Anterior cruciate ligament (ACL) injuries are highly frequent in the athletes and in the general population enjoying leisure sports. According to the United States epidemiology survey, more than 200,000 injuries a year are reported (Frank & Jackson, 1997; Prodromos, Han, Rogowski, Joyce, & Shi, 2007). The causes of ACL injuries are reported to be about 70% of body non-contact injuries that occur during landing or fast turnover after a landing from jump rather than physical contact with the others (Arendt, Agel, & Dick, 1999; Griffin et al., 2000). The non-contact ACL injuries result in a sudden deceleration for unidirectional and landing, unstable landing, and failure to turn.

Anterior cruciate ligament reconstruction (ACLR) is required as a typical treatment method to restore the previous level of exercise capacity after injury, and ACLR helps restore the structural problems and instability of the knee joint. However, despite the structural restoration of the knee through ACLR, ACL patients are at risk of re-injuring 15 times within one year and 6 times within two years after reconstruction compared with healthy individuals (Paterno, Rauh, Schmitt, Ford, & Hewett, 2012, 2014). In addition, about 6~27% of the patients who underwent the surgery had a second ACL injury within 10 years after the first operation (Pinczewski et al., 2007; Salmon, Russell, Musgrove, Pinczewski, & Refshauge, 2005; Wright et al., 2007). ACL re-injuries occur not only on the ipsilateral side but also on the contralateral side of the knee, and ACL rupture of the knee is reported to occur 6 times more often in female than in male (Paterno et al., 2012; Salmon et al., 2005; Shelbourne, Sullivan, Bohard, Gray, & Urch, 2009; Wright et al., 2007). These results suggest that it is necessary to evaluate not only the injured knee joint but also the uninjured knee joint.

One of the causes of ACL re-injury is the biomechanical change of the patient's lower limb after return to activity (Clarke, Kenny, & Harrison, 2015; Frank et al., 2013; Goerger et al., 2014; Jin, 2013). Patients with ACLR showed low internal knee extension moment (KEM) and anterior tibial shear force (ATSF) when compared to healthy subjects at landing. Additionally patients with ACLR showed greater internal knee varus moments (KVM) and vertical ground reaction force (vGRF) at landing compared to healthy individuals. (Clarke et al., 2015; Frank et al., 2013; Goerger et al., 2014, Lim, Ryu, & Kim, 2013). Small KEM and ATSF are understood to not potentially increase the direct tension of the ACL. However, since this is a result of abnormal function of the quadriceps muscle in patients with ACLR, small KEM and ATSF are not considered to be positive phenomenon.

To prevent ACL re-injury, functional tests such as quadriceps and hamstring muscle strength measurement and hopping test are performed to evaluate the body's recovery level at the time of patient return (Logerstedt et al., 2014). A general evaluation criterion for functional testing is that the functional difference between the injured leg and the healthy leg is less than 15%. Despite performing functional tests of both legs for the decision to return after ACLR, biomechanical differences of bilateral knee joints during both landing and cutting have not been investigated.

The purpose of this study was to compare the biomechanical differences of both knee joints during double leg jump landing (DLJL) and single leg jump cut (SLJC) in women who underwent ACLR. The results of this investigation would help to prevent ACL re-injury by providing the biomechanical basis of patients with ACLR.

1. Participants

A total of 18 females between the ages of 18 and 30 years (Age: 19.9±1.2 yrs, Height: 165±0.5 cm, Mass: 64.2±10.3 kg) who underwent ACLR were recruited and volunteered to participate in the investigation. Informed written consent was obtained from all subjects. Following enrollment, subjects completed the individual's health examination questionnaire, the Tegner activity scale, International knee documentation committee 2000, and Knee outcome survey-Active daily living scale (KOS-ADLS). Subjects were eligible to participate if they reported participating in moderate-vigorous physical activity at least 150 minutes per week, had an experience participating sports including jumping and cutting such as soccer and volleyball. In addition, subjects were included if they were at least 12 months and no more than 5 years post-unilateral ACLR. Subjects were excluded from the study if they had experience of operation of the lower back and lower limbs, cardiovascular and nervous system diseases, and experience of ACLR within 6 months. This study was approved by the University institution review board. The physical characteristics of the subject and the results of the questionnaire are shown in Table 1.

2. Procedure

1) Instrument

Kinematic and kinetic data were collected using a 9-camera motion capture system (Vicon, Lake Forest, CA, USA) interfaced with two force plates (Bertec Corp, Columbus, OH, USA).

2) Measurement preparation

A reflection marker set was attached to the following anatomical locations bilaterally for biomechanical variable measurement during DLJL and SLJC; Left and right 1st and 5th metatarsal heads, heel of the shoe, lateral and medial malleolus, and anteromedial tibia shaft, lateral and medial epicondyle of femur, anterior thigh, greater trochanter of hip, anterior superior iliac spine (ASIS), posterior superior iliac spine (PSIS), acromion process, and lumbar spine (joint between 5th lumbar and 1st sacral spinous process). Calibration wand was utilized to set the spatial coordinates of the measurement tools and static traces were performed on the subjects. After performing the static measurement, the reflection markers attached to the medial epicondyle of the femur and the medial malleolus were removed. After the preparation for measurement was completed, participants performed landing and cutting tasks.

3) Double leg jump landing (DLJL)

Subjects were stood on a 30 cm height box located 50% of the subjects' height from the force plate. Subjects jumped to the direction of the force plate, and they landed on the force plate with both feet in each force plate then immediately jumped vertically as high as possible (Figure 1-A). Subjects were provided with a minimum of 3 practice trials to familiarize with the landing task and then completed 3 successful trials.

4) Single leg jump cut (SLJC)

Subjects were stood on the floor marked 50% of the height from the force plate. A 17 cm height hurdles was placed midway between the force plate and the jump position (25% of the height) (Frank et al., 2013). Subjects jumped over the hurdle then they landed one foot (testing foot) then immediately cut to the other direction of the landed foot (e.g. Right foot landing then cut left side) (Figure 1-B). Subjects were provided with a minimum of 3 practice trials to familiarize with the landing task and then completed 3 successful trials. Subjects performed the jump cut task bilaterally counterbalanced by limb.

Figure 1. (A) Double leg jump landing (B) Single leg jump cut

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3. Data sampling, processing, and reduction

1) Kinematic and kinetic data

Kinematic and kinetic data were sampled at 120 Hz and at 1,560 Hz, respectively. Reflective markers were labeled, and a macro was utilized to predict the location for missing markers for segments. Three-dimensional marker coordinates and force plate data were imported to TheMotion- Monitor software (Innovative Sports Training, Inc., Chicago, IL, USA).

Center of the hip joint defined by using Bell method (Bell, Pedersen, & Brand, 1990), and knee joint center was defined between the medial and lateral epicondyle of the femur. Ankle joint center was defined between the medial and lateral malleolus. Joint angles were calculated based on a right-hand rule using Euler angles with a Y (flexion/extension), X' (adduction/abduction), Z" (internal/external) rotation sequence. Angles were defined about the knee as the shank relative to the thigh. Kinematic data were filtered using a fourth-order low-pass Butterworth filter at 12 Hz, time synchronized to kinetic data and re-sampled at 1,560 Hz.

Kinetic data was filtered using 4th Order Low-pass Butterworth Filter at 12 Hz. To calculate internal joint moments and forces, kinetic data was combined with kinematic and anthropometric data using inverse dynamic approach within theMotionMonitor software (Gagnon & Gagnon, 1992). Initial contact was defined when vertical ground reaction force (vGRF) was greater than 10 N. Biomechanical variables measured in the current study were as follow; knee flexion angle at initial contact (KFIC), peak knee flexion angle (PKF), knee valgus angle at initial contact (KVIC), peak knee valgus angle (PKV), knee extension moment (KEM), anterior tibial shear force (ATSF), vGRF. Custom LabView software (LabVIEW, National Instrument, Austin, TX, USA) was used to identify all variables at initial contact or the peak value between initial contact and the time of peak knee flexion. Peak KEM and KVM were normalized to the product of body weight and height, and ATSF and vGRF were normalized to body weight.

4. Statistical analysis

SPSS 23.0 (SPSS Inc., Chicago, IL, USA) statistical program was used to calculate the subjects' basic information (age, height, weight, etc.). Paired T-test was performed to compare the biomechanical differences of both knees at the time of the double-landing and cutting after landing. The statistical significance level (alpha) was set at 0.05.

1. Double leg jump landing (DLJL)

During DLJL, the healthy knee showed greater KVIC compared to the injured knee (Injured: 2.93±2.59, Healthy: 4.20±2.46, t=2.957, p= 0.009). There was a significant difference in ATSF with greater in the injured knee (Injured: 1.41±0.39, Healthy: 1.30±0.35, t=2.201, p=0.042). Table 2 summarized the biomechanical differences between the injured and the healthy knee during DLJL.

2. Single leg jump cut (SLJC)

During SLJC, injured knee showed greater KEM compared to healthy knee (Injured: 0.51±0.19, Healthy: 0.47±0.17, t=2.761, p=0.013). However, there was no significant differences between the knees in the other variables. Table 3 summarized the biomechanical differences between the injured and the healthy knee during SLJC.

The purpose of this study was to compare the biomechanical differences between injured and healthy knee joints during a DLJL and a SLJC in females who returned to daily life and sports activities after ACLR. The investigation showed that while the healthy knee exhibited greater KVIC compared to the injured knee (p=0.009), ATSF showed greater in the injured knee compared to the healthy knee during DLJL (p=0.04). During SLJC, the injured knee exhibited greater KEM compared to the healthy knee (p=0.01).

The purpose of this study was to compare the biomechanical differences between injured and healthy knee joints during a DLJL and a SLJC in females who returned to daily life and sports activities after ACLR. The investigation showed that while the healthy knee exhibited greater KVIC compared to the injured knee (p=0.009), ATSF showed greater in the injured knee compared to the healthy knee during DLJL (p=0.04). During SLJC, the injured knee exhibited greater KEM compared to the healthy knee (p=0.01).

After the ACLR, it has been well-reported that the healthy knee was frequently injured (Pinczewski et al., 2007; Wright et al., 2007). Many previous studies have discussed the asymmetry of sagittal plane movements of the knee (Di Stasi, Logerstedt, Gardinier, & Snyder-Mackler, 2013; Paterno et al., 2014). For example, an analysis of the patient's gait after ACLR showed that KFIC and KEM exhibited greater in the injured knee compared to the healthy knee (Di Stasi et al., 2013). Additionally, the injured knee exhibited greater vGRF than the healthy knee in patients with 2 years post-ACLR (Paterno et al., 2014). However, there is a limited evidence on biomechanical asymmetry in frontal plane movement, therefore it would be significant findings in frontal plane movement imbalance between the knees.

In contrast to the KVIC, the ATSF was greater at the injured knee during DLJL. ATSF is greater when the angle of the knee joint is smaller and the force acting on the quadriceps muscle is greater (Chappell et al., 2005; Markolf et al., 1995). In the current study, there was no statistical difference, but the flexion angle at the landing point of the injured knee was lesser than the healthy one, and the KEM was greater in the injured knee. It is considered that the movement characteristics of the knee in the subjects of the current study may have resulted in greater ATSF in the injured knee. Applying a program that prevents the risk of re-injury of ACL by practicing landing with the same angle bilaterally would have a positive effect on patients.

It was confirmed that the KEM in the injured knee was greater than healthy knee during SLJC. In previous studies, it was reported that when the KEM was large, the plantar-flexor moment and the hip joint moment was decreased (Lee & Lim, 2014, Shimokochi, Yong Lee, Shultz, & Schmitz, 2009). In the present study, it is assumed that subjects who returned after ACLR may have a different force distribution strategy during one leg movement tasks. However, since the current study investigated only knee joint biomechanics, the future study for analyzing ankle and hip joint biomechanics during SLJC will be warranted.

In this study, we compared the biomechanical differences between the injured knee and the healthy knee during DLJL and SLJC in a woman who returned after ACLR. The conclusions are as follows.

1. During DLJL, the KVIC was greater in the healthy knee compared to the injured knee.

2. During DLJL, the ATSF was greater in the injured knee compared to the healthy knee.

3. During SLJC, the KEM was greater in the injured knee compared to the healthy knee.

The subjects of this study returned to the physical activity. However, the current study found a biomechanical asymmetry of the knee joint in two movements that mimic the sports activity. This biomechanical asymmetry of the knee joint may cause the ACL re-injury, secondary injury and chronic pain in the healthy knee, and long-term knee pain. Therefore, it would be necessary to compare the biomechanical differences of both knees when performing rehabilitation exercise after ACLR and to return to activity.