The mechanism of hamstring injuries - a systematic review.

BACKGROUND: Injuries to the hamstring muscles are among the most common in sports and account for significant time loss. Despite being so common, the injury mechanism of hamstring injuries remains to be determined. PURPOSE: To investigate the hamstring injury mechanism by conducting a systematic review. STUDY DESIGN: A systematic review following the PRISMA statement. METHODS: A systematic search was conducted using PubMed, EMBASE and the Cochrane Library. Studies 1) written in English and 2) deciding on the mechanism of hamstring injury were eligible for inclusion. Literature reviews, systematic reviews, meta-analyses, conference abstracts, book chapters and editorials were excluded, as well as studies where the full text could not be obtained. RESULTS: Twenty-six of 2372 screened original studies were included and stratified to the mechanism or methods used to determine hamstring injury: stretch-related injuries, kinematic analysis, electromyography-based kinematic analysis and strength-related injuries. All studies that reported the stretch-type injury mechanism concluded that injury occurs due to extensive hip flexion with a hyperextended knee. The vast majority of studies on injuries during running proposed that these injuries occur during the late swing phase of the running gait cycle. CONCLUSION: A stretch-type injury to the hamstrings is caused by extensive hip flexion with an extended knee. Hamstring injuries during sprinting are most likely to occur due to excessive muscle strain caused by eccentric contraction during the late swing phase of the running gait cycle. LEVEL OF EVIDENCE: Level IV.

Editorial Commentary: When Is Too Small, Too Small? Allograft Augmentation of Autologous Hamstring Grafts During Anterior Cruciate Ligament Reconstruction.

Soft tissue allograft augmentation of small hamstring autografts, so-called hybrid grafts, has been proposed as an option during anterior cruciate ligament reconstruction (ACL-R). However, notable concerns exist with both small autograft use and allograft use during ACL-R, particularly in young, active patients. We currently choose to augment hamstring autografts with diameters <8.0 mm, adding only enough allograft to create a hybrid graft with an overall diameter no larger than 8.5 mm. Based on the available evidence, surgeons continue to seek the tipping point where the benefit of additional allograft tissue exceeds the consequence of its use.

Matrix metalloproteinase activity and prostaglandin E2 are elevated in the synovial fluid of meniscus tear patients.

PURPOSE: Meniscus tears are a common knee injury and are associated with the development of post-traumatic osteoarthritis (OA). The purpose of this study is to evaluate potential OA mediators in the synovial fluid and serum of meniscus tear subjects compared to those in the synovial fluid of radiographic non-OA control knees. MATERIALS AND METHODS: Sixteen subjects with an isolated unilateral meniscus injury and six subjects who served as reference controls (knee Kellgren-Lawrence grade 0-1) were recruited. Twenty-one biomarkers were measured in serum from meniscus tear subjects and in synovial fluid from both groups. Meniscus tear subjects were further stratified by tear type to assess differences in biomarker levels. RESULTS: Synovial fluid total matrix metalloproteinase (MMP) activity and prostaglandin E2 (PGE2) were increased 25-fold and 290-fold, respectively, in meniscus tear subjects as compared to reference controls (p < 0.05). Synovial fluid MMP activity and PGE2 concentrations were positively correlated in meniscus tear subjects (R = 0.83, p < 0.0001). In meniscus tear subjects, synovial fluid levels of MMP activity, MMP-2, MMP-3, sGAG, COMP, IL-6, and PGE2 were higher than serum levels (p < 0.05). Subjects with complex meniscus tears had higher synovial fluid MMP-10 (p < 0.05) and reduced serum TNFα and IL-8 (p < 0.05) compared to other tear types. CONCLUSIONS: Given the degradative and pro-inflammatory roles of MMP activity and PGE2, these molecules may alter the biochemical environment of the joint. Our findings suggest that modulation of PGE2 signaling, MMP activity, or both following a meniscus injury may be targets to promote meniscus repair and prevent OA development.

Lower Extremity Movement Differences Persist After Anterior Cruciate Ligament Reconstruction and When Returning to Sports.

OBJECTIVE: To examine how landing mechanics change in patients after anterior cruciate ligament reconstruction (ACL-R) between 6 months and 12 months after surgery. DESIGN: Case-series. SETTING: Laboratory. PARTICIPANTS: Fifteen adolescent patients after ACL-R participated. INTERVENTIONS: Lower extremity three-dimensional motion analysis was conducted during a bilateral stop jump task in patients at 6 and 12 months after ACL-R. Joint kinematic and kinetic data, in addition to ground reaction forces, were collected at each time point. MAIN OUTCOME MEASURES: During the stop jump landing, the peak joint moments and the initial and peak joint motion at the ankle, knee, and hip were examined. The peak vertical ground reaction force was also examined. RESULTS: Interactions were observed for both the peak knee (P = 0.03) and hip extension moment (P = 0.07). However, only the hip extension moment was symmetrical level at 12 months. Statistically significant (P < 0.05) side-to-side differences existed for the ankle angle at initial contact, peak plantarflexion moment, peak hip flexion angle, and peak impact vertical ground reaction force independent of time. CONCLUSIONS: The findings of this study suggest that sagittal plane moments at the knee and hip demonstrate an increase in symmetry between 6 months and 1 year after ACL-R surgery, however, symmetry of the knee extension moment is not established by 12 months after surgery. The lack of change in the variables across time was unexpected. As a result, it is inappropriate to expect a change in landing mechanics solely as a result of time alone after discharge from rehabilitation.

Gender Differences in Plantar Loading During an Unanticipated Side Cut on FieldTurf.

OBJECTIVE: To determine whether force-time integral (FTI) and maximum force (MF) are significantly different between genders when performing an unanticipated side cut on FieldTurf. DESIGN: Thirty-two collegiate athletes (16 men and 16 women) completed 12 unanticipated cutting trials, while plantar pressure data were recorded using Pedar-X insoles. SETTING: Controlled Laboratory Study. PARTICIPANTS: Division I cleated sport athletes with no previous foot and ankle surgery, no history of lower extremity injury in the past 6 months, and no history of metatarsal stress fracture. INTERVENTIONS: None. MAIN OUTCOME MEASURES: Maximum force and the FTI in the total foot, medial midfoot (MMF), lateral midfoot (LMF), medial forefoot (MFF), middle forefoot (MiddFF), and the lateral forefoot (LFF). RESULTS: Males had a greater FTI beneath the entire foot (TF) (P < 0.001). Females had a significantly higher MF beneath the LMF (P = 0.001), MiddFF (P < 0.001), and LFF (P = 0.001). Males had a significantly greater MF beneath the MMF (P = 0.003) and greater FTI beneath the MMF (P < 0.001) and MFF (P = 0.002). CONCLUSIONS: Significant differences in plantar loading exist between genders with males demonstrating increased loading beneath the TF in comparison with females. Females had overall greater loading on the lateral column, whereas males had greater loading on the medial column of the foot. CLINICAL RELEVANCE: The results of this study indicate that plantar loading is different between genders; therefore, altering cleated footwear to be gender specific may result in more optimal foot loading patterns. Optimizing cleated shoe design could decrease the risk for metatarsal stress fractures.

Time to get rid of the clock: intraobserver and interobserver reliability in determination of the o'clock position of the femoral tunnel in ACL reconstruction.

This study evaluates intraobserver and interobserver agreement in reporting the o'clock position of the femoral tunnel during anterior cruciate ligament (ACL) reconstruction. Four PGY2 residents, four PGY5 residents, and four sports medicine orthopedic surgeons reported the o'clock position of the femoral ACL tunnel in 10 arthroscopic pictures on two occasions 3 months apart. Intraobserver agreement was determined using the intraobserver correlation coefficient (r > 0.576 for 0.05 significance level). Interobserver agreement between members of each group and between reviewer groups was evaluated with the intraclass correlation coefficient (ICC > 0.75 considered good agreement). Poor interobserver agreement was demonstrated between the attending and PGY2 groups (ICC = 0.1685), between the attending and PGY5 groups (ICC = 0.2982), and between the PGY5 and PGY2 groups (ICC = 0.267). Attending surgeons, PGY5s, and PGY2s demonstrated poor interobserver agreement amongst themselves (ICC = 0.2244, 0.471, and 0.0859, respectively). PGY2s and PGY5s demonstrated good intraobserver agreement, but attending surgeons demonstrated poor intraobserver agreement. Attending orthopedic surgeons and residents of different levels of training interpret the o'clock position of the femoral tunnel differently. Greater years of experience does not improve intraobserver or interobserver agreement on the o'clock position. The clock face terminology for femoral tunnel placement may not be a reliable descriptor for scientific investigations or clinical instruction.

Tram-track appearance of the posterior cruciate ligament (PCL): correlations with mucoid degeneration, ligamentous stability, and differentiation from PCL tears.

OBJECTIVE: The purpose of our study was to describe the MRI findings in the posterior cruciate ligament (PCL) analogous to mucoid degeneration in the anterior cruciate ligament (ACL); to correlate MRI findings in the PCL with ligamentous stability; to differentiate the PCL tram-track appearance from the appearance of PCL tears; and to emphasize the coexistence of PCL and ACL mucoid degeneration, cruciate ganglia, and meniscal cysts. CONCLUSION: The tram-track PCL appearance commonly coexists with ACL mucoid degeneration; ganglia; and, less frequently, meniscal cysts. Both PCL tears and MRI findings suggestive of PCL mucoid degeneration show ligament thickening and increased PCL signal intensity. Tram-track PCLs are usually asymptomatic and typically have no ligamentous instability.

Intercondylar synovial septum in two patients with nail-patella syndrome.

This case report describes the arthroscopic findings in two patients with nail-patella syndrome (NPS). In both cases, a midline synovial septum was encountered that completely subdivided the knee into medial and lateral compartments. One patient required two subsequent arthroscopic procedures, and the synovial septum was found to have recurred even after it had been resected at the initial surgery. The etiology and clinical significance of this anatomic anomaly are unknown, however, surgeons should be aware of its existence and the potential difficulties it may present during knee arthroscopy in patients with NPS.

Hemi-bucket-handle tears of the meniscus: appearance on MRI and potential surgical implications.

OBJECTIVES: To describe a type of meniscus flap tear resembling a bucket-handle tear, named a "hemi-bucket-handle" tear; to compare its imaging features with those of a typical bucket-handle tear; and to discuss the potential therapeutic implications of distinguishing these two types of tears. MATERIALS AND METHODS: Five knee MR examinations were encountered with a type of meniscus tear consisting of a flap of tissue from the undersurface of the meniscus displaced toward the intercondylar notch. A retrospective analysis of 100 MR examinations prospectively interpreted as having bucket-handle type tears yielded 10 additional cases with this type of tear. Cases of hemi-bucket-handle tears were reviewed for tear location and orientation, appearance of the superior articular surface of the meniscus, presence and location of displaced meniscal tissue, and presence of several classic signs of bucket-handle tears. RESULTS: A total of 15/15 tears involved the medial meniscus, had tissue displaced toward the notch, and were mainly horizontal in orientation. The superior surface was intact in 11/15 (73.3%). In 1/15 (6.7%) there was an absent-bow-tie sign; 6/15 (40%) had a double-PCL sign; 14/15 (93.3%) had a double-anterior horn sign. CONCLUSION: We describe a type of undersurface flap tear, named a hemi-bucket-handle tear, which resembles a bucket-handle tear. Surgeons at our institution feel this tear would likely not heal if repaired given its predominantly horizontal orientation, and additionally speculate the tear could be overlooked at arthroscopy. Thus, we feel it is important to distinguish this type of tear from the typical bucket-handle tear.

Graft size and patient age are predictors of early revision after anterior cruciate ligament reconstruction with hamstring autograft.

PURPOSE: To evaluate whether decreased hamstring autograft size and decreased patient age are predictors of early graft revision. METHODS: Of 338 consecutive patients undergoing primary anterior cruciate ligament (ACL) reconstruction with hamstring autograft, 256 (75.7%) were evaluated. Graft size and patient age, gender, and body mass index at the time of ACL reconstruction were recorded, along with whether subsequent ACL revision was performed. RESULTS: The 256 patients comprised 136 male and 120 female patients and ranged in age from 11 to 52 years (mean, 25.0 years). The mean follow-up was 14 months (range, 6 to 47 months). Revision ACL reconstruction was performed in 18 of 256 patients (7.0%) at a mean of 12 months after surgery (range, 3 to 31 months). Revision was performed in 1 of 58 patients (1.7%) with grafts greater than 8 mm in diameter, 9 of 139 patients (6.5%) with 7.5- or 8-mm-diameter grafts, and 8 of 59 patients (13.6%) with grafts 7 mm or less in diameter (P = .027). There was 1 revision performed in the 137 patients aged 20 years or older (0.7%), but 17 revisions were performed in the 119 patients aged under 20 years (14.3%) (P < .0001). Most revisions (16 of 18) were noted to occur in patients aged under 20 years with grafts 8 mm in diameter or less, and the revision rate in this population was 16.4% (16 of 97 patients). Age less than 20 years at reconstruction (odds ratio [OR], 18.97; 95% confidence interval [CI], 2.43 to 147.06; P = .005), decreased graft size (OR, 2.20; 95% CI, 1.00 to 4.85; P = .05), and increased follow-up time (OR, 1.07; 95% CI, 1.02 to 1.12) were associated with increased risk of revision. CONCLUSIONS: Decreased hamstring autograft size and decreased patient age are predictors of early graft revision. Use of hamstring autografts 8 mm in diameter or less in patients aged under 20 years is associated with higher revision rates. LEVEL OF EVIDENCE: Level III, retrospective comparative study.

Prevention of ACL injury, part I: injury characteristics, risk factors, and loading mechanism.

The anterior cruciate ligament (ACL) injury is one of the most common injuries in sports. ACL injuries are not only costly from financial and health services consumption standpoints, but also can have devastating consequences on patients' activity levels and quality of life. Tremendous efforts have been made over the past two decades toward the goal of preventing ACL injuries. A substantial number of studies have been performed to determine the characteristics of ACL injury events, identify risk factors for ACL injury, and develop prevention strategies. The purpose of this review was to objectively summarize the current literature regarding the characteristics of ACL injury, ACL loading mechanisms, and risk factors for injury to provide a comprehensive understanding of the current state of research and how our current level of knowledge may inform clinical practice in this area.

Prevention of ACL injury, part II: effects of ACL injury prevention programs on neuromuscular risk factors and injury rate.

Prevention strategies have been developed based on existing knowledge in an attempt to alter neuromuscular control and lower extremity biomechanics in order to reduce anterior cruciate ligament (ACL) injury rates. These strategies have included different training programs ranging from injury education to multicomponent training. Many training programs have been demonstrated as resulting in altered lower extremity movement patterns. The effects of current training programs on ACL injury rate, however, are inconsistent. This review was focused on the effects of current ACL injury training programs on neuromuscular risk factors and ACL injury rate. Recommendations were made based on the available evidence for clinicians and coaches to implement ACL injury prevention programs.

Effects of a knee extension constraint brace on lower extremity movements after ACL reconstruction.

BACKGROUND: Patients have high reinjury rates after ACL reconstruction. Small knee flexion angles and large peak posterior ground reaction forces in landing tasks increase ACL loading. QUESTIONS/PURPOSES: We determined the effects of a knee extension constraint brace on knee flexion angle, peak posterior ground reaction force, and movement speed in functional activities of patients after ACL reconstruction. PATIENTS AND METHODS: Six male and six female patients 3.5 to 6.5 months after ACL reconstruction participated in the study. Three-dimensional videographic and force plate data were collected while patients performed level walking, jogging, and stair descent wearing a knee extension constraint brace, wearing a nonconstraint brace, and not wearing a knee brace. Knee flexion angle at initial foot contact with the ground, peak posterior ground reaction force, and movement speed were compared across brace conditions and between genders. RESULTS: Wearing the knee extension constraint brace increased the knee flexion angle at initial foot contact for each activity when compared with the other two brace conditions. Wearing the knee extension constraint brace also decreased peak posterior ground reaction force during walking but not during jogging and stair descent. CONCLUSIONS: Although the knee extension constraint brace did not consistently reduce the peak posterior ground reaction force in all functional activities, it consistently increased knee flexion angle and should reduce ACL loading as suggested by previous studies. These results suggest the knee extension constraint brace has potential as a rehabilitation tool to alter lower extremity movement patterns of patients after ACL reconstruction to address high reinjury rates.

Wrisberg variant of the discoid lateral meniscus with flipped meniscal fragments simulating bucket-handle tear: MRI and arthroscopic correlation.

An MRI diagnosis of the Wrisberg variant discoid lateral meniscus should be considered in patients presenting with an anteriorly flipped posterior horn fragment without a definable peripheral rim. We present four cases discovered on arthroscopy that were thought to resemble bucket-handle tears on preoperative MRI. Posterior hypermobility poses a surgical challenge as excessive debridement without careful attention to underlying meniscal morphology may lead to further instability. Although this diagnosis can be difficult to make on MRI, alerting the orthopedic surgeon preoperatively may influence repair technique and meniscus conservation.

Novel drug OMS103HP reduces pain and improves joint motion and function for 90 days after arthroscopic meniscectomy.

PURPOSE: This phase 2 study compared OMS103HP (Omeros, Seattle, WA) with control (lactated Ringer's) irrigation solution in patients undergoing arthroscopic partial meniscectomy. METHODS: This was a prospective, multicenter, double-blind, randomized, vehicle-controlled, parallel-group study. Safety and postoperative pain, range of motion, and self-reported function were evaluated for 90 days. Statistical results were based on univariate analysis of variance and repeated-measures analyses. RESULTS: Mean visual analog scale (VAS) pain scores within 24 hours after discharge from the recovery room showed more pain in the control group beginning at 2 hours and peaking at 8 hours. Univariate analysis of variance of mean VAS scores over the 24-hour period did not meet statistical significance. Repeated-measures analysis yielded a statistically significant difference (P = .004) for time-by-treatment interaction, showing a clear drug benefit over time based on VAS scores. There were statistically significant differences at day 7 between the groups in passive flexion without pain (P = .022). The proportion of patients achieving flexion of 95° or greater, 110°, and 125° was greater for the OMS103HP group. The Knee Injury and Osteoarthritis Outcome Score (KOOS) showed statistically significant differences (P ≤ .05) between the OMS103HP and control groups for 4 of 5 outcomes (symptoms, pain, sport and recreation, and knee-based quality of life but not activities of daily living). All scores showed a treatment effect through day 90. The overall incidence of adverse events and abnormal laboratory values for the OMS103HP and control groups was similar. Serious adverse events occurred in 1 control patient. CONCLUSIONS: In this study of patients with meniscal tears who underwent simple debridement, the use of OMS103HP resulted in reduced acute postoperative pain (measured by VAS over the first 24 hours postoperatively), reduced pain during recovery (measured by the KOOS pain subscale, which measures both background levels of pain and exacerbations caused by movements or activities), improved postoperative knee motion, and improved functional outcomes as assessed with the KOOS Knee Survey. Clinical benefits of OMS103HP were consistent and sustained throughout 90 days of postoperative follow-up. LEVEL OF EVIDENCE: Level I, prospective, randomized, controlled trial.

Hamstring graft preparation using a needleless suture loop.

Placement of running, locking stitches during the harvest and preparation of hamstring tendons can be time consuming and requires placement of suture in the most distal and proximal 2 to 3 cm of the tendons. The authors have regularly used a needleless suture loop technique to both harvest and prepare autograft gracilis and semitendinosis tendons and to tension quadrupled hamstring tendon grafts during anterior cruciate ligament reconstruction. This cost-effective technique reduces exposure to needles, the time required to harvest and prepare the grafts, and the amount of suture in the tibial tunnel, while providing adequate fixation to tension the tendons as needed.

Are Weightbearing Restrictions Required After Microfracture for Isolated Chondral Lesions of the Knee? A Review of the Basic Science and Clinical Literature.

CONTEXT: A strict rehabilitation protocol is traditionally followed after microfracture, including weightbearing restrictions for 2 to 6 weeks. However, such restrictions pose significant disability, especially in a patient population that is younger and more active. EVIDENCE ACQUISITION: An extensive literature review was performed through PubMed and Google Scholar of all studies through December 2018 related to microfracture, including biomechanical, basic science, and clinical studies. For inclusion, clinical studies had to report weightbearing status and outcomes with a minimum 12-month follow-up. STUDY DESIGN: Clinical review. LEVEL OF EVIDENCE: Level 3. RESULTS: Review of biomechanical and biology studies suggest new forming repair tissue is protected from shear forces of knee joint loading by the cartilaginous margins of the defect. This margin acts as a shoulder to maintain axial height and allow for tissue remodeling up to at least 12 months after surgery, well beyond current weight bearing restriction trends. A retrospective case-control study showed that weightbearing status postoperatively had no effect on clinical outcomes in patients who underwent microfracture for small chondral (<2 mm2) defects. In fact, 1 survey showed that many orthopaedic surgeons currently do not restrict weightbearing after microfracture. CONCLUSION: This clinical literature review suggests that weightbearing restrictions may not be required after microfracture for isolated tibiofemoral chondral lesions of the knee. STRENGTH OF RECOMMENDATION TAXONOMY: C.

Comparison of plain radiography, computed tomography, and magnetic resonance imaging in the evaluation of bone tunnel widening after anterior cruciate ligament reconstruction.

Bone tunnel widening poses a problem for graft fixation during revision anterior cruciate ligament (ACL) reconstruction. Large variability exists in the utilization of imaging modalities for evaluating bone tunnels in pre-operative planning for revision ACL reconstruction. The purpose of this study was to identify the most reliable imaging modality for identifying bone tunnels and assessing tunnel widening, and specifically, to validate the reliability of radiographs, MRI, and CT using intra- and inter-observer testing. Data was retrospectively collected from twelve patients presenting for revision ACL surgery. Five observers twice measured femoral and tibial tunnels at their widest point using digital calipers in coronal and sagittal planes. Measurements were corrected for magnification. Tunnel identification, diameter measurements, and cross-sectional area (CSA) calculations were recorded. A categorical classification of tunnel measurements was created to apply clinical significance to the measurements. Using kappa statistics, intra- and inter-observer reliability testing was performed. CT demonstrated excellent intra- and inter-observer reliability for tunnel identification. Intra- and inter-observer reliability was significantly less for MRI and radiographs. CT revealed superior reliability versus either radiographs or MRI for CSA analysis. Intra-observer kappa scores for tibial CSA using CT, radiographs, and MRI were 0.66, 0.5, and 0.37, respectively. Inter-observer kappa scores for tibial CSA using CT, radiographs, and MRI were 0.65, 0.39, and 0.32, respectively. Our results demonstrate CT is the most reliable imaging modality for evaluation of ACL bone tunnels as proven by superior intra- and inter-observer testing results when compared to MRI and radiographs. Radiographs and MRI were not reliable, even for simply identifying the presence of a bone tunnel.

Integrated injury prevention program improves balance and vertical jump height in children.

Implementing an injury prevention program to athletes under age 12 years may reduce injury rates. There is limited knowledge regarding whether these young athletes will be able to modify balance and performance measures after completing a traditional program that has been effective with older athletes or whether they require a specialized program for their age. The purpose of this study was to compare the effects of a pediatric program, which was designed specifically for young athletes, and a traditional program with no program in the ability to change balance and performance measures in youth athletes. We used a cluster-randomized controlled trial to evaluate the effects of the programs before and after a 9-week intervention period. Sixty-five youth soccer athletes (males: n = 37 mass = 34.16 +/- 5.36 kg, height = 143.07 +/- 6.27 cm, age = 10 +/- 1 yr; females: n = 28 mass = 33.82 +/- 5.37 kg, height = 141.02 +/- 6.59 cm) volunteered to participate and attended 2 testing sessions in a research laboratory. Teams were cluster-randomized to either a pediatric or traditional injury prevention program or a control group. Change scores for anterior-posterior and medial-lateral time-to-stabilization measures and maximum vertical jump height and power were calculated from pretest and post-test sessions. Contrary with our original hypotheses, the traditional program resulted in positive changes, whereas the pediatric program did not result in any improvements. Anterior-posterior time-to-stabilization decreased after the traditional program (mean change +/- SD = -0.92 +/- 0.49 s) compared with the control group (-0.49 +/- 0.59 s) (p = 0.003). The traditional program also increased vertical jump height (1.70 +/- 2.80 cm) compared with the control group (0.20 +/- 0.20 cm) (p = 0.04). There were no significant differences between control and pediatric programs. Youth athletes can improve balance ability and vertical jump height after completing an injury prevention program. Training specificity appears to affect improvements and should be considered with future program design.

In vivo attachment site to attachment site length and strain of the ACL and its bundles during the full gait cycle measured by MRI and high-speed biplanar radiography.

The purpose of this study was to measure in vivo attachment site to attachment site lengths and strains of the anterior cruciate ligament (ACL) and its bundles throughout a full cycle of treadmill gait. To obtain these measurements, models of the femur, tibia, and associated ACL attachment sites were created from magnetic resonance (MR) images in 10 healthy subjects. ACL attachment sites were subdivided into anteromedial (AM) and posterolateral (PL) bundles. High-speed biplanar radiographs were obtained as subjects ambulated at 1 m/s. The bone models were registered to the radiographs, thereby reproducing the in vivo positions of the bones and ACL attachment sites throughout gait. The lengths of the ACL and both bundles were estimated as straight line distances between attachment sites for each knee position. Increased attachment to attachment ACL length and strain were observed during midstance (length = 28.5 ±â€¯2.6 mm, strain = 5 ±â€¯4%, mean ±â€¯standard deviation), and heel strike (length = 30.5 ±â€¯3.0 mm, strain = 12 ±â€¯5%) when the knee was positioned at low flexion angles. Significant inverse correlations were observed between mean attachment to attachment ACL lengths and flexion (rho = -0.87, p < 0.001), as well as both bundle lengths and flexion (rho = -0.86, p < 0.001 and rho = -0.82, p < 0.001, respectively). AM and PL bundle attachment to attachment lengths were highly correlated throughout treadmill gait (rho = 0.90, p < 0.001). These data can provide valuable information to inform design criteria for ACL grafts used in reconstructive surgery, and may be useful in the design of rehabilitation and injury prevention protocols.