Purpose
The purpose of this study was two-fold; to validate the ground reaction forces derived from a motion capture system with values determined directly from a force plate, and to determine if the accompanying software permitted ready access to the complex biomechanical calculations necessary. Subjects came to the laboratory and performed three trials of body weight squats on a force plate while simultaneously being recorded with a motion capture system. Force values calculated from the motion capture system were compared to values calculated from a force plate (independent variables). The dependent variables of peak force, lowest force, and mean force values were calculated and compared between the force plate and motion capture system. In addition, correlations of the synchronized force-time data points were performed between the force plate and motion capture system values for all trials.
Subjects
Five healthy men (X ± SD; age = 21.0 ± 4.3 years; body mass = 92.9 ± 15.3 kg; height = 1.85 ± 0.10 m) volunteered for this investigation. Each subject completed a pre-exercise health status questionnaire and signed a written informed consent document before testing. Prior to any exercise testing, each subject was scanned with a DXA to determine segmental masses (GE Prodigy, Madison, WI). These data were then used for later analyses. None of the subjects reported any current or ongoing neuromuscular diseases or musculoskeletal injuries specific to the ankle, knee, or hip joints. This study was approved by the University’s institutional review board for human subjects research.
Procedures
Subjects were required to perform 3 body weight squats separated by 30s of rest. While on the uni-axial force plate (RoughDeck, Rice Lake Weighing Systems, Rice Lake, WI), the participants were instructed to perform a squat with a controlled velocity to a parallel depth and back to the starting position. Parallel was defined as the inguinal fold at the proximal end to the thigh being below the level of the superior border of the patella. Although the velocity was controlled, each subject self-selected the velocity during the movement. Participants used a shoulder width stance with feet flat on the ground during the squat movement. In addition, participants extended their arms forward during the movement, which allowed the arms to be parallel to the ground throughout the movement.
Prior to placement on the force plate, subjects put on a form fitting full body suite on which 43 markers were placed with Velcro (i.e., ankles, knees, hips, etc.) to form rigid bodies to track joint and segment positions (see Figs. 2 & 3). All the experimental tests were performed with a customized motion capture system (MCS) using Optitrack Flex V100R2 hardware and Arena Motion Capture Software (Optitrack System, Natural Point, Inc., Corvallis, OR). The implemented acquisition system was composed of five infrared synchronized cameras that were mounted around the capture volume. Calibration procedures were performed prior to experimental testing in accordance to the Optitrack Flex V100R2 user guide (http://docplayer.net/7564823-Tracking-tools-2-4-0-user-s-guide.html), which included both dynamic and static calibrations.
The signal from the force plate was recorded with a Biopac data acquisition system (MP150WSW, Biopac Systems, Inc., Santa Barbara, CA) during each assessment. The force signal was sampled at 1000 Hz using a 16-bit analog-to-digital converter interfaced with a desktop computer. Since the MCS sampled at 100 Hz, the force plate data’s sampling rate was adjusted to 100 Hz offline (LabVIEW V 8.5, National Instruments Corp., Austin, TX) to match the MCS. GRFs (N) were calculated for each data point for the force plate and MCS data. Proprietary methods were used to calculate GRFs from the MCS data [15]. Ground reaction peak force (PF) and lowest ground reaction force (LF) were calculated as the highest and lowest 0.10s epoch measured by the force plate and MCS, respectively. The data point at which the peak force was observed was used to synchronize the signals from the MCS and the force plate. Specifically, the 100 data points (1 s) prior to PF and the 100 data points immediate after the PF as calculated by the two devices, were matched. The data point matching resulted in a synchronized two-second epoch, for which ground reaction mean force (MF) was calculated.
Statistical analysis
All data are reported as \( \overset{-}{\mathrm{X}}\pm \mathrm{S}\mathrm{D} \). Linear regression was performed on the 2s epoch of force plate and MCS values (collapsed across trials and subjects) with the Pearson correlation (R), coefficient of determination (R
2), and standard error of the measurement (SEM) calculated. For each data point in the two-second synchronized epoch, the GRF as calculated by the force plate was compared with the GRF calculated by the MCS. Three 2-way repeated measure ANOVAs (condition [force plate vs. MCS] vs. trial [1 vs. 2 vs. 3]) were used to examine differences between the force plate and MCS for PF, LF, and MF. Significance was set at p < 0.05.