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ACL

The (ACL) is one of two cruciate ligaments in the human knee joint, forming a diagonal band of dense, fibrous that attaches the posterior aspect of the lateral femoral condyle to the anterior of the tibial plateau. Composed primarily of fibers arranged in fascicles, it functions to restrain anterior tibial translation relative to the , limit internal tibial , and provide rotational stability during weight-bearing activities such as pivoting or cutting motions. ACL injuries represent one of the most prevalent disruptions, particularly in sports involving rapid deceleration, jumping, or directional changes like soccer, , and , with non-contact mechanisms accounting for the majority of cases through valgus loading and tibial rotation. Symptoms typically include immediate pain, , and a of , often diagnosed via clinical tests like the Lachman maneuver and confirmed with MRI imaging, which reveals partial or complete tears. Treatment varies by patient activity level and concomitant injuries, with surgical using autografts or allografts common for active individuals to restore function, though conservative can suffice for select low-demand cases, highlighting ongoing debates over surgical necessity and long-term risks post-injury. Epidemiological data indicate higher incidence in females due to biomechanical and neuromuscular factors, underscoring prevention strategies like targeted programs.

Medicine and anatomy

Anterior cruciate ligament

The (ACL) is a primary stabilizer consisting of dense, fibrous that originates from the posterolateral aspect of the intercondylar notch on the lateral femoral condyle and inserts onto the anterior tibial plateau, anterior to the anterior horn of the . It measures approximately 32-38 mm in length and 10-12 mm in width in adults, with a twisted, band-like structure comprising two functional bundles: the anteromedial bundle, which tightens in flexion, and the posterolateral bundle, which tightens in extension. The is intra-articular but extrasynovial, covered by a that nourishes it via , as it lacks direct blood supply from vascular structures. The ACL's primary biomechanical role is to resist anterior tibial translation relative to the , limiting forward displacement of the tibia by up to 85% during knee flexion-extension activities, while secondarily restraining internal tibial rotation and varus-valgus angulation. It achieves this through tension that peaks at 90-120 degrees of flexion, interacting with secondary stabilizers like the menisci and collateral ligaments to maintain knee under load. Disruption of the ACL leads to anterior laxity exceeding 5 mm on Lachman testing and a positive anterior drawer sign, compromising joint stability during pivoting or decelerating maneuvers common in sports. ACL injuries occur at an estimated annual incidence of 68.6 cases per 100,000 person-years globally, with approximately 400,000 reconstructions performed yearly in the United States alone, reflecting a rate of about 1 in 3,500 individuals. Non-contact mechanisms account for roughly 70% of ruptures, often involving valgus loading with internal tibial rotation during landing or cutting, and are disproportionately prevalent in females (2-8 times higher risk than males) due to factors including wider pelvic geometry, higher quadriceps-to-hamstring strength ratios, and neuromuscular control differences. Sports such as soccer, , and show elevated rates, with professional athletes experiencing 0.86 injuries per 10,000 athlete-exposures. Acute tears present with a "pop" sensation, hemarthrosis, and instability, while chronic deficiencies increase risk by 4-6 fold over 10-15 years due to altered tibiofemoral loading. Diagnosis relies on clinical exams like the (sensitivity 0.24-0.48, specificity 0.97-0.99) confirmed by MRI, which detects complete tears with 86-97% accuracy. Nonoperative management suits low-demand patients but yields high failure rates (up to 50% instability recurrence); surgical , using autografts (e.g., or ) or allografts, restores stability in 80-90% of cases, with return-to-sport rates of 60-80% at 9-12 months post-op. Outcomes include 84% improvement in knee-related at one year, though re-rupture risks 5-10% and contralateral tears 15-20%, influenced by graft choice and adherence. Early (within 6 months) may minimize meniscal damage but requires careful timing to avoid .

Computing and technology

Access control list

An is a mechanism in computer security that implements by listing the entities (such as users, groups, or processes) permitted to access a and the specific access rights granted or denied to each. Each entry in an ACL, known as an access control entry (ACE), identifies a and defines permissions like read, write, execute, or deny operations. ACLs enforce , where resource owners or administrators define rules, with evaluation typically proceeding sequentially from the first matching ACE until a decision is reached, defaulting to denial if no match occurs. The ACL model traces its origins to the operating system, where the first implementation appeared in 1965 to secure filesystem resources by allowing per-file permission lists beyond simple owner-group-other modes. This approach addressed limitations in earlier systems by enabling fine-grained, user-specific controls, influencing subsequent designs in and Windows environments. Over time, ACLs evolved to support complex scenarios, including in hierarchical structures like directories, where child objects derive permissions from parents unless overridden. In operating systems, ACLs provide granular permissions beyond basic models. Windows NTFS employs ACLs for securable objects, distinguishing discretionary ACLs (DACLs) for user-defined access from system ACLs (SACLs) for auditing; each DACL contains ACEs ordered for explicit denies before allows to prioritize security. systems, such as , support ACLs as an extension to traditional permissions, permitting additional user or group entries with masks to limit maximum rights, thus avoiding proliferation of group assignments. In networking, ACLs function as packet filters on routers, switches, and firewalls to permit or block traffic based on rules evaluated in order. Standard ACLs, numbered 1-99 or 1300-1999 in , match solely on source or range, suitable for simple topology edge filtering but risking unintended blocks due to lack of specificity. Extended ACLs, numbered 100-199 or 2000-2699, offer precision by matching source and destination , protocols (e.g., , , ICMP), ports, and flags, enabling targeted controls like denying specific HTTP traffic while allowing others; they are placed close to the source to minimize unnecessary packet traversal. Named ACLs provide by assigning descriptive labels instead of numbers, while reflexive and dynamic variants add temporary or lock-and-key behaviors for advanced session . Misconfigurations in networking ACLs can lead to security gaps or performance degradation from rule explosion, underscoring the need for ordered, minimal rules with logging for verification.

Association for Computational Linguistics

The (ACL) is the premier international scientific and professional society dedicated to advancing research in and , encompassing computational models of linguistic phenomena such as syntax, semantics, and discourse. It supports the development of technologies addressing human language challenges, including , , and . Founded in 1962 as the Association for Machine Translation and Computational Linguistics (AMTCL), the organization shifted focus amid evolving priorities in the field and was renamed the ACL in 1968, reflecting a broader emphasis on computational approaches beyond early efforts. The ACL organizes an annual meeting, typically held each summer, which serves as a primary venue for presenting peer-reviewed and fostering collaboration among researchers. It also sponsors regional and specialized conferences, including the North American Chapter (NAACL), European Chapter (EACL), Conference on Empirical Methods in (EMNLP), and International Conference on Computational Linguistics (COLING), providing platforms for disseminating advancements in areas like neural language models and multilingual processing. Key publications include the journal, published quarterly by since the organization's early years, featuring original research on theoretical and applied aspects of the field. Additionally, the ACL sponsors the Transactions of the Association for Computational Linguistics (TACL), an open-access journal by that covers innovative work across NLP subdomains. The ACL Anthology, a digital archive, hosts over 114,000 papers from ACL-sponsored events, enabling free access to historical and contemporary scholarship. The organization maintains Special Interest Groups (SIGs) to promote focused research in niche areas, such as SIGDAT for linguistic data and corpus-based methods, SIGLEX for , SIGPARSE for natural language parsing, SIGEDU for educational applications, and SIGDIAL for discourse and . These groups organize workshops and dedicated tracks at conferences. The ACL further recognizes contributions through awards, including the Lifetime Achievement Award, as exemplified by the 2025 honor to Kathy McKeown for advancements in and summarization.

Entertainment and media

Austin City Limits

is an music program produced by Austin PBS and broadcast on stations nationwide. It features live performances by musicians across genres including country, blues, rock, folk, and indie, recorded in . The series is recognized as the longest-running music program in television history, having aired continuously since its debut. The program originated from a pilot episode taped on October 17, 1974, at the KLRU-TV studios, featuring as the inaugural performer; this episode aired as the series premiere on March 22, 1975. It was conceived by KLRU program director Bill Arhos, producer Paul Bosner, and director Bruce Scafe to highlight Austin's vibrant music scene and promote public television viewership. Performances are now filmed at ACL Live at the Moody Theater, a dedicated 2,500-seat venue opened in 2011. Over its 50 seasons as of 2024, the show has hosted more than 900 artists, earning distinctions such as the in 2012—the only television program to receive this honor—and designation as and Roll Hall of Fame in 2011. Inspired by the television series, the debuted in September 2002 as a two-day event in , expanding to its current format of two consecutive three-day weekends attracting over 450,000 attendees annually. Organized by , the festival features a diverse lineup of more than 100 acts across multiple stages, emphasizing Austin's role as the "Live Music Capital of the World," though it operates independently from the TV production.

Transportation and infrastructure

Atlantic Coast Line Railroad

The (ACL) was a prominent Class I railroad in the , operating primarily along the Atlantic seaboard from , to central Florida, with key extensions into , , and . Its core route, known as the "A Line," spanned approximately 900 miles from to Port Tampa, facilitating both passenger and freight transport critical to regional agriculture, phosphates, timber, and industries. By the mid-20th century, the ACL controlled a network of about 5,472 route miles, excluding subsidiaries, and maintained connections to major northern lines for through service to and . The ACL's origins trace to early 19th-century lines, including the Petersburg Railroad chartered in 1830, with the "Atlantic Coast Line" name emerging in 1871 under financier William T. Walters. It was formally consolidated on April 23, 1900, from over a dozen predecessor railroads, initially comprising more than 1,500 miles focused on coastal and routes like the Wilmington and Weldon. Expansion accelerated in 1902 with the acquisition of the Plant System, which provided access to , and ports such as Savannah and ; this added southern and eastern mileage south of Savannah. Further growth included a 51% stake in the Louisville & Nashville Railroad in 1903 for western freight links, the 1927 purchase of the Atlanta, Birmingham & Atlantic (renamed Atlanta, Birmingham & Coast, later integrated in 1946), and a joint lease of the in 1925 for Appalachian coal traffic. The system absorbed the Charleston & Western Carolina in 1959, enhancing operations. Operationally, the ACL emphasized efficiency and profitability, investing $268 million between 1939 and 1951 in upgrades, signaling, and dieselization—fully transitioning from by 1955 with a roster of over 500 units, including streamliners and freight locomotives. Passenger services featured high-speed trains like the , offering New York-to-Miami runs in under 24 hours, while freight handled diverse commodities through coordinated manifests with affiliates like the Clinchfield. Financially conservative management ensured stability, with only brief Depression-era deficits and no bankruptcies, bolstered by stakes in profitable partners yielding substantial dividends. The railroad's involvement was indirect but lucrative, profiting from and manifest flows via controlled lines rather than extensive owned trackage there. Facing postwar competition from highways and airlines, the ACL merged with longtime rival on July 1, 1967, creating the to consolidate parallel routes and achieve ; this entity later formed in 1980. The merger preserved much of the ACL's infrastructure, though passenger services declined sharply post-1960s under .

Other uses

Aircraft landing (aviation term)

is the phase of flight in which an transitions from airborne flight to ground contact, typically on a , involving descent, alignment with the landing surface, and deceleration to a stop. This process requires precise control of , descent rate, and directional alignment to ensure safety, with the defining landing as commencing from the until the exits the or stops. A standard normal landing procedure encompasses five phases: base leg (establishing position for final approach), final approach (stabilizing descent toward the threshold at a glide path of approximately 3 degrees), roundout or (gradually increasing pitch to reduce descent rate just above the ), (main wheels contacting the surface at or near stall speed), and after-landing roll (deceleration using brakes, spoilers, and reverse if equipped). Factors influencing the approach include conditions, aircraft weight, length, and , with pilots maintaining a stabilized approach—constant , descent rate, and configuration—to minimize risks such as undershooting or overshooting the . Types of landings vary by operational needs and environmental conditions. Normal landings suit prepared runways under benign weather, emphasizing gradual touchdown and rollout. Short-field landings maximize deceleration to fit constrained runways, involving higher approach speeds, prompt flaring, and aggressive braking post-touchdown. Soft-field landings address uneven or yielding surfaces like grass or mud, requiring minimal wheel loading during touchdown via high-angle-of-attack attitudes and gentle power application to avoid bogging down. Other variants include power-off (dead-stick) landings simulating engine failure, crosswind landings countering lateral wind drift with coordinated aileron and rudder inputs, and touch-and-go maneuvers for training, where the aircraft briefly contacts the runway before reapplying power for immediate takeoff. Emergency landings demand rapid and adaptation. Forced landings occur without power or runway availability, prioritizing selection for minimal damage. Precautionary landings involve electing to land due to deteriorating conditions like fuel exhaustion risks, while ditching refers to water landings, often requiring flotation gear activation. Belly landings dispense with gear extension to avoid collapse risks, and crash landings imply uncontrolled high-impact contacts, as in gear failures. Data from the indicates landing phases account for a significant portion of accidents, often due to improper timing or speed mismanagement, underscoring the need for recurrent training and adherence to stabilized approach criteria.

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