Knee hyperextension, also called genu recurvatum or back knee, is commonly seen in women, people with ligamentous laxity, stroke and cerebral palsy patients. This faulty posture would result in excessive tension of the passive tissues such as anterior cruciate ligament (ACL) and posterior capsule of the knee. Subjects may also develop compensations at hip and ankle joint, as well as lower extremity malalignment. Muscles surrounding the knee could also become dysfunctional when performing functional tasks requiring stability during terminal knee extension, during which uncontrolled knee hyperextension could easily be utilized to lock the joint for stability in gait and stair climbing. In athletes, landing from a jump on an extended knee is one of the common reasons resulting in ACL injury. Little is known about the injury rate of athletes with knee hyperextension who participate in sports involving jump-landing activities.

The aim of the study is to explore if knee hyperextension is associated with poor lower extremity alignment and dynamic control and injury rate in athletes requiring jump-landing activities.

One of the study hypothesis is that athlete with knee hyperextension can find more compensatory lower extremity alignments and poor control in dynamic movement than control group.

The other hypothesis is with or without knee hyperextension, the parameter of lower extremity alignment and dynamic control can predict injury rate in jump landing athlete.

Investigators expect to recruit 100 subjects into 2 group- control group and knee hyperextension group (KH group). In knee hyperextension group, investigators will include college athletes having knee hyperextension alignment, no lower extremity injuries in past three months, and participating in jump-landing activities. Control group will match the gender, age, height, weight and sports to KH group. Inclusion criteria is the same as KH group, except the knee hyperextension angle is <5⁰ in control group. Every subjects will receive the first time assessment and follow up by filling the injury-follow-up form every months for about four months.

At the first assessment, investigators will measure lower extremity alignments include pelvic angle in sagittal plane, hip anteversion, tibiofemoral angle, knee hyperextension angle in supine and standing position, tibial rotation angle, and navicular drop; lower extremity muscle flexibility include rectus femoris, hamstrings, gastrocnemius, soleus, iliotibial band; lower extremity muscle strength include hip, knee, muscle group; dynamic control task will record tibio-femoral acceleration, angle change ground reaction force and EMG muscle firing during vertical jump and drop jump.

Inclusion Criteria:

• age ≥ 20 years old
• knee hyperextension ≥ 5⁰
• participate in jump- landing sports
• training at least 3 days per week
• no lower extremity injury last 3 months

Exclusion Criteria:

• having knee surgery before
• In the past three months have occurred lower extremity musculoskeletal injury leading the person can't participate in training or competition for at least three days

college athletes in Yang-Ming University and University of Taipei