Validity of Velocity Measurements of a Motorized Resistance Device During Change of Direction

The aim of this study was to determine validity of velocity measurements of a motorized resistance device (MRD) during change of direction (CoD). Eight male (age: 22.1 ± 4.2 yrs; weight: 83.3 ± 17.1 kg; height: 181.6 ± 12.6 cm) and three female participants (age: 21.7 ± 1.5 yrs; mass: 69.7 ± 2.4 kg; height: 167.0 ± 3.6 cm) completed the modified 505 CoD test (m505) with turning off the left and right foot while exposed to external loads (3, 6, and 9 kg) provided by the MRD. Three-dimensional kinematic data were measured (200 Hz) for all tests using a full-body marker set with an additional marker placed on the pulley used to attach the carabiner (CAR) at the end of the line from the MRD to the participant. Average velocity of overall center of mass (COM_vel), pelvis (COM_{pelvis_vel}), and pulley (CAR_vel) was then calculated and compared to the velocity measured by MRD (MRD_vel) in 0.5 s intervals 1.5 s before and after CoD. Average velocities from these intervals were then compared using correlational, Bland-Altman analysis, coefficient of variation (CV), and statistical parametric mapping (SPM). Mostly, excellent correlations were observed and ranged from 0.93 to 1.00, 0.53 to 1.00 and 0.93 to 1.00 for the 3, 6, and 9 kg load conditions, respectively. CV values ranged from 0.3 to 3.2%, 0.8 to 4.3%, and 1.5 to 7.7% for the CAR_vel, COM_{pelvis_vel}, and COM_vel comparisons, respectively. The biases for CAR_vel comparisons ranged from -0.027 to 0.05 m/s, -0.246 to 0.128 m/s and -0.486 to 0.082 m/s across all load conditions and time intervals for the CAR_vel, COM_{pelvis_vel}, and COM_vel comparisons, respectively. SPM analysis indicated significant differences between MRD_vel and COM_vel and COM_{pelvis_vel} over short time periods during the CoD, but no difference between MRD_vel and CAR_vel. The velocity measurements obtained by a MRD during a m505 test are valid as low biases, low CV's, and high correlations are observed for the MRD_vel to CAR_vel comparison. As single points of measurement (i.e., laser) has been proven useful to assess other athletic tasks (i.e., sprint running), the single point CAR_vel comparison is an appropriate comparison for validating MRD_vel measurements during the m505 test.