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Topic:Biomechanics

Biomechanics in horses refers to the study of the mechanical principles that govern movement and physical function in equine species. This field encompasses the analysis of gait, locomotion, and the forces exerted by and upon the horse's musculoskeletal system. Biomechanical studies often involve the use of motion capture technology, force plates, and computer modeling to assess how horses move and how various factors, such as conformation, training, and health status, influence their performance and soundness. Research in equine biomechanics contributes to understanding injury prevention, rehabilitation, and performance optimization. This page compiles peer-reviewed research studies and scholarly articles that explore the mechanics of movement, the impact of external and internal forces, and the applications of biomechanical analysis in equine care and management.
Left and right-side echocardiographic measurement of pulmonary artery stiffness in thoroughbred racehorses.
Journal of equine veterinary science    January 19, 2026   Volume 158 105784 doi: 10.1016/j.jevs.2026.105784
Bozzola C, Stucchi L, Sala G, Schinardi L, Stancari G, Zucca E.Background: Pulmonary Artery Stiffness (PAS) is a non-invasive echocardiographic index of pulmonary artery elasticity. In horses, PAS has been measured only from the right parasternal short-axis view, while the influence of alternative imaging windows remains unknown.Aims/objectivesTo assess the repeatability and reproducibility of Acceleration Time (AT), Maximal Frequency Shift (MFS), and PAS measurements from the left parasternal angled view of the right ventricular inflow/outflow, and to compare these parameters with those obtained from the right parasternal short-axis view in Thoroughbred ...
Multiplanar intersegmental angular velocity in the assessment of topline movement in horses.
American journal of veterinary research    January 19, 2026   1-10 doi: 10.2460/ajvr.25.09.0331
Ursini TL, Clayton HM, Levine D, Richards J.To measure intersegmental movement in the sagittal, dorsal, and transverse planes of the cranial thoracic to caudal thoracic, caudal thoracic to lumbar, and lumbar to sacral segments using range of motion and angular velocity as measures of quality of movement. Unassigned: 6-degrees-of-freedom spinal motion was measured at the walk and trot in 3 sound Thoroughbred and Thoroughbred cross horses, and the data were pooled, giving a total of 54 gait cycles at walk and 33 at trot. These were compared against 8 cycles at walk and 13 at trot from 1 Thoroughbred horse that was confirmed as having mode...
Risk assessment for condylar stress fracture in elite racing Thoroughbreds using standing computed tomography-based virtual mechanical testing.
Equine veterinary journal    January 18, 2026   Volume 58, Issue 3 674-681 doi: 10.1002/evj.70145
Brown NL, Irandoust S, Thom EJ, Whitton RC, Henak CR, Muir P.Condylar stress fracture of the third metacarpal bone (MC3) is a common catastrophic injury in Thoroughbred racehorses and is associated with parasagittal groove (PSG) subchondral osteolysis. Standing computed tomography (sCT) imaging enables sensitive identification of this fatigue-induced early subchondral bone injury (SBI), but there is no objective method for identifying racehorses at heightened risk of condylar stress fracture. Objective: To estimate PSG first principal strain in elite Thoroughbred racehorses that have undergone subjective risk assessment using sCT fetlock screening. Meth...
Validation of a handheld smartphone markerless gait-analysis tool using an estimated groundline in horses.
Equine veterinary journal    January 16, 2026   doi: 10.1002/evj.70149
Key K, Kirkegaard J, Berg K, Andresen KR, Skov Hansen S.A handheld smartphone-based computer vision algorithm (RealHorse® [RH]) offers accessible alternatives for equine gait analysis but requires validation against a gold-standard three-dimensional multicamera optical motion capture system (Qualisys® [QS]). Objective: To evaluate the accuracy and precision of RH in measuring vertical displacement signals (VDS) at the eye, withers, back and croup in horses trotting on a straight line and on a circle. Methods: Cross-sectional comparative validation study of a markerless computer vision algorithm. Methods: Fifty-nine horses were recorded while trot...
Functional and biochemical inflammatory responses to low-dose intra-articular recombinant equine IL-1β: a pilot study.
Frontiers in veterinary science    January 16, 2026   Volume 12 1746738 doi: 10.3389/fvets.2025.1746738
Korac L, St George L, MacNicol J, McCrae P, Jung L, Golestani N, Karrow N, Cánovas A, Pearson W.Low-dose intra-articular injection of recombinant equine interleukin-1β (reIL-1β) may offer a useful model for studying early onset or subclinical joint inflammation in horses. This pilot study aimed to determine the lowest intra-articular dose of reIL-1β required to produce biochemical evidence of synovitis, and to correlate synovitis biomarkers with functional, upper-body asymmetry parameters. Saline (control) and 50, and 75 ng reIL-1β were injected into the left or right intercarpal joint of three ( = 3) horses in a three-way crossover design. Synovial fluid was collected by aseptic art...
Comparative tensile properties of the equine vagina, penile sheath, and scrotum.
Journal of biomechanics    January 16, 2026   Volume 197 113169 doi: 10.1016/j.jbiomech.2026.113169
Collins AE, Wayne JS, Ferrando CA, De Vita R.This experimental study characterizes the elastic behavior of male and female equine genital tissues using uniaxial tensile testing, with strain measurements obtained via digital image correlation. Dog-bone-shaped tissue specimens were excised from mares and geldings (n=23 from female specimens and n=42 from male specimens) with all specimens aligned along the circumferential direction (CD) of the vagina, penis, and scrotum. The results include load-displacement data, stress-strain data, and tangent moduli for the penile sheath, vaginal canal, and scrotum, with strain measured in both the CD a...
Natural progression of tarsal osteochondrosis in Standardbred pacers and trotters.
Veterinary surgery : VS    January 3, 2026   doi: 10.1111/vsu.70073
McCoy AM, Lopp-Schurter CT, Bishop RC, Narotsky A, Grogger K, Kemper AM.To determine the natural progression of tarsal osteochondrosis (OC) in a cohort of Standardbred foals and assess the impact of gait preference (trotting vs. pacing). Methods: Longitudinal observational cohort study. Methods: Client-owned Standardbred foals (n = 148). Methods: Tarsal radiographs were taken every 2 months from 2 to 12 months of age and foals were video monitored to document time spent pacing or trotting. Differences between groups were assessed using χ analysis. Survival analysis was used to determine if lesion healing differed between groups over time. Results: Of 148 ...
Weekly riding frequency has a greater impact than rider skill level on western riders’ identification of stirrup asymmetry.
Journal of equine veterinary science    January 2, 2026   Volume 157 105758 doi: 10.1016/j.jevs.2025.105758
Jolley EN, Higgins AH, Logan AA.Asymmetry of stirrups and rider experience level has shown to impact rider symmetry and position. Objective: This study aims to determine if novice and advanced riders can detect stirrup asymmetry in the same frequency, hypothesizing that advanced riders would detect asymmetry in a greater frequency. Methods: Ten stock-type horses and 10 (5 advanced, 5 novice) riders, paired together to test random stirrup treatments. Riders were assigned to a horse for 2 rides, one with asymmetric stirrups, the other with symmetric stirrups, but blinded to their treatments. At the end of both rides, they took...
Group and Individual Changes in Spinal Mobility During a 12-Week Rehabilitation Program Including Swimming in Horses with Axial Musculoskeletal Lesions.
Animals : an open access journal from MDPI    December 30, 2025   Volume 16, Issue 1 103 doi: 10.3390/ani16010103
Pécresse B, Moiroud C, Hanne-Poujade S, Hatrisse C, De Azevedo E, Coudry V, Jacquet S, Audigié F, Chateau H.Locomotor disorders involving the spine are a major cause of impaired performance and early retirement in sport horses. Swimming is increasingly incorporated into rehabilitation protocols, but its effects on spinal biomechanics remain poorly understood. This prospective study evaluated changes in thoracolumbar mobility in sixteen sport horses diagnosed with cervical or thoracolumbar axial musculoskeletal lesions over a 12-week rehabilitation program comprising 4 weeks of land-based training followed by 8 weeks during which swimming sessions were incorporated three times per week. Weekly measur...
Evaluating the Accuracy of a Vision-Based Algorithm for Groundline Estimation in Trotting Horses Using Multiple Camera Angles.
Veterinary medicine and science    December 30, 2025   Volume 12, Issue 1 e70739 doi: 10.1002/vms3.70739
Key K, Berg K, Kirkegaard J, Andresen KR, Hansen SS.Equine lameness diagnosis largely relies on subjective visual assessments, which can be biased. Although marker-based methods, force plates and inertial measurement units (IMUs) provide objective measurements, they require specialized setups. Vision-based algorithms offer a portable, markerless alternative, but their accuracy needs thorough testing. Objective: To evaluate a custom vision-based algorithm for estimating the groundline across multiple camera angles, including handheld use in horses trotting on a treadmill. Methods: Experimental comparative study. Methods: Eight Standardbred trott...
Upwards or onwards? Assessment of objective gait quality parameters in three European horse breeds at walk and trot.
Journal of equine veterinary science    December 30, 2025   Volume 157 105764 doi: 10.1016/j.jevs.2025.105764
Gmel AI, Haraldsdóttir EH, Rosa TV, Lamas LP, Neuditschko M, Weishaupt MA.Europe has many horse breeds differing in conformation, gaits and genetics due to diverging breeding goals. Research studies comparing objective gait parameters between different horse breeds are limited and date back a decade. Objective: In this study, objective gait parameters were measured in three European breeds, namely Lusitanos (LUS; n = 52), Franches-Montagnes (FM; n = 311) and Swiss Warmblood (WB; n = 40) using an inertial measurement system (sensors at poll, withers, pelvis and all four canon bones) at walk and trot. Results: At walk and trot, LUS moved significantly slower than WB a...
Comparative biomechanical analysis of equine accessory carpal bone fracture repair: Cortical screws in lag fashion versus X-plate technique.
Veterinary surgery : VS    December 21, 2025   Volume 55, Issue 3 620-630 doi: 10.1111/vsu.70071
Gernhardt J, Reuter T, Mählmann K, Schulze N, Lischer CJ.To compare the feasibility and biomechanical stability of two surgical techniques for fixation of vertical plane fractures of the accessory carpal bone (ACB). Methods: Randomized experimental ex vivo study. Methods: Eight equine accessory carpal bones were included in a control group. A total of 20 equine cadaveric forelimbs were randomly assigned into two groups (n = 10 per group). Methods: Vertical plane fractures were created palmar to the extensor sulcus using an oscillating saw. In Group 1 (CS), fractures were stabilized with two 4.5 mm cortical screws in lag fashion. In Group 2 (XP...
Assessment of core stability in equestrian riders.
Folia medica Cracoviensia    December 17, 2025   Volume 65, Issue 3 71-83 doi: 10.24425/fmc.2025.156685
Skwierawska Z, Trybulec B, Jagielski P, Wodka-Natkaniec E.Core stability allows to control and properly perform movements of all body. Optimal core stability level depends on deep muscles capacity and neuromuscular control. Poor core stability is a risk factor for injury, especially during physical activity. Objective: The purpose of this study was to examine the differences in core stability between sport and recreational horse riders and people, who don't do any sports. We hypothesized that horse riders demonstrate better core stability performance. Methods: 75 people aged 15-30 (20 recreational horse riders, 20 sport horse riders and 35 non-horse ...
Standing CT-based finite element models efficiently identify regions of high mechanical strain in equine metacarpal subchondral bone.
Scientific reports    December 11, 2025   Volume 16, Issue 1 1166 doi: 10.1038/s41598-025-30921-6
Malekipour F, Whitton RC, Muir P, Lee PV.Stress fractures are common in racehorses, with the metacarpophalangeal (MCP) joint being the most frequently affected site as it is subjected to high-magnitude and high-rate cyclic loads during training and racing. These loads lead to repeated compressive stresses, resulting in subchondral bone (SCB) sclerosis, fatigue microcracks, and matrix damage that can progress to parasagittal fractures or palmar osteochondral disease (POD). The present study developed joint-specific 3D FE models and slice-based FE models using standing CT images for three trained racehorses, each presenting distinct SC...
Association Between Stride Parameters and Racetrack Curvature for Thoroughbred Chuckwagon Horses.
Sensors (Basel, Switzerland)    December 4, 2025   Volume 25, Issue 23 7376 doi: 10.3390/s25237376
van den Broek M, Chan ZYS, De Bruyne C, Garcia-Alamo K, Skotarek Loch S, Pfau T.Increased risk of musculoskeletal injury in galloping racehorses has been linked to decreased stride length and reduced speed over consecutive races prior to the injury. As racetrack curvature influences horses' maximal speed, we hypothesized it also affects stride parameters. During training sessions, twenty-eight wagon-pulling Thoroughbred Chuckwagon horses were equipped with Global Navigation Satellite System (GNSS) loggers, allowing for identification of speed, stride length (SL) and stride frequency (SF), and average speed, SL and SF were calculated for consecutive 100 m sections. Effects...
Biomechanical comparison of the bone-screw-fastener to conventional cortical buttress screw in a simulated ex vivo model of equine midbody proximal sesamoid bone fracture repair.
Veterinary surgery : VS    November 25, 2025   Volume 55, Issue 5 953-964 doi: 10.1111/vsu.70060
O'Brien TJ, Johnson JW, Kawcak CE, Gadomski BC, Carpenter RS, Nelson BB.To compare implant failure and gap displacement characteristics of simulated medial mid-body proximal sesamoid bone (PSB) fractures repaired with bone-screw-fasteners (BSF) or cortical screws (CS) in single or double screw configurations. Methods: Ex vivo experimental study. Methods: A total of 14 paired equine cadaver forelimbs. Methods: Medial mid-body PSB osteotomies were created in each forelimb. Surgical repair was performed using either: (1) single 3.5 mm BSF (BSF), (2) single 4.5 mm CS (CS), (3) two 3.5 mm BSFs (BSF), or (4) two 3.5 mm cortical screws (CS) (n = 7 repairs/gro...
Preliminary Biometric Study on Symmetry of Hoof Solear Aspect in Forelimbs in Four Horse Breeds.
Animals : an open access journal from MDPI    November 21, 2025   Volume 15, Issue 23 3369 doi: 10.3390/ani15233369
Stachurska A, Wnuk E, Łuszczyński J, Donderowicz W.Correct conformation and symmetry of bilateral hooves are associated with a properly balanced hoof, which has a significant impact on the biomechanics of movement and reduces the risk of injury. The aim of the study was to compare solear aspect dimensions of bilateral hooves in forelimbs (hoof width, length, frog width, length, medial and lateral diagonals) with regard to sex, breed and age of horses. The dimensions were measured with a calliper in 100 horses of four breeds. The results showed that the breed factor significantly affected all dimensions. Age was significant for some variables, ...
Could “wild horses” drag you away? Quantifying muscular architecture in the forelimbs of extant, non-domestic equids (Perissodactyla: Equidae).
Anatomical record (Hoboken, N.J. : 2007)    November 20, 2025   doi: 10.1002/ar.70092
De Ridder T, Aerts P, MacLaren JA.Equid evolution is characterized by a high diversity of extinct species and morphologies, whereas extant equids share a superficially similar, monodactyl morphology. This inferred musculoskeletal similarity of modern equid limbs remains unexplored, and it is often assumed that domestic horse limbs are representative for wild equids (e.g., zebras, onagers, etc.). Our aim was to quantitatively describe the muscle architecture and arrangement of all forelimb muscles in extant wild Equus species to test this assumption, and investigate any differences between the species. We hypothesized that ther...
Statistical approaches for estimating forelimb ground reaction forces in foals during walking and trotting.
Journal of biomechanics    November 16, 2025   Volume 194 113078 doi: 10.1016/j.jbiomech.2025.113078
Opolz MD, Sipes GC, Moshage SG, McCoy AM, Kersh ME.Equine models are useful in biomechanics research due to their similarity in musculoskeletal tissue to humans, their athletic nature, and rapid skeletal development which permits ontogenetic studies. However, a continuing challenge in musculoskeletal models for large animal biomechanics is measuring the ground reaction force (GRF) during locomotion and therefore few reports of biomechanical measures such as joint torques. Here we evaluate two statistical approaches for estimating forelimb ground reaction forces in foals (n = 3). Longitudinal motion capture, GRF, and subject mass data during ...
Impact of the technology to monitor horse behaviour and health: a scoping review.
Journal of equine veterinary science    November 14, 2025   Volume 155 105734 doi: 10.1016/j.jevs.2025.105734
Giannone C, Atallah E, Dalla Costa E, Benetti E, Santolini E, Tassinari P, Bovo M.Technology for monitoring behaviour and health of horses has evolved significantly, presenting new opportunities and challenges in equine management and healthcare. This scoping review consolidates recent findings on wearable sensors and non-contact technologies, discussing their benefits and limitations. Among these advancements, heart rate monitors stand out as a key point in equine monitoring. By tracking heart rate variability, devices help monitor cardiac autonomic regulation, optimise training regimens and allow early detection of cardiac issues, although accuracy can be compromised by i...
The effect of repeated shocks on the low back during horse riding.
Journal of biomechanics    November 13, 2025   Volume 194 113067 doi: 10.1016/j.jbiomech.2025.113067
Smit NA, Bos JE, van Dieën JH, Kingma I.Repeated shocks, such as those occurring during horse riding, may result in a risk of overloading the low back. This study investigated accelerations and angular changes in the low back during horse riding, using Inertial measurement units (IMUs) on the pelvis, and L4, L1, and T11 spine levels during 30 min of walking and 10 min of cantering in twelve female participants. The root mean squared (RMS) linear acceleration of each IMU and the transmission (signal transfer) between each pair of IMUs were calculated from the measured accelerations. Additionally, angular motions between IMU's were ...
Proof of concept of a wearable IoT-based system for monitoring respiratory rate and surface temperature in horses.
Journal of equine veterinary science    November 8, 2025   Volume 155 105729 doi: 10.1016/j.jevs.2025.105729
Farias BJP, Furtado DA, Barbosa do Nascimento JW, Neto JPL, de Morais FTL, Santos TLS, Vasconcelos AS, Silva RC, Alves JIP, Mcmanus C, Silveira RMF....This study presents the development and proof of concept of a wearable monitoring system designed to measure respiratory rate (RR) and coat surface temperature (CST) in horses. The device integrates an embedded system based on an ESP-32 microcontroller equipped with sensors for RR (strain gauge) and CST (DS18B20) Data are transmitted in real time via Wi-Fi and stored locally on SD cards. The system was validated against conventional manual methods under field conditions. Agreement between methods was evaluated using Intraclass Correlation Coefficients (ICC) and Bland-Altman plots. Results show...
Quantitative assessment of gait alterations and variability in sulky-driven Standardbred trotters after a standardised exercise test.
Equine veterinary journal    November 6, 2025   doi: 10.1111/evj.70114
Aarts RM, Smit IH, Ferraz GC, Rhodin M, Serra Braganca FM, Hernlund E.Assessment of equine metabolic and locomotory responses to quantified levels of physical exercise is needed to support the creation of tailored protocols for optimal training adaptation. The locomotory response to exercise in horses is not fully understood. Objective: To investigate the effect of a standardised exercise test (SET) on upper-body and limb kinematics and stride-to-stride variability in Standardbred Warmblood trotters. Methods: Experimental repeated-measures field study. Methods: Thirteen Standardbred trotters, equipped with nine inertial measurement units (IMUs), performed an inc...
Reliability, agreement and variability of a markerless computer vision algorithm for equine gait analysis under field conditions.
Equine veterinary journal    November 4, 2025   doi: 10.1111/evj.70109
Key K, Berg K, Kirkegaard J, Andresen KR, Skov Hansen S.Computer vision-based algorithms offer accessible alternatives for equine gait analysis but require thorough assessment under diverse conditions. Objective: To evaluate a proprietary vision-based algorithm's reliability in measuring vertical displacement signals (VDS) at the eye, withers and croup, alongside groundline estimation, for horses trotting on straight lines and circles under field conditions. Methods: Cross-sectional comparative study evaluating agreement, variability and reliability of a markerless computer vision algorithm. Methods: We obtained 67 handheld iPhone recordings from 3...
Determinants of stride parameters in Thoroughbreds racing in Japan.
American journal of veterinary research    October 28, 2025   Volume 87, Issue 2 ajvr.25.07.0273 doi: 10.2460/ajvr.25.07.0273
Takahashi Y, Pfau T, Tsuruoka F, Yoshida T, Edwards WB, Mukai K.To identify the race- and horse-level factors affecting stride parameters during Thoroughbred races in Japan. Unassigned: Global Navigation Satellite System sensors were attached to 921 horses (1,189 starts) participating in 83 races, with distances ranging from 1,000 to 1,800 m, held from April through July 2024. Stride frequency and stride length were calculated from speed spectrograms at 3 racing phases (phase 1, 200 m after gate open; phase 2, 10 m after reaching the final straight stretch; and phase 3, 130 m before the finishing line). Additionally, 10 variables (race distance, surface ty...
Ultrasonography and shear wave elastography of equine skin: a preliminary study.
American journal of veterinary research    October 16, 2025   Volume 87, Issue 1 ajvr.25.05.0186 doi: 10.2460/ajvr.25.05.0186
Braga C, Prazeres J, Borges J, Ribeiro G.To noninvasively characterize skin thickness and elasticity in healthy horses using B-mode ultrasound and shear wave elastography (SWE) and to verify whether there are differences between the skin of stabled and outdoor horses. Unassigned: 10 clinically healthy horses were enrolled in this study. Five animals were housed exclusively indoors (indoor group), and 5 had temporary or permanent access to an outdoor environment (outdoor group). All horses underwent bilateral ultrasonographic examination of 3 anatomical regions: neck, withers, and girth region caudal to the elbow. Skin thickness was a...
Dynamic Mobilization Exercises Improve Activity and Stride Parameters Measured with Accelerometry in Sedentary Horses.
Animals : an open access journal from MDPI    October 10, 2025   Volume 15, Issue 20 2943 doi: 10.3390/ani15202943
Saitua A, Pérez-Umbría J, García-Álamo K, Muñoz A.Dynamic mobilization exercises (DME) are an effective strategy to prevent musculoskeletal injuries and promote back health in sport horses. Previous studies focused mainly on multifidus muscle cross-sectional area, with limited data on locomotion and adaptation timing. This study evaluated locomotor changes using accelerometry, over 8 weeks of DME application in 14 sedentary horses: a DME group ( = 8) performing 10 different DME (3 neck flexions, 1 neck extension and 3 lateral bending exercises to each side), 5 repetitions of each DME per session, 3 sessions/week, and a control group ( = 6), t...
Mechanical characterization of the equine linea alba and finite element modeling of suture patterns effects on its closure.
Journal of the mechanical behavior of biomedical materials    October 10, 2025   Volume 173 107228 doi: 10.1016/j.jmbbm.2025.107228
Bollens J, Gheysen L, Verkade M, Stael J, Martens A, Segers P.Postoperative incisional complications are common in horses following abdominal surgery, which typically involves an incision through the abdominal wall along the linea alba. The linea alba is a fibrous band running in the craniocaudal direction along the ventral abdomen. This incision is closed with sutures, where the choice of suture pattern and surgical technique has shown to influence the rate of complications. Therefore, this study investigated how different suture patterns and variations influence the stresses in the tissue by combining experimental and computational biomechanics. The me...
The Effect of Hippotherapy Simulator-Assisted Therapy on Motor and Functional Outcomes in Children with Cerebral Palsy.
Medicina (Kaunas, Lithuania)    October 9, 2025   Volume 61, Issue 10 1811 doi: 10.3390/medicina61101811
Günay Yazıcı C, Özden F, Çoban O, Tarakçı D, Aydoğdu O, Sarı Z.: Horse riding simulators (HRS) provide rhythmic, repetitive, and multidirectional movements analogous to horseback riding, which may facilitate postural control, balance, and functional abilities in children with cerebral palsy (CP). This study aimed to investigate the effects of the HRS application on the muscle tone of the lower extremity, gross motor function, trunk postural control, balance, gait functions, and functional independence in children with CP. : A quasi-experimental study included 30 children with cerebral palsy (17 hemiparetic, 13 diparetic; mean age, 9.3 ± 3.2 years). All p...
A mathematical model of metacarpal subchondral bone adaptation, microdamage and repair in racehorses.
Journal of the Royal Society, Interface    October 1, 2025   Volume 22, Issue 231 20250297 doi: 10.1098/rsif.2025.0297
Pan M, Malekipour F, Pivonka P, Morrice-West AV, Flegg JA, Whitton RC, Hitchens PL.Fractures of the distal limb in Thoroughbred racehorses primarily occur because of accumulation of bone microdamage from high-intensity training. Mathematical models of subchondral bone adaptation of the third metacarpal lateral condyles are capable of approximating existing data for Thoroughbred racehorses in training or at rest. To improve upon previous models, we added a dynamic resorption rate and microdamage accumulation and repair processes. Our ordinary differential equation model simulates the coupled processes of bone adaptation and microdamage accumulation, and is calibrated to data ...