REF-1315 · Hughes et al., 2018 · Bone · Paper · PubMed

Why build running slowly when my aerobic fitness is fine?

In military women, new impact loading made the tibia stronger within 8-14 weeks, yet shaft cortical density dipped then (remodelling). Bone scans, not injuries. B-.

Main caveat: Marker outcomes. Density, structure and modelled strength, not fractures or performance.
limited, mixed evidenceMeasured on non-athletes and recreational people

Hughes and colleagues scanned the distal tibia (metaphysis, 4% of tibial length; diaphysis, 30%) with high-resolution peripheral quantitative CT and measured serum bone markers in 91 female recruits before and after 8 weeks of US Army basic combat training, adjusting for race/ethnicity, age and BMI.

At the metaphysis, cortical thickness, trabecular thickness and number, bone volume fraction and total and trabecular density rose by 1-2%, while trabecular separation, cortical tissue mineral density and cortical density fell by 0.3-1.0%; at the diaphysis cortical density and tissue mineral density fell by 0.7%.

Estimated strength (micro-finite element) rose 2.5% at the metaphysis and 0.7% at the diaphysis. Sclerostin fell 5.7%, and bone formation and resorption markers and 25(OH)D rose 10-28%. The authors read this as new bone formation together with intracortical remodelling.

The companion followed 51 women through the 44-week British Army Officer Commissioning Course with scans at weeks 1, 14, 28 and 44: trabecular density at the metaphysis rose 3.0% by week 44, cortical area 4.8% and cortical thickness 4.0%; at the diaphysis cortical density fell 0.3% from week 1 to 14 and then recovered, while failure load rose 2.5% by week 44; trabecular microarchitecture changed only from week 28, later than density and geometry.

The authors link the week-14 cortical dip to the reported median time to stress fracture on that course (102 days).

Our reading: under unaccustomed impact loading bone gets stronger on balance from the first weeks, but the cortex — where tibial stress fractures occur — goes through a phase of remodelling that lasts about two to three months; the dip is small (below the companion's own least significant change) and estimated strength did not fall, so it is a window to respect, not proof of a weaker bone.

Protocol

Hughes 2018 (abstract): n = 91 female recruits who volunteered and completed the observational study, age 21.5 ± 3.3 years; HR-pQCT of the distal tibial metaphysis (4%) and diaphysis (30%) and serum markers before and after 8 weeks of BCT; linear mixed models controlling for race/ethnicity, age and BMI; strength by micro-finite element analysis.
O'Leary 2021 (full text): all women starting British Army Officer training between May 2017 and January 2018 invited; 77 eligible, 61 volunteered and completed baseline, 51 completed the study — two medically discharged, six removed from training for injury (one tibial stress fracture, one medial tibial stress syndrome, the rest overuse injuries of the knee, back or shoulders), one left voluntarily, one unavailable.Three 14-week terms; aerobic endurance training, strength and conditioning, military fitness training, drill, progressive loaded marching and field exercises. HR-pQCT of the right tibia (4% and 30%) and whole-body DXA at weeks 1, 14, 28 and 44; blood at weeks 1, 28 and 44. Least significant change for cortical density at the 30% site 0.4%.A minimum of 48 participants was planned for 90% power.
Load and energy (companion, from the same women's earlier reports): total daily energy expenditure 3332 ± 424, 3849 ± 363 and 3041 ± 286 kcal a day at weeks 9-10, 19-20 and 35-36; oligomenorrhoea or amenorrhoea in 25%, 65% and 43% of non-users of hormonal contraception across the three terms.
Funding and interests: Hughes 2018 — funder records (Crossref, Europe PMC) list the U.S. Department of Defense's Defense Health Program and the U.S. Army Research Institute of Environmental Medicine; the paper's conflict statement was not read. O'Leary 2021 — funded by the UK Ministry of Defence (Army); the authors declare no competing interests. Military employers funded both; no device maker or federation.

Key Results

OutcomeHughes 2018: 8 weeks, 91 recruitsO'Leary 2021: 44 weeks, 51 cadets
Trabecular density, distal tibia (4%)+1-2%+3.0% by week 44 (P < 0.001)
Cortical density, distal tibia (4%)−0.3 to −1.0%unchanged
Cortical density, shaft (30%)−0.7% (P < 0.001)−0.3% weeks 1-14 (P < 0.001), then recovered
Cortical thickness, distal tibia+1-2%+4.0% by week 44
Trabecular microarchitecture, distal tibiathickness and number up, separation down, 1-2%number +3.5%, thickness +2.1%, separation −3.1% by week 44; changes evident only from week 28
Estimated strength+2.5% (4%), +0.7% (30%)failure load +2.5% at 30% by week 44; unchanged at 4%
Serum markerssclerostin −5.7%; formation and resorption markers and 25(OH)D +10-28%no change in formation or resorption markers (measured weeks 1, 28, 44)
Size against precision (companion): the week-14 cortical dip at the shaft (−0.3%) was smaller than the least significant change for that measure (0.4%); the authors say some changes were close to or below it.
Stress fractures: 1.6% of the companion cohort, lower than earlier reports on the same course (11.2% in the Discussion; 11.4% in the Introduction) — the authors suggest training changes or recruitment bias; the woman with a tibial stress fracture left the study, so she is not in the scans.
Whole-body bone density: unchanged in the companion; rib aBMD fell from week 1 to 14.

Mechanisms

Measured: trabecular density and structure and cortical thickness rose while cortical density fell (8 weeks) or dipped and recovered (14 weeks); estimated strength rose at the shaft.
Authors' interpretation (Hughes): rising formation markers indicate new bone formation; rising resorption markers with falling cortical density are consistent with intracortical remodelling. (O'Leary): temporary falls in cortical density support a role of intracortical remodelling in stress fracture; the cortical dip coincided with the reported median time to stress fracture on the course (102 days); military training stayed osteogenic for 44 weeks despite periods of low energy availability and menstrual disturbance.
Inference: the cortex at the tibial shaft — where most tibial stress fractures in military training occur (companion) — spends roughly the first two to three months of new impact loading remodelling, whatever the heart rate says; this matches SYNTH-75's inference that the first 8-12 weeks of running are the tendon's adaptation window (its cyclist row's practice is 6-8 weeks before fast, hilly or long running) but does not say how fast to add running.
Inference: estimated strength did not fall at any point measured, and the dip is within measurement precision for an individual; "vulnerable window" is a reading of remodelling biology plus injury timing, not a measured drop in strength.

Practical Application

Why build running slowly when my aerobic fitness is fine? In military women, new impact loading made the tibia stronger within 8-14 weeks, yet shaft cortical density dipped then (remodelling). Bone scans, not injuries. B-.
Give new running load a runway of about 2-3 months before fast, long or hilly running (inference from bone markers, longer than SYNTH-75's practice of no fast running, hills or long runs in the first 6-8 weeks; its 8-12-week figure is an inference about tendon, and these scans favour the longer end): this applies to the aerobically fit newcomer too — a cyclist's engine does not load the tibia (REF-896; REF-1313 for injuries in men from non-axial sports).
Bone pain is a stop signal, not a training response (not from this paper; the base's practice in REF-622): localised bony tenderness, or pain at rest or at night, goes to a clinician (SYNTH-73).
Do not read these figures as a dose: the loads were military (boots, marching, very high energy expenditure), the subjects women, the outcomes scans; no ramp rate or weekly limit follows from them.
Fuel and menstrual function still matter (inference): the companion's bone adapted despite periods of low energy availability, but the women with stress fractures left the study, so the scans say nothing about who breaks.

Limitations

Marker outcomes. Density, structure and modelled strength, not fractures or performance; no non-exercising control (O'Leary says this is typical of such studies).
Hughes 2018 read from the abstract; its precision limits, attrition and conflict statement were not available.
Survivor bias (companion): only women who completed training were scanned; the stress-fracture case left the study (authors).
Precision: the companion's key early cortical dip (−0.3%) is below its least significant change (0.4%); some changes sit near it (authors).
Population and load: young military women, boots, load carriage and very high energy expenditure; TRANSFER to runners, to men and to masters athletes is inference.
Funding and interests: Hughes 2018 — U.S. Department of Defense Defense Health Program and USARIEM (funder records); O'Leary 2021 — UK Ministry of Defence (Army), no competing interests.
How this entry was checked and changed
Why this level

uncontrolled pre-post designs (rubric B) whose outcome is a bone marker, not injury or performance (B-); Hughes 2018 was read from the abstract only (the full text is not open), the O'Leary 2021 companion in full (accepted manuscript); military women in arduous mandated programmes, no non-exercising control, and the companion's early cortical dip is smaller than its own least significant change

Full text (checked 2026-10-27)
O'Leary 2021 (companion): seventy-seven women were invited and screened eligible, sixty-one volunteered, fifty-one women completed the study; six were removed from training for injury, including one tibial stress fracture (full text)
O'Leary 2021: trabecular vBMD increased from week 1 to 44 at the 4% site (3.0%, P < 0.001); cortical vBMD decreased from week 1 to 14 at the 30% site (−0.3%, P < 0.001) (full text)
O'Leary 2021: this decrease was close to the LSC (0.4%) reported for this measure (full text)
O'Leary 2021: training increased failure load at the 30% site from week 1 to 44 (2.5%, P < 0.001); cortical area (4.8%) and cortical thickness (4.0%) increased at the 4% site (full text)
O'Leary 2021: the incidence of stress fracture in this cohort was 1.6%; the median time to stress fracture on this course was reported as 102 days (full text)
O'Leary 2021: total daily energy expenditures 3332 ± 424, 3849 ± 363 and 3041 ± 286 kcal per day at weeks 9 to 10, 19 to 20 and 35 to 36 (full text)
O'Leary 2021: the study was funded by the UK Ministry of Defence (Army); the authors have no competing interests to declare (full text)