Short-duration exercise and confinement alters bone mineral content and shape in weanling horses

Effects of different winter paddock management of Thoroughbred weanlings and yearlings in the cold region of Japan on physiological function, endocrine function and growth

Effects of different winter paddock management of Thoroughbred weanlings and yearlings in Hokkaido, Japan, which is extremely cold in winter, on physiological function, endocrine function and growth were investigated. They were divided into two groups; those kept outdoors for 22 hr in the paddock (22hr group) and those kept outdoors for 7 hr in daytime with walking exercise for 1 hr using the horse-walker (7hr+W group), and the changes in daily distance travelled, body temperature (BT), heart rate (HR), HR variability (HRV), endocrine function and growth parameters were compared between the two groups from November at the year of birth to January at 1 year of age. The 7hr+W group could travel almost the same distance as the 22hr group by using the horse-walker. The 22hr group had a lower rate of increase in body weight than the 7hr+W group in January. In addition, lower in BT and HR were observed, and HRV analysis showed an increase in high frequency power spectral density, indicating that parasympathetic nervous activity was dominant. And also, changes in circulating cortisol and thyroxine were not observed despite cold environment. On the other hand, the 7hr+W group had higher prolactin and insulin like growth factor than the 22hr group in January, and cortisol and thyroxine were also increased. Physiological and endocrinological findings from the present study indicate that the management of the 7hr+W group is effective in promoting growth and maintaining metabolism during the winter season.

Metacarpophalangeal Joint Pathology and Bone Mineral Density Increase with Exercise but Not with Incidence of Proximal Sesamoid Bone Fracture in Thoroughbred Racehorses

Proximal sesamoid bone (PSB) fracture is the leading cause of fatal musculoskeletal injury in Thoroughbred racehorses in Hong Kong and the US. Efforts are underway to investigate diagnostic modalities that could help identify racehorses at increased risk of fracture; however, features associated with PSB fracture risk are still poorly understood. The objectives of this study were to (1) investigate third metacarpal (MC3) and PSB density and mineral content using dual-energy X-ray absorptiometry (DXA), computed tomography (CT), Raman spectroscopy, and ash fraction measurements, and (2) investigate PSB quality and metacarpophalangeal joint (MCPJ) pathology using Raman spectroscopy and CT. Forelimbs were collected from 29 Thoroughbred racehorse cadavers (n = 14 PSB fracture, n = 15 control) for DXA and CT imaging, and PSBs were sectioned for Raman spectroscopy and ash fraction measurements. Bone mineral density (BMD) was greater in MC3 condyles and PSBs of horses with more high-speed furlongs. MCPJ pathology, including palmar osteochondral disease (POD), MC3 condylar sclerosis, and MC3 subchondral lysis were greater in horses with more high-speed furlongs. There were no differences in BMD or Raman parameters between fracture and control groups; however, Raman spectroscopy and ash fraction measurements revealed regional differences in PSB BMD and tissue composition. Many parameters, including MC3 and PSB bone mineral density, were strongly correlated with total high-speed furlongs.

A Review of Three Decades of Research Dedicated to Making Equine Bones Stronger: Implications for Horses and Humans

Much research has been conducted in an attempt to decrease skeletal injuries in athletic horses. The objective of this literature review is to compile the findings of over three decades of research in this area, make practical recommendations, and describe how research can develop over the years. An initial study investigating the role of bioavailable silicon in the diets of horses in race training produced the unexpected finding of decreased bone mineral content of the third metacarpus subsequent to the onset of training. Further studies revealed this decrease to be associated with stall housing eliminating high-speed exercise, leading to disuse osteopenia. Only relatively short sprints (between 50 and 82 m) were necessary to maintain bone strength and as few as one sprint per week provided the needed stimuli. Endurance exercise without speed fails to elicit the same benefits to bone. Proper nutrition is also required for optimal bone health, but without the right exercise, strong bone cannot be maintained. Several pharmaceuticals may have unintended consequences capable of impairing bone health. Many of the factors influencing bone health in horses also exist in humans including a sedentary lifestyle, improper nutrition, and pharmaceutical side-effects.

The Impact of Circular Exercise Diameter on Bone and Joint Health of Juvenile Animals

Circular exercise is used in many equestrian disciplines and this study aimed to determine if circle diameter impacts juvenile animal forelimb bone and joint health. On day 0, 24 calves at 9 weeks of age were assigned the following exercise treatments: small circle (12 m clockwise), large circle (18-m clockwise), treadmill, or non-exercised control. Exercise was initiated at 1.1−1.5 m/s for 5 min/d and increased 5 min weekly until reaching 30 min/d. On day 49, synovial fluid was collected from multiple joints, cartilage was collected from the proximal surface of fused third and fourth metacarpi (MC III and IV), and forelimbs underwent computed tomography scans. A statistical analysis (PROC mixed) was performed in SAS 9.4. The inside leg of the small circle treatment had a larger MC III and IV dorsopalmar external diameter than the outside (p = 0.05). The medial proximal phalanx had a greater mediolateral diameter than the lateral proximal phalanx of the small circle treatment (p = 0.01). Fetlock nitric oxide was greater in the large circle and treadmill treatments (p < 0.0001). Cartilage glycosaminoglycan concentration was greater in the outside leg of the small circle exercise treatment than the inside leg (p = 0.03). Even at slow speeds, circular exercise diameter can impact joint and bone health, but faster speeds may have greater alterations.

Comparison of Bone Mineral Content of the Equine Third Metacarpal to Total Radiographic Bone Aluminum Equivalents From Unprocessed Digital Radiographs

Digital radiographs are common for estimating bone mineral content (BMC) in horses with radiographic bone aluminum equivalents (RBAE). Processing algorithms are used to produce clearer images of digital radiographs in clinical settings, but this distorts RBAE. This study compared RBAE from digital radiographs to BMC from bone ash, and physical and digital morphology measurements. Digital radiographs were taken of six third metacarpals (MCIII) from equine cadavers with an aluminum step wedge penetrometer. A 2-cm transverse section of bone was digitally analyzed and excised from each MCIII. For each image, a linear regression was created using the step wedge thicknesses (R² > 0.99) and used to obtain total and individual RBAE. Outer and inner diameters of both dorsal-palmar (DP) and lateral-medial (LM) aspects, as well as cortical thicknesses, were measured physically and digitally. Bone samples were ether-extracted, oven-dried, and ashed. Statistics were analyzed as correlations between variables. Physical and digital measurements of dorsal, medial, and lateral cortices, as well as DP outer diameter, tended to be similar (r > 0.74, P < .09) but palmar cortex measurements did not (r = 0.41, P = .42). The LM outer and DP/LM inner diameters were similar between the two methods (r > 0.95, P < .05). Total RBAE was strongly correlated to BMC (r > 0.93, P < .01). These results suggest that using digital radiographs to make morphological measurements is accurate and confirm the strong relationship between total RBAE and bone ash. However, this study emphasizes the need to use unprocessed radiographs for analysis.

Training Young Horses: The Science behind the Benefits

Simple Summary

Of common debate among equine professionals and enthusiasts alike, is whether entering race training at two years of age is detrimental or beneficial to the animal’s career and growth. This literature review evaluates epidemiological studies to elucidate that two-year-old horses are not at greater risk of injury compared to older horses. Horses which enter race training at two years of age are, in fact, found to have greater earnings and longer race careers. This review also tackles the impact that exercising an animal at two years of age or younger has on bone, articular cartilage, and tendons. Numerous studies on growing animals have found confinement to be detrimental to normal musculoskeletal growth. However, exercise of dynamic nature in moderate distances, such as that attained with pasture access or prescribed sprints, is beneficial to musculoskeletal development and may prevent injuries when entering race training. Based on scientific evidence, the research cited in this review supports the training and racing of two-year-old horses and advises caution in the use of medications such as corticosteroids.

Abstract

Conflicting research and anecdotal evidence have created disagreement among equestrians as to whether two-year-old horses should be trained and raced. The objective of this literature review is to evaluate epidemiological studies, as well as physiological data on equine bone, articular cartilage, and tendons to better determine the impact of training and racing two-year-old horses. The evaluation of numerous studies on the topic provides evidence that a horse which is trained or raced as a two-year-old has a lower risk of injury and better adapted tissues for the rigors of racing. Unfortunately, the current prolific use of pain-mitigating substances in the racing industry does place horses, including young cohorts, at greater risk of injury, and should be used with caution.

Sprint Exercise of Juvenile Animals Does Not Impact Cartilage Glycosaminoglycan or Synovial Fluid Neopeptide Collagenase Cleavage of Type I and II Collagen Content

Short sprints performed during growth can increase bone strength, mineral density, and cortical widths, but their impact on joint health is not fully understood. Some studies have found joints of young animals are damaged by forced sprints, while others found confinement hindered joint development. This study aimed to determine the impact of short sprints on synovial fluid neopeptide collagenase cleavage of type I and II collagen (C1,2C) and cartilage glycosaminoglycan (GAG) content. Calves were used as a model for young horses in this terminal study. Twenty-four Holstein bull calves were assigned to treatment groups of 1, 3, 5, or 0 days/week of sprinting (n = 6). A sprint consisted of an individual calf being walked from its stall and down a 71-m concrete aisle, at the end of which the calf was released, and encouraged to sprint towards the barn. Calves remained on treatment for 6 weeks and were euthanized on day 42. Immediately posteuthanasia synovial fluid was sampled from the middle carpal joint and cartilage discs sampled from 9 locations on the carpal bones. Analysis was performed in SAS 9.4 with fixed effects of treatment and bone. There were no differences in C1,2C or GAG concentrations between treatments (P = .73 and P = .96, respectively), but there was a difference between individual carpal bone GAG concentration (P = .03). Short sprints for 6 weeks during growth did not impact middle carpal joint C1,2C or carpal bone GAG content, and should be permitted given the benefits to bone health.

Effects of aquatic conditioning on cartilage and bone metabolism in young horses

While beneficial in rehabilitation, aquatic exercise effects on cartilage and bone metabolism in young, healthy horses has not been well described. Therefore, 30 Quarter Horse yearlings (343 ± 28 kg; 496 ± 12 d of age) were stratified by age, body weight (BW), and sex and randomly assigned to 1 of 3 treatments for 140-d to evaluate effects of aquatic, dry, or no exercise on bone and cartilage metabolism in young horses transitioning to an advanced workload. Treatments included nonexercise control (CON; n = 10), dry treadmill (DRY; n = 10), or aquatic treadmill exercise (H2O; n = 10; water: 60% wither height, WH). Horses were housed individually (3.6 × 3.6 m) from 0600 to 1800 hours, allowed turnout (74 × 70 m) from 1800 to 0600 hours, and fed to meet or exceed requirements. During phase I (days 0 to 112), DRY and H2O walked on treadmills 30 min/d, 5 d/wk. Phase II (days 113 to 140) transitioned to an advanced workload 5 d/wk. Every 14-d, WH, hip height (HH), and BW were recorded. Left third metacarpal radiographs on days 0, 112, and 140 were analyzed for radiographic bone aluminum equivalence (RBAE). Every 28-d, serum samples were analyzed for osteocalcin and C-telopeptide crosslaps of type I collagen (CTX-1), and synovial fluid samples were analyzed for prostaglandin E2, collagenase cleavage neopeptide (C2C), collagenase of type I and type II collagen, and carboxypeptide of type II collagen using ELISAs. All data were analyzed using PROC MIXED of SAS, including random effect of horse within treatment, and repeated effect of day. Baseline treatment differences were accounted for using a covariate. There were treatment × day interactions (P < 0.01) where OC and CTX-1 remained consistent in both exercise groups while inconsistently increasing in CON. There were no treatment differences (P > 0.30) in RBAE, BW, or HH, but all increased over time (P < 0.01). There were no treatment × day interactions of synovial inflammation or markers of cartilage metabolism; however, there was an effect of day for each marker (P<0.03). Changes in biomarkers of cartilage turnover in horses exercised at the walk, whether dry or aquatic, could not be distinguished from horses with access to turnout alone. This study indicates that early forced exercise supports consistent bone metabolism necessary for uniform growth and bone development, and that there are no negative effects of buoyancy on cartilage metabolism in yearlings transitioned from aquatic exercise to a 28-d advanced workload.

Calves, as a model for juvenile horses, need only one sprint per week to experience increased bone strength

Previous research has determined that maintaining young animals in stalls is detrimental to their bone health, while the addition of 50 to 82-m sprints 5 d/week aids in counteracting the reduction of bone strength from confinement. The current research aims to determine if 1 or 3 d/week of sprinting affords the same benefits to bone as 5 d/week of sprinting compared to animals confined with no sprinting. Twenty-four Holstein bull calves were obtained from the Michigan State University Dairy Cattle Teaching and Research Center. At 9 wk of age, calves were randomly assigned to treatments of 1, 3, or 5 d/week of sprint exercise, or to the confined control group sprinted 0 d/week. Each treatment had 6 calves. Individual sprinting bouts included a single sprint down a 71-m concrete aisle. For the duration of the 6-wk study, calves were housed at the MSU Beef Cattle Teaching and Research Center in stalls which afforded calves room to stand, lay down, and turn around. Serum was collected weekly via jugular venipuncture to obtain concentrations of osteocalcin (OC) and C-telopeptide crosslaps of type I collagen (CTX-1)—markers of bone formation and degradation, respectively. Sprints were videotaped weekly to determine stride frequency and sprint velocity. On day 42, calves were humanely euthanized at the Michigan State University Meat Lab and both front limbs were immediately harvested. Computed tomography scans and mechanical testing were performed on the left fused third and fourth metacarpal bones. Serum OC concentration was greatest for calves sprinted 5 d/week (P < 0.001). Calves sprinted 5 d/week had both greater stride frequency (P < 0.05) and lower sprint velocity (P < 0.05). All exercise treatments experienced greater dorsal cortical widths compared to control animals (P < 0.01). Through mechanical testing, fracture forces of all sprinting treatments were determined to be greater than the control treatment (P < 0.02). Results from this study support that sprinting 1, 3, or 5 d/week during growth can increase bone health and cause favorable alterations in bone markers. While all exercise treatments had over a 20% increase to fracture force, calves sprinted 1 d/week sprinted only 426 m over the 6-wk study and still experienced over a 20% increase in bone strength compared to confined calves. This study demonstrates the remarkably few strides at speed needed to enhance bone strength and emphasizes the danger to skeletal strength if sprinting opportunities are not afforded.

Influence of Long-Term Furosemide Use on Bone Mineral Content, Bone Metabolism Markers, and Water Weight Loss in Horses

Furosemide is used to reduce the incidence of exercise-induced pulmonary hemorrhage in racehorses. Previous research suggests furosemide negatively impacts calcium balance, which may have long-term implications for bone health. Eleven healthy horses, either control (CON, n = 5) or treatment (FUR, n = 6), were used to test furosemide's effects on bone mineral content (BMC), bone metabolism biomarkers, and weight loss after administration. Treatment horses received IV furosemide at 1 mg⋅kg^-1 BW once weekly for seven weeks, and blood was collected before and at 24 hours after administration for biomarker analysis. All horses were weighed before and at 2, 4, 8, 24, and 48 hours after administration. Radiographs of the left third metacarpal were taken every 28 days for BMC determination using radiographic bone aluminum equivalence. After administration, FUR lost more BW than CON (P < .05 for all) as quickly as 2 hours after administration (CON: -0.4 ± 0.3%, FUR: -2.2 ± 0.3%), and these losses remained greater than CON at 4 hours (CON: -1.0 ± 0.3%, FUR: -3.3 ± 0.3%) and 8 hours (CON: 0.0 ± 0.3%, FUR: -1.2 ± 0.3%). FUR lost more BW on day 0 than CON (P = .03), but on day 28 and day 49, FUR BW losses were no greater than CON (P > .10). No treatment effects were observed for BMC nor pyridinoline and osteocalcin concentrations (P > .10). Reduced BW changes over time in FUR but not CON warrant further investigation to establish the efficacy of frequent furosemide administration over long periods of time.

Omeprazole and its impact on mineral absorption in horses

Omeprazole is commonly used to treat and prevent ulcers in horses. As a proton-pump inhibitor, and with absorption of calcium dependent on gastric acidity, there is concern that reduction in acidity may interfere with calcium absorption. The present study was conducted to evaluate whether daily omeprazole administration can influence bone health in horses. Prior to commencement of treatment, 10 Standardbred geldings underwent endoscopic examinations for gastric ulcers and to determine gastric pH. They were fed a grass-hay diet. Three-day total collections of faeces and urine were performed to evaluate mineral balance. The left third metacarpus was radiographed to establish baseline values of mineral content by using a radiographic photo-densitometry technique to calculate radiographic bone aluminium equivalence, and blood samples were taken for examination of markers of bone metabolism. The horses were pair-matched and randomly assigned to either a treatment or the control group. Treated horses received omeprazole at the preventative dose (1 mg/kg bodyweight) and control horses received no medication for the duration of the 8-week study. They were housed in box stalls and had daily turnout on drylots. Endoscopy was performed again at the completion of the study, total collections were performed again at the middle and end of the study and blood samples were taken every 2 weeks. No meaningful treatment effects were found for ulcers, gastric pH, mineral balance, radiographic bone aluminium equivalence, or markers of bone formation. Daily administration of omeprazole did not appear to have any negative effects on indices of skeletal health measured in the study.

Bone mass and bone quality are altered by hypoactivity in the chicken

Disuse induces a rapid bone loss in adults; sedentarity is now recognized as a risk factor for osteoporosis. Hypoactivity or confinement also decrease bone mass in adults but their effects are largely unknown and only few animal models have been described. We have used 10 chickens of the rapidly growing strain 857K bred in a large enclosure (FREE group); 10 others were confined in small cages with little space to move around (HYPO group). They were sacrificed at 53 days and femurs and tibias were evaluated by texture analysis, dual energy X-ray densitometry, microcomputed tomography (microCT) and histomorphometry. Hypoactivity had no effect on the length and diameter of the bones. Bone mineral density (BMD), microCT (trabecular bone volume and trabecular microarchitecture) and texture analysis were always found significantly reduced in the animals of the HYPO group. BMD was reduced at both femur and tibia diaphysises; BMD of the metaphysis was significantly reduced in the femur but not in the tibia. An increase in osteoid volume and surfaces was noted in the HYPO group. However, there was no alteration of the mineral phase as the osteoid thickness did not differ from control animals. Bone loss was much more pronounced at the lower femur metaphysis than at the upper metaphysis of the tibia. At the tibia, only microarchitectural changes of trabecular bone could be evidenced. The confined chicken represents a new method for the study of hypodynamia since these animals do not have surgical lesions.

Medullary cavity diameter of metacarpal and metatarsal bones in cats. A cadaveric radiographic and computed tomographic analysis

OBJECTIVES

To compare radiography and computed tomography (CT) for the evaluation of medullary cavity diameters of metacarpal and metatarsal bones in cats and to analyse their correlation with intramedullary pin size.

METHODS

Radiography and CT were performed in cadavers (n = 12). The mid-diaphysis was determined and the medullary cavity diameter was measured by two observers. Each bone was osteotomized at the mid-diaphysis, Kirschner wires were inserted, and pin size was documented. Intra- and interobserver variability was evaluated (hierarchical analysis of variance). Radiography and CT were compared (correlation analysis, a t-test for dependent samples, a Bland-Altman analysis) and pin size, radiography and CT were compared (regression and correlation analysis).

RESULTS

Intra-observer variability was low for radiographs and CT, with an estimated median variation of 0.10 mm for radiographs and 0.12 mm for CT. Inter-observer variability was low, with a median variation of 0.03 mm for radiographs and 0.21 mm for CT. There was a high correlation between radiography and CT for the assessment of the medullary cavity diameter. A high correlation was found between pin size and measurement on radiographs, and between pin size and measurements on CT. In two cats, pin insertion was difficult or impossible.

CLINICAL SIGNIFICANCE

Mid-diaphyseal measurements are applicable methods for the assessment of the medullary cavity diameter in feline metacarpal and metatarsal bones regarding intramedullary pins. In some cats the medullary cavity diameter may be too small for intramedullary pinning techniques, which can be identified preoperatively on radiographs.

Comparison of two software packages for determining radiographic bone aluminium equivalent values

Radiographic photodensitometry has been used to track changes in bone mineral content in the distal limb of horses through traditional film radiographs. With increasing popularity of digital radiographs, the technique has been adapted for digital technology. The objective of this study was to validate and describe the analysis of digital radiographs and to compare traditional densitometric analysis to digital analysis. Twelve traditional and 12 digital radiographs were taken of the left third metacarpal (MCIII) of 12 horses with an aluminium stepwedge penetrometer attached to the radiographic cassette. Both medial-lateral and dorsal-palmar views were represented. The traditional and digital radiographs were each analysed using two different methods to compare results. Analysis involved determining optical density at each step of the aluminium stepwedge, plotting optical density verses mm Al, and then determining the equation of the subsequent line. The optical density of each cortice was inserted into the equation and radiographic bone aluminium equivalents (RBAE) in mm Al for each cortice was determined. Total RBAE was also examined 1 cm distal to the nutrient foramen of the MCIII. Data were analysed using Proc CORR (SAS v. 9.1) and are reported as mean ± standard deviation. Total RBAE on the dorsal-palmar view from traditional radiographs averaged 286±154 mm Al for digital analysis and 292±173 mm Al for densitometric analysis (R2=0.98). Total RBAE on medial-lateral view from the digital radiographs averaged 539±137 mm Al for digital analysis and 530±165 mm Al for densitometric analysis (R2=0.95). The results demonstrate high correlations between analyses, thereby validating that digital analysis yields similar results as densitometric analysis. Digital radiographs, with increased clarity as compared to traditional films, should allow greater ability to detect treatment differences in research trials, and monitor changes associated with training or nutrition.

Calcium absorption and bone density in immature horses fed two levels of crude protein

Research in other animal models indicate that high protein diets increase urinary calcium (Ca) excretion and may lead to a negative Ca balance and reduced bone density. This study was undertaken to determine the effects of two common levels of dietary protein intake on physiological parameters, Ca absorption, and bone density in immature horses. Sixteen 10-month old horses were blocked by age and sex into two dietary treatments. The control diet (C) was formulated using common ingredients to provide 100% of the National Research Council (NRC) for crude protein (CP), while the high protein diet (H) was formulated at 150% of NRC recommendations. Lower than expected dietary intake resulted in mean protein intake of 769±16 g/d in C and 978±18 g/d in H, equivalent to 91% and 116% of NRC, respectively. Blood, urine, and faeces were collected during the 112-day study to determine pH and mineral balance. Radiographs of the left third metacarpal were used to estimate bone density via radiographic bone aluminum equivalence (RBAE). Although urine pH decreased over time (P<0.001), no change in blood or urine pH was observed due to diet. Faecal pH, normalised to day 0, was significantly lower in H (P<0.02). Faecal Ca loss was greater in H (P<0.005); while Ca absorption and absorption as a percent of intake were lower for H (P<0.02). RBAE of dorsal and palmar cortices increased over time (P<0.001), but no differences were observed between diets. While excess dietary protein decreased faecal pH, increased faecal Ca excretion, and decreased Ca absorption, there appeared to be no effect on bone density over the course of this study.

Changes in shape of the Standardbred distal phalanx and hoof capsule in response to exercise

The aims of this study were to determine whether the equine distal phalanx changes in shape in response to exercise and to relate any osseous changes to those in the hoof capsule. Eighteen mature Standardbred horses were randomly divided into exercise and control groups. Exercised horses were jogged on a straight track at individual mean speeds between 4 and 8 m s− 1for 10–45 min, 4 days per week for 16 weeks. Both groups were similarly shod and pastured on the same field. Before and after the training period, each horse had digital photographs and magnetic resonance images (MRI) made of the right forehoof. Five linear measurements of the distal phalanx were recorded from the MRI and 24 measurements of the hoof capsule were made on the digital photographs. Small but significant changes in bone width (P = 0.039) were found in the controls and in two sagittal measurements of bone length (P = 0.039, 0.001, respectively) for the exercise group. These changes were slight and did not correlate with changes in shape of the hoof capsule, suggesting that the bone acts as a stable platform for supporting the capsule and withstanding loads.

The response of bone, articular cartilage and tendon to exercise in the horse

Horses can gallop within hours of birth, and may begin training for athletic competition while still growing. This review cites studies on the effects of exercise on bone, tendon and articular cartilage, as detected by clinical and research imaging techniques, tissue biochemical analysis and microscopy of various kinds. For bone, alterations in bone mineral content, mineral density and the morphology of the mineralized tissue are the most common end-points. Apparent bone density increases slightly after athletic training in the cortex, but substantially in the major load paths of the epiphyses and cuboidal bones, despite the lower material density of the new bone, which is deposited subperiosteally and on internal surfaces without prior osteoclastic resorption. With training of greater intensity, adaptive change is supervened by patho-anatomical change in the form of microdamage and frank lesions. In tendon, collagen fibril diameter distribution changes significantly during growth, but not after early training. The exact amount and type of protracted training that does cause reduction in mass average diameter (an early sign of progressive microdamage) have not been defined. Training is associated with an increase in the cross-sectional area of some tendons, possibly owing to slightly greater water content of non-collagenous or newly synthesized matrix. Early training may be associated with greater thickness of hyaline but not calcified articular cartilage, at least in some sites. The age at which adaptation of cartilage to biomechanical influences can occur may thus extend beyond very early life. However, cartilage appears to be the most susceptible of the three tissues to pathological alteration. The effect of training exercise on the anatomical or patho-anatomical features of connective tissue structures is affected by the timing, type and amount of natural or imposed exercise during growth and development which precedes the training.