From a stationary standing position on top of a block that was placed over a force platform, the subjects stepped down onto an adjacent force platform. The force platforms (AMTI OR6–7; Advanced Mechanical Technologies Inc., Boston, MA) measured (at 100 Hz) the contact forces between the foot and the ground. A five-camera, three-dimensional, motion-analysis system (Vicon 250; Oxford Metric Ltd., Oxford, UK) was used to simultaneously record (at 50 Hz) body segment kinematics as participants completed each step down. Three block heights were used, equating to those of a curb (7.5 cm), a stair riser (15.0 cm), and stepping from a bus (22.0 cm)—obstacles frequently encountered in daily life.
24 Blocks were constructed from medium-density fiberboard of 1.8-cm thickness, which were bonded together to create a solid block with standing area 46.4 × 50.8 cm. Each block was covered with colored vinyl material that matched the surrounding floor. Room illuminance, measured at head height, was approximately 300 lux, and the luminance of the floor and top surface of the step was 30 cd/m
2 measured with a photometer (CS-100; Minolta Co. Ltd., Osaka, Japan).
The starting position on top of the block was the feet positioned a comfortable width apart and the tips of the shoes aligned directly behind the leading edge of the block. After approximately 5 seconds in this position (looking straight ahead), the subjects were instructed to step down in a single step at their own comfortable speed coming to a stationary standing position on the lower level with their feet side by side. The subjects were free to choose where they looked when stepping. They undertook a familiarization trial at each block height wearing their own spectacles. For each block height (low, medium, and high), they repeated the trials while wearing single-vision distance, PAL, or bifocal spectacles. They were not informed which pair of spectacles they had been given. All trials were repeated three times, with the order of spectacle condition and block height randomized (height was “blocked” in three's, because of the practicalities associated with changing the step), totalling 27 trials. The subjects led with the same self-selected limb in all trials. Any trial that was not completed according to these instructions was discarded and repeated. An assistant stood close by to ensure that the subjects did not fall if they should stumble. The subjects had a seated rest each time block height was changed to minimize the onset of fatigue.
For each subject, data were collected during a single 2-hour testing session. The subjects wore their own shorts, t-shirt, and low-heeled, comfortable shoes. The five cameras, which were either wall or ceiling mounted, where positioned with approximately 70° separation and encircled the stepping area. Reflective spherical markers (25-mm diameter), with their instantaneous positions tracked by the camera system, where placed on the feet (superior aspects of the second metatarsal head, lateral malleoli, and posterior aspect of the calcanei), upper and lower legs (lateral aspects of each shank and thigh and lateral femoral condyles), pelvis (anterior superior iliac spines and sacrum), upper and lower arms (medial and lateral aspects of the wrists, lateral humeral epicondyles, and acromions), trunk (xiphoid process, jugular notch, and spinous processes of the seventh cervical and tenth thoracic vertebrae), and head (anterolateral and posterolateral aspects).
The 3-D marker trajectory data were filtered and processed as previously reported,
12 to define a 3-D linked-segment model of the subject incorporating whole-body center of mass (CM) location. Knee, ankle, and head flexion–extension angular displacement data; 3-D ground contact force data from each force platform (including magnitude and the co-ordinates of its instantaneous location); and the 3-D co-ordinate data for the whole-body CM, knee, ankle, and all foot markers were exported (at 50 Hz) for further analysis.