Lower Extremity Kinematics During Single-Leg Functional Tasks

Participants wore spandex shorts, a form-fitting shirt, and their own exercise shoes. Forty-two spherical, retro-reflective markers (14mm diameter) were placed, bilaterally, on the lower extremity, pelvis, and trunk (Fig 1). Specifically, markers were placed over the following landmarks: acromion processes, xiphoid process, spinous process of the seventh cervical vertebra (C7), superior aspects of the iliac crests, anterior superior iliac spines, sacrum (midpoint between the posterior superior iliac spines), greater trochanters, lateral and medial femoral epicondyles, lateral and medial malleoli, posterior aspect of the calcanei, and first and fifth metatarsal heads. Plastic shells that contained four, non-collinear markers each were positioned laterally over the thigh and shank [39 (link)]. Marker shells were attached to the lower extremity segments via neoprene wraps and hook and loops fasteners. Once the markers were placed over the appropriate anatomical landmark, a static calibration trial was recorded. Following the static trial, the medial knee and ankle markers were removed so they would not encumber participants during the movement trials.
Marker motion was recorded using motion capture system (Nexus, Vicon Motion Systems Ltd, Centennial, CO) with ten Vicon MX-T20 cameras sampling at 100 Hz. Each camera was calibrated to have less than 0.15 mm residual error. Participants performed three different single leg weight bearing tasks (Fig 2) in the following order: step down from a 16 cm step (SD16), step down from a 24 cm step (SD24), and single leg squat (SLS). Order was not randomized as this was part of a larger clinical study. For each trial of each task, the starting and ending position was standing on both legs with feet in a self-selected position. For the step down tasks, participants stood with both feet on top of a wooden box. From the starting position, they were instructed to stand on one leg, lower the non-stance limb until the heel lightly touched the floor and then return to standing with both feet on the box. For the single leg squat task, participants were instructed shift their weight onto one leg, squat as low as possible with their non-stance limb extended anteriorly, and return to standing on both legs. The position of the non-stance limb was selected to be similar to the step down task. Participants had an opportunity to practice each task. Participants were given approximately 10 seconds between each single trial of each task, and approximately two to three minutes between tasks. The same leg was always tested first. A metronome set at 60 beats per minute was used to help standardize movement speed. Participants were instructed to try to move “down on a beat and up on a beat.” Participants were given feedback to help maintain a consistent movement speed for each individual trial; however, strict adherence to the metronome was not enforced. Upper extremities had to be maintained either at their sides or out to the side. Trials in which subjects lost their balance or used their upper extremities for support on the surrounding bars were recollected. Five trials were collected on each lower extremity for each task. Only the data during right stance was analyzed for this study.