The performance of five participants during single leg squat and single leg landing tasks was assessed using 3-D motion assessment and a qualitative scoring system. This group comprised of three male and two female subjects (mean age 20.6 ± 1.3 years; height 1.78 ± 0.1 m; weight 78 ± 7 kg) who gave written informed consent to participate and the project was approved by the host university research and ethics committee. All participants were physically active, participating in at least 3 hours training per week and had no current or previous (in the last 2 years) lower limb, low back or pelvic injuries.

Single leg squat test (SLS) task: Participants were asked to take a single leg stance on the force plate, then to squat to at least 45° knee flexion and no greater than 60°, over a period of five seconds. Knee flexion angle was checked during practice trials using a standard goniometer (Gaiam-Pro) then observed by the same examiner throughout the trials. There was also an electronic counter used to mark the five second period with the first count initiating the movement, the third indicating the lowest point of the squat and the fifth indicating the end. Trials were only accepted if the participant squatted within the desired range of knee flexion [9]. Data was collected from three trials which met the inclusion criteria.Single leg landing (SLL) task: Participants dropped from a 30 cm step, standing on the leg to be tested (right in all cases), participants were instructed to hop and land on a mark 30 cm in front of the step on the force platform. Participants had to ensure the contralateral leg made no contact with any other surface. Participants were required to hold the landing for at least two seconds before stepping off the force plate. Trials were only accepted if the subjects landed on the mark and held the position for 2 seconds [9]. Data was collected from three trials which met the inclusion criteria.
3D analyses:A twelve-camera OQUS (Qualisys, Gothenburg, Sweden) motion analysis system sampling at 100 Hz, and a force platform (AMTI BP400600, USA) sampling at 1000 Hz, was used to collect the kinematic data. Prior to testing reflective markers were attached to the participants at the anterior superior iliac spines, posterior superior iliac spines, iliac crest, greater trochanters, medial and lateral femoral condyles, medial and lateral malleoli, posterior calcanei, and the head of the first, second and fifth metatarsals. These markers were used to define the anatomical reference frame and centres of rotations of the joints. Five rigid plates, each consisting of four non-collinear markers, were secured with elastic bandages on the anterolateral aspect of the thigh, shank and around the pelvis. These rigid bodies were used as tracking markers to track the movement of each segment during the movement trial. The use of a rigid marker set of four non-collinear markers for tracking purposes has previously been shown to be the optimal configuration in comparison to using individual skin markers and other rigid arrays [10]. To track the motion of the thoracic spine, a rigid plate with three attached markers, was attached to the sternum and in order to define an anatomical reference frame for this segment, markers were attached to C7, the spinous process of the sixth thoracic vertebra (T6), the suprasternal notch and the xiphoid process. The calibrated anatomical systems technique (CAST) was employed to determine the movement of each segment and anatomical significance during the movement trials [11]. A static trial was carried out initially to allow for later identification of the anatomical and tracking markers in the Qualysis software prior to extraction to post-processing software and define the subjects neutral (anatomical) zero position which are referred back to this position. Post-processing calculation of the kinematic and kinetic time series data was conducted using Visual3D motion capture software (Version 4.21, C-Motion Inc., Rockville, MD, USA). Motion and force plate data were filtered using a Butterworth 4th order bi-directional low-pass filter with cut-off frequencies of 12 Hz for kinematic data and 25 Hz for force plate data. Three trials were recorded and mean data from the three trials of each task was used for comparison to the qualitative score.

Qualitative assessment: Qualitative assessment of the two tasks was made from digital video footage captured simultaneously during the 3D assessment. A digital video camera (Sony Handycam DCR-HC37) sampling at 25 Hz was wall mounted at a height of 60 cm and 10 metres away from the force plates. Digital video footage was recorded at a standard 10x optical zoom throughout each trial in order to standardize the camera position.

A qualitative scoring system was devised by the primary author (LH) based on the previously reported scoring systems of Crossley, et al. [1] and Whatman, et al. [3]. It involved dichotomous scoring of the movement strategy occurring in individual body regions (arm, trunk, pelvis, thigh, knee, foot). Scoring was defined as a zero for appropriate strategy and one for inappropriate movements, for each region with best overall score being 0 and worst 10 points. The scoring sheet is shown in Table 1. Typical errors are shown in Table 2. Optimal behaviour involved minimal deviation or body movement from that prescribed, that is arm do not move, trunk is slightly flexed, but held still, pelvis stays in mid position with minimal tilt, thighs stay parallel and approximately vertically orientation, patellae point towards middle of foot and foot demonstrates minimal wobble.

Analysis: A single examiner (LH) (blind to 3-D data) assessed the videos of the single leg squat and single leg landing of each subject; each video was viewed three times at standard speed and then scored using the qualitative scoring sheet. A second investigator (AM) then analysed the findings of qualitative assessment and compared them to those of the 3-D assessment. Prior to viewing the qualitative scores the same investigator (AM) reduced the 3-D kinematic data for each participant and each joint motion, into dichotomous scores (0=alignment/motion of segment/joint within range of normative data; 1= alignment/ motion exceeds range of normative data) corresponding to the movement individual segment movement strategy within the qualitative scoring sheet this reflected the method used by Onate et al. [3]. The normative range was based on those reported in the review of Fox et al. [12].

Statistical analysis: To assess the agreement between 3-D score and qualitative score, a kappa statistical analysis was used.