Determine the vertical ground reaction forces and knee mechanics with different gait inclinations in the sheep model.

Our current understanding of knee mechanics and anterior cruciate ligament (ACL) function is predominately based on data recorded during simulations of clinical examinations or the application of nonphysiologic loads and motions. These methodologies provide little information on knee and ACL mechanics during activities of daily living (ADLs). Additionally, researchers have not directly measured knee kinetics, knee contact pressures, and ACL forces, and it is unknown how these parameters change with different activities. This study quantified the effects of activity level on vertical ground reaction forces, knee kinematics, and joint and ligament forces during in vivo motions. Five female Suffolk sheep were walked twice weekly on a treadmill during level (0°), inclined (+6°), and declined (-6°) gait for 12 weeks. Electromagnetic (EM) trackers were surgically implanted onto the left distal femur and the left proximal tibia, and in vivo motions were recorded for all activities. Following sacrifice, the in vivo motions were applied to their respective knees using a serial robot with a multi-axis load cell. In vitro simulations were repeated to measure (a) total knee forces, (b) contact pressure maps, and (c) ACL-only forces. Declining the gait surface led to increased posterior translation during the swing phase and decreased flexion at hoof-strike, decreased medial contact pressure at push-off, decreased ACL force at hoof-strike and increased ACL force at push-off. This study established a system that can be used to examine knee mechanics and ACL forces during ADLs for different knee states to define design requirements for ACL reconstruction techniques.

Effect of implanting a soft tissue autograft in a central-third patellar tendon defect: biomechanical and histological comparisons.

Previous studies by our laboratory have demonstrated that implanting a stiffer tissue engineered construct at surgery is positively correlated with repair tissue stiffness at 12 weeks. The objective of this study was to test this correlation by implanting a construct that matches normal tissue biomechanical properties. To do this, we utilized a soft tissue patellar tendon autograft to repair a central-third patellar tendon defect. Patellar tendon autograft repairs were contrasted against an unfilled defect repaired by natural healing (NH). We hypothesized that after 12 weeks, patellar tendon autograft repairs would have biomechanical properties superior to NH. Bilateral defects were established in the central-third patellar tendon of skeletally mature (one year old), female New Zealand White rabbits (n = 10). In one limb, the excised tissue, the patellar tendon autograft, was sutured into the defect site. In the contralateral limb, the defect was left empty (natural healing). After 12 weeks of recovery, the animals were euthanized and their limbs were dedicated to biomechanical (n = 7) or histological (n = 3) evaluations. Only stiffness was improved by treatment with patellar tendon autograft relative to natural healing (p = 0.009). Additionally, neither the patellar tendon autograft nor natural healing repairs regenerated a normal zonal insertion site between the tendon and bone. Immunohistochemical staining for collagen type II demonstrated that fibrocartilage-like tissue was regenerated at the tendon-bone interface for both repairs. However, the tissue was disorganized. Insufficient tissue integration at the tendon-to-bone junction led to repair tissue failure at the insertion site during testing. It is important to re-establish the tendon-to-bone insertion site because it provides joint stability and enables force transmission from muscle to tendon and subsequent loading of the tendon. Without loading, tendon mechanical properties deteriorate. Future studies by our laboratory will investigate potential strategies to improve patellar tendon autograft integration into bone using this model.

Effect of surgery to implant motion and force sensors on vertical ground reaction forces in the ovine model.

Activities of daily living (ADLs) generate complex, multidirectional forces in the anterior cruciate ligament (ACL). While calibration problems preclude direct measurement in patients, ACL forces can conceivably be measured in animals after technical challenges are overcome. For example, motion and force sensors can be implanted in the animal but investigators must determine the extent to which these sensors and surgery affect normal gait. Our objectives in this study were to determine (1) if surgically implanting knee motion sensors and an ACL force sensor significantly alter normal ovine gait and (2) how increasing gait speed and grade on a treadmill affect ovine gait before and after surgery. Ten skeletally mature, female sheep were used to test four hypotheses: (1) surgical implantation of sensors would significantly decrease average and peak vertical ground reaction forces (VGRFs) in the operated limb, (2) surgical implantation would significantly decrease single limb stance duration for the operated limb, (3) increasing treadmill speed would increase VGRFs pre- and post operatively, and (4) increasing treadmill grade would increase the hind limb VGRFs pre- and post operatively. An instrumented treadmill with two force plates was used to record fore and hind limb VGRFs during four combinations of two speeds (1.0 m/s and 1.3 m/s) and two grades (0 deg and 6 deg). Sensor implantation decreased average and peak VGRFs less than 10% and 20%, respectively, across all combinations of speed and grade. Sensor implantation significantly decreased the single limb stance duration in the operated hind limb during inclined walking at 1.3 m/s but had no effect on single limb stance duration in the operated limb during other activities. Increasing treadmill speed increased hind limb peak (but not average) VGRFs before surgery and peak VGRF only in the unoperated hind limb during level walking after surgery. Increasing treadmill grade (at 1 m/s) significantly increased hind limb average and peak VGRFs before surgery but increasing treadmill grade post op did not significantly affect any response measure. Since VGRF values exceeded 80% of presurgery levels, we conclude that animal gait post op is near normal. Thus, we can assume normal gait when conducting experiments following sensor implantation. Ultimately, we seek to measure ACL forces for ADLs to provide design criteria and evaluation benchmarks for traditional and tissue engineered ACL repairs and reconstructions.

Technical aspects of anterior cruciate ligament reconstruction for the general orthopaedic surgeon.

Anterior cruciate ligament reconstruction is the sixth most common procedure performed by orthopaedic surgeons. The goals of the procedure are to restore knee stability and patient function. These goals are dependent on proper graft positioning and incorporation. Anterior cruciate ligament reconstruction involves a technically complicated series of steps, all of which affect graft healing and clinical outcome. A wide variety of graft choices and surgical techniques are currently available for use. It is important for orthopaedic surgeons performing anterior cruciate ligament reconstructions to be aware of the indications for graft selection, techniques for correct graft placement, and the biologic implications related to these factors.

Evaluation of the Effectiveness of Newer Helmet Designs with Emergent Shell and Padding Technologies Versus Older Helmet Models for Preserving White Matter Following a Season of High School Football.

We aimed to objectively compare the effects of wearing newer, higher-ranked football helmets (HRank) vs. wearing older, lower-ranked helmets (LRank) on pre- to post-season alterations to neuroimaging-derived metrics of athletes' white matter. Fifty-four high-school athletes wore an HRank helmet, and 62 athletes wore an LRank helmet during their competitive football season and completed pre- and post-season diffusion tensor imaging (DTI). Longitudinal within- and between-group DTI metrics [fractional anisotropy (FA) and mean/axial/radial diffusivity (MD, AD, RD)] were analyzed using tract-based spatial statistics. The LRank helmet group exhibited significant pre- to post-season reductions in MD, AD, and RD, the HRank helmet group displayed significant pre- to post-season increases in FA, and both groups showed significant pre- to post-season increases in AD (p's < .05 [corrected]). Between-group analyses revealed the pre- to post-season increase in AD was significantly less for athletes wearing HRank compared to LRank (p < .05 [corrected]). These data provide in vivo evidence that wearing an HRank helmet may be efficacious for preserving white matter from head impact exposure during high school football. Future prospective longitudinal investigations with complimentary imaging and behavioral outcomes are warranted to corroborate these initial in vivo findings.

Biomechanical and Functional Outcomes After Medial Patellofemoral Ligament Reconstruction: A Pilot Study.

BACKGROUND: Previous studies have acknowledged the medial patellofemoral ligament (MPFL) as the primary stabilizer of the patella, preventing lateral displacement. MPFL reconstruction (MPFL-R) restores stability and functionality to the patellofemoral joint and has emerged as a preferred treatment option for recurrent lateral patellar instability. PURPOSE: To objectively measure biomechanical characteristics of athletes cleared for return to sport after MPFL-R compared with healthy controls. STUDY DESIGN: Cohort study; Level of evidence, 3. METHODS: A prospective case-control study design was employed on 31 total athletes. Sixteen athletes (6 male, 10 female; mean age, 16.1 ± 2.74 years; 385 ± 189 days after surgery and 235 ± 157 days after return to sport) underwent MPFL-R and were medically cleared to return to sport. This group was matched by age, sex, and activity level to 15 healthy athletes with no history of lower extremity injuries. Athletes and controls completed validated questionnaires as well as hopping, jumping, and cutting tests with 3-dimensional motion analysis and underwent strength, flexibility, laxity, and balance assessments. RESULTS: Participants in the MPFL-R group scored significantly lower (worse) on the International Knee Documentation Committee (IKDC) (89.2 ± 7.6 vs 98.1 ± 2.0, respectively; P = .0005) and significantly higher (worse) on the Tampa Scale for Kinesiophobia (TSK) (32.4 ± 5.0 vs 25.4 ± 6.5, respectively; P = .006) than those in the control group, but there was no difference in the Kujala score (95.6 ± 5.3 vs 98.8 ± 3.0, respectively; P = .06). Participants in the MPFL-R group demonstrated reduced hip and ankle flexion relative to those in the control group (P < .05). Participants in the MPFL-R group also took significantly longer to complete the 6-m timed hop test relative to those in the control group (P < .05). No statistically significant differences were found in anthropometrics, knee extension or flexion strength, hamstring flexibility, hip abduction strength, or joint laxity between the MPFL-R and control groups. CONCLUSION: The current data indicate that MPFL-R generally restores functional symmetry, while subtle deficits in global power may remain after being released to full activity. Clinicians should ensure that athletes are fully rehabilitated before returning to sport after MPFL-R by emphasizing functional multijoint exercises.

Anterior Cruciate Ligament Injury Risk in Sport: A Systematic Review and Meta-Analysis of Injury Incidence by Sex and Sport Classification.

OBJECTIVE: To evaluate sex differences in incidence rates (IRs) of anterior cruciate ligament (ACL) injury by sport type (collision, contact, limited contact, and noncontact). DATA SOURCES: A systematic review was performed using the electronic databases PubMed (1969-January 20, 2017) and EBSCOhost (CINAHL, SPORTDiscus; 1969-January 20, 2017) and the search terms anterior cruciate ligament AND injury AND (incidence OR prevalence OR epidemiology). STUDY SELECTION: Studies were included if they provided the number of ACL injuries and the number of athlete-exposures (AEs) by sex or enough information to allow the number of ACL injuries by sex to be calculated. Studies were excluded if they were analyses of previously reported data or were not written in English. DATA EXTRACTION: Data on sport classification, number of ACL injuries by sex, person-time in AEs for each sex, year of publication, sport, sport type, and level of play were extracted for analysis. DATA SYNTHESIS: We conducted IR and IR ratio (IRR) meta-analyses, weighted for study size and calculated. Female and male athletes had similar ACL injury IRs for the following sport types: collision (2.10/10 000 versus 1.12/10 000 AEs, IRR = 1.14, P = .63), limited contact (0.71/10 000 versus 0.29/10 000 AEs, IRR = 1.21, P = .77), and noncontact (0.36/10 000 versus 0.21/10 000 AEs, IRR = 1.49, P = .22) sports. For contact sports, female athletes had a greater risk of injury than male athletes did (1.88/10 000 versus 0.87/10 000 AEs, IRR = 3.00, P < .001). Gymnastics and obstacle-course races were outliers with respect to IR, so we created a sport category of fixed-object, high-impact rotational landing (HIRL). For this sport type, female athletes had a greater risk of ACL injury than male athletes did (4.80/10 000 versus 1.75/10 000 AEs, IRR = 5.51, P < .001), and the overall IRs of ACL injury were greater than all IRs in all other sport categories. CONCLUSIONS: Fixed-object HIRL sports had the highest IRs of ACL injury for both sexes. Female athletes were at greater risk of ACL injury than male athletes in contact and fixed-object HIRL sports.

Effects of cell-to-collagen ratio in stem cell-seeded constructs for Achilles tendon repair.

The objective of the present study was to test the hypotheses that implantation of cell-seeded constructs in a rabbit Achilles tendon defect model would 1) improve repair biomechanics and matrix organization and 2) result in higher failure forces than measured in vivo forces in normal rabbit Achilles tendon (AT) during an inclined hopping activity. Autogenous tissue-engineered constructs were fabricated in culture between posts in the wells of silicone dishes at four cell-to-collagen ratios by seeding mesenchymal stem cells (MSC) from 18 adult rabbits at each of two seeding densities (0.1 x 10(6) and 1 x 10(6) cell/mL) in each of two collagen concentrations (1.3 and 2.6 mg/mL). After 5 days of contraction, constructs having the two highest ratios (0.4 and 0.8 M/mg) were damaged by excessive cell traction forces and could not be used in subsequent in vivo studies. Constructs at the lower ratios (0.04 and 0.08 M/mg) were implanted in bilateral, 2 cm long gap defects in the rabbit's lateral Achilles tendon. At 12 weeks after surgery, both repair tissues were isolated and either failed in tension (n = 13) to determine their biomechanical properties or submitted for histological analysis (n = 5). No significant differences were observed in any structural or mechanical properties or in histological appearance between the two repair conditions. However, the average maximum force and maximum stress of these repairs achieved 50 and 85% of corresponding values for the normal AT and exceeded the largest peak in vivo forces (19% of failure) previously recorded in the rabbit AT. Average stiffness and modulus were 60 and 85% of normal values, respectively. New constructs with lower cell densities and higher scaffold stiffness that do not excessively contract and tear in culture and that further improve the repair stiffness needed to withstand various levels of expected in vivo loading are currently being investigated.

Effects of mechanical stimulation on the biomechanics and histology of stem cell-collagen sponge constructs for rabbit patellar tendon repair.

The objective of this study was to determine how mechanical stimulation affects the biomechanics and histology of stem cell-collagen sponge constructs used to repair central rabbit patellar tendon defects. Autogenous tissue-engineered constructs were created for both in vitro and in vivo analyses by seeding mesenchymal stem cells from 10 adult rabbits at 0.14x10(6) cells/construct in type I collagen sponges. Half of these constructs were mechanically stimulated once every 5 min for 8 h/day to a peak strain of 4% for 2 weeks. The other half remained in an incubator without mechanical stimulation for 2 weeks. Samples allocated for in vitro testing revealed that mechanically stimulated constructs had 2.5 times the linear stiffness of nonstimulated constructs. The remaining paired constructs for in vivo studies were implanted in bilateral full-thickness, full-length defects in the central third of rabbit patellar tendons. Twelve weeks after surgery, repair tissues were assigned for biomechanical (7 pairs) and histologic (3 pairs) analyses. Maximum force, linear stiffness, maximum stress, and linear modulus for the stimulated (vs. nonstimulated) repairs averaged 70% (vs. 55%), 85% (vs. 55%), 70% (vs. 50%), and 50% (vs. 40%) of corresponding values for the normal central third of the patellar tendons. The average force-elongation curve for the mechanically stimulated repairs also matched the corresponding curve for the normal patellar tendons, up to 150% of the peak in vivo force values recorded in a previous study. Construct and repair linear stiffness and linear modulus were also positively correlated (r = 0.6 and 0.7, respectively). Histologically both repairs showed excellent cellular alignment and mild staining for decorin and collagen type V, and moderate staining for fibronectin and collagen type III. This study shows that mechanical stimulation of stem cell-collagen sponge constructs can significantly improve tendon repair biomechanics up to and well beyond the functional limits of in vivo loading.

The effect of autologous mesenchymal stem cells on the biomechanics and histology of gel-collagen sponge constructs used for rabbit patellar tendon repair.

The objective of this study was to introduce mesenchymal stem cells (MSCs) into a gel-sponge composite and examine the effect the cells have on repair biomechanics and histology 12 weeks postsurgery. We tested two related hypotheses-adding MSCs would significantly improve repair biomechanics and cellular organization, and would result in higher failure forces than peak in vivo patellar tendon (PT) forces recorded for an inclined hopping activity. Autogenous tissue-engineered constructs were created by seeding MSCs from 15 adult rabbits at 0.1 x 10(6) cells/mL in 2.6 mg/mL of collagen gel in collagen sponges. Acellular constructs were created using the same concentration of collagen gel in matching collagen sponges. These cellular and acellular constructs were implanted in bilateral full-thickness, full-length defects in the central third of patellar tendons. At 12 weeks after surgery, repair tissues were assigned for biomechanical (n = 12 pairs) and histological (n = 3 pairs) analyses. Maximum force and maximum stress for the cellular repairs were about 60 and 50% of corresponding values for the normal central third of the PT, respectively. Likewise, linear stiffness and linear modulus for these cellular repairs averaged 75 and 30% of normal PT values, respectively. By contrast, the acellular repairs exhibited lower percentages of normal PT values for maximum force (40%), maximum stress (25%), linear stiffness (30%), and linear modulus (20%). Histologically, both repairs showed strong staining for collagen types III and V, fibronectin, and decorin. The cellular repairs also showed cellular alignment comparable to that of normal tendon. This study shows that introducing autogenous mesenchymal stem cells into a gel-collagen sponge composite significantly improves tendon repair compared to the use of a gel-sponge composite alone in the range of in vivo loading.

Jump-land characteristics and muscle strength development in young athletes: a gender comparison of 1140 athletes 9 to 17 years of age.

BACKGROUND: Many authors have speculated that altered neuromuscular control and strength of the lower extremity are responsible for the gender disparity in knee ligament injury rates. HYPOTHESIS: Significant increases in normalized quadriceps and hamstrings strength and limb symmetry on single-legged hop test occur with age. No gender differences in strength occur until age 14 years, after which boys generate greater peak torques than do girls. Age and gender do not influence lower limb alignment on a drop-jump test. STUDY DESIGN: Cross-sectional study; Level of evidence, 3. METHODS: We studied the effects of age and gender in 1140 athletes, 9 to 17 years old, on muscle strength and neuromuscular control during functional activities. Isokinetic quadriceps and hamstrings strength was measured at 300 deg/s. Limb symmetry was assessed with single-legged hop tests. A video drop-jump test determined lower limb alignment in the coronal plane. RESULTS: Extension peak torques significantly increased with age; maximum strength was noted in girls at age 13 years and in boys at age 14 years (P < .001). Although maximum flexion strength occurred in boys at age 14 years (P < .001), girls had only slight increases from ages 9 to 11 years (P = not significant). Boys aged 14 to 17 years had significantly greater normalized isokinetic strength than did age-matched girls. No age or gender effects existed in limb alignment on the drop-jump test or limb symmetry on single-legged hop testing. CONCLUSION: Maximum hamstrings strength was noted in female athletes by age 11 years, compared with age 14 years in male athletes, and a distinct lower limb valgus alignment existed in the majority of all athletes on landing. The absence of a gender difference in lower limb alignment on landing suggests other factors may be responsible for the gender disparity in knee ligament injury rates.

Effects of cell-to-collagen ratio in mesenchymal stem cell-seeded implants on tendon repair biomechanics and histology.

Autogenous tissue-engineered constructs were fabricated at four cell-to-collagen ratios (0.08, 0.04, 0.8, and 0.4 M/mg) by seeding mesenchymal stem cells (MSCs) from 16 adult rabbits at one of two seeding densities (0.1 x 10(6) and 1 x 10(6) cells/mL) in one of two collagen concentrations (1.3 and 2.6 mg/mL). The highest two ratios (0.4 and 0.8 M/mg) were damaged by excessive cell contraction and could not be used in subsequent in vivo studies. The remaining two sets of constructs were implanted into bilateral full-thickness, full-length defects created in the central third of the patellar tendon (PT). At 12 weeks after surgery, repair tissues were assigned for biomechanical (n = 13) and histological (n = 3) analyses. A second group of rabbits (n = 6) received bilateral acellular implants with the same two collagen concentrations. At 12 weeks, repair tissues were also assigned for biomechanical (n = 4) and histological (n = 2) analyses. No significant differences were observed in any structural or material properties or in histological appearance among the two cell-seeded and two acellular repair groups. Average maximum force and maximum stress of the repairs were approximately 30% of corresponding values for the central one-third of normal PT and higher than peak in vivo forces measured in rabbit PT from one of our previous publications. However, average repair stiffness and modulus were only 30 and 20% of normal PT values, respectively. Current repairs achieved higher maximum forces than in previous studies and without ectopic bone, but will need to achieve sufficient stiffness as well to be effective in the in vivo range of loading.

Assessment of lower limb neuromuscular control in prepubescent athletes.

BACKGROUND: Although neuromuscular indices have been extensively studied in adolescents and adults, limited data exist for prepubescent children. HYPOTHESIS: No differences exist between prepubescent boys and girls in lower limb strength, symmetry on single-legged hop testing, and limb alignment during drop-jump testing. STUDY DESIGN: Cross-sectional study (prevalence); Level of evidence, 1. METHODS: The authors tested 27 female and 25 male athletes who were aged 9 to 10 years and matched for both body mass index and years of organized sports participation. In a drop-jump screening test, the distance between the right and left hips, knees, and ankles was measured as an indicator of lower limb axial alignment in the coronal plane. The distance between the knees and ankles was normalized by the hip separation distance. Quadriceps and hamstrings strengths were measured isokinetically at 180 deg/s. Lower limb symmetry was determined from 2 single-legged hop function tests. RESULTS: Boys demonstrated greater mean absolute and normalized knee and ankle separation distances on the drop-jump test. Even so, 76% of boys had a normalized knee separation distance of 60% or less of the hip separation distance, as did 93% of girls, indicating a distinctly valgus alignment. There were no differences between the sexes in quadriceps and hamstrings peak torques, hamstrings/quadriceps ratio, time to peak torque, total work, or lower limb symmetry values. CONCLUSIONS: A high percentage of the prepubescent athletes studied had a distinctly valgus lower limb alignment during the drop-jump test and a lack of lower limb symmetry during the hop tests. These same indices have been hypothesized to increase the risk for knee ligament injuries in older athletes. Neuromuscular training may be needed to address these issues in children.

In vivo forces used to develop design parameters for tissue engineered implants for rabbit patellar tendon repair.

Previous studies in tissue engineering have shown that suspending undifferentiated mesenchymal stem cells in collagen gels and wrapping them about a suture causes alignment of cells and contraction of constructs in culture in a form that is suitable for implantation for tendon repair. Little is known about the patterns of these in vivo signals that might improve tendon repair biomechanics. Three hypotheses were tested in this study using the rabbit patellar tendon (PT) model: (1) peak in vivo forces and the rates of rise and fall in these forces will increase significantly with increasing levels of activity; (2) the PTs safety factor for all activities will be in the range of values found for tendons (2.5-3); (3) rabbits will not "favor" the operated limb at the time of evaluation but maintain similar vertical ground reaction forces in both limbs during quiet standing (QS). In vivo rabbit PT forces were measured during QS and while the animal hopped on a treadmill whose speed (0.04 and 0.13 m/s) and inclination (0 degrees and 12 degrees) were controlled. Implantable force transducers were surgically placed in one PT and data collected three days post surgery in each of eight New Zealand White rabbits. Peak tensile forces increased significantly with inclination of the treadmill and the rates of rise and fall in tendon force increased significantly with both speed and inclination (p<0.001). Such design criteria should be useful in mechanically stimulating cell-gel constructs for tendon repair.

Characterization of in vivo Achilles tendon forces in rabbits during treadmill locomotion at varying speeds and inclinations.

The objective of this study was to test the hypothesis that increasing the speed and inclination of the treadmill increases the peak Achilles tendon forces and their rates of rise and fall in force. Implantable force transducers (IFT) were inserted in the confluence of the medial and lateral heads of the left gastrocnemius tendon in 11 rabbits. IFT voltages were successfully recorded in 8 animals as the animals hopped on a treadmill at each of two speeds (0.1 and 0.3 mph) and inclinations (0 degrees and 12 degrees). Instrumented tendons were isolated shortly after sacrifice and calibrated. Contralateral unoperated tendons were failed in uniaxial tension to determine maximum or failure force, from which safety factor (ratio of maximum force to peak in vivo force) was calculated for each activity. Peak force and the rates of rise and fall in force significantly increased with increasing treadmill inclination (p<0.001). Safety factors averaged 30.8+/-7.5 for quiet standing, 7.0+/-2.9 for level hopping, and 5.2+/-0.7 for inclined hopping (mean+/-SEM). These in vivo force parameters will help tissue engineers better design functional tissue engineered constructs for rabbit Achilles tendon and other tendon repairs. Force patterns can also serve as input data for mechanical stimulation of tissue-engineered constructs in culture. Such approaches are expected to help accelerate tendon repair after injury.

Neuromuscular function in athletes following recovery from a recent acute low back injury.

STUDY DESIGN: Observational case control design. OBJECTIVES: To examine muscle response to sudden trunk loading in athletes with and without a recent history of acute low back injury (LBI). BACKGROUND: Impaired neuromuscular function is associated with chronic low back pain. This study examined whether such impairment persists after recovery from an acute LBI. METHODS AND MEASURES: Seventeen athletes who had a recent history of acute LBI and 17 matched healthy controls were tested. At the time of testing (mean = 56 days postinjury, range = 7-120 days postinjury), all athletes were symptom free and had returned to regular competition. Subjects performed isometric exertions in trunk flexion, extension, and left and right lateral bending against a trunk restraining cable. Upon reaching the target isometric force, the cable was released to impose sudden loading on the lumbar spine. Surface EMG signals from 12 major trunk muscles were recorded. The shut-off and switch-on latencies and number of muscles responding to sudden loading were compared between the 2 groups. RESULTS: In all 4 testing directions, the athletes with a recent history of acute LBI shut off significantly fewer muscles and did so with delayed latency. On average, the injured subjects shut off 4.0 out of 6.0 (SD = 1.3) muscles compared to 4.6 out of 6.0 (SD = 1.3) muscles in the control group. The average muscle shut-off latency was 71 (SD = 31) milliseconds for the injured and 50 (SD = 21) milliseconds for the control subjects. No differences were found in number or latency of muscles switching on. CONCLUSIONS: These objective measures of neuromuscular function indicated an altered muscle response pattern to sudden trunk loading in athletes following their clinical recovery from a recent acute LBI.

Effect of aging on male and female master athletes' performance in strength versus endurance activities.

Aging has traditionally been associated with several functional declines within the musculoskeletal system. In this study, we characterized and compared age-related changes in performance in power and endurance activities (ie, power-lifting, stationary rowing). From age 25 to age 85, men's and women's rowing performance decreases 29%; from age 25 to age 55, men's rowing performance decreases 0.12% per year, and women's performance decreases 0.23% per year. The pattern of decline in power-lifting performance is characterized by earlier onset and more rapid progression. During the fourth decade, men's and women's power-lifting performance decreases 3% per year; thereafter, their performance decreases 1% per year. The difference between the age-related change rates for rowing and power-lifting is statistically significant (P = .05). The older athletes' performance in both activities suggests that the capacity for significant function in activities requiring power and endurance persists throughout life in individuals who train and who remain in good health. That strength decreases more rapidly than endurance capacity points to the importance of resistance exercises for minimizing muscle weakness in an aging population.

Primary and secondary restraints of human and ovine knees for simulated in vivo gait kinematics.

Knee soft tissue structures are frequently injured, leading to the development of osteoarthritis even with treatment. Understanding how these structures contribute to knee function during activities of daily living (ADLs) is crucial in creating more effective treatments. This study was designed to determine the role of different knee structures during a simulated ADL in both human knees and ovine stifle joints. A six degree-of-freedom robot was used to reproduce each species' in vivo gait while measuring three-dimensional joint forces and torques. Using a semi-randomized selective cutting method, we determined the primary and secondary structures contributing to the forces and torques along and about each anatomical axis. In both species, the bony interaction, ACL, and medial meniscus provided most of the force contributions during stance, whereas the ovine MCL, human bone, and ACLs of both species were the key contributors during swing. This study contributes to our overarching goal of establishing functional tissue engineering parameters for knee structures by further validating biomechanical similarities between the ovine model and the human to provide a platform for measuring biomechanics during an in vivo ADL. These parameters will be used to develop more effective treatments for knee injuries to reduce or eliminate the incidence of osteoarthritis.

The role of mechanical loading in tendon development, maintenance, injury, and repair.

Tendon injuries often result from excessive or insufficient mechanical loading, impairing the ability of the local tendon cell population to maintain normal tendon function. The resident cell population composing tendon tissue is mechanosensitive, given that the cells are able to alter the extracellular matrix in response to modifications of the local loading environment. Natural tendon healing is insufficient, characterized by improper collagen fibril diameter formation, collagen fibril distribution, and overall fibril misalignment. Current tendon repair rehabilitation protocols focus on implementing early, well-controlled eccentric loading exercises to improve repair outcome. Tissue engineers look toward incorporating mechanical loading regimens to precondition cell populations for the creation of improved biological augmentations for tendon repair.

The native cell population does not contribute to central-third graft healing at 6, 12, or 26 weeks in the rabbit patellar tendon.

Investigators do not yet understand the role of intrinsic tendon cells in healing at the tendon-to-bone enthesis. Therefore, our first objective was to understand how the native cell population influences tendon autograft incorporation in the central-third patellar tendon (PT) defect site. To do this, we contrasted the histochemical and biomechanical properties of de-cellularized patellar tendon autograft (dcPTA) and patellar tendon autograft (PTA) repairs in the skeletally mature New Zealand white rabbit. Recognizing that soft tissues in many animal models require up to 26 weeks to incorporate into bone, our second objective was to investigate how recovery time affects enthesis formation and graft tissue biomechanical properties. Thus, we examined graft structure and mechanics at 6, 12, and 26 weeks post-surgery. Our results showed that maintaining the native cell population produced no histochemical or biomechanical benefit at 6, 12, or 26 weeks. These findings suggest that PTA healing is mediated more by extrinsic rather than intrinsic cellular mechanisms. Moreover, while repair tissue biomechanical properties generally increased from 6 to 12 weeks after surgery, no further improvements were noted up to 26 weeks.