Marker Motion Capture

aximise ubject image resolution. Due to the position of the force platform in the

from video recordings. Four to six meras would have been necessary to do this satisfactorily for the current study. the same optical axis. The cameras were set up as far away from the subject as possible, within the confines of the laboratory, and then zoomed in to m

s

laboratory, the respective cameras were 13.5 m and 19.5 m away from the force platform at an elevation of 1.06 m above floor level. Cameras were also rolled 90° in order to optimise the field of view, and maximise the resolution of the image, for the standing activities performed by the subject.

Although all trial activities were essentially confined to the subject’s sagittal plane and the motion analysis was planned to be only two-dimensional, motion capture in 3-D would have been preferable. Two gen-lockable cameras is the absolute minimum number required for 3-D reconstruction of a single marker, however, more cameras are required to achieve 3-D reconstruction for all markers in a full body motion analysis (Chiari et al., 2005). The decision to conduct a 2-D data capture process was based on the capabilities and limitations of the available resources. Although automatic digitising software was used (Peak Motus Version 4.3.3; Peak Performance Technologies, Inc., Englewood, Colorado, U.S.A.), this early version of the software requires a great deal of operator intervention to accurately recognise and digitise markers

ca

Even if enough cameras had been available, the possibility of using more than two cameras was ruled out by the labour-intensive intervention process required to ensure the success of the automatic digitising software for the large number of long-duration trials used in this study. The use of two cameras bilaterally was

amera shutter speeds were set to 0.001 s. Lighting was kept dim and the iris of e brightest and yet sharpest (not overexposed) LED marker images possible, on a uniformly dark background. This was to enable successful automatic digitising to be conducted during marker data

connected to an Event Synchronisation Unit (Peak Performance Technologies,

the Event Synchronisation Unit (ESU). The composite video output signal from each camera was also connected to the ESU, where a visual identifier was added

bilateral video data to be synchronised later by visual inspection. The video

time code generators. Finally, the video signals were fed into Panasonic digital video cameras (Panasonic Matsushita Electric Corporation of America, Seraucus, NJ, U.S.A.) where the signals were converted from composite video to digital video and recorded onto digital videotapes.

A calibration rod, with two LED markers of the type shown in Fig. 8c placed 2.032 m apart, was video-recorded to allow subsequent data scaling. It was deemed the best approach, given the available resources and considering that all trial activities were designed to be confined to the subject’s sagittal plane.

C

each camera adjusted to produce th

processing. The bilateral cameras were gen-locked. A thumb switch was

Inc., Englewood, CO., U.S.A.), which enabled the generation of a TTL signal by

to each video signal every time the TTL signal was generated, thus enabling

.3.3

Kinetic Data Capture

e measured by an AMTI LG6-4-1

d prior to ach trial, signals were zeroed. Signals were sampled at 1000 Hz with an

used to control the recording of kinetic data. A 50 Hz vertical Each of these LEDs was positioned at the same height as the surface of the force platform and 8 mm beyond the ends of the platform. These LEDs acted as the reference points for marker data processing (see section 4.4.2).

4

3-D ground reaction force and moment data wer

force platform, of length 1.219 m, and amplified with an AMTI SGA6-4 amplifier (Advanced Mechanical Technology, Inc., Newton, MA., U.S.A.). The surface of the force platform was bare and surrounding sections of the laboratory false floor were removed to ensure the subject made no inadvertent contact beyond the force platform (see Fig. 5, background). While the force platform was unloade

e

AMLAB II 16-bit analogue-to-digital converter data acquisition system (AMLAB Technologies, Lewisham, N.S.W., Australia). Software and computer processor and storage limitations prevented the use of a higher sampling rate. The gain was set to 4000 and the excitation was 10 V for all activities (except the jumps, for which the gain was 2000), in order to maximise the data resolution without clipping any peak force values. The calibration matrix provided by the force platform manufacturer was used to convert the voltage signal into newtons. A real-time analogue anti-alias low-pass filter with a cut-off frequency of 500 Hz was applied to the six signals prior to digital conversion and subsequent saving of the data on a personal computer hard drive.

The thumb switch TTL signal from the ESU was connected to the AMLAB system and was

nchronisation signal from the ESU identified the instant when the gen-locked data. Recorded with the force platform data, the video synchronisation signal allowed subsequent manual alignment of the

Each type of movement activity performed by the subject was designed to provide appropriate data for analysing one or more of the optimisation objective functions investigated in this research. More detailed descriptions of the specific movement activities adopted for this research follow in ensuing chapters, accompanying the descriptions of the optimisation problems for which they were designed.

Common to all movement patterns was the way they were contrived to minimise segment movement out of the sagittal plane with respect to the segment’s longitudinal axis and to minimise segmental rotation about that axis. With respect to the latter, the aim was to restrict each segment’s orientation about its longitudinal axis to that depicted in Fig. 5, thereby fulfilling the assumptions of the model adopted for this research (see section 4.4.1). The trunk was kept as straight and rigid as possible in all movement activities to reflect the trunk rigidity assumption of the model. The subject was instructed to restrict forward flexion of the humerus to less than 60 degrees from the anatomical position to avoid excessive scapular and clavicular elevation and misalignment of the shoulder sy

cameras were sampling video

kinematic data to within 0.001 s of the kinetic data.