Bone

What it is

Most of the adult skeleton is made of bone, a hard and dense connective tissue¹. The skeleton supports the body, helps it move, protects the organs, makes blood cells, and stores minerals and fat¹. Mineral crystals give bone its hardness and strength¹. Collagen (a strong, rope-like protein) fibres give it flexibility, so it is not brittle¹.

Compact or cortical bone (the dense, solid outer layer of a bone) is the denser, stronger type and forms the shafts of long bones¹. Spongy or trabecular bone is a lattice of thin struts, each formed along lines of stress¹. A review describes trabecular bone as about 50–90% porous (full of spaces), against about 5–10% for cortical bone⁶.

Osteoclasts (cells that break down and remove bone) keep clearing old bone, while osteoblasts (cells that build new bone) keep laying down new bone¹. Osteocytes are the main and most common cell of mature bone, linked to each other by long branches¹. A review describes the cycle of bone removal and rebuilding, called remodelling (old bone replaced with new), as taking place over months³.

How it responds to load

From strain to signal

A review describes bone as adapting to the loads it usually carries⁶. It sets its structure by the size, rate, frequency, spread and type of strain (how far something stretches as a share of its resting length)⁶. A position statement’s review explains that impact and muscle forces deform bone². This strain switches on osteocytes². They signal osteoblasts and osteoclasts to adapt the bone, though much of this is known from animal studies².

What kind of loading counts

A 1998 review proposed that changing loads drive bone adaptation rather than static ones, and a short spell of loading starts it⁷. It added that bone cells get used to routine loading and respond less⁷. The 2016 position statement’s review says bone responds most to loads that are changing, moderate to high, brief, quick and unusual in direction². It states that static loads and repeated low loads do not build bone².

In mechanostat theory (the idea that bone is gained or lost as strain crosses set levels), strain below a minimum level leads to bone loss⁶. Strain above it leads to new bone⁶. A position statement’s review says the strain needed to start building bone is set by the strain a bone usually meets². So inactive children may respond to low-impact loading, while more active children need higher loads².

More is not always better

One review reports that bone cells lose 95% of their responsiveness after about 20 loading cycles in a row⁸. Over 90% returns after 4–8 hours of rest⁸. Another review reports that the same loading split into separate bouts can be more effective⁶.

Loading too light and rare may lead to bone loss, and loading too heavy and frequent to a stress reaction, a review notes⁶. In 14 crew members aged 40 to 55 on space station missions, hip bone density fell by 1.4–1.5% a month⁹.

How it gets injured

Bone stress injuries

An international expert consensus describes bone stress injuries, often called stress reactions and stress fractures, as overuse injuries to bone¹⁰. A review describes built-up tiny damage starting as a stress reaction (an early injury, before a crack forms)³. It can progress to a stress fracture (a small crack from repeated loading), then a complete fracture³.

Loading normally causes microscopic damage, which remodelling repairs⁸. One review argues an injury follows when damage builds faster than bone can repair or resist it⁸. Another notes the exact mechanism is not settled³. The same review reports evidence that rapid, large rises in training load can raise the risk³.

Who gets them

One review argues that total load outrunning the bone’s ability to adapt is the likely necessary cause of every bone stress injury³. It says single risk factors predict them poorly³.

In subgroups of athletes, studies have linked them with female sex and competitive sport³. Problematic low energy availability (too little energy from food left for the body’s needs after training) is also linked³. Low vitamin D or iron, irregular menstrual cycles and errors in the training plan are also listed³. The same review reports these figures:

• In running sports, they make up 15–20% of musculoskeletal injuries³.
• Female runners’ rate is 1.8–2.3 times that of male runners³.

Group of American athletes	Bone stress injuries per 100,000 athlete-exposures (one athlete in one practice or game)
High school athletes	1.53
National Collegiate Athletic Association athletes	5.73

A 2025 consensus statement (a report setting out what an expert panel agrees on) describes the Female Athlete Triad¹¹. It has three linked parts, energy status, reproductive function and bone health, set off by energy deficiency¹¹. The statement cites consistent links between parts of the Triad and a higher risk of bone stress injury¹¹.

How a broken bone heals

A textbook describes healing in stages, starting with a fracture haematoma (the blood clot that forms at a broken bone soon after the break)¹. Next, calluses of cartilage and bone form around the break and steady it¹. Later, the callus cartilage is replaced by bone, and remodelling often leaves no outward sign¹.

How training builds it

Bigger, not only denser

Among 113 national-level female athletes, those in impact sports had larger tibia (shinbone) shafts, thicker cortical bone and somewhat denser trabecular bone than 30 non-athletes¹². Swimmers did not show this¹². In 12 former national-level male tennis players, the playing arm had extra bone mineral and strength in the humerus (upper arm bone)¹³. This came mainly from larger bone size, not denser bone¹³.

Bone can adapt its size, shape and inner architecture to routine loads, gaining strength and stiffness without gaining mass⁶. Bone mineral density (how much mineral is packed into a given amount of bone) leaves out shape and architecture, which add greatly to strength⁶.

Bone can adapt its size, shape and inner architecture to routine loads, gaining strength and stiffness without gaining mass.

Which exercise: findings vary by group and site

Two large reviews, in different groups, point to different kinds of exercise^4,5:

• A 2026 network meta-analysis (one comparing many options at once) pooled 124 randomised trials in adults aged 40 and over⁴.
• It found, on low to moderate certainty evidence, that brisk walking or jogging and combined aerobic and resistance exercise probably help bone density⁴.
• A Cochrane review (a systematic review done to Cochrane’s standards) pooled 43 trials in postmenopausal women⁵.
• At the femoral neck (just below the ball of the hip), progressive leg resistance training appeared to have the largest effect⁵.
• At the spine, combined programmes had the largest effect⁵.

Other reviews looked at particular groups or types of exercise^14,15:

Group studied	What the review found
Adults 65 and over, 14 randomised trials	Resistance training raised hip density by about 2.8%, but not lower back density14
Postmenopausal women, controlled trials of 6 months or more	Resistance exercise had a significant, low-to-moderate effect on femoral neck density16
Men 45 and over, 10 randomised trials	A significant but mild gain in bone density17
Premenopausal (before menopause) women, six studies	Jumping raised density at the femoral neck, with less benefit at the lower back18
Children to older adults, 28 randomised trials of impact exercise15	Gains in some measures, such as at the lower tibia, but no effect at the lower back15

The LIFTMOR randomised trial studied 101 postmenopausal women with low bone mass¹⁹. Eight months of supervised high-intensity resistance and impact training raised lower back density by 2.9%¹⁹. A low-intensity home exercise group saw a 1.2% fall¹⁹. In these closely supervised, otherwise healthy women, the training produced one minor adverse event¹⁹.

Density is not fractures

The Cochrane review also found a small but significant effect on bone density, and no effect on the number of fractures⁵. A 2022 meta-analysis (a study that pools earlier studies) of 20 trials in adults 45 and over found fewer low-trauma fractures in exercise groups²⁰. The drop was about twice as large in mainly supervised programmes, and not significant in unsupervised ones²⁰. The 2026 network meta-analysis suggested, with low certainty, that two types of exercise may lower fracture risk⁴. These were mixed aerobic exercise and mind-body exercise (exercise that joins movement with breathing and focus, such as yoga or tai chi)⁴.

On dose, the 2026 network meta-analysis found the link between exercise dose and bone density appeared to be generally an inverted U, rising then falling⁴. A 2016 position statement notes that how to measure the activity that builds bone is still not fully understood².

Across the lifespan

Building bone while growing

Long bones grow in length at the growth plate, a layer of cartilage replaced by bone at about 18–21 years¹. A 2016 position statement reports that bone mineral builds slowly through childhood and fast around puberty². In children of European ancestry, about 39% of the body’s bone mineral is gained in the 4 years around that peak². By 4 years after it, 95% of adult bone mass is reached².

Sources differ on when bone peaks. A textbook puts peak bone density at about age 30¹. The position statement reports it varies by site². In a Canadian study of women, it fell at 16–19 years at the hip and 33–40 at the lower spine².

Exercise in childhood

The 2016 statement graded the evidence for physical activity building peak bone mass in youth as grade A, its strongest². It saw this especially in late childhood and around puberty². Trials give a mixed picture:

• In 22 exercise trials in children aged 3–18, exercise brought 0.6–1.7% more bone gain a year, mainly before puberty²¹.
• In that analysis, there were no differences among children in puberty²¹.
• A 2023 meta-analysis of impact-exercise trials found no overall effect on bone structure in children and adolescents¹⁵.
• A meta-analysis of 10 supervised exercise trials found stronger loaded leg bones in children but not in adults²².

A review states that loading during growth enlarges bones, a gain that lasts long term⁸. It finds no consistent evidence of size changes after the skeleton matures⁸. In professional baseball players, extra bone mass built by throwing in youth was gradually lost after throwing stopped²³. About half the extra size and a third of the extra strength lasted for life²³. The 2025 consensus statement says energy deficiency in adolescent female athletes can cause low bone density and low peak bone mass¹¹.

Menopause and older age

From about age 50, females lose bone faster than males¹. A textbook links this to menopause, when the ovaries stop making estrogen, a hormone that promotes bone building¹.

Osteoporosis (a disease in which bones become thin and weak) happens when bone removal outpaces bone building, and is common with ageing¹. A review attributes most of the strength bone loses with age to decay of its fine structure, driven largely by rising porosity⁶. Another review proposes that bone responds less to loading with age partly because the spaces holding osteocytes change shape²⁴. It notes that data on those changes are limited²⁴.

Connections

The library has entries on the femur, tibia, pelvis, humerus, patella and scapula. In one study of 90 throwing athletes, arm-to-arm differences in the humerus were larger in more mature groups²⁵.

The largest forces bone routinely carries come from muscle⁶. Long bones grow in length from a plate of cartilage¹. The training methods studied above have their own pages: resistance training and jump-based plyometric training^14,18.

Sources

1. Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, Korol O, Johnson JE, Womble M, DeSaix P (2022). Anatomy and Physiology 2e. OpenStax. Link
2. Weaver CM, Gordon CM, Janz KF, Kalkwarf HJ, Lappe JM, Lewis R, O'Karma M, Wallace TC, Zemel BS (2016). The National Osteoporosis Foundation's position statement on peak bone mass development and lifestyle factors: a systematic review and implementation recommendations.. Osteoporosis international. doi:10.1007/s00198-015-3440-3 PMID 26856587
3. Crunkhorn ML, Etxebarria N, Toohey LA, Charlton P, Watson K, Drew M (2025). The Natural History of Bone Stress Injuries in Athletes: From Inception to Resolution.. Sports medicine. doi:10.1007/s40279-025-02280-9 PMID 40721688
4. Lu C, Li L, Zhang L, Gao Q, Meng J, Xue Y, Jiang H, Yuan T, Qian H, Bao N, Luo Z, Yang X (2026). Effect of exercise on bone health in middle aged and older adults: hierarchical network meta-analysis of randomised trials.. BMJ. doi:10.1136/bmj-2026-100561 PMID 42716561
5. Howe TE, Shea B, Dawson LJ, Downie F, Murray A, Ross C, Harbour RT, Caldwell LM, Creed G (2011). Exercise for preventing and treating osteoporosis in postmenopausal women.. Cochrane database of systematic reviews. doi:10.1002/14651858.CD000333.pub2 PMID 21735380
6. Hart NH, Nimphius S, Rantalainen T, Ireland A, Siafarikas A, Newton RU (2017). Mechanical basis of bone strength: influence of bone material, bone structure and muscle action.. Journal of musculoskeletal & neuronal interactions. PMID 28860414
7. Turner CH (1998). Three rules for bone adaptation to mechanical stimuli.. Bone. doi:10.1016/s8756-3282(98)00118-5 PMID 9823445
8. Warden SJ, Edwards WB, Willy RW (2021). Preventing Bone Stress Injuries in Runners with Optimal Workload. Current osteoporosis reports. doi:10.1007/s11914-021-00666-y PMID 33635519
9. Lang T, LeBlanc A, Evans H, Lu Y, Genant H, Yu A (2004). Cortical and trabecular bone mineral loss from the spine and hip in long-duration spaceflight.. Journal of Bone and Mineral Research. doi:10.1359/JBMR.040307 PMID 15125798
10. Hoenig T, Hollander K, Popp KL, Fredericson M, Kraus EA, Warden SJ, Tenforde AS, Bone Stress Injury Authorship Group (2025). International Delphi consensus on bone stress injuries in athletes. British journal of sports medicine. doi:10.1136/bjsports-2024-108616 PMID 39638438
11. De Souza MJ, Williams NI, Misra M, Nattiv A, Joy E, Barrack M, Ricker EA, Gorrell S, Koltun KJ, O'Donnell E, Mallinson RJ, Salamunes ACC, Woodruff K, Fredericson M, Plessow F (2026). 2025 Update to the Female Athlete Triad Coalition Consensus Statement Part 1: State of the Science and Introduction of a New Adolescent Model.. Sports medicine. doi:10.1007/s40279-025-02333-z PMID 41474493
12. Nikander R, Sievänen H, Uusi-Rasi K, Heinonen A, Kannus P (2006). Loading modalities and bone structures at nonweight-bearing upper extremity and weight-bearing lower extremity: a pQCT study of adult female athletes. Bone. doi:10.1016/j.bone.2006.04.005 PMID 16731064
13. Haapasalo H, Kontulainen S, Sievänen H, Kannus P, Järvinen M, Vuori I (2000). Exercise-induced bone gain is due to enlargement in bone size without a change in volumetric bone density: a peripheral quantitative computed tomography study of the upper arms of male tennis players.. Bone. doi:10.1016/s8756-3282(00)00331-8 PMID 10962345
14. O'Bryan SJ, Giuliano C, Woessner MN, Vogrin S, Smith C, Duque G, Levinger I (2022). Progressive Resistance Training for Concomitant Increases in Muscle Strength and Bone Mineral Density in Older Adults: A Systematic Review and Meta-Analysis.. Sports medicine. doi:10.1007/s40279-022-01675-2 PMID 35608815
15. Ng CA, Gandham A, Mesinovic J, Owen PJ, Ebeling PR, Scott D (2023). Effects of Moderate- to High-Impact Exercise Training on Bone Structure Across the Lifespan: A Systematic Review and Meta-Analysis of Randomized Controlled Trials.. Journal of bone and mineral research. doi:10.1002/jbmr.4899 PMID 37555459
16. Shojaa M, von Stengel S, Kohl M, Schoene D, Kemmler W (2020). Effects of dynamic resistance exercise on bone mineral density in postmenopausal women: a systematic review and meta-analysis with special emphasis on exercise parameters.. Osteoporosis International. doi:10.1007/s00198-020-05441-w PMID 32399891
17. Lu X, Wei J, Liu Y, Lu Y (2023). Effects of exercise on bone mineral density in middle-aged and older men: A comprehensive meta-analysis.. Archives of osteoporosis. doi:10.1007/s11657-023-01317-8 PMID 37548809
18. Zhao R, Zhao M, Zhang L (2014). Efficiency of jumping exercise in improving bone mineral density among premenopausal women: a meta-analysis.. Sports medicine (Auckland, N.Z.). doi:10.1007/s40279-014-0220-8 PMID 24981245
19. Watson SL, Weeks BK, Weis LJ, Harding AT, Horan SA, Beck BR (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. doi:10.1002/jbmr.3284 PMID 28975661
20. Hoffmann I, Shojaa M, Kohl M, von Stengel S, Becker C, Gosch M, Jakob F, Kerschan-Schindl K, Kladny B, Clausen J, Lange U, Middeldorf S, Peters S, Schoene D, Sieber C, Tholen R, Thomasius F, Bischoff-Ferrari HA, Uder M, Kemmler W (2022). Exercise Reduces the Number of Overall and Major Osteoporotic Fractures in Adults. Does Supervision Make a Difference? Systematic Review and Meta-Analysis.. Journal of bone and mineral research. PMID 36082625
21. Specker B, Thiex NW, Sudhagoni RG (2015). Does Exercise Influence Pediatric Bone? A Systematic Review.. Clinical orthopaedics and related research. doi:10.1007/s11999-015-4467-7 PMID 26208606
22. Nikander R, Sievänen H, Heinonen A, Daly RM, Uusi-Rasi K, Kannus P (2010). Targeted exercise against osteoporosis: A systematic review and meta-analysis for optimising bone strength throughout life.. BMC Medicine. doi:10.1186/1741-7015-8-47 PMID 20663158
23. Warden SJ, Mantila Roosa SM, Kersh ME, Hurd AL, Fleisig GS, Pandy MG, Fuchs RK (2014). Physical activity when young provides lifelong benefits to cortical bone size and strength in men.. Proceedings of the National Academy of Sciences of the United States of America. doi:10.1073/pnas.1321605111 PMID 24706816
24. Hemmatian H, Bakker AD, Klein-Nulend J, van Lenthe GH (2017). Aging, Osteocytes, and Mechanotransduction.. Current osteoporosis reports. doi:10.1007/s11914-017-0402-z PMID 28891009
25. Warden SJ, Weatherholt AM, Gudeman AS, Mitchell DC, Thompson WR, Fuchs RK (2017). Progressive skeletal benefits of physical activity when young as assessed at the midshaft humerus in male baseball players.. Osteoporosis International. doi:10.1007/s00198-017-4029-9 PMID 28396902

Every source the library cites

Changes

• September 30, 2026 First published.