230 EQUINE EXERCISE PHYSIOLOGY 6Equine vet. J., Suppl. 34 (2002) 230-235

Summary

Scintigraphy has been used in numerous clinical settings toexamine horses to determine the origin of lameness problems,but it has not been used previously to monitor prospectivelythe skeletal responses of a group of similarly-trainedracehorses. Our hypothesis was that in naive Thoroughbred(TB) racehorses, initial treadmill training induces increasedradiopharmaceutical uptake in high-motion joints and in thedorsal third metacarpal bone (MC3). Eight previously-untrained TB racehorses underwent sequential skeletalscintigraphic examinations as they exercised daily for 9 weekson an inclined treadmill. At the end of Weeks 0 (pre-training),3 (trotting at 4.2 m/s and initial galloping), 6 (galloping at 7.5 m/s), and 9 (sprinting 600 m at 12.5 m/s), horses received140 mCi 99m Technetium-methylene diphosphonate i.v.followed by a standard skeletal scintigraphic forelimbexamination 2 h later. Views were graded for increasedradiopharmaceutical uptake by 3 co-investigators who wereblinded to horse identification, breed, sex, date, and clinicalfindings. Results were compared before and after training foreach skeletal location using the Mann-Whitney Rank SumTest with the level of significance set at P<0.05. Initialtreadmill training resulted in increased radiopharmaceuticaluptake in the carpus (P = 0.031), metacarpophalangeal joint(P = 0.021), proximal phalanx (P = 0.035), and distal phalanx(P = 0.003). Training did not affect dorsal MC3radiopharmaceutical uptake (P>0.05).

Introduction

Skeletal scintigraphy has been used in numerous clinical settingsto examine horses to determine the origin of lameness problems.It has been shown previously to have tremendous value in thediagnosis of various lameness conditions in the horse includingfractures of the dorsal cortical third metacarpal (MC3) bone(Koblick et al. 1989; Foreman et al. 1991), proximal palmarMC3 (Pleasant et al. 1992), tibia (Mackey et al. 1987; Johnsonet al. 1988; Pilsworth and Webbon 1988), humerus (Mackey etal. 1987), radius (Mackey et al. 1987), and distal phalanx(Keegan et al. 1993). Other lameness conditions in which it hasproven valuable include bucked shins (Koblick et al. 1989;Foreman et al. 1991), tarsocrural collateral ligament injury

(Boero et al. 1988), navicular disease (Keegan et al. 1996),metacarpophalangeal arthritis (Martinelli et al. 1994; Arthur andConstantinide 1995), metatarsophalangeal arthritis (Ross 1998)and muscle injury (Morris et al. 1991; Swor et al. 2001). The useof scintigraphy prospectively to monitor skeletal responses totraining in a group of similarly trained Thoroughbred (TB)racehorses has not been previously reported.

Skeletal scintigraphy has been used in previous studies tocharacterise the frequency, distribution, and severity of corticalbone response in dorsal MC3 disease in racing and nonracinghorses (Koblick et al. 1988; Foreman et al. 1991). Scintigraphyhas been shown to be 100% sensitive and 93.5% specific in theappropriate diagnosis of dorsal MC3 disease in TB racehorses(Foreman et al. 1991). Risk factors for increased dorsalmetacarpal/metatarsal scintigraphic changes in that study(Foreman et al. 1991) included breed (TB), forelimb (vs.hindlimb) and age (younger vs. older TB) . Thoroughbreds alsohad a surprisingly high rate of increased radiopharmaceuticaluptake in the dorsal aspect of the third metatarsal (MT3) bonewhen compared to the hindlimbs of Standardbreds andnonracing horses (Foreman et al. 1991).

Despite the clear value of scintigraphy in the diagnosis ofmany lamenesses, including dorsal MC3 disease, sequentialscintigraphs obtained longitudinally as TB racehorses trainhave not been reported previously. In this experiment,untrained TB horses underwent sequential scintigraphicexaminations as they trained for 9 weeks on an inclinedtreadmill prior to beginning other studies. Our hypothesis wasthat, in previously-untrained TB racehorses, initial treadmilltraining induces increased radiopharmaceutical uptake in high-motion joints such as the carpus and the metacarpophalangealjoint and in the dorsal third MC bone.

Materials and methods

All materials and methods used for this study were performedunder the approval and authority of the University of IllinoisInstitutional Animal Care and Use Committee.

Subjects

Eight previously-untrained TB horses (4 geldings, 4 fillies, meanage 3.75 years, range 3–5 years) were housed and trained in aclimate-controlled laboratory and fed hay and oats twice daily andwater ad libitum. All subjects were bred to be racehorses, werepasture-raised and acquired before they had any race training ofany kind. All were judged to be clinically sound based on physical,

Forelimb skeletal scintigraphy responses in previouslyuntrained Thoroughbreds undergoing initial treadmill trainingJ. H. FOREMAN*, S. K. KNELLER, A. R. TWARDOCK†, M. D. CHAMBERS† and O. J. INOUE

Departments of Veterinary Clinical Medicine and †Veterinary Biosciences, College of Veterinary Medicine,University of Illinois, 1008 West Hazelwood Drive, Urbana, Illinois 61802, USA.

Keywords: horse; exercise; bone; scintigraphy; radiopharmaceutical

*Author to whom correspondence should be addressed. Presentaddress: The Equine Centre, 4850 Davenport Creek Road, SanLuis Obispo, California 93401, USA.

J. H. Foreman et al. 231

lameness, neurological, upper airway endoscopic, complete bloodcount, serum biochemistry, fecal and serum equine infectiousanemia agar-gel immunodiffusion examinations.

Exercise

Horses were exercised daily on a high speed treadmill1 inclinedat 4% slope with no rider or lead weight added. Horses weretrained for 9 weeks according to a regimen which had beenstudied previously in non-naive TB racehorses training on aracetrack with a jockey mounted (Foreman et al. 1990a,b).Training velocities were 1.8 m/s (walk), 4.2 m/s (trot), 7.5 m/s(gallop in Weeks 1–6), 8.0 m/s (gallop in Weeks 7–9), and 12.5 m/s (sprint in Weeks 7–9). Each day horses walked 400 m tobegin and again to end the exercise period. In Weeks 1–6, horsesworked 6 days/week and were rested on the seventh day of eachweek. Daily distance and/or intensity were increased each weekaccording to the following schedule: Week 1 (trot 2400 m daily);Week 2 (trot 4000 m); Week 3 (trot 3200 m, gallop 1200 m);Week 4 (trot 2800 m, gallop 1600 m); Week 5 (trot 1600 m,gallop 2400 m); and Week 6 (trot 1600 m, gallop 3200 m). InWeeks 7–9, horses worked on 5 day cycles (n = 4), with 3 daysof trotting (1600 m daily) and galloping (3200 m daily) followedby sprinting (‘breezing’) exercise on the fourth day and rest onthe fifth day of each cycle (Foreman et al. 1990a,b). On sprintingdays, horses trotted 1600 m, galloped 800 m and sprinted 600 m.

Scintigraphy

At the end of Weeks 0, 3 (trotting and initial galloping), 6(galloping) and 9 (sprinting), horses each received 140 mCi 99m

Technetium (Tc)-methlyene diphosphonate (MDP) i.v. Standardskeletal scintigraphic forelimb surveys were performed 2 h laterusing a large-field-of-view gamma camera (Technicare 410)2.Lateral views were made from the distal radius to the digit. Onepalmar view was made of each forefoot and digit by manuallyextending the forelimb digit over the gamma camera which wasplaced in a horizontal plane for this view only.

Examinations were saved on films which were laterrandomised as to date and horse. Films were masked toprevent all examiners from knowing date or horseidentification. All views were examined subsequently andgraded for areas of increased radiopharmaceutical uptake by 3co-investigators (SKK, ART and MDC) who were blinded tohorse identification, breed, sex, date of examination, clinicalfindings of lameness and the results of the other film readers’examinations. Previous validation work on scintigraphicdiagnosis has shown that 4 radiologists were in agreementregarding positive vs. negative results for 99 of 101portosystemic scintigraphic studies (Sammi et al. 2001).Calculation of shunt fraction by the 4 radiologists was fraughtwith more variance, but the findings of increased uptakeversus no uptake (analogous to the methods employed ourstudy) were consistently in agreement between film readers(Sammi et al. 2001).

Graded locations included the carpus; dorsal MC3; palmarMC3; MC2 and 4; the metacarpophalangeal joint; the proximal,middle, and distal phalanges; and the navicular bone (distalsesamoid). Uptakes in those locations were graded as 0 (normal,no increased radiopharmaceutical uptake), 1 (mildly increaseduptake), 2 (moderately increased), or 3 (severely increaseduptake, intensity consistent with a fracture). If increased uptakewas observed, areas of increased uptake were characterised aseither focal or diffuse. This scintigraphic grading system hasbeen reported previously (Keegan et al. 1993).

Statistical analysis

A skeletal location was recorded as increased in uptake only ifa minimum of 2 of the 3 film readers independently reportedincreased uptake. Results of grading for each location beforeand after training were compared using the Mann-WhitneyRank Sum Test (SigmaStat 2.03)3 with the level of significanceset at P<0.05.

Results

Data from scintigraphs performed in weeks 0 (pretraining) and 9(post-training) are summarised in Table 1. Examples of a normalpretraining metacarpophalangeal joint and digit are depicted inFigures 1a and 2a. Post-training scintigraphs of the same jointand digit are depicted in Figures 1b and 2b.

Carpus

Increased uptake was observed after training in 8 of 16 carpi(P = 0.031). Increased carpal uptake consisted mainly ofdiffusely increased uptake in the distal row, primarily in thethird carpal bone. This increase was usually evident on boththe dorsal and lateral views, although increases on the dorsalviews were more common. Carpal uptake did notprogressively increase over the 4 examinations in all affectedhorses (n = 4), but was increased when examinations werecompared before and after training (P<0.05), primarilybecause 3 horses with no previous carpal activity hadincreased uptake only at the Week 9 examination.

TABLE 1: Summary of forelimb scintigraphy results beforeand after 9 weeks of initial treadmill training of 8 naïveThoroughbred horses. Numbers in each column represent thenumber of horses or sites with increased radiopharmaceuticaluptake detected in each area of interest by a minimum of 2 of3 examiners blinded to horse identification, age, sex, date ofscintigraphic examination and clinical findings

Week 0 Week 9No. No. No. No.

Region of interest horsesa sitesb horsesa sitesb

Carpus 1 1 4 8*MC2/4 1 1 2 2MC3 (dorsal) 0 0 0 0MC3 (palmar linear) 1 1 1 2Metacarpophalangeal 1 1 6* 8*jointProximal and middle 2 2 5 8phalanges (all)c

Proximal phalanx 0 0 4 7*(palmar linear)d

Distal phalanx 2 3 6* 12*Navicular bone 2 3 5 8

aMaximum number of horses possible with increased uptake = 8;bmaximum number of sites possible with increased uptake = 16;cincludes the remainder of the proximal phalanx distal to the mostproximal portion immediately in contact with themetacarpophalangeal joint. The most proximal portion wastabulated in the summary of increased uptake in themetacarpophalangeal joint; dsubset of the previous line (allproximal and middle phalangeal increased uptake), in which onlyincreased uptake in a narrow palmar linear pattern was tabulated.*Significantly increased (P<0.05) over pretraining as determinedby Mann-Whitney Rank Sum Test.

232 Forelimb responses to training

Metacarpus

Training did not affect dorsal MC3 radiopharmaceutical uptakeat any time (P>0.05). Rare increases in uptake in a palmar linearpattern were observed in MC3 but were not significantlyincreased over the 9 week training period (P>0.05). Minimalchanges were observed in MC2 and 4, but they were notsignificantly increased over the training period (P>0.05).

Metacarpophalangeal joint

Increased uptake was observed after training in themetacarpophalangeal joint, with 6 of 8 horses (P = 0.021) and 8of 16 joints (P = 0.029) affected. Sequential scintigraphsdemonstrated gradually increasing uptake in themetacarpophalangeal joint over the 3 training measurementperiods (Weeks 3, 6 and 9). Increased uptake occurred in thedistal end of MC3, the proximal aspect of the proximal phalanx(Fig 1b) and/or the proximal sesamoids (Figs 1b and 2b).

Proximal and middle phalanges

Increased uptake in the remainder of the proximal and middlephalanges was uncommon. When only linear palmar increaseduptake in the proximal phalanx (Fig 1b) was analysed separately,there was a significant increase with training, with 7 of 16proximal phalanges (P = 0.035) affected.

Distal phalanx

Increased uptake was observed after training in 6 of 8 horses (P = 0.031) and 12 of 16 distal phalanges (P = 0.003). Nearly allthese changes were progressive over the 9 week training periodand, usually, consisted of diffuse grade 2 uptake in one or both

wings of the distal phalanx, with increased uptake evident onboth the lateral (Fig 1b) and palmar (Fig 2b) views.

Navicular bone

Increases in navicular bone uptake were common but were notsufficiently frequent to be increased significantly aftertraining (P>0.05).

Discussion

Training-induced changes manifested earliest in themetacarpophalangeal joints, where changes could be seen as earlyas 3 weeks in some horses, with a gradual progression in severityof uptake over the 9 week training period. This finding is notsurprising since this joint sustains considerable training stress inTB racehorses. For example, catastrophic injuries in this jointaccounted for half of the fatal appendicular fractures observed atCalifornia Thoroughbred racetracks over a 2 year period (Johnsonet al. 1994). Previous scintigraphic studies have demonstratedhigh frequencies of distal palmar MC3 lesions in TBmetacarpophalangeal joints (Arthur and Constantinide 1995) anddistal plantar MT3 lesions in Standardbreds (Ross 1998). Anothersummary of only scintigraphic palmar metacarpophalangeal jointlesions in racehorses showed that more intenseradiopharmaceutical uptake was negatively correlated withsuccessful return to racing form after injury (Martinelli et al.1994). Previous work in 21-month-old training naïve TB fillies (n = 6) showed that 8 weeks of treadmill training resulted in a

Fig 1b: Lateral view of post-training right front metacarpophalangealjoint and digit from a 4-year-old Thoroughbred gelding (Horse 7). Theproximal sesamoids (small downward arrow) are more diffusely intensein uptake (grade 2) than is distal MC3. There is a focal area (small right-handed arrow) of increased uptake (grade 2) in the proximal palmaraspect of the proximal phalanx at the metacarpophalangeal joint. Thereis a narrow linear band (large right-handed arrow) of increased uptakeon the palmar aspect of the proximal phalanx (grade 2). The distalphalanx (small upward arrow) is diffusely increased in uptake (grade 2).

Fig 1a: Lateral view of pretraining right front metacarpophalangealjoint and digit from a 4-year-old Thoroughbred gelding (Horse 7).Radiopharmaceutical uptake by distal MC3, the proximal aspect ofthe proximal phalanx, and the proximal sesamoids is uniform inintensity (arrow).

J. H. Foreman et al. 233

mean 8.2% increase in range of motion in the left frontmetacarpophalangeal joint (Corley and Goodship 1994). Ourstudy also documented that the bones comprising themetacarpophalangeal joint were stressed early in TB race training,by simple trotting work in our treadmill horses and that thosestresses accumulated further as horses progressed to galloping andthen to sprinting. While the structure of the metacarpophalangealjoint is designed to absorb considerable shock from the concussionof the limb hitting the ground or treadmill surface, the bonystructures making up the joint sustain considerable stress,resulting in considerable impetus to remodel with training (Figs 1band 2b), with 75% of the horses and 50% of the forelimb fetlocksaffected by 9 weeks of training in our study.

Carpal changes took longer in our study, with only one of 4affected horses manifesting changes prior to the lastexamination. These late appearing carpal increases may be dueto a gradual accumulation of training stress over the 9 weekperiod, finally resulting in some apparent bone remodelingmanifested as increased radiopharmaceutical uptake. Morelikely, these late developing changes illustrate that the sprintingexercise was more stressful to the carpus than was the earliertrotting and galloping work. Corley and Goodship (1994)showed previously that 8 weeks of initial treadmill cantering at7–11 m/s resulted in a 17.2% increase in the range of motion ofthe left carpus in previously naive young TB fillies. If canteringresults in such dramatic kinematic adaptations, it is only logicalto surmise that early sprinting (Weeks 7–9, galloping at 8.0 m/sand sprinting at 12.5 m/s) in young, naive horses would alsorequire biomechanical adaptation. Such adaptation might bemanifested as the late appearing carpal changes observed in ourhorses at Week 9 examination.

It has been estimated that as many as 70% of racing TBdevelop clinically apparent dorsal MC disease some time during

their racing careers (Richardson 1984). One recent study of 2-year-old TB racehorses in training (n = 226) reported an 86%incidence of bucked shins (56 of 65 horses not lost to follow-up),although considerably more horses (n = 161) were lost to follow-up than were included in the final report (Boston and Nunamaker2000). It is possible that our horses’ slightly older mean age of3.75 years contributed to their apparent lack of MC3 remodelling.Younger Thoroughbreds appear to be at risk for development ofdorsal MC3 injury (Foreman et al. 1990?) at least in part owingto differences in the geometric properties (minimum moment ofinertia) of the dorsal cortex of MC3 when compared to age-matched Standardbreds (Nunamaker et al. 1989). Nunamaker etal. (1990) have shown that as TB racehorses age, dorsal MCstrain decreases, implying either growth-related and/or training-induced adaptation to dorsal MC3 strain. Three-year-old TB andSTB metacarpi are not different when compared by fully reversedcyclic bending in vitro (Nunamaker et al. 1991). The implicationthroughout all these data sets, including ours, is that if TBracehorses were trained at ages later than the typical 2-year-oldstarter, then perhaps age-related MC maturation would minimisethe remarkably high incidence of dorsal MC disease currentlyseen in TB racehorses (Richardson 1984; Foreman et al. 1991;Boston and Nunamaker 2000). Conversely, some argue thatearlier introduction of 2-year-olds to shorter, more frequentintervals of high velocity work (15–16 m/s for 600 m) would alsocontribute to minimising the incidence of MC3 disease byallowing earlier exposure and adaptation to the velocitiesrequired to induce significant dorsal metacarpal strain (Bostonand Nunamaker 2000).

Velocity-related strain is another important factor in thematuration of dorsal MC bones. Previous studies on treadmillinduction of MC3 bone density have shown minimal

Fig 2a: Palmar view of pretraining right front metacarpophalangealjoint and digit from the same 4-year-old Thoroughbred gelding (Horse 7)as in Figure 1. Radiopharmaceutical uptake by distal MC3 and theproximal aspect of the proximal phalanx is uniform in intensity (largearrow). The sesamoids are not easily discernible since they are notincreased in uptake. The distal phalanx (small arrow) is uniformthroughout in intensity.

Fig 2b: Palmar view of post-training right front metacarpophalangealjoint and digit from the same 4-year-old Thoroughbred gelding (Horse 7)as in Figure 1. The proximal sesamoids (arrows) are easily discerniblesince they are focally increased in uptake (grade 1) and are readilydistinguishable from distal MC3 and the proximal aspect of the proximalphalanx. The distal phalanx is not uniform throughout in intensity, butinstead has a diffuse area (circled) of increased uptake (grade 2) in themedial wing, corresponding to the diffuse grade 2 uptake seenthroughout the distal phalanx in the lateral view (Fig 1b).

234 Forelimb responses to training

(Buckingham and Jeffcott 1991) to measurable (McCarthy andJeffcott 1991) increases in MC3 radiographic density with 7.8 and 12–14.5 m/s maximal training velocities, respectively.Davies and McCarthy (1994) demonstrated velocity-relatedhigher strains in younger TB dorsal MC3s when measuredduring treadmill training at speeds up to 16.5 m/s. The velocitywhich is normally reported as causing significantly increaseddorsal MC strain is approximately 15–16 m/s (Boston andNunamaker 2000), slightly faster than our horses sprinted, so itis possible that horses simply did not train fast enough. Ourtraining regimen mimicked that used typically by many TBtrainers in the field (Foreman et al. 1990a,b), although we werecautious about introducing too much speed too soon since thesehorses were training for other future research projects for whichthey were required to be sound (Foreman et al. 1995, 1996).Despite our top training speed of 12.5 m/s, velocities in thatrange (less than 15–16 m/s) still result in increased strain asvelocity increases (Davies and McCarthy 1994). Even moderatepasture turnout exercise results in measurably increased MC3bone density in young horses when compared to housed cohorts(Bell et al. 2001) and it was not, therefore, unreasonable toexpect some increased MC uptake in our horses even whentraining below the 15–16 m/s figure often cited as necessary toproduce significant MC strain (Boston and Nunamaker 2000).

Another possible explanation for our failure to demonstrateincreased MC3 uptake is the lack of rider weight borne by thehorses as they trained. Previous research on horses exercising ontreadmills showed that adding weight in a single bout of exercisecaused increased heart rate and plasma lactate and also affectedseveral measured kinematic variables (Sloet van Oldruitenborgh-Oosterbaan et al. 1995). In that study, there were no detectabledifferences between 90 kg of live weight of an experienced rider ordead weight in lead attached to the saddle. Either form of addedweight caused the subjects to work harder on the treadmill thanwhen they were not carrying any weight. It was hoped in the designof our study that the added 4% slope would compensate for the lackof weight borne by the horses as had previously been demonstratedwith a treadmill slope of 3.5% by Barrey et al. (1992).

Another possible explanation for our failure to demonstrateMC3 changes is that inclined treadmill locomotion may notadequately simulate racetrack locomotion for TB racehorses asthey train. Previous research on treadmill habituation has shownthat Dutch Warmblood horses adapt consistently to trotting onhigh-speed treadmills by the third time they are asked to trot onthe moving treadmill belt (Buchner et al. 1994) and sohabituation should not have influenced the training results overthe entire 9 week training session. However, it is possible that themovement of the treadmill belt in a caudad direction, as horsescanter, gallop or sprint at higher speeds, causes differences indorsal MC strain than that seen when galloping over ground.Ground reaction forces have been shown to be different simplybetween different equine treadmill designs (Roepstorff et al.1994). Recent work has shown that increasing treadmill sloperesults in earlier recruitment of trunk musculature whencompared to trotting on a flat treadmill (Robert et al. 2001).These apparent differences in inclined treadmill vs. groundlocomotion may also help to explain the inordinately highfrequency of increased palmar linear proximal phalangeal uptakeseen in our horses when compared to the lower frequency of thislesion seen typically in TB racehorses (Arthur and Constantinide1995). This change is more commonly seen in jumping than inTB racehorses, probably owing to the stress they sustain whenlanding at an angle to the ground after jumping an obstacle(Ehrlich et al. 1998; Twardock 2001).

Uptake in the distal phalanx was substantially increased with

training. The distal phalanx is clearly sustaining a considerableamount of concussion as these horses train. The increasedconcussion of the foot against the treadmill surface as opposed toa sand or grass outdoor training surface might contribute to thehigh frequency of distal phalangeal uptake in our study. Somehave speculated that rubber-belted treadmills induce considerablefriction between the landing foot and the moving belt, resulting inpersistently increased foot temperatures (T. Ivers, personalcommunication). These increased foot temperatures might induceincreased blood flow and result in increased delivery ofradiopharmaceutical to the distal phalanx. Despite the suddenbreakover which must be induced by the belt as it pulls the footcaudally toward the end of the stride, the toe region of the frontfeet was rarely affected. The wings of the distal phalanx weremuch more commonly affected than the toe (Fig 2b).

In our study, scintigraphy proved to be a simple, noninvasivemethod of monitoring bony response to training over a 9 weekperiod. It was concluded that initial treadmill training of TB horsesin this study resulted in increased bone activity in the carpus,metacarpophalangeal joint and proximal and distal phalanges, butnot in the dorsal metacarpus. The changes observed in themetacarpophalangeal joint, proximal phalanx, and distal phalanxwere progressive with increasing training stress over the 9 weekstudy period. We submit that treadmill training may be an effectivemethod for further studies on joint stress in high-motion forelimbjoints such as the carpus or metacarpophalangeal joint, but it didnot in this study stimulate increased dorsal MC3 exercise-inducedbone remodelling as would be expected in racetrack training ofracing age Thoroughbreds.

Acknowledgements

This study was supported by the Maria Caleel Fund for EquineSports Medicine Research. Funds for daily feeding and trainingwere provided by the United States Department of AgricultureHatch Funds and by the American Horse Shows Association.The equine treadmill at the University of Illinois was supportedwith funds provided by the Illinois Department of AgricultureThoroughbred and Standardbred Breeders Funds.

Manufacturers’ addresses

1Sato I, Equine Dynamics, Uppsala, Sweden.2QR Systems, San Antonio, Texas, USA.3SPSS Inc., Chicago, Illinois, USA.

References

Arthur, R.M. and Constantinide, D. (1995) Results of 428 nuclear scintigraphicexaminations of the musculoskeletal system at a Thoroughbred racetrack.Proc. Am. Ass. equine Practnrs. 41, 84-87.

Barrey, E., Galloux, P., Valette, J.P. and Auvinet, B. (1992) Comparison of heart rate,blood lactate, and stride length and frequency during incremental exercise testsin overground vs. treadmill conditions. Proc. Ass. equine sports Med. 11, 36-39.

Bell, R.B., Nielsen, B.D., Waite, K., Rosenstein, D. and Orth, M. (2001) Dailyaccess to pasture turnout prevents loss of mineral in the third metacarpus ofArabian weanlings. J. anim. Sci. 79, 1142-1150.

Boero, M.J., Kneller, S.K., Baker, G.J., Metcalf, M.R. and Twardock, A.R. (1988)Clinical, radiographic, and scintigraphic findings associated with enthesitis ofthe lateral collateral ligaments of the tarsocrural joint in Standardbredracehorses. Equine vet. J., Suppl. 6, 53-59.

Boston, R.C. and Nunamaker, D.M. (2000) Gait and speed as exercise componentsof risk factors associated with onset of fatigue injury of the third metacarpalbone in 2-year-old Thoroughbred racehorses. Am. J. vet. Res. 61, 602-608.

Buchner, H.H.F., Savelberg, H.H.C.M., Scharmhardt, H.C., Merkens, H.W. andBarneveld, A. (1994) Habituation of horses to treadmill locomotion. EquineVet. J., Suppl. 17, 13-15.

Buckingham, S.H.W. and Jeffcott, L.B. (1991) Skeletal effects of a long termsubmaximal exercise programme on Standardbred yearlings. In: Equine

J. H. Foreman et al. 235

Exercise Physiology 3, Eds: S.G.B. Persson, A. Lindholm and L.B. Jeffcott,ICEEP Publications, Davis, California. pp 411-418.

Corley, J.M. and Goodship, A.E. (1994) Treadmill training induced changes tosome kinematic variables measured at the canter in Thoroughbred fillies.Equine vet. J., Suppl. 17, 20-24.

Davies, H.M.S. and McCarthy, R.N. (1994) Surface strain on the dorsomedialmetacarpus of yearling Thoroughbreds at different speeds and gaits on atreadmill. Equine vet. J., Suppl. 17, 25-28.

Ehrlich, P.J., Seeherman, H.J., O’Callaghan, M.W., Dohoo, I.R. and Brimacombe,M. (1998) Results of bone scintigraphy in horses used for show jumping,hunting, or eventing: 141 cases (1988-1994). J. Am. vet. med. Ass. 213, 1460-1467.

Foreman, J.H., Bayly, W.M., Allen, J.R., Matoba, H., Grant, B.D. and Gollnick,P.D. (1990a) Muscle responses of Thoroughbred horses to conventional racetraining and detraining. Am. J. vet. Res. 50, 909-913.

Foreman, J.H., Bayly, W.M., Grant, B.D. and Gollnick, P.D. (1990b) Standardizedexercise test and daily heart rate responses of Thoroughbred horses undergoingconventional race training and detraining. Am. J. vet. Res. 50, 914-920.

Foreman, J.H., Hungerford, L.L., Twardock, A.R. and Baker, G.J. (1991)Scintigraphic appearance of dorsal metacarpal and metatarsal stress changes inracing and nonracing horses. In: Equine Exercise Physiology 3, Eds: S.G.B.Persson, A. Lindholm and L.B. Jeffcott, ICEEP Publications, Davis,California. pp 402-410.

Foreman, J.H., Grubb, T.L., Benson, G.J., Frey, L.P., Foglia, R.A. and Griffin, R.L.(1995) Physiological effects of shortening steeplechase in a 3-day event.Equine vet. J., Suppl. 20, 73-77.

Foreman, J.H., Grubb, T.L., Benson, G.J., Frey, L.P., Foglia, R.A. and Griffin, R.L.(1996) Acid-base and electrolyte effects of shortening steeplechase in a 3-dayevent. Equine vet. J., Suppl. 22, 85-90.

Johnson, P.J., Allhands, R.V., Baker, G.J., Boero, M.J., Foreman, J.H., Hyppa, T.,and Huhn, J.C. (1988) Incomplete tibial fractures in the horse: Two casereports. J. Am. vet. med. Ass. 192, 522-524.

Johnson, B.J., Stover, S.M., Daft, B.M., Kinde, H., Read, D.H., Barr, B.C.,Anderson, M., Moore, J., Woods, L., Stoltz, J. and Blanchard, P. (1994) Causesof death in racehorses over a 2-year period. Equine vet. J. 26, 327-330.

Keegan, K.G., Twardock, A.R., Losonsky, J.M. and Baker, G.J. (1993)Scintigraphic evaluation of fractures of the distal phalanx in horses: 27 cases(1979-1988). J. Am. vet. med. Ass. 202, 1993-1997.

Keegan, K.G., Wilson, D.A., Lattimer, J.C., Twardock, A.R. and Ellersieck, M.R.(1996) Scintigraphic evaluation of 99mTc-methylene diphosphonate uptake inthe navicular area of horses with lameness isolated to the foot by anesthesia ofthe palmar digital nerves. Am. J. vet. Res. 57, 415-421.

Koblick, P.D., Hornof, W.J. and Seeerman, H.J. (1988) Scintigraphic appearance ofstress-induced trauma of the dorsal cortex of the third metacarpal bone in racingThoroughbred horses: 121 cases (1978-1986). J. Am. vet. med. Ass. 192, 390-395.

Mackey, V.S., Trout, D.R., Meagher, D.M. and Hornof, W.J. (1987) Stress fracturesof the humerus, radius, and tibia in horses. Vet. Radiol. 28, 26-31.

Martinelli, M.J., Chambers, M.D., Baker, G.J. and Semevolos, S.A. (1994) A

retrospective study of increased bone scintigraphic uptake in the palmar-plantar fetlock and its relationship to performance: 50 horses (1989-1993).Proc. Am. Ass. equine Practnrs. 40, 53-54.

McCarthy, R.N. and Jeffcott, L.B. (1991) Treadmill exercise intensity and itseffects on cortical bone in horses of various ages. In: Equine ExercisePhysiology 3, Eds: S.G.B. Persson, A. Lindholm and L.B. Jeffcott, ICEEPPublications, Davis, California. pp 419-428.

Morris, E., Seeherman, H.J., O’Callaghan, M.W., Schelling, S.H., Paradis, M.R.,and Steckel, R.S. (1991) Scintigraphic identification of skeletal muscledamage in horses 24 hours after strenuous exercise. Equine vet. J. 23, 347-352.

Nunamaker, D.M., Butterweck, D.M. and Provost, M.T. (1989) Some geometricproperties of the third metacarpal bone: A comparison between theThoroughbred and Standardbred racehorse. J. Biomech. 22, 129-134.

Nunamaker, D.M., Butterweck, D.M. and Provost, M.T. (1990) Fatigue fractures inThoroughbred racehorses: Relationships with age, peak bone strain, andtraining. J. orthop. Res. 8, 604-611.

Nunamaker, D.M., Butterweck, D.M. and Black, J. (1991) In vitro comparison ofThoroughbred and Standardbred racehorses with regard to local fatigue failureof the third metacarpal bone. Am. J. vet. Res. 52, 97-100.

Pilsworth, R.C. and Webbon, P.M. (1988) The use of radionuclide bone scanningin the diagnosis of tibial ‘stress’ fractures in the horse: A review of five cases.Equine vet. J., Suppl. 6, 60-65.

Pleasant, R.S., Baker, G.J., Muhlbauer, M.C., Foreman, J.H. and Boero, M.J.(1992) Stress reactions and stress fractures of proximal palmar aspect of thethird metacarpal bone in horses: 58 cases (1980-1990). J. Am. vet. med. Ass.201, 1918-1923.

Richardson, D.W. (1984) Dorsal cortical fractures of the equine metacarpus. Comp.cont. Educ. pract. Vet. 6, 248-254.

Robert, C., Valette, J.P. and Denoix, J.-M. (2001) The effects of treadmillinclination and speed on the activity of three trunk muscles in the trottinghorse. Equine vet. J. 33, 466-472.

Roepstorff, L., Johnston, C. and Drevemo, S. (1994) The influences of differenttreadmill constructions on ground reaction forces as determined by the use ofa force-measuring horseshoe. Equine vet. J., Suppl. 17, 71-74.

Ross, M.W. (1998) Scintigraphic and clinical findings in the Standardbredmetatarsophalangeal joint: 114 cases (1993-1995). Equine vet. J. 30, 131-138.

Sammi, V.F., Kyles, A.E., Long, C.D., Mellema, L.M., Pollard, R.E., Kass, P.H.,and Hornoff, W.J. (2001) Evaluation of interoperator variance in shunt fractioncalculation after transcolonic scintigraphy for diagnosis of portosystemicshunts in dogs and cats. J. Am. vet. med. Ass. 218, 1116-1119

Sloet van Oldruitenborgh-Oosterbaan, M.M., Barneveld, A., and Scharmhardt,H.C. (1995) Effects of weight and riding on workload and locomotion duringtreadmill exercise. Equine vet. J., Suppl. 18, 413-417.

Swor, T.M., Schneider, R.K., Ross, M.W., Hammer, E.J. and Tucker, R.L. (2001)Injury to the origin of the gastrocnemius muscle as a possible cause oflameness in four horses. J. Am. med. Ass. 219, 215-219.

Twardock, A.R. (2001) Equine bone scintigraphic uptake patterns related to age,breed, and occupation. Vet. Clin. N. Am.: Equine Pract. 17, 75-94.