Ball flight kinematics, release variability and in-season performance in elite baseball pitching.

The purpose of this study was to quantify ball flight kinematics (ball speed, spin rate, spin axis orientation, seam orientation) and release location variability in the four most common pitch types in baseball and relate them to in-season pitching performance. Nine NCAA Division I pitchers threw four pitching variations (fastball, changeup, curveball, and slider) while a radar gun measured ball speed and a 600-Hz video camera recorded the ball trajectory. Marks on the ball were digitized to measure ball flight kinematics and release location. Ball speed was highest in the fastball, though spin rate was similar in the fastball and breaking pitches. Two distinct spin axis orientations were noted: one characterizing the fastball and changeup, and another, the curveball and slider. The horizontal release location was significantly more variable than the vertical release location. In-season pitching success was not correlated to any of the measured variables. These findings are instructive for inferring appropriate hand mechanics and spin types in each of the four pitches. Coaches should also be aware that ball flight kinematics might not directly relate to pitching success at the collegiate level. Therefore, talent identification and pitching evaluations should encompass other (e.g., cognitive, psychological, and physiological) factors.

Comparative effectiveness of alternative clinical pathways for primary hip and knee joint replacement patients: a pragmatic randomized, controlled trial.

OBJECTIVE: Total hip replacement (THR) and total knee replacement (TKR) (arthroplasty) surgery for end-stage osteoarthritis (OA) are ideal candidates for optimization through an algorithmic care pathway. Using a comparative effectiveness study design, we compared the effectiveness of a new clinical pathway (NCP) featuring central intake clinics, dedicated inpatient resources, care guidelines and efficiency benchmarks vs. the standard of care (SOC) for THR or TKR. METHODS: We compared patients undergoing primary THR and TKR who received surgery in NCP vs. SOC in a randomised controlled trial within the trial timeframe. 1,570 patients (1,066 SOC and 504 NCP patients) that underwent surgery within the study timeframe from urban and rural practice settings were included. The primary endpoint was improvement in Western Ontario and McMaster University osteoarthritis index (WOMAC) overall score over 12 months post-surgery. Secondary endpoints were improvements in the physical function (PF) and bodily pain (BP) domains of the Short Form 36 (SF-36). RESULTS: NCP patients had significantly greater improvements from baseline WOMAC scores compared to SOC patients after adjusting for covariates (treatment effect=2.56; 95% confidence interval (CI) [1.10-4.01]). SF-36 BP scores were significantly improved for both hip and knee patients in the NCP (treatment effect=3.01, 95% CI [0.70-5.32]), but SF-36 PF scores were not. Effects of the NCP were more pronounced in knee patients. CONCLUSION: While effect sizes were small compared with major effects of the surgery itself, an evidence-informed clinical pathway can improve health related quality of life (HRQoL) of hip and knee arthroplasty patients with degenerative joint disorder in routine clinical practice for up to 12 months post-operatively. CLINICALTRIALS.GOV IDENTIFIER: NCT00277186.

Osteoblast response to ovariectomy is enhanced in intrinsically high aerobic-capacity rats.

The role of exercise in promoting bone health is typically attributed to increased mechanical loading, which induces functional adaptation. Recent evidence suggests that habitual aerobic exercise has influence at the cellular level as well. The effect of aerobic capacity on osteoblast-lineage cell differentiation and function as well as skeletal phenotype is unknown. Using a rat model of high-capacity and low-capacity runners (HCRs and LCRs, respectively), in which an intrinsic functional genomic difference in aerobic capacity exists between nontrained animals, this study evaluated the effects of aerobic capacity on measures of bone mass and strength as well as osteoblast activity following ovariectomy. The ovariectomized rat emulates the clinical features of the estrogen-depleted human skeleton and represents a valuable model for studying short-term upregulation of osteoblast activity. We hypothesized that intrinsically high aerobic capacity would augment osteoblast response, which would mitigate the deleterious effects of hormone withdrawal. Femora and tibiae were assessed by micro-computed tomography, mechanical testing, and dynamic histomorphometry. HCRs had enhanced femoral tissue mineral density and estimated elastic modulus relative to LCRs. At 4 weeks postovariectomy, HCRs demonstrated a more robust osteoblast response. Markers of bone formation were upregulated to a greater extent in HCRs than LCRs, suggesting a role for aerobic capacity in governing osteoblast activity. Results from this and future studies will help to identify the influence of cellular aerobic metabolism on bone health, which may lead to new strategies for targeting diseases of the skeleton.

Influence of cortical canal architecture on lacunocanalicular pore pressure and fluid flow.

Bone is a dynamic tissue that undergoes structural modification in response to its mechanical environment, but how bone cells sense and respond to loading conditions remains incompletely understood. Current theories focus on strain-induced fluid flow for the primary means of mechanotransduction. To examine the influence of age-related cortical rarefaction on lacunocanalicular fluid characteristics, coupled fluid flow and mechanical computational models of bone specimens representing young, mid-age and aged samples were derived artificially from the same original micro-computed tomography image data. Simulated mechanical loading was applied to the bone models to induce pressure-driven interstitial fluid flow. Results demonstrated a decrease in pore pressure and fluid velocity magnitudes with age as a result of increased cortical porosity. Mean canal separation, as opposed to canal size, was implicated as a primary factor affecting age-related fluid dynamics. Future investigations through refinement of the model may implicate fluid stasis or inadequate nutrient transport experienced by osteocytes as a key factor in the initiation of cortical remodelling events.

Bisphosphonates reduce bone mineral loss at ligament entheses after joint injury.

OBJECTIVE: To examine the effects of anterior cruciate ligament (ACL) insufficiency, and subsequent bisphosphonate (BP) antiresorptive therapy, on the bone mineral interface at the enthesis of remaining ligamentous restraints. METHODS: We measured bone mineral geometry (and subsequent adaptation) at the medial collateral ligament (MCL) origin, using micro-computed tomography (muCT). Groups of normal control, 6 and 14 wk anterior cruciate ligament transected (ACLX), and 6 wk ACLX-BP (risedronate) dosed rabbits were evaluated. Samples were then processed histologically, and the results of mineral adaptation and progression of osteoarthritis (OA) compared to joint laxity values obtained from previous biomechanical testing of the MCL-complex. RESULTS: muCT defined the MCL origin as a symmetrical, metaphyseal depression that contained soft-tissue elements, including fibrocartilage and ligament--as seen in subsequent histological sections. In contrast, the insertions from ACLX animals lost significant bone mineral, with an MCL-insertion volume 1.2 times that of normal controls at 6 wk ACLX, which further increased to 2.3 times that of normal controls at 14 wk ACLX. Significant differences were also measured between 6 and 14 wk ACLX and age-matched normal controls in volume of cortical bone containing the MCL insertion. However, there were no significant differences in the percentage of cortical bone to underlying trabecular bone at the MCL insertion. When comparing muCT mineral adaptation at the MCL-enthesis with historical MCL-complex laxity data, the values for laxity after ACLX increased proportionately as bone mineral at the insertion was lost, and subsequent use of the BP risedronate reduced both mineral loss and MCL-complex laxity. CONCLUSION: Compared to the untreated ACLX condition, administering bisphosphonate immediately after loss of the ACL conserved bone mineral at the MCL enthesis, suggesting the potential to therapeutically influence joint-complex laxity and OA progression.

Structure, function and adaptation of bone-tendon and bone-ligament complexes.

Mechanical and physiological processes contribute to joint tissue adaptations during growth and exercise and after injury. Those adaptations are often in response to the mechanotransductive stimuli linked to the transmission of forces across these load-bearing structures. Muscle-tendon interactions have been explored during skeletal loading and describe the relation of sarcomere shortening at the expense of tendon lengthening(1,2). The effects of load transmission through the bone-tendon and bone-ligament complexes, however, have not been studied as extensively, although both disuse and exercise will alter the stiffness of these significant structures. Recently, however, renewed interest has emerged about the pathogenesis underlying enthesopathies and enthesitis, and investigators are beginning to reveal the intricacies of bone-tendon and bone-ligament complexes(3,4,5). Here, we summarize the structure and function of the types of entheses between bone-tendon and bone-ligament, and relate how mechanical loading leads to functional adaptation, and at times, entheseal pathophysiology.

Antiresorptive therapy conserves some periarticular bone and ligament mechanical properties after anterior cruciate ligament disruption in the rabbit knee.

The purpose of this study was to assess, in an osteoarthritic (OA) model, whether bisphosphonate (BP) antiresorptive therapy altered periarticular bone and bone-ligament biomechanics and OA progression. We surgically transected the anterior cruciate ligament (ACLX) in two groups of rabbits; the first group was dosed with BP (risedronate, 0.01 mg/kg s.c. daily for 6 wk), the second group remained untreated, and a third group of normal (unoperated) control rabbits was also evaluated. We measured distal femoral bone mineral density (BMD, Dual Energy X-ray Absorptiometry [DEXA]), medial collateral ligament (MCL) laxity, and bone mechanical function (bone cores mechanically tested in compression). These measures were related to cartilage/joint gross morphology, histology, and measures of vascular volume (gelatin-dye perfusion) for evidence of inflammatory angiogenesis and early OA. BMD by DEXA in 6 wk ACLX animals was 18% less than normal controls (p<0.05). In contrast, BP dosing conserved periarticular BMD; risedronate-treated rabbits had distal femoral BMD only 5% less and not significantly different than normal controls. When the same bone cores were compressed to failure, both ACLX and BP-dosed animals were significantly weaker than normal controls (p<0.05). However, the bone energy to failure and elastic modulus of BP-dosed animals was conserved and not significantly different from normal controls 6 wk after ACLX. Blocking bone resorption with BP also resulted in a significantly improved bone-ligament structural complex. MCL-complex laxity was significantly less in BP-dosed animals (1.2 times that of normal controls) compared to untreated ACLX animals (1.7 times that of normal controls; p<0.05). Blocking bone resorption with risedronate did not suppress osteophytosis and inflammatory angiogenesis, which were significantly increased in the periarticular bone of both untreated and BP treated ACLX animals. Thus, administering BP immediately after ACL loss conserved some periarticular bone and MCL-complex properties in an early OA model.

Angiogenesis in the distal femoral chondroepiphysis of the rabbit during development of the secondary centre of ossification.

In the developing chondroepiphyses of long bones, the avascular cartilaginous anlage is invaded by numerous blood vessels, through the process of angiogenesis. The objective of this study was to investigate the chronology of this vascular invasion with the spontaneous calcification of the cartilaginous epiphysis during development of the secondary ossification centre in the rabbit distal femur. The time-course of chondroepiphyseal vascular invasion was determined histologically and standardized for eight gestational and four postnatal intervals by plotting kit body mass against crown-rump length. Similarly, microcomputed tomography (micro-CT) helped to visualize calcification at those same gestational and postnatal intervals. To confirm the angiogenic nature of the avascular chondroepiphysis, such samples were assayed on the chick chorio-allantoic membrane (CAM). Neovascular outgrowths from the CAM were apparent 48 h following introduction of an 18-day (gestational) chondroepiphyseal sample. Chondroepiphyseal samples were assayed for the potent developmental angiogenic factors bFGF and VEGF, with the mRNA expression for both these mediators being confirmed using RT-PCR. As angiogenesis and calcification during chondroepiphyseal development occur in a defined tissue environment initially devoid of blood vessels and mineral, those processes provided a unique opportunity to study their progression without complication of injury-related inflammation or extant vasculature and mineral. Furthermore, the discovery of angiogenic, angiostatic or mineral-regulating mediators specific to developing connective tissue may prove useful for analysing the regulation of vascular and mineral pathogenesis in articular tissues.

Ankle and first metatarsophalangeal joint dorsiflexion in children with clubfoot.

It has been hypothesised that the stiffness of the plantar aponeurosis after clubfoot surgery affects push-off. Because the first metatarsophalangeal (MTP) joint motion relies on the plantar aponeurosis, it was important to determine whether there was a lack of first MTP joint motion in children with clubfoot that also affected push-off. By examining the motion of the first MTP joint using a motion analysis system and passive motion techniques, then correlating these with gait characteristics, the authors found that the first MTP joint was not affected in children with clubfoot. The authors found that a motion analysis system could be used to determine range of motion accurately.

Evidence of chondrocyte repopulation in adult ovine articular cartilage following cryoinjury and long-term transplantation.

OBJECTIVE: To characterize the response of articular chondrocytes to a specific cryoinjury that leads to cluster formation following long-term transplantation. DESIGN: Osteochondral dowels from 20 adult sheep were cryopreserved to optimize the recovery of chondrocytes immediately after thawing. The dowels were transplanted as allografts and observed at 3 and 12 months. Chondrocyte distribution and viability was assessed using paravital dyes after transplantation. Chondrocyte phenotype was assessed by in situ hybridization and immunohistochemistry to detect type II collagen. An anticentrosome antibody was used to identify cells undergoing cell cycle progression towards mitosis. RESULTS: All cryopreserved grafts showed the presence of spheroidal clusters of chondrocytes 1 year after transplantation while the host cartilage adjacent to the graft appeared morphologically normal. The average size of the clusters increased from four cells at 3 months to 12 cells at 1 year. The chondrocytes in the clusters displayed newly formed type II collagen protein and mRNA. Some cells within clusters were observed with two centrosomes, indicative of cells progressing through the S phase of the cell cycle. CONCLUSION: Adult articular chondrocytes retain the ability to repopulate the matrix, an ability which is demonstrated with this specific cryoinjury. This may be an initial stage of cartilage regeneration.

Estimation of spinal deformity in scoliosis from torso surface cross sections.

STUDY DESIGN: Correlation of torso scan and three-dimensional radiographic data in 65 scans of 40 subjects. OBJECTIVES: To assess whether full-torso surface laser scan images can be effectively used to estimate spinal deformity with the aid of an artificial neural network. SUMMARY OF BACKGROUND DATA: Quantification of torso surface asymmetry may aid diagnosis and monitoring of scoliosis and thereby minimize the use of radiographs. Artificial neural networks are computing tools designed to relate input and output data when the form of the relation is unknown. METHODS: A three-dimensional torso scan taken concurrently with a pair of radiographs was used to generate an integrated three-dimensional model of the spine and torso surface. Sixty-five scan-radiograph pairs were generated during 18 months in 40 patients (Cobb angles 0-58 degrees ): 34 patients with adolescent idiopathic scoliosis and six with juvenile scoliosis. Sixteen (25%) were randomly selected for testing and the remainder (n = 49) used to train the artificial neural network. Contours were cut through the torso model at each vertebral level, and the line joining the centroids of area of the torso contours was generated. Lateral deviations and angles of curvature of this line, and the relative rotations of the principal axes of each contour were computed. Artificial neural network estimations of maximal computer Cobb angle were made. RESULTS: Torso-spine correlations were generally weak (r < 0.5), although the range of torso rotation related moderately well to the maximal Cobb angle (r = 0.64). Deformity of the torso centroid line was minimal despite significant spinal deformity in the patients studied. Despite these limitations and the small data set, the artificial neural network estimated the maximal Cobb angle within 6 degrees in 63% of the test data set and was able to distinguish a Cobb angle greater than 30 degrees with a sensitivity of 1.0 and specificity of 0.75. CONCLUSIONS: Neural-network analysis of full-torso scan imaging shows promise to accurately estimate scoliotic spinal deformity in a variety of patients.

Periarticular cancellous bone changes following anterior cruciate ligament injury.

To understand more fully the early bone changes in an experimental model of osteoarthrosis, we quantified periarticular bone mineral density and bone mechanical properties in anterior cruciate ligament transected (ACLX) knee joints (4, 10, 32, and 39 wk post-ACLX) compared with contralateral joints and unoperated normal joints of skeletally mature animals. Maximal stress and energy were significantly reduced in ACLX cancellous bone from the medial femoral condyles at 4 wk postinjury. All mechanical properties (e.g., yield stress and elastic modulus) declined after 4 wk and were significantly reduced at 10 wk. ACLX bone mineral density was significantly reduced at all measured time points. Ash content was significantly reduced at 10 and 32 wk. Changes in the lateral condyles were similar but less pronounced than in the medial condyles. These bony changes accompanied the earliest articular cartilage molecular changes and preceded changes in the articular cartilage gross morphology. We suggest that these early changes in bone mechanical behavior contribute to the progression of osteoarthrosis and pathogenic changes in the joint.

Kinematics and kinetics of the hip, knee, and ankle of children with clubfoot after posteromedial release.

Clubfoot is a bony deformity characterized by inversion, adduction, and equinus that often require surgical intervention. This study assessed the gait kinetics and kinematics of children with unilateral and those with bilateral clubfoot, comparing them with age-and gender-matched normal (control) children. Patient satisfaction also was examined using a questionnaire, and muscle strength was evaluated at the ankle and knee. In evaluating the kinematics, it was important to note that deviations occurred at the ankle of children with clubfoot. Differences in kinematics and kinetics at the hip and knee between normal children and those with clubfoot resulted from lack of motion at the ankle. Furthermore, the strength of ankle plantarflexors was weak, which reduced plantarflexion during push-off. This restricted motion may have been caused by residual bony deformities and muscle tightness resulting from the original condition that contributed to muscle stiffness during gait.

Evaluation of laser-Doppler perfusion imaging for measurement of blood flow in cortical bone.

Most techniques currently available to measure blood flow in bone are time consuming and require destruction of the tissue, but laser-Doppler technology offers a less invasive method. This study assessed the utility of laser-Doppler perfusion imaging (LDI) to measure perfusion in cortical bone. Twelve mature New Zealand White rabbits were assigned to one of three groups: normal control, constriction (norepinephrine), or dilatation (nitroprusside). The left and right medial tibiae were consecutively scanned at red (634-nm) and near-infrared (810-nm) wavelengths to examine the repeatability of LDI output. The pharmacological intervention groups were injected with the respective drug, and LDI measurements at 810 nm were obtained concurrently with colored microsphere-determined flow in all of the groups. LDI effectively quantified blood flow in cortical bone and detected physiologically induced changes in perfusion. A significant positive correlation was found between microsphere-determined flow and LDI output (r = 0.6, P < 0.05). Repeatability of consecutive LDI measurements was within 5%. The effectiveness of LDI to measure perfusion in bone suggests this method has potential for investigating the role of blood flow in bone metabolism and remodeling.

Physiological and mechanical adaptation of periarticular cancellous bone after joint ligament injury.

The relation between blood flow and bone mineral density (BMD) of periarticular bone was examined in an in vivo model of joint instability. Eighty mature New Zealand White rabbits were randomly assigned to experimental [anterior cruciate ligament transection (ACLX)], sham-operated control, or age-matched normal control groups. Experimental rabbits underwent unilateral transection of the right anterior cruciate ligament, and the nonoperated left [contralateral (Cntra)] limb was a within-animal control. BMD and blood flow to the periarticular bone in the femoral condyles were assessed in each group at 2, 4, 6, 14, and 48 wk postsurgery, using quantitative computed tomography scanning and entrapment of colored microspheres. BMD was significantly lower (5%) in the ACLX compared with Cntra limbs. Periarticular bone blood flow in the ACLX limbs was significantly greater than in the Cntra limb (29%) in the early stages (6 wk) after injury. Up to 48 wk post-ACLX, a significant correlation was found between increased blood flow and decreased BMD in the periarticular bone of the femoral condyles in the ACLX limbs. This correlation suggested that heightened blood flow may be linked to mechanisms of bone adaptation in joints after ligament injury.

Functional adaptation of bone to exercise and injury.

Bone adapts to altered physical stimuli, dietary changes, or injury. Dietary calcium and vitamins play important roles in maintaining skeletal health, but high-fat diets are pervasive in western cultures and may contribute to the increasing prevalence of osteoporosis and incidence of related hip fractures. Exercise helps maintain bone mass and counter osteoporosis, but exercise can also have detrimental effects-particularly for immature bone. Some negative exercise effects may also be linked to diet. For example, insufficient dietary protein during exercise can impair bone development and remodeling. Bone remodeling is a potent example of tissue repair. Chronically altered loading after a joint injury, however, can result in remodeling processes that can be detrimental to the joint. Anterior cruciate ligament injury, for example, commonly leads to osteoarthritis. Early changes in the periarticular cancellous bone may play a role in the development of knee osteoarthritis. Although these factors influence skeletal health, the mechanisms remain unclear by which bone interprets its environment and responds to mechanical stimuli or injury. To understand why different levels of exercise are beneficial or detrimental or why altered joint loading leads to changes in periarticular bone structure, underlying mechanisms must be understood by which bone interprets its mechanical environment.

Bone affinity of a bisphosphonate-conjugated protein in vivo.

Growth factors capable of stimulating bone formation are potential therapeutic agents for osteoporosis treatment. It is essential, however, that a targeting mechanism is incorporated into the growth factors to deposit them at osseous tissue with minimal distribution to extraskeletal sites. To this end, a strategy has been developed in which a bone-seeking molecule, 1-amino-1,1-diphosphonate methane (aminoBP), was chemically conjugated to a model protein, bovine serum albumin (BSA). This study was carried out to assess the bone affinity of the conjugates in a tibia injection model. Using ovariectomized (OVX) rats, initial (3 h) retention of BSA and aminoBP-BSA were found to be equivalent when injected into the medullary cavity of tibia. After 1 day, an 8- and 12-fold higher tibiae retention of the protein was obtained in normal and OVX rats as a result of aminoBP conjugation. A similar result ( approximately 12-fold difference) was also obtained in OVX rats after 3 days. We concluded that aminoBP conjugation to BSA imparted a high bone affinity and enhanced bone retention of proteins in normal and OVX rats.

Early regional adaptation of periarticular bone mineral density after anterior cruciate ligament injury.

The present study measured early-stage adaptation of bone mineral (BMD) in the periarticular cancellous bone of the canine knee (stifle) joint after anterior cruciate ligament (ACL) transection (ACLX). Regional changes in BMD in the tibia and femur were analyzed by using quantitative computed tomography (qCT) at 3 wk and 12 wk after unilateral ACLX to determine whether there were focal points for BMD changes and whether these changes occurred early after the induced knee injury. BMD decreased rapidly after ACLX, and the more pronounced response was in the femur. In the 3-wk group, there were decreases in BMD in the tibia and the femur, and these changes were significant in the posterior-medial region of the femur, which showed a decrease of BMD in the ACLX limb (-0.048 +/- 0.011 g/cm(3)). In the 12-wk group, all regions in the tibia and femur exhibited significant decreases in BMD, and the average decrease was greatest in the posterior-medial region of the femur (-0.142 +/- 0.021 g/cm(3)). The regions of pronounced periarticular cancellous BMD adaptation corresponded to observed focal cartilage defects. Early decreases in BMD in the injured knee may be related to altered loading and kinematics in the knee and may be an important link in the pathogenesis of posttraumatic osteoarthritis.

Early morphometric and anisotropic change in periarticular cancellous bone in a model of experimental knee osteoarthritis quantified using microcomputed tomography.

OBJECTIVE: To quantify early stage microstructural changes of periarticular cancellous bone in a canine anterior cruciate ligament transection model for experimental osteoarthritis. DESIGN: Unilateral transection of the anterior cruciate ligament was performed in 10 animals. Bone structure changes were quantified in five animals at 3-week post-transection and five animals at 12-week post-transection. An additional two non-operated animals were used as controls. BACKGROUND: Changes in trabecular architecture of the periarticular cancellous bone in early stage post-traumatic osteoarthritis is not well understood. Previous studies have found alterations in bone mineral density in experimental osteoarthritis suggesting adaptation of the trabecular structure. Early change of the periarticular bone following a ligament injury may contribute to the long-term development of osteoarthritis. METHODS: ++. Bone cores from the medial condyles of the femoral and tibial pairs were scanned with a three-dimensional microtomographic system. Structural indices were quantified including bone volume ratio, bone surface ratio, trabecular thickness, trabecular separation, trabecular number, as well as structural anisotropy determined by the mean-intercept-length method.Results. Significant structural changes were observed at 3-week post-transection, and were more prominent at 12-week post-transection. These changes were accompanied by decreasing anisotropy. CONCLUSIONS: Periarticular cancellous bone microstructure is significantly altered in experimental osteoarthritis. These changes occurred as early as 3-week post-transection, and were large at 12-week post-transection. RELEVANCE: The pathogenesis of post-traumatic osteoarthritis is poorly understood, but it is clear that this disease involves the entire organ system of the joint, including the cartilages, synovium, ligaments, and bones. This study focuses on the changes that occur in the bones during the early stages following a joint injury, and contributes to a better overall understanding of the disease aetiology.

High-impact exercise and growing bone: relation between high strain rates and enhanced bone formation.

We investigated whether high-impact drop jumps could increase bone formation in the middiaphyseal tarsometatarsus of growing rooster. Roosters were designated as sedentary controls (n = 10) or jumpers (n = 10). Jumpers performed 200 drop jumps per day for 3 wk. The mechanical milieu of the tarsometatarsus was quantified via in vivo strain gauges. Indexes of bone formation and mechanical parameters were determined in each of twelve 30 degrees sectors subdividing the middiaphyseal cortex. Compared with baseline walking, drop jumping produced large peak strain rates (+740%) in the presence of moderately increased peak strain magnitudes (+30%) and unaltered strain distributions. Bone formation rates were significantly increased by jump training at periosteal (+40%) and endocortical surfaces (+370%). Strain rate was significantly correlated with the specific sites of increased formation rates at endocortical but not at periosteal surfaces. Previously, treadmill running did not enhance bone growth in this model. Comparing the mechanical milieus produced by running and drop jumps revealed that jumping significantly elevated only peak strain rates. This further emphasized the sensitivity of immature bone to high strain rates.