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ANIMAL GROWTH, PHYSIOLOGY, AND REPRODUCTION |
* Department of Animal Sciences,
and
Large Animal Clinical Medicine, and
and
Department of Agricultural Engineering, Michigan State University, East Lansing 48824-1225
Abstract
The ability of short-duration high-intensity exercise to stimulate bone formation in confinement was investigated using immature Holstein bull calves as a model. Eighteen bull calves, 8 wk of age, were assigned to one of three treatment groups: 1) group-housed (GR, which served as a control), 2) confined with no exercise (CF), or 3) confined with exercise (EX). The exercise protocol consisted of running 50 m on a concrete surface once daily, 5 d/wk. Confined calves remained stalled for the 42-d duration of the trial. Blood samples were taken to analyze concentrations of osteocalcin and deoxypyridinoline, markers of bone formation and resorption. At the completion of the trial, calves were humanely killed, and both forelegs were collected. The fused third and fourth metacarpal bone was scanned using computed tomography for determination of cross-sectional geometry and bone mineral density. Three-point bending tests to failure were performed on metacarpal bones. The exercise protocol resulted in the formation of a rounder bone in EX as well as in increased dorsal cortex thickness compared with those in the GR and CF. The exercised calves had a significantly smaller medullary cavity than CF and GR (P < 0.01) and a larger percentage of cortical bone area than CF (P < 0.01). Dorsal, palmar, and total bone mineral density was greater in EX than in CF (P < 0.05), and palmar and total bone mineral densities were greater (P < 0.05) in EX than in GR. There was a trend for the bones of EX to have a higher fracture force than CF (P < 0.10). Osteocalcin concentrations normalized from d 0 were higher in EX than CF (P < 0.05). Therefore, the exercise protocol altered bone shape and seemed to increase bone formation comparison with the stalled and group-housed calves.
Key Words: Bone Development • Bovine • Confinement • Exercise
Introduction
The skeletal system senses changes to its loading environment and responds by adjusting bone mass and geometry. Unfortunately, management practices of many domestic livestock species may jeopardize skeletal integrity. Confinement housing decreases bone strength as a result of decreased loading (Knowles and Broom, 1990
; Marchant and Broom, 1996). The confinement of weanling and yearling horses in stalls decreases bone mineral content of the third metacarpus (Hoekstra et al., 1999; Bell et al., 2001). This may be especially detrimental to the young equine if it is then placed into a strenuous training regimen. Therefore, the relationships among confinement, bone strength, and skeletal injury need to be explored.
Only a few loading cycles, or individual loading events, stimulate an osteogenic response and ameliorate the reduction in bone mass observed with immobilization. Only four cycles per day maintained bone mineral density in immobilized turkey ulnae (Rubin and Lanyon, 1984), and 36 cycles prevented disuse osteoporosis in rat tibiae (Inman et al., 1999). Although externally applied loads in these experiments establish a defined strain regimen, they also create unusual, nonphysiological strains on the bone.
We suggest the completely in vivo and practical approach of using confinement, rather than immobilization, to produce disuse osteoporosis and then to ameliorate these effects via a controlled, short-term exercise program, precluding any unusual or artificial effects from a loading apparatus applied to the limb. The noninvasive exercise approach subjects the bone to elevated but physiologically normal strain patterns that would be impossible with external mechanical loading. Therefore, the goal of this study was to determine the extent of bone resorption that occurs after 6 wk of stall confinement and to determine whether short-term high-intensity exercise of a physiological nature can be used to promote bone strength in the immature animal.
Materials and Methods
Animals and Management
Eighteen Holstein bull calves, obtained from the Michigan State University Dairy Teaching and Research Center, were used in a randomized complete block experiment designed to test the effect of housing type and exercise on bone density and strength. The project was approved by the Michigan State University All University Committee on Animal Use and Care. Calves born at the Michigan State University Dairy Teaching and Research center were age-matched in groups of three, so that the average age of the three calves was 56.2 d (±2.6 d). If calves did not meet this criterion, they were excluded from the project. Individuals from each age-matched group were then randomly assigned to one of three treatments, resulting in six calves per treatment group. Two groups were housed in tie stalls (0.65 m x 1.55 m) that allowed the calves only to stand and lie down. One group remained in the tie stalls for the 6-wk duration of the project, with no access to exercise (CF), whereas the second group of stalled calves received controlled exercise (EX). The exercise regimen consisted of short-duration high-intensity running five times per week. Calves were removed from their tie stalls, led to an adjoining barn 32 m from their stalls and verbally encouraged to run through a 50-m concrete alley once per day. This approximated 25 strides, or cycles, per running bout at approximately 4 m/s (calculated from timing the running bouts). They were then returned to their stalls, with the entire distance traveled being 164 m. The final treatment (GR) served as a control; these calves were housed together in an 8.5-m x 7.3-m pen and allowed to exercise at will and interact with their pen mates. Calves were initially weaned on a milk replacer diet at 7 wk of age and thereafter had ad libitum access to a commercially available pelleted calf grower and allowed free access to water. Before the project, all calves were housed together in a pen of a size similar to that of the group-housed experimental calves. Calves were weighed on d 0, 21, and 42. After 42 d, calves were killed with a captive bolt, and both forelimbs were removed above the carpi to allow for collection of the fused third and fourth metacarpal bone. The left and right ninth ribs were collected as representative of bones not bearing weight, which should not have been affected by the treatments and thus could serve as a control for external factors that did not include loading effects.
Behavioral Observation
To determine the influence of voluntary activity on bone measures, observations of behavior were made over 24 h on d 0, 21, and 42. Calves were videotaped in either their tie-stalls or in the group setting with an extended play recorder, which allowed 24 h to be recorded on one tape. Behaviors were observed and recorded for four randomly chosen 15-min periods per 24 h. Because the primary objective of this project was to determine the influence of activity on bone parameters, behavior observations were limited to the activities that would load the bone and therefore impact bone strength. Therefore, no observations were made of ingestive or eliminative behavior or social interactions. Behavioral data were recorded with the Observer software (Noldus Information Technologies, Sterling, VA). Behaviors for stalled calves were defined as either standing or lying because they were primarily restricted to these activities. The behavior of the calves in tie stalls was analyzed for duration of the two activities, as well as the frequency at which the calves altered from one posture to the next. The frequency of lying down and standing up was determined as the number of occurrences of the behavior pattern that occurred in the observation period. Duration was recorded as the number of minutes in each of the four 15-min periods for which the animal stood or lay down summed together. The total duration of the behaviors was recorded as a proportion or percentage of time for which all occurrences of the behavior lasted over the observation session. Data were averaged over the six calves in each group. Similar observations were made for GR. Voluntary activity of the group-housed animals was further defined to include bouts of standing, walking, lying, trotting, loping, and jumping. Walking bouts were defined as four consecutive steps or one stride. If four consecutive steps were not taken, this behavior was characterized as simply standing. Trotting consisted of performing a definite two-beat gait and loping as a three-beat gait. Jumping was also included as a possible behavior, but was defined as an event vs. a state, and thus was not timed.
Measurements and Sample Collection
Blood was collected via jugular venipuncture into unheparinized Vacutainers for analysis of concentrations of osteocalcin and deoxypyridinoline, markers of bone formation and resorption, respectively. To determine how quickly bone responds to alterations of loading regimens, blood was drawn daily at 0800 for the initial 7 d, followed by once per week for the remainder of the project. Blood was allowed to coagulate at 20°C for 90 min and then centrifuged at 1,340 x g for 12 min for serum separation. Serum samples were frozen at –20°C for later analysis. Serum total pyridinoline concentrations were analyzed using Pyrilinks-D ELISA (Quidel Corp., San Diego, CA). Serum samples were diluted in a 1:4 ratio with double-distilled water. Osteocalcin concentrations were analyzed using Novacalcin, an ELISA kit obtained from Metra Biosystems Inc. (Mountainview, CA). Serum samples were diluted in a 1:30 ratio in order to obtain concentrations within the linear range of the standard curve. All assays were performed according to manufacturer’s instructions.
Computed Tomography
Following death, intact limbs, with the soft tissue intact, were imaged by computed tomography for determination of cross-sectional geometry and bone mineral density (BMD). Limbs were placed on a QCT phantom pad with hydroxyapatite bone standards for BMD analysis and scanned with a GE 9800 CT scanner (General Electric Medical Systems, Milwaukee, WI) at 80 kV, 70 mA, with a 2-s scan time. An initial scout view was taken to determine the midpoint between the proximal margin of the fused third–fourth metacarpal bone (metacarpal III–IV) and the distal physis. A single 10-mm-thick transverse image was then acquired at this selected location, which was chosen because it corresponded with the smallest diameter of the bone and thus the location at which the break was predicted to begin during the three-point bending test. This allowed evaluation of both bone density and cross-sectional area at the same location.
The endosteal and periosteal margins of the bone were traced on the CT computer, which then calculated the area within the region of interest to provide total, cortical, and medullary cavity cross-sectional area. The diameter of the bone was also measured, including the dorsopalmar bone diameter, dorsopalmar medullary diameter, lateromedial bone diameter, and lateromedial medullary diameter (Figure 1). These distances were measured at the line that bisected the bone for the dorsopalmar diameter and at the widest distance across the bone for the lateromedial diameter. Finally, the width of the individual cortices—dorsal, palmar, medial, and lateral—were measured, again in the same plane in which the previous measurements were made.
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Mechanical Testing
Ultimate bending strength, modulus of elasticity, and fracture force of the metacarpi were determined using three-point bending tests conducted on a universal testing machine (model 4202; Instron Corp., Canton, MA) according to ASAE standards (2000). A crosshead speed of 10 mm/min was used with supports set at 10 cm apart. Three-point bending to failure was performed on left metacarpi alone, whereas deformation to 4 mm was performed on the right metacarpi as well. Ultimate bending strength (stress) was calculated by the equation
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where
The moment of inertia for a hollow ellipse was calculated as follows (ASAE, 2000).
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Modulus of elasticity was determined by taking the mean of the data from deformation tests performed on both the right and left limbs. The force/deformation curve between 2 and 4 mm of deformation was used to calculate the slope of the straight line (F/
). This portion was chosen because it fell in the linear portion of the curve, which provided an r2 = 0.99. Modulus of elasticity, E, was calculated by
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where = deformation, m.
The modulus of elasticity of the ninth rib also was determined by three-point bending in order to test that the treatments imposed on the calves were not affected by factors other than loading. The crosshead speed and distance between supports were same as those used for the tests on the metacarpi. The curved portion of the ribs was cut 18 cm from the costochondral junction to provide a relatively flat sample for bending tests. Modulus of elasticity was calculated by the same formula as above, with the exception that the moment of inertia was calculated using the equation for a quadrant of an ellipse (ASAE, 2000; Figure 1
):
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Ash Determination
One-centimeter cross sections of bone were cut at the midpoint between the proximal margin of metacarpal III–IV and the distal physis in the same location as was scanned for computed tomography. An additional 5-mm cross section was made proximal to the carpus and further sectioned into 5-mm cubes to include the same cortical regions (dorsal, palmar, medial, and lateral) analyzed for BMD with the CT. Bone volume was determined by suspending the intact bone slice within a beaker filled with deionized water. The volume of the bone was calculated via Archimedes’ principle, so that the density of the bone was calculated using the following formula: density = (A – B)/P, where P is the weight of water, A is the weight of the bone out of water, and B is the weight of the bone submerged in water. A 1-cm section from the midpoint of the left rib was removed and ashed as described previously for the metacarpal bone samples. Fat was removed from all bone samples via ether extraction with a Soxhlet apparatus (Corning Inc., Acton, MA). Samples were then dried at 150°C for 8 h, weighed to determine the fat-free weight, placed in crucibles, and ashed in a muffle furnace at 600°C for 12 h. Ash was then weighed and either expressed as a percentage of the dry fat-free weight or based on the volume of the bone as determined with Archimedes’ principle.
Ca and P Concentrations
A 5-mm section of bone was sliced just distal to the midsection and then sectioned to be representative of the dorsal, palmar, medial, and lateral cortices of the bone, in the same location at which BMD was determined. Samples were lyophilized for 24 h, weighed, and then hydrolyzed in 1 mL of 6 M HCl for 18 h at 105°C, cooled, and stored at 4°C. For analysis of Ca and P, 500-µL samples were removed and dried completely via an ATR Vacuum Concentrator (Appropriate Technical Resources, Laurel, MD). The dried samples were reconstituted in 1 mL of 6 N HNO3, transferred to acid-washed volumetric flasks, and diluted to 25 mL with double-distilled water. Samples were diluted 200-fold in a 1% (wt/vol) lanthanum chloride matrix. Calcium concentrations were determined by flame atomic absorption absorptiometry (Unicom 989 AA spectrophotometer; Thermo Elemental, Franklin, MA). Phosphorus concentrations were determined by diluting the reconstituted samples 10-fold in deionized water and measured with a colorimetric assay against a standard curve of known phosphorus concentrations (DU 7400 spectrophotometer; Beckman Coulter, Holton, CA).
Statistical Analyses
Data were analyzed for the effects of treatment according to the GLM procedure of SAS (SAS Inst., Inc., Cary, NC). When treatment effects were significant, means were compared by LSD. A test for homogeneity of variance was performed using the Bartlett and Forsythe test. When variance was heterogeneous, the mixed procedure of SAS was used to analyze the data, and individual SEM for each treatment were then included in the tables. If the variance was homogeneous, pooled SEM were provided. To visualize changes over time in relation to initial values, some data were normalized or subtracted from d-0 values where appropriate. Correlations between BMD values, moment of inertia, and results from the mechanical testing were also calculated using Pearson’s product moment correlation. Durations of behaviors averaged over the three observation periods were tested for differences between groups using Fisher’s exact test.
Results
The average age of the calves at the beginning of the project was 56.2 ± 2.6 d. After 6 wk, there was a trend for the EX calves to have gained more weight than GR, 43.9 kg vs. 34.9 kg (P = 0.109), though not in comparison with CF (37.7 kg).
Behavioral Observation
Analysis of behavioral tapes made on d 0, 21, and 42 revealed that the group-housed calves rarely underwent any high-intensity exercise. When observational data were averaged over the 3 d, 63.4% of the group-housed calves’ time was spent lying down and 35.0% standing, whereas movement that would significantly load the bone was limited to 1.58% walking and only 0.02% trotting. In comparison, CF calves spent 35% of the time standing and 65% lying down, with EX calves standing 27% and lying 73% of the day. The frequency of postural shifts decreased over time in the confined calves, further reducing the load placed on the bone.
Computed Tomography
There were no differences between the right and left legs in the CT data; therefore, mean values were used for statistical analysis. Neither confinement nor exercise altered the total cross-sectional area of the bone; however, the medullary cavity of the bone was significantly smaller in the forcedly exercised calves vs. those in either CF or GR treatment (P = 0.006; Table 1; Figure 2). Cortical bone areas were also not different between groups, but when cortical area was expressed as a percentage of the total cross-sectional area, the confined animals had a smaller percentage of cortical bone compared with either of the calves on treatment GR or EX (P = 0.009; Table 1).
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). Interestingly, lateromedial bone diameter was the widest in GR and smallest in EX (P = 0.042), whereas CF was not different from either EX or GR (Table 2). Lateromedial medullary diameter was greater in CF and GR (owing to the overall larger medullary cavity) than EX (P = 0.003). There was no difference in dorsopalmar bone diameter, but there was a trend (P = 0.059) for the dorsopalmar medullary diameter to be greater in CF than in EX. The group-housed calves were not different from either group. There were no differences in the individual cortical diameters between groups, with the exception of the dorsal cortex, which was greater in EX than in either CF or GR (P = 0.003).
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). There was also a trend for the EX and GR to have greater bone mineral density in the lateral cortex (P = 0.053), whereas medial BMD did not differ between groups.
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Treatment differences seen in bone geometry and density were not reflected in mechanical properties. Although there was a trend for the exercised calves to have a higher fractural force than CF (P = 0.80), when the ultimate bending strength and apparent modulus of elasticity were calculated, which are basic material properties independent of geometry, there were no differences between groups (Table 4). Others have found bone mineral density (Les et al., 1994) or ash content (El Shorafa et al., 1979; Lawrence et al., 1994) to be predictive of mechanical properties, such as failure stress. In this study, there was no significant correlation between BMD and fractural force, ultimate bending strength, or modulus of elasticity. However, if bone mineral density values were compared with the calculated moment of inertia, there was a significant correlation between BMD in the medial cortex and moment of inertia (P = 0.034), whereas there was a weak correlation between total BMD and moment of inertia (P = 0.092). In general, any observed treatment effects on mechanical properties were manifested more in geometric terms (consistent with an effect on fractural force), rather than fundamental material properties.
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Serum Bone Markers
Mean serum osteocalcin concentrations were not affected by treatment (Table 5). However, when values were normalized from d 0, the EX group had higher concentrations of osteocalcin in relation to CF (P = 0.036), indicative of greater bone formation. Even though EX began the project with lower osteocalcin concentrations than CF, they later increased to almost consistently higher values than those seen in CF.
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Ca and P Concentrations
Phosphorus or Ca concentrations in the bone sections did not differ with treatment. Although Ca concentrations did not differ according to the region of the bone, P concentrations tended to differ by cortex (P = 0.080), with the medial and lateral cortices having higher concentrations of P than the palmar region (Table 6). Ratios of Ca to P were not different between treatments or cortices of the bone. Similarly, bone density of cross-sectional slices when calculated by Archimedes’ principle, percentage of ash expressed on a fat-free dry-weight basis, or density of bone calculated from ash weight and bone volumes did not differ with treatment. Differences between cortices in percentage ash were seen (P < 0.001) as well as a trend for ash weight expressed in relation to calculated bone volume (P = 0.681; Table 7). Ash percentage was greater in the dorsal and medial cortex vs. lateral and palmar ash percentage, with the palmar ash percentage significantly less than any other aspect of the bone. When ash weight was expressed relative to bone volume, the dorsal cortical samples were greater in density vs. the palmar sections.
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Bone resorption resulting from immobilization occurs quickly, with decreases in bone mineral density, stiffness, and ultimate load occurring after only 6 wk in rats (Inman et al., 1999). In the present study, stall confinement for a duration of 6 wk did not seem to adversely affect most properties of the bone when compared with group-housed calves. Although bone geometry and density were the least favorable in the confined calves, they were not different from the group-housed calves for most measurements, with the exception of the percentage of cortical area of the bone. Confinement of these calves probably did not alter the loading pattern of the bone beyond that experienced by the calves allowed free exercise. Owing to their own voluntary activity, the control animals (GR) did not differ from the exercise-restricted animals (CF) in the amount of voluntary activity experienced. Thus, the mechanical loading placed on the confined calves was not decreased greatly beyond the group-housed animals. The group-housed calves’ voluntary activity was minimal, with the majority of their time spent lying down. In fact, CF spent almost the same percentage of time lying down as GR, explaining the absence of relative differences in BMD. Short periods of walking in GR would presumably not appreciably load the skeleton beyond that of standing in the stalled calves because of the slow speeds at which they walked. This is similar to the results seen by Skerry and Lanyon (1995), in which walking did not maintain bone mass in immobilized limbs. In humans, as much as 4 h of walking daily are needed to prevent bone loss (Anderson and Cohn, 1985). Therefore, in light of the limited activity of GR, it is difficult to make conclusive statements in regards to our EX group compared with the control condition. It is also possible that the environment of GR was not stimulating enough to elicit greater amounts of activity. The authors suggest a pasture, rather than a pen setting, might elicit a greater treatment difference when compared with CF.
Although the confinement protocol did not result in reduction of bone mass, short-duration high-intensity exercise positively influenced both bone mass and structure beyond that seen in the calves allowed free exercise. It was anticipated that short bouts of running exercise might alleviate the reduction in bone mass that occurs with disuse; however, the improvement beyond that of the group-housed calves was unexpected. Again, this is presumably due to the rather sedentary pattern of activity in GR. Sudden jumps or movements, though not occupying an appreciable percentage of daily activity, may be more important than the influence of standing or walking. Roosters running on a treadmill sustained loads of 500 microstrains (unitless measure of the amount of deformation/original length of the bone; 1 million microstrains = 1 strain), whereas intermittent high-magnitude strains greater than 1,000 microstrains were recorded during normal background activity (Konieczynski et al., 1998). Similarly in sheep, high-magnitude strains of 1,150 microstrains are recorded during startling responses (Skerry and Lanyon, 1995). Whalen and Carter (1988) predicted that bone mass is more dependent on stress magnitude than number of cycles, and as little as a single loading cycle applied to the foot of a sedentary individual equivalent to that produced by running could lead to a density change of 61%. Therefore, our hypothesis that daily running exercise as short as a 50-m distance can result in bone hypertrophy was substantiated.
The increased growth rate in EX could have contributed to the differences in bone mass and density, as a larger body mass will, of course, produce more strain. Unfortunately, feed intake, and thus growth rate, was not controlled in this experiment. However, as calves had ad libitum access to feed, feed availability should not have been a contributing factor. In humans, bone density in the calcaneus is predicted to be proportional to the square root of the body weight (Whalen and Carter, 1988). In 5-mo-old foals subjected to confinement or confinement with sprint training, growth rate had a significant effect on BMD of the lateral radius, with the faster growing foals having greater BMD (Firth et al., 1999). However, in mature cows, there was no difference in metacarpal weight or bone cortex composition between cows with large differences in age and weight (Field et al., 1999). If similar studies are undertaken, it would be of value to control feed intake so that the calves gain weight at a similar rate. This would eliminate the question of a heavier overall body weight contributing to greater bone mass or density due to the increased loading.
The exercise protocol certainly seemed to cause adaptation in metacarpal III–IV, producing a rounder, denser bone with less medullary cavity. In this study, the medulla of CF and GR metacarpal III–IV was larger than EX (23 and 15% greater, respectively), yet the total cross-sectional area did not differ. Other studies have found geometric adaptation of bone to occur while not changing the cross-sectional area of the bone. In rats, immobilization did not change total cross-sectional area but did alter cortical width, increasing the marrow cavity and decreasing the percentage of cortical area (Ma, 1999). Presumably, this would indicate that such modification occurred through endocortical bone resorption in the sedentary groups. Indeed, Ma (1999) found a decreased bone formation rate on the endosteal as well as the periosteal surface. The normal cyclical process of bone remodeling was not entirely halted because the increased resorption on the endosteal surface was accompanied by some new bone formation on the eroded surfaces. However, as mentioned previously, the confined calves were not very different from GR, and thus predictions of bone resorption occurring in the sedentary animals would be unsubstantiated. Alternatively and more likely, the greater cortical mass in EX is due to increased bone formation. Similar to our results, Loitz and Zernicke (1992) found that exercised roosters laid down bone endosteally along the anterior-posterior plane, which decreased the anterior-posterior endosteal diameter and increased cortical thickness. However, most studies have shown that new bone formation occurs on the periosteal surface of the bone rather than the endocortical surface (Raab-Cullen et al., 1994). We did not attempt to resolve whether this adaptation occurred via endocortical bone resorption or by greater bone formation, which would require labeling of the active surfaces or serial CT images. However, Raab et al. (1991) observed mineral apposition rate to increase in formerly sedentary sows subjected to exercise; the increase in osteocalcin seen in the EX indicates the occurrence of greater bone formation as opposed to bone resorption.
These data suggest that only short periods of high-intensity exercise are needed to cause adaptation in the young growing animal. However, the changes in bone structure and density were not highly reflected by the mechanical testing. Woo et al. (1981) also found that although exercise caused a reduction of the medullary cavity of the femur of swine and increased ash content, there were no differences in the mechanical properties of the bone. Loitz and Zernicke (1992) also found that whereas exercise altered bone geometry in roosters there was no difference in elastic modulus. Thus, exercise changed the quantity but not the actual quality of the tissue. Perhaps had the present study been continued for a longer period, it would have resulted in mechanically stronger bone tissue.
Changing geometry of the bone does change its resistance to bending and thus alters the strain experienced. Although true that bone density is the greatest factor governing strength of trabecular bone, the primary factor influencing the strength of long bones is the moment of inertia or shape (Whalen et al., 1993). Therefore, changing the shape of bone may result in a stronger structure than would result from merely an increase in bone density. Also, bone may adapt to decreased usage by reducing bone mass without overly affecting bone strength. The confined calves overall had a numerically larger total bone area with statistically more medullary area compared with EX. This larger, more hollow cylinder seemed to be mechanically similar to the smaller, more dense bone of the exercised calves.
In the present study, although BMD, as predicted by CT, differed between treatments, the Ca and P concentration were unaffected by treatment. This is somewhat difficult to explain as numerous studies have reported a high correlation between CT and analytical measurements of mineral concentrations. Phosphorus concentration did tend to differ between regions of the bone as did the percentage of ash. Similar to the CT BMD values, percentage of ash was greatest in the dorsal cortex and the least in the palmar cortex. However, no differences between treatments in physical measurements were observed. Owing to the small size of the cross-sectional image of the bone, the region of interest selected in each cortex for BMD determination was below the manufacturer’s recommendations and could have influenced the accuracy of this technique. Therefore, while useful in many studies with larger animals, with small bones, physical measurements may provide more reliable assessment of mineral content of bone than CT. Why differences between treatments existed in CT data is unknown, but they may be more related to differences in geometry between treatment groups.
Implications
The usefulness of the bovine model for studies concerning bone physiology and the effects of various management programs seems promising. This study indicated that subjecting young animals to an exercise regimen may be beneficial in strengthening the skeleton, especially when animals are placed into confinement where natural movement may be limited during the critical growth period. The exercise programs do not seem to need to be of any great duration, as our simple running program of once daily for 50 m was effective. It is also possible that an even shorter exercise protocol or only exercise every other day may be sufficient. In species where soundness and the ability to withstand a large amount of exercise are critical, such early training may be very beneficial. Whether such an exercise system is useful for other species needs to be tested.
1 Correspondence: 410 S. Third St., River Falls, WI 54022 (phone: 715-425-3704; fax:715-425-3785; e-mail: kristina.hiney{at}uwrf.edu).
Received for publication July 24, 2003. Accepted for publication February 17, 2004.
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  D. T. Shaw, D. W. Rozeboom, G. M. Hill, M. W. Orth, D. S. Rosenstein, and J. E. Link Impact of supplement withdrawal and wheat middling inclusion on bone metabolism, bone strength, and the incidence of bone fractures occurring at slaughter in pigs J Anim Sci, May 1, 2006; 84(5): 1138 - 1146. [Abstract] [Full Text] [PDF]
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