis Reports found that aged bone shows a blunted mechanoadaptive response compared to younger bone, meaning the same amount of loading that would trigger new bone growth in a 30-year-old often produces little to no response in an older adult's skeleton (Javaheri & Pitsillides, 2019).
This is the opposite of what most people assume. The common instinct is "I'm older, so I should go gentler." But if aging bone requires a stronger signal to respond at all, gentler loading may be exactly what fails to produce any real benefit. This is backed up directly by exercise research: a 2024 systematic review of high-velocity resistance training in older adults found meaningful improvements in bone mineral density at the spine, hip, and femoral neck, but only in programs that trained at least twice a week with real intensity behind the movement, and with the same reviewers noting that lower-intensity programs consistently failed to move the needle on bone density at all (Haque, Schlacht, & Skelton, 2024).
In plain terms: light weights and high reps might be a comfortable way to feel like you're doing something. But if the goal is actually building bone, the evidence says intensity isn't optional it's the priority.
Addressing the Fear Directly: "Won't Heavy Weights Hurt Me?"
This is the fear underneath almost everything above, and it deserves a direct answer, not just a dismissal.
The most important study on this exact question is the LIFTMOR trial, which took postmenopausal women with low bone mass (osteopenia and osteoporosis) the exact population most people assume should avoid heavy lifting, and had them perform eight months of supervised high-intensity resistance and impact training at up to 85% of their one-rep max. The results: significant improvements in bone mineral density at the spine and hip, along with better physical function and, critically, no increase in fractures or serious injury compared to a low-intensity control group (Watson et al., 2018).
A dedicated follow-up study went even further, specifically checking for the exact injury most people fear from heavy lifting with fragile bones - vertebral compression fractures. Not only did the high-intensity group show no evidence of new vertebral fractures, their posture actually improved, with a measurable reduction in spinal curvature (kyphosis) compared to the control group (Watson et al., 2019).
To be clear: this doesn't mean anyone should walk into a gym and load a barbell to 85% of their max on day one. It means the assumption that heavier loading is inherently dangerous for older or lower-bone-density individuals isn't supported by the strongest evidence we have, when that loading is applied progressively, with proper supervision and technique, which is precisely how a program should be built in the first place.
Who Should Be Cautious With Heavy Loading
None of this means heavy resistance training is automatically right for everyone. The women in the LIFTMOR trial were carefully screened before they were ever allowed to participate - people with a recent fracture, an active joint injury, uncontrolled cardiovascular disease, or an unresolved musculoskeletal condition were excluded from the study entirely. That screening is a real part of why the results were so safe. Of the 406 women who volunteered, only about 1 in 4 actually qualified.
In practice, this means heavy loading should be approached cautiously, or modified for anyone with an unhealed injury, a recent surgery, uncontrolled heart or vascular conditions, or joint issues that haven't been properly evaluated. It doesn't mean these individuals can't strength train, or that they're stuck avoiding resistance work altogether. It means the starting point, pace, and exercise selection need to be built around their specific situation, ideally with input from both a doctor and a trainer who can properly assess where they're starting from before deciding how - and how quickly - to load up.
The truth sits between the two extremes: heavy, appropriately progressed loading is one of the most effective tools we have for aging bone, but "appropriate" is doing real work in that sentence. It's not a green light for everyone regardless of health history, it's a case for taking loading seriously enough to build it correctly, with the right screening and guidance, rather than avoiding it altogether or approaching it carelessly.
What About Building Muscle First?
The other version of this fear is less about injury and more about sequencing: the idea that someone needs to spend months building a "base" of muscle and strength with light weights before they're ready for anything heavier. It's a reasonable-sounding idea. It's just not how the adaptation actually works.
A meta-analysis of resistance exercise trials in aging adults found that meaningful gains in lean body mass came specifically from resistance training performed at a real training intensity, not from extended periods of light-load conditioning beforehand (Peterson, Sen, & Gordon, 2011). Muscle and bone respond to the same core signal: a load that's genuinely challenging relative to a person's current capacity, applied consistently over time, and increased as the body adapts. There isn't a separate "prep phase" required before that process can begin, the appropriately loaded training is the prep phase, the muscle-building phase, and the bone-building phase, all at once, done progressively.
What "Appropriate" Actually Means
None of this is a case for recklessness. The research above worked because the loading was progressive and supervised, meaning a 68-year-old starting out isn't handed a heavy barbell on their first day. It means a program is built to move someone toward a genuinely challenging intensity over time, with form, pacing, and load managed by someone watching closely enough to know exactly when to add weight and when to hold steady.
That's really the heart of it: the fear of "heavy" usually comes from imagining an all-or-nothing jump to something unsafe. The reality, and the actual research, describes something much more controlled - a gradual, well-supervised climb toward the intensity that bone and muscle actually need to rebuild. Skipping that climb out of caution doesn't protect you. It just means the signal your bones are waiting for never arrives.
If you've been holding back on strength training out of fear of going too heavy, that instinct is worth revisiting, with the right guidance; "heavy enough to matter" and "safe" aren't in conflict. They're the same program.
Curious what appropriately progressed strength training looks like for your specific situation? Reach out, I'd love to talk through it with you.
P.S.
Strength training should always be on your rules. If you're working with a personal trainer know that you are in charge. The trainer's job is to help you reach your goals safely. If there is ever anything you do not feel safe doing or are not yet ready for or comfortable with, let them know.
You're reading Stewarding Health. A weekly newsletter for those stewarding their health in this life, for the next. If you're not subscribed to receive my weekly email, subscribe here!
Meet Danny George, B.S. Exercise Science, CPT, PPSC, PN-L1
Hi, I'm Danny George, a Christian, husband, father, and personal trainer who is passionate about helping people build strength, confidence, and healthier lives.
Professionally, I hold a Bachelor’s degree in exercise science with a minor in nutrition, and I'm a certified personal trainer, nutrition coach, and health coach. For over a decade, I've had the privilege of working with clients to create lasting transformations -physically, mentally, and spiritually. I've been featured in the Owensboro Times, a member of the Owensboro Chamber of Commerce, and have received over fifty 5-star reviews on Google.
Outside of coaching, you’ll find me enjoying life with my wife and our two amazing kids. They’re a big part of my “why,” and the reason I'm passionate about helping others invest in their health, so they can show up strong for the people they love most.
Sources Cited
Javaheri, B., & Pitsillides, A.A. (2019). Aging and mechanoadaptive responsiveness of bone. Current Osteoporosis Reports, 17(6), 560–569.
Haque, I., Schlacht, T.Z., & Skelton, D.A. (2024). The effects of high velocity resistance training on bone mineral density in older adults: A systematic review. Bone, 179, 116986.
Watson, S.L., Weeks, B.K., Weis, L.J., Harding, A.T., Horan, S.A., & Beck, B.R. (2018). High-intensity resistance and impact training improves bone mineral density and physical function in postmenopausal women with osteopenia and osteoporosis: The LIFTMOR randomized controlled trial. Journal of Bone and Mineral Research, 33(2), 211–220.
Watson, S.L., Weeks, B.K., Weis, L.J., Harding, A.T., Horan, S.A., & Beck, B.R. (2019). High-intensity exercise did not cause vertebral fractures and improves thoracic kyphosis in postmenopausal women with low to very low bone mass: The LIFTMOR trial. Osteoporosis International, 30(5), 957–964.
Peterson, M.D., Sen, A., & Gordon, P.M. (2011). Influence of resistance exercise on lean body mass in aging adults: A meta-analysis. Medicine & Science in Sports & Exercise, 43(2), 249–258.
Comments