THE EFFECT OF EXERCISE ON THE EQUINE THIRD M E T A C A R P A L BONE:

A R E V I E W

Kevin Sherman DVM, Patrick Colahan DVM

Injury of the third metacarpal bone is common in racing Thoroughbreds, and catastrophic failure of the bone during racing is a leading cause of their humane destruction. The study of exer- cise on bone started with the work of Galileo in the seventeenth century, but remains poorly understood. The fol- lowing discussion focuses on recent research concerning the effects of exer- cise on bone strength, especially con- eerning the equine third metacarpal bone.

Longbone strength is determined by both its material strength and overall dimensions and has been investigated by direct and indirect techniques. Di- rect techniques generally involve test- ing of bone segments or complete bones to failure (mechanical breaking strength). Indirect techniques involve measuring factors related to bone strength including mineral content, den- sity, porosity, cross sectional dimen- sions and transcortical ultrasound ve- locity.

Saville and Whyte t were among the first to evaluate the effect of run- ning exercise on bone in a controlled study. Rats were subjected to alternat- ing periods of running and rest for up to

Authors' addresses: RR1, Box 24E, Brimfield, MA 01010

10 weeks. Although the bones increased in size with exercise no change in over- all bone strength or bone material strength could be shown. Similar ob- servations have been made concerning exercise induced bone hypertrophy in retrospective studies of adult human athletes. King et al. 2, examining the throwing arm of professional baseball pitchers radiographically, found uni- lateral humeral hypertrophy as a con- sistent feature. Jones et al. 3 radio- graphically demonstrated significant hypertrophy of the playing arm versus the passive arm humerus in adult tennis players.

Kiiskinen 4 showed that the inten- sity of exercise and individual maturity may also be important when evaluating the effect of exercise on bone. Several hundred mice were divided into groups receiving moderate and intensive tread- mill exercise for variable periods of up to 22 weeks. Femora from mice sub- jected to moderate training showed an increase in dry weight where mice sub- jected to intensive training had de- creased volume and length of long bones. This decreased volume and length may have been related to the program intensity however, as the mor- tality rate of intensive program sub- jects was more than twice that of con- trois. The changes in bone weight and volume tended to be transient when training was continued beyond matu- rity.

Woo et al. 5 examined the effect of prolonged (12 months) running exer- cise on the femur of immature swine. They found that prolonged exercise resulted in little change in bone compo- sition or biochemical content but caused a significant increase in cortical thick- ness and cross sectional area. They

suggested that prolonged exercise has significant effects on bone quantity, but not on its quality.

Schryver ° was among the first to examine the effect of exercise on equine bone. He reported the effect of trotting exercise on longbone strength in 18 month-old ponies. He found signifi- cant exercise induced increases in ra- dial and femoral cortical segment break- ing strength. In two following studies, by McCarthy and Jeffcott 7,s the effect of treadmill exercise on the equine third metacarpal bone was examined. In- creased dorsal cortical thickness re- suiting from maximal exercise was documented in both Standardbred and Thoroughbred yearling horses.

Nunamaker et al. have examined the third metacarpal bone of Thorough- bred and Standardbred racehorses to determine why Thoroughbreds have a higher incidence of dorsal metacarpal disease. They found that second mo- ments of area, a cross sectional prop- erty related to stiffness, increased more as result of age and training in Thor- oughbreds when compared to Standard- breds. 9 No significant difference could be shown between fatigue failure char- acteristics from third metacarpal bone samples taken from the two breeds, t° In a separate study they showed that increasing age and training may de- crease deformation (strain) of the Thor- oughbred third metacarpal bone at rac- ing speeds, tt They hypothesize that excessive third metacarpal bone defor- mation in young Thoroughbreds trig- gers the development of bucked shins.12

Because of the difficulty involved with directly measuring the effect of exercise on equine bone strength sev- eral non invasive methods have been devised. Pratt TM suggested that ultra-

Volume 14, Number 4, 1994 187

Figure 1, Diagram of the method used to determine ultrasound velocity through the third metacarpal bone. Redrawn from Jeffcott LB, McCartney RN: Ultrasound as a tool for assessment of bone quality in the horse. Vet Rec 116:337-342, 1985.

sound velocity is related to bone strength. In a study of 6-8 month-old Thoroughbreds by Glade et al., 14 a sig- nificant correlation was shown between third metacarpal bone ultrasound ve- locity and mechanical breaking strength.

Studies on the effect of exercise on third metacarpal bone ultrasound velocity have had conflicting results. Jeffcott et al. ~s, measuring metacarpal bone ultrasound velocity in 16 to 47 month-old Thoroughbred racehorses, demonstrated a significant progressive increase in ultrasound velocity associ- ated with training (Figure 1). However, McCarthy and Jeffcott 7 found that two- year-old Standardbreds subjected to maximal treadmill training had pro- gressively decreasing third metacarpal bone ultrasound velocities. In additional studies of adult horses 16, and yearlings, e McCarthy and Jeffeott demonstrated significant increases in third metacar- pal ultrasound velocity as a result of treadmill exercise.

Recently a non invasive technique has been developed which allows de- termination of longbone cross sectional area using ultrasound. A significant

transmit transducer

~u l la

cortex

I receive transducer

Figure 2, Cross sectional diagram of third metacarpal bone showing ultrasound pathway (shaded area) used in estimation of cross sectional area. Redrawn from Jeffcott LB, Buckingham SHW, McCarthy RN, Cleeland JC, Scotti E: Non-invasive measurement of bone: a review of clinical and research applications in the horse. Eq Vet J Supp 6, pp.71-79, 1986.

correlation could not be demonstrated between training and third metacarpal bone cross sectional area in a study of racing Thoroughbreds. 17 Because of the pathway of the ultrasound beam, the technique cannot detect cross sec- tional area changes caused by varia- tions in dorsal cortical thickness how- ever (Figure 2). Because training in- duced changes occur predominantly at the dorsal aspect, the technique is of questionable value in measuring exer- cises effect on third metacarpal bone cross sectional area.

Bone density (mass per unit area) and bone mineral content are related ideas that are indirectly related to bone strength. Traditionally bone mineral content is expressed as ashed bone weight per volume of bone tissue. Min- eral content has also been determined indirectly by examination of percent x- ray transmission (Vose and Kubalale). Results of this technique have been shown to be closely correlated with mechanical breaking strength. Most recently single photon absorbtiometry has been used as a non invasive tech- nique to evaluate bone density and bone mineral content.

Several of the previously de-

scribed studies examined the effect of exercise on bone density, mineral con- tent and porosity. Saville and Whyte t found no effect of treadmill exercise on bone mineral content in rats. Simi- larly, Woo et al. s found no change in bone density or mineral content re- sulting from exercise in swine. How- ever, two studies using single photon absorbtiometry have shown adult hu- man athletes have significantly greater bone density than do non athletes (Nilsson and Westlin 19, Williams et al.~). McCarthy and Jeffcott7, a and Buckingham and Jeffcott 21 used single photon absorbtiometry to evaluate the effect of treadmill exercise on equine bone mineral content. Mineral con- tent increased because of maximal treadmill exercise in yearling horses but not in submaximally exercised yearlings, maximally exercised 2 year- olds or adults. They concluded that both age and intensity of exercise have effects on equine bone mineral content. McCarthy and Jeffcott have also demonstrated a reduction in third metacarpal bone porosity in yearling e and two-year-old 7 Standardbreds sub- jected to treadmill exercise. However, histologic examination of the third

188 JOURNAL OF EQUINE VETERINARY SCIENCE

metacarpal bone from young racehorses that died of colic or other non orthope- dic diseases by Nunamaker et al.1 a have suggested that third metacarpal bone porosity may increase during training. The authors of this review have made similar observations from cross sec- tional microradiographs of Thorough- bred third metacarpal bones.

Clearly several factors influence the way in which bone responds to exercise. The work of Kiiskinen 4, and McCarthy and Jeffcott 7,s have shown that individual maturity and exercise intensity are important in the response of bone to exercise. Training method may also be important, as reports by Jeffcott et al. 15 and McCarthy and Jeffcott r have shown treadmill training reduces third metacarpal ultrasound velocity where track training does the opposite.

Several important changes have been shown as a result of exercise in the equine third metacarpal bone. Thick- ening of the dorsal cortex has been shown in yearling Standardbreds and Thoroughbreds as a result of intensive treadmill training. Second moments of area increase and load deformation de- creases in the third metacarpal bone of youngThoroughbreds as a result of age and training. Intensive, but not moder- ate, treadmill training results in an in- crease in yearling metacarpal bone min- eral content but not in two-year-olds or adults. Treadmill training has been shown to result in decreased cortical porosity in yearling and two-year-old Standardbreds.

Several additional factors con- cerning the effect of training and racing on the Thoroughbred third metacarpal bone need further research. The effect of training and racing on the overall strength of the third metacarpal bone is still not understood. Increases in bone material strength noted in ponies sub- jected to trotting exercise6 may not hold true in the third metacarpal bone of training and racing Thoroughbreds.

Changes in bone strength resulting from treadmill versus track training are still unknown. Clearly more research is needed if an attempt is to be made to reduce the number of injuries involv- ing the third metacarpal bone of Thor- oughbred racehorses.

References

1. Baville PD, Whyte MP: Positive effect of running exercise in the rat. C/in Orthop 65:81-88,1969.

2. King J, Brelsford H, Tullos H: Analy- sis of the pitching arm of the professional baseball pitcher. C/in Orthop and Re/Res 67:116123, 1969.

3. Jones HH, Priest JD, Hayes WC, Tichenor CC, Nagel DA: Humoral hypertro- phy in response to exercise. J Bone & Joint Burg 59a:204208,1977.

4. Kiiskinen A: Physicaltraining and con- nective tissues in young mice-physical prop- erties of achilles tendons and long bones. Growth 41:123-137,1977.

5. Woo SL, Kuei SC, Amiel D, Gomez MA, Hayes WC, White FC, Akeson WH: The effect of prolonged physical training on the properties of long bone:a study of Wolff's law. J Bone & Joint Burg 63a:780787,1981.

6. Schryver H: Bending properties of cortical bone in the horse. Am J Vet lies 39: 25-28,1978.

7. McCarthy R, Jeffcott L: Treadmill ex- ercise intensity and it's effect on cortical bone in horses of various ages. Equine Exercise Physio/ogy 3 Ed. SGB Persson, A Lindholm and LB Jeffcott, ICEEP Publica- tions, Davis CA pp 419-428 1991.

8. McCarthy R, Jeffcott L: Effect oftread- mill exercise on cortical bone in the third metacarpus of young horses. Research in Vet sci 2:28-37,1992.

9. Nunamaker DM, Butterweck DM, Pro- vost MT: Some geometric properties of the third metacapal bone: a comparison be- tween the Thoroughbred and Standardbred racehorse. J Biomechanics 22:129-134, 1989.

10. Nunamaker DM, Butterweck DM, Black J: In vitro comparison of Thorough- bred and Standardbred racehorses with re- gard to laocal fatigue fracture of the third metacarpal bone. Am J Vet Res 152:97- 100, 1991.

11. Nunamaker DM, Butterweck DM, Pro- vost MT: Fatigue fractures in Thoroughbred racehorses: Relationship with age, peak bone strain and training. J Orthop Res 18:604-611, 1990.

12. Nunamaker DM, Provost MT: The bucked shins complex revisited. Proceed-

ings of the 37th annual convention of the AAEP and cumulative index 1956-1991 pp 757-762.

13. Pratt G: An in vivo method of ultra- sonically evaluating bone strength. Proc AAEP 26:295-305,1980.

14. Glade M, Luba N, Schryver H: Effect of age and diet on the development of me- chanical strength by the third metacarpal and metatarsal bones of young horses. J Anita Sci 63:1432-1444, 1986.

15. Jeffcott L, Buckingham S, McCartney R: Non-invasive measurement of bone qual- ity in the horses and changes associated with exercise. Equine Exercise Physio/ogy 2. Ed. JR Gillespie and NE Robinson, ICEEP Publications, Davis CA pp 615-630 1987.

16. McCarthy RN, Jeffcott LB: Monitor- ing the effects of treadmill exercise on bone by non-invasive means during a progres- sive fitness programme. Eq Vet J supp 6:88-92, 1988.

17. Buckingham SH, McCarthy RN, Anderson GA, McCartney RN, Jeffcott LB: Ultrasolnd speed in the metacarpal cortex-a survey of 347 Thoroughbrads in training. Eq VetJ 4:i84 186, 1992.

18. Vose G and Kubala A: Bone strength- its relationship to x-ray determined ash con- tent. Hum Biol 31:262-270,1959.

19. NUsson BE, Westlin NE: Bone Den- sity in Athletes. C/in Orthop 77:179-182, 1971.

20. Williams JA, Wagner J, Wasnich R, Heilbrun L: The effect of long distance run- ning upon appendicular bone mineral con- tent. IVied and Sci in Sports and Exercise 16:223-227, 1984.

21. Buckingham SHW, Jeffcott LB: Skel- etal effects of a long term submaximal exer- cise programme on Standardbred yearlings. Equine Exercise Physiology 3 Ed. SGB Persson, A Lindholm and LB Jeffcott, ICEEP Publications, Davis CA pp 411-418 1991.

ARE YOU ON THE MOVE? Don't get Iostl

SEND YOUR NEW ADDRESS AS WELL AS

YOUR OLD ADDRESS TO Journal of Equine

Veterinary Science PO Box 1209

Wlldomar, CA 92595

Volume 14, Number 4, 1994 ira