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J Appl Physiol 97: 1859-1865, 2004. First published July 16, 2004; doi:10.1152/japplphysiol.00438.2004
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Adaptations to free-fall impact are different in the shafts and bone ends of rat forelimbs

J. M. Welch,1 C. M. Weaver,1 and C. H. Turner2

1Foods and Nutrition, Purdue University, West Lafayette 47907-2059; and 2Indiana University School of Medicine, Indianapolis, Indiana 46202

Submitted 27 April 2004 ; accepted in final form 12 July 2004


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ACKNOWLEDGMENTS
 REFERENCES
 
Impact exercise can have beneficial effects on the growing skeleton. To understand what changes it promotes in the shafts and ends of weight-bearing bones, we measured the effects of impact from repetitive free falls in growing rats. Fischer 344 female rats, 6.5 wk old, were assigned to one of three groups (n = 10 each). Controls were not dropped, whereas those subjected to impact were dropped from 30 or 60 cm. Rats in both free-fall groups were dropped 10 times per day for 8 wk. Leg bones were mechanically tested, and their cross-sectional area (CSA), cross-sectional moments of inertia, and volumetric bone mineral density (BMD) were measured by peripheral quantitative computed tomography. In the shafts of the forelimbs, but not the hindlimbs, free-fall impact resulted in greater ultimate breaking force, minimum and maximum second moments of area, and CSA but not BMD. In the bone ends of the forelimb and tibial bones, trabecular BMD increased but CSA did not. Landing from 30 and 60 cm produced peak impact forces of 12.0 and 16.7 times the standing forefoot weight for each front leg and of 4.5 and 7.7 times the standing hind foot weight for each hind foot. Overall, free-fall impact affected the forelimbs by increasing trabecular bone density in the bone ends and improving the strength at the shaft as a result of geometric improvements. These results indicate that adaptation to impact may occur by different mechanisms in bone end and shaft regions.

exercise; bone strength; ground reaction force; peripheral quantitative computed tomography; rats

OSTEOPOROSIS IS CURRENTLY a disease of epidemic proportions in Western countries. Aging bone is predisposed to fracture when thinning of the diaphysis is not offset by a sufficient increase in shaft diameter (27). Deterioration also occurs in the predominately trabecular bone ends, where age-related loss of bone is greater than in the shaft (28). The attainment of substantial bone strength by the end of growth may reduce the proportion of the population afflicted with osteoporosis many decades later (9). Specific exercise regimes are effective methods of optimizing peak bone mass and geometry. High strain rates produced by impact or torque from sports such as gymnastics and tennis increase bone mineral density (BMD) (23) and improve bone geometry (3, 19) in children. Although studies have reported that the benefits of exercise to bones may eventually be lost in both rats (24) and men (16) who subsequently resume a sedentary lifestyle, greater bone strength gained from exercise during growth is retained in women who continue their sports at reduced activity levels (18, 38).

We were interested in determining how the shaft and bone ends of growing leg bones adapt to impact forces. Our longer term goal was to develop a simple impact-loading protocol that could be used to investigate exercise-nutrient interactions. The ideal exercise protocol to evaluate this would be markedly osteogenic without alterations in body weight, bone length, or calcium absorption, which can be effects of aerobic protocols on young rats (13, 34, 37). A jumping protocol that requires rats to jump upward onto a platform from an electrical stimulus increased bone formation and has been implemented in several studies (11, 17, 29, 33, 36). However, because impact forces from landing exceed those from an upward jump of equal distance, and we wished to more closely mimic drop jumping performed by children, we tested the effectiveness of a protocol in which rats were dropped from specific heights.

Therefore, the purposes of this study were twofold: 1) to devise and test a simple method of mechanically loading both the bone shafts and ends of the long bones of growing rats and 2) to determine whether mechanical loading can improve the strength, density, or geometry in long bone ends as well as shaft regions in the appendicular skeleton in the rat. We hypothesized that the free-fall method of loading would increase the strength of leg bones in the shaft and the parameters indicative of greater strength in the bone ends.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 ACKNOWLEDGMENTS
 REFERENCES
 
Animals.   Thirty female Fischer 344 rats, aged 32 days, were purchased from Harlan Sprague Dawley (Indianapolis, IN) and housed individually in stainless steel cages with wire floors (25 x 20 x 18 cm3). Room temperature was maintained at 22–24°C with lights on a 12:12-h on-off cycle. Standard rat chow (C. H. Distributors, Lebanon, IN) and distilled water were provided ad libitum. To equalize the quantity of daily exercise between the control and free-fall rats, all rats were allowed an equal time period of daily exercise. Also, periods of free play activity more closely mimicked the activity levels of children for which our rats were models. Small groups of rats were placed in a circular, 75-cm-wide, aluminum pen that contained enrichment devices designed to encourage play behavior. An added benefit to the devices is that rats provided with such devices more accurately reflect human responses to disease (10). Animal protocols and care were approved by the Purdue Animal Care and Use Committee of Purdue University in accordance with the National Research Council's Guide for the Care and Use of Laboratory Animals. All rats received regular veterinary checks and were monitored daily for injury or evidence of soreness. Daily behavioral records were maintained.

Pretreatment.   Rats were habituated to their new care and handling regimen for 2 wk. During the first week, the rats were handled and weighed daily. At the end of the first week, they were randomly assigned to 1 of 3 treatment groups of 10 rats per group: rats dropped from heights of 30 cm (F30; free fall 30 cm) and 60 cm (F60; free fall 60 cm) and control rats that were not dropped. The mean weights of the groups did not differ significantly at the time of randomization. Rats in the F30 and F60 groups were gradually introduced to the free-fall procedure during the second pretreatment week. The number of repetitions and height of drop were gradually increased until the specifications of the protocol were reached.

Treatment.   Rats began 8 wk of full treatment at 6.5 wk of age, a period of age that spans early puberty into breeding age in this strain of rat (1). From the beginning of this treatment period, each rat assigned to a free-fall group was dropped from its designated height 10 times at a frequency of 1 drop per 11 s, 5 days a week, for 8 wk by the same handler. The 30-cm drop height was selected because a similar height of upward jumping produced a bone response (22), whereas the 60-cm drop height was expected to exceed the effects produced by all previous impact exercise models. The 11-s rest between drops was within the optimal 10- to 14-s rest insert for rats reported by others (7), and the number of drops per day had produced osteogenic effects in upward jumping rats (35). Each repetition began by the rat being lifted horizontally until its feet reached the specified height; the rat was then released so that it landed on all feet approximately simultaneously onto a bare floor. The protocol was visually monitored, as well as photographed and videotaped periodically, to verify the accuracy of the landings. Because we found that four rats could be dropped in sequence every 11 s, the daily loading for each group of four rats was completed in 110 s, which translates to <30 s per rat. All rats were weighed twice a week.

Preparation of specimens.   Rats were killed in prerandomized order rather than by treatment group to prevent a possible time of day bias. They were first injected with the anesthetic ketamine plus xylazine (90 mg/kg + 10 mg/kg), exsanguinated, and then placed in a jar with CO2 to ensure death. Bilateral humeri, ulnae, radii, femora, and tibiae with fibulae attached were collected from each rat. Bones from the left side were fixed in 10% neutral buffered formalin at 4°C for 7 days and then placed in 70% ethanol and stored at 4°C for later scanning by peripheral quantitative computed tomography (pQCT). Bones from the right side were wrapped in saline-soaked gauze and stored at –20°C for future mechanical testing. Storing bones by these methods minimizes changes in the relevant properties of these tissues (30).

pQCT Measurements.   The length of each bone was measured with digital calipers (model 500-474, Mitutoyo). Cross-sectional sites (1.0 mm thick) on the bones were scanned by pQCT (model Research SA+ of Norland Stratec XCT, Stratec Electronics, Pforzheim, Germany) by using a 0.46-mm collimation (4 x 105 counts/s) and a 0.08-mm voxel size. To identify the interior morphology at the location of each scan, representative bones were longitudinally bisected and slices mounted for viewing. Locations of sites scanned and the relationships between scan locations and bone morphology are depicted in Fig 1.



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  		Fig. 1. Locations of peripheral quantitative computed tomography scans on the humerus (A), ulna (B), radius (C), femur (D), tibia (E), and fibula with tibia attached (F). Bone morphology at each site is depicted. Scans at the proximal ulna transected the olecranon process.

 
Tomographs generated by pQCT were analyzed by BonAlyse software (version 1.3, Jyväskylä, Finland). Each bone scan was measured for volumetric BMD and cross-sectional area (CSA) of total bone. To separate soft tissue from bone, and trabecular from cortical bone, the thresholds of 375 and 900 mg/cm3 were used, respectively (26). Separation was performed by use of the software's mode P setting, which separates cortical and trabecular bone by use of a contour-detection algorithm equivalent to the Stratec software's contour mode 1, peel mode 2. Marrow was excluded in analyses and smoothing was not used. Diaphyseal sites were measured for BMD and CSA of the total bone as well as for trabecular and cortical fractions. The geometry of each diaphyseal scan was also assessed by calculation of the minimum and maximum second moments of area (Imin and Imax, respectively), which estimate the resistance at the scan location to bending in the weakest and strongest directions, respectively, by taking into account both the amount and distribution of cortical bone in that location (25). The coefficients of variation (CV) of the proximal ulna for this laboratory were 4.0% for CSA and 1.9% for total BMD, whereas the CVs for the shaft were 4.4% for CSA, 1.8% for cortical BMD, 3.8% for Imin, and 2.0% for Imax.

Biomechanical testing.   Mechanical tests were performed on the radius and femur by using a three-point bending to failure test (MTS Alliance RT/5 kN equipped with TestWorks 4 software, MTS Systems, Eden Prairie, MN). Bones were prepared for this test by thawing at room temperature (21–23°C) immediately before placement in the saline bath. Bones were placed on two pins separated by 15 mm in a 37 ± 1°C saline bath for 2 min before commencement of the test. Radii were positioned with the anterior of the bone facing downward, whereas femora were tested with their posterior side down; both were broken with a loading rate of 3.3 mm/s. The ulnae were broken at a loading rate of 0.50 mm/s by using an axial compression system that has been described elsewhere (25). Breaking force (N), stiffness (N/mm), and work to failure (N·mm) were calculated from force vs. displacement curves.

Ground reaction force determination.   Additional female Fischer 344 rats were utilized to measure the ground reaction forces (GRF) produced by rats landing from the two drop heights. Rats (67 days old and 139 g), four per height, were randomized to drop height on the basis of body weight. All GRF measurements were made on the same day. A 17.78 x 17.78-cm platform was attached above three 2.27-kg load cells (model P/N AL311AT, Honeywell Sensotec, Columbus, OH). The load cells were connected to a data acquisition card (National Instruments, Austin, TX) placed in a personal computer. Sampling frequency was 100 kHz. Rats were placed on an ink pad and then dropped 10 times each from 30 or 60 cm onto graph paper fitted to the force platform. Peak vertical forces were calculated by combining load cell results with individual paw placement in a custom program built in Labview 7 (National Instruments). The GRFs of standing rats were also measured to determine the proportion of body mass borne by the forefeet and hind feet when the rats were standing. Peak vertical forces were then standardized for fore and hind body mass on the basis of the standing data. Contralateral legs were assumed to bear equal force. Additionally, the landing stance, as determined by paw placement, at the two drop heights was investigated by comparing distances between forepaws, hind paws, and forepaws to hind paws at the moment of landing.

CVs for the peak landing forces were calculated for each animal using the pooled variance from its 10 drops. The CVs for the peak landing forces from the 30 cm height were as follows: total (4 ft) 5.2%, forefeet (2 ft) 10.9%, and hind feet (2 ft) 20.1%. From 60 cm, the peak landing force CVs for the total, fore, and hind forces were 10.2, 14.7, and 19.3%, respectively. CVs for paw distances were calculated the same way. The average CVs for the distance between both forepaws at landing, both hind paws, and the forepaws to hind paws were 11.9, 20.0, and 14.3% at 30 cm and 17.5, 17.2, and 8.9% at 60 cm, respectively. The precision calculated for the force plate measurements may not be representative of those for the free-fall procedure, because some of the rats were less adaptable than others to placement of their paws on the ink pad, which is required for the GRF measurements. The low sample size did not permit these animals to be evenly distributed within the groups.

Statistical tests.   Data are reported as means ± SE unless otherwise specified. Differences among groups were assessed for significance by ANOVA using SPSS software (version 11.0, Chicago, IL). Pairwise comparisons between means were performed by the Tukey's honestly significantly different test. Statistical significance was accepted at P < 0.05. Imin and Imax were calculated as percentage differences between the means of the control and treatment. SE error for all results, which are presented as percent differences, was calculated to include the variance in the control groups (2). Landing stance was compared by using ANOVA with body weight as the covariate.


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 ACKNOWLEDGMENTS
 REFERENCES
 
Effects on animals.   All rats that began the free-fall impact trial completed it without injury, as determined from daily inspections and veterinary opinion. Additionally, the rats in the free-fall groups exhibited no aversion to handling or the free-fall procedure, no vocalization on landing, and no decreased play behavior immediately after their daily treatment. The average weights of the rats did not differ between treatment groups, and no fluctuations in group weights were apparent during the trial (Fig. 2). The average lengths of the bones at the conclusion of the trial also did not differ between treatment groups.



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  		Fig. 2. Mean body weights for control (Con), 30-cm free-fall (F30), and 60-cm free-fall (F60) groups through the treatment periods. There were no significant differences between groups at any time.

 
Effects on bone strength and fracture site.   Bone strength results are presented in Table 1. The free-fall impact treatment resulted in increased strength in the ulnae and radii with no significant differences between the F30 and F60 groups in most properties tested. The site of fracture in the ulnae moved distally in the F60 group, from 44 and 45% from the distal end in the control and F30 groups, respectively, to 38% in the F60 rats (P < 0.5) (Fig. 3). The minimum width of the ulnae from the F60 rats was greater than those from the F30 rats at the 45% distal site (P < 0.05) but not at the 38% site.


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  		Table 1. Effects of free-fall impact on mechanical properties in the ulna, radius, and femur of growing rats

 


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  		Fig. 3. Ulnar breaking point: mean distance of fracture from the distal end of the ulna using the axial compression test. *Significant difference from Con, P < 0.05. {dagger}Significant difference between F30 and F60, P < 0.05.

 
Effects on BMD and CSA.   The effects of free-fall impact on total BMD and total CSA are reported in Table 2. Total BMD was increased in metaphyseal regions of all bones by the 60 cm drop height and also by the 30-cm height in the humerus. This was due to an increase in trabecular BMD in proximal locations (Table 3) but not in distal sites. In the shaft, BMD was not affected by free-fall impact (Table 2), and no differences in cortical BMD between free-fall and control rats were found at any bone end site (Table 3). Free-fall impact from 60 cm increased total CSA of all forelimb, but not hindlimb, bones (Table 2). The 30-cm drop height resulted in greater CSA in the shafts of the ulna and humerus. In metaphyseal areas, the 60-cm, but not the 30-cm, height increased total CSA in the distal radius and proximal ulna.


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  		Table 2. Effects of free-fall impact on CSA and BMD in the humerus, radius, ulna, femur, tibia, and fibula of growing female rats as determined by pQCT

 

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  		Table 3. Effects of free-fall impact on trabecular and cortical bone mineral density in proximal and distal sites of the appendicular skeleton in rats as determined by pQCT

 
Effects on cross-sectional moments of inertia.   The Imin of the diaphysis of all forelimb bones was consistently greater for both the F30 and F60 groups than for the controls (Fig. 4A). Differences in Imin between the F60 rats and their controls were greatest in the ulna (58%), humerus (34%), and then radius (21%). The 60-cm height improved significantly on the 30 cm results only in the ulna. Free-fall impact showed a less consistent increase in the mean Imax in the midshafts of all three forelimb bones (Fig. 4B). The increase in ulnar Imax measured in the F30 group was significantly less than that in the F60 group (P < 0.001). The difference in humeri Imax between drop heights was not significant.



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  		Fig. 4. Minimum second moments of area (Imin; A) and maximum second moments of area (Imax; B). Values are mean percent difference (± SE) between Con and F30 and F60 groups. Significant difference from Con: *P < 0.05; **P < 0.01; ***P < 0.001. Significant difference between F30 and F60: {dagger}P < 0.05; {ddagger}P < 0.001.

 
GRFs at the drop heights.   Peak impact forces exerted on a forepaw of a rat dropped from 30 cm and 60 cm were, respectively, 12.0 and 16.7 times the mass borne by a forepaw when standing (Fig. 5). By comparison, each hind paw showed a peak impact force of 4.5 and 7.7 times that of the standing paw mass. Peak impact forces produced by the 60-cm drop height were significantly greater than from 30 cm in both the forepaws and hind paws (P < 0.001). Paw placement at landing did not differ significantly between heights.



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  		Fig. 5. Peak vertical impact forces produced by 1 forefoot and one hind foot per body weight. Values are means ± SD. {circ}, Forelimb; {square}, hindlimb. ***Significant difference between 30- and 60-cm drop heights, P < 0.001. {ddagger}Significant difference between fore and hind forces, P < 0.001.

 

   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
ACKNOWLEDGMENTS
 REFERENCES
 
This study showed that landing from a height of 30 cm increased bone strength in the forelimbs of growing rats by altering the geometry of the shaft and possibly by increasing strength in the bone ends by increasing the trabecular density in those sites. The ulna and radius increased in diaphyseal strength, which was predictable from increases in both second moments of area and CSA (25) because increases in these parameters decrease bending stress in response to impact, due to increased rigidity of the bone. However, changes in BMD did not occur in the shaft. This adaptation in the shaft to loading by modifications in geometry rather than density has also been reported in young human jumpers (21) and tennis players (3, 19). We suggest that the preferential adaptation in diaphyseal geometry over density occurs because a modest increase in BMD would confer less strength to an axially loaded bone than would improvements in geometry.

Unlike the shaft, metaphyseal regions adapted to repetitive impact with increases in BMD. This increased density was solely due to increased trabecular BMD, likely a result of increased thickness of the individual trabeculae (5, 14). This adaptation by increased trabecular BMD in the bone ends would prevent interference with the functioning of joints and movement that could be caused by periosteal expansion (15). Effects of impact-style exercise on cortical and trabecular BMD in ends of appendicular bones have not, to our knowledge, been previously reported in growing rats. Mechanically loaded ulnae of adult Sprague-Dawley rats showed little change in total BMD (1.5%) and no change in cortical or trabecular BMD or CSA at a similar site on the ulnar olecranon (26). However, these dissimilarities from our results may be due to differences in the maturity of the rats, rat strain, or protocol between the studies. Our results are similar to those reported for young tennis competitors, who showed increased bone density in the distal radius (19).

Substantial changes produced in shafts of the forelimb, but not hindlimb, bones reflected the greater forces placed on them by the free-fall impact than are habitual for a rat. Standing rats bore only 42% of their mass on their forefeet, yet when dropped from 30 and 60 cm, 66 and 61% of the force, respectively, was borne by the forefeet. This translated into a two- to threefold greater impact to the forefeet than the hind feet. Additionally, the 2.2-fold larger size of hind paws distributed the stress from impact over a proportionately larger area than the front feet (stress = force/area). The lesser forces borne by the hind feet at impact were then transmitted to the larger bones of the hindlimbs, further diminishing the stimuli received by the femur and tibia. We presume that the failure of the free-fall impact method to stimulate the diaphysis of the femur and tibia was due to insufficient stress on the hindlimbs. It is possible that, by dropping the rats hind feet first, or increasing the number of drops per day, the free-fall method could produce changes in these bones.

Our peak impact forces suggest that a single-limb GRF >7.7 but ≤12.0 times mass is required to produce osteogenic effects from 10 impacts per day in growing Fischer 344 female rats. In comparison, human studies have shown that neither walking, with a GRF of 1.4 times body weight (8), nor running, with a GRF of 2.7 times body weight (4), is particularly osteogenic in humans, whereas jumping from 61-cm boxes, with an average GRF of 8.8 times body weight, had an osteogenic effect in children after 7 mo of jumping (6). Furthermore, elite athletes competing in the sport of triple jumping produced an average maximal GRF of 17.3 times body weight and displayed substantive geometric and bone increases of 20–55% in a variety of parameters (8). Interestingly, their GRF was similar to that produced by the forefeet of our rats when landing from 60 cm.

The strength in the forelimbs was not improved by the increase in drop height from 30 to 60 cm. This indicates that a threshold for bone stimulation was reached by 30 cm, above which the strength of the whole shaft was not improved. A similar threshold of adaptation was reported for rats that jumped upward 40 cm, 5 times per day, yet did not improve bone strength with more jumps (35) and/or with greater height (22). There is an apparent discrepancy between the results for ultimate breaking force and pQCT measurements at the ulnar shaft. The strength data clearly show no improvement with added drop height, whereas the moment of inertia and CSA data show significant improvements. However, the fracture site of the ulnas from the F60 rats had moved distally in response to a thickening on the midshaft area where the control and F30 bones had broken. Thus the breaking strength and scan data were from different locations; nevertheless, breaking strength for the whole ulna was not improved.

In the ulnae and radii of the 60-cm drop group, we observed significant increases in CSA and Imin, but not in mechanical strength, suggesting that the new bone formed might be of poor quality and resemble woven bone. Others have reported the formation of woven bone on the periosteal surface of mechanically loaded rat bones (12, 20, 31), and, unlike lamellar bone, this new woven bone is not fully mineralized for 14 wk (32). Our rats were terminated immediately after 8 wk of treatment, so it is possible that incompletely mineralized woven bone had formed on the periosteal surface of the bones of rats in the 60 cm group. Additionally, Hsieh and Silva (12) reported that loading adult rat bone to fatigue resulted in an immediate decrease in mechanical strength and eventually also decreased BMD. We did not observe a significant decrease in either ultimate strength or energy to failure, nor was BMD reduced by the 60-cm dropping. However, the increase in ulnar stiffness in the F60 group without concomitant increases in ultimate force and energy to failure suggests increased brittleness, and this might be caused by disorganized woven bone formation. However, we did not examine the bones histologically, so we did not definitively distinguish lamellar from woven bone in the 60 cm group.

The free-fall protocol was developed for use when studying the interactions of impact loading with systemic treatments. The intent was to modify bones through mechanical stimulation with as little perturbation as possible to the health, energy expenditure, nutrient intake or absorption, growth, or limb length of the rat. Greater calcium absorption has been reported as a function of aerobic running exercise in both rats (37) and people (39). However, our protocol took only 110 s per day of discontinuous exercise per rat, so it is unlikely that it had any aerobic effect. Additionally, growth, as measured by bone length and body weight, did not differ between dropped rats and their controls.

Both the upward jumping technique of Umemura et al. (34) and our free-fall impact system subjected young rats of the same strain, gender, and age to 400 loads over 8 wk. However, free-fall impact imparted greater osteogenic effects to the forelimb bones than were reported in the hindlimbs of the upward jumping rats. The method of Umemura et al. resulted in 14% (35) and 15% (36) increases in femoral CSA and in 13% (35) and 18% (36) increases in breaking force, which is somewhat less than the 20% increase in shaft CSA and 41% in breaking force that we measured in the ulnae our free-fall rats. Many factors between the method of Umemura et al. (34) and our free-fall method could have resulted in these differences. Breaking forces, which the free-fall method is composed of entirely, exceed propulsive forces, which predominate in the upward jumping method (8). Also, the trajectories of the rats in the two methods differ, with our rats dropped directly downward, whereas the upward-jumping rats traveled on a angle. The feet of our rats traveled downward either 30 or 60 cm, whereas the height jumped upward by the rats in the studies of Umemura et al. was not specified but would be somewhat less than 40 cm, because their rats grasp the top of their box, set at 40 cm, with their front feet and then climb up. Additionally, we used an 11-s rest insert between jumps, which may be more optimal than the ~3 s between jumps reported by Umemura et al. (35). A subsequent study from Umemura's group reported a better outcome from a 30-s rest between jumps than the 3-s intervals (36), perhaps due to increased stimulation of osteocytes by fluid flow in the bone caniculi (7). The benefit of the upward-jumping system over the free-fall system is clearly its ability to stimulate the femur. However, the free-fall protocol provides an alternative impact-loading method that greatly stimulates both the shafts and ends of the bones in the front legs, uses few repetitions at precise heights, and is correlated with peak vertical impact forces.

Impact from 10 drops per day resulted in improvements in strength, geometry, and density in the forelimbs of growing rats. A threshold of bone response by 30 cm occurred for bone strength in the ulna and radius, for shaft Imin in all forelimb bones, for shaft CSA in the humerus and ulna, and for trabecular BMD in the bone ends of the proximal humerus and ulna. Although the 60-cm height did not result in additional improvements in bone shaft strength, some improvements were measured in ulnar Imin and Imax, and CSA in the shaft. In conclusion, free-fall impact stimulated adaptive mechanisms that differed between shaft and bone ends, with geometric changes occurring in the shafts, whereas trabecular density increased in the bone ends. It therefore is possible, but remains to be tested, that a few peak impact forces of 8–12 times body weight per day during growth can lead to long-term improvements in human bone, especially if some impact is perpetuated through the adjoining years.


   ACKNOWLEDGMENTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ACKNOWLEDGMENTS
REFERENCES
 
We thank K. Condon for preparation of bone specimens, J. Durst for bone diagrams, J. Wade and B. Hillberry for force platform construction, D. Burr and T. Järvinen for critical review of the manuscript, and D. Welch for data assistance.


   FOOTNOTES
 

Address for reprint requests and other correspondence: C. M. Weaver, Purdue University, Foods and Nutrition, 1264 Stone Hall, 700 W State St., West Lafayette, IN 47907-2059 (E-mail: weavercm{at}purdue.edu).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ACKNOWLEDGMENTS
 REFERENCES
 

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