Comparative Force Outputs From Non-Centered Accelerometers During Loaded Barbell Jump Squats

Abstract

The direct acquisition of ground reaction force data during isoinertial resistance tasks typically requires a force platform. As this is not always practical, alternative devices have been used to estimate dynamic force by directly measuring other variables and then using inverse dynamics to calculate it. For example, linear position and linear velocity transducers have been tethered to bars and used for this purpose. Recently, a dynamometer incorporating an accelerometer and no tether became available for this application. However, since barbell equipment inherently allows for asymmetrical movement through multiple planes, it follows that force output may differ depending upon bar orientation, especially deviations from horizontal. Furthermore, differences related to dynamometer placement and/or variations in the lifting movement pattern could be problematic if training-induced changes are to be monitored. To determine if peak force differed during barbell jump squats using non-centered accelerometer placements. 52 subjects (27 women & 25 men) with squat training experience were recruited from a university population. Subjects performed duplicate loadspectrum countermovement jump squats (CMJ) and static jump squats (SJ) (20%, 30% & 40% of their back squat 1RM) on two separate days in a counterbalanced sequence. A triaxial accelerometer device was directly affixed to each end of the barbell midway between the lateral-most aspect of the shoulder and the thumb side of each hand. Acceleration data were downsampled from 1.5 KHz to 500Hz by averaging every 3 data points. These data were then low-pass filtered (4th order Butterworth) with a cutoff frequency of 10Hz via proprietary software. Force was determined via inverse dynamics with mass considered the sum of body and barbell weights. Peak force data from the two dynamometers were always obtained from the same trial. Paired t-tests were used for data analysis. Dynamometers provided similar peak force values for both CMJ and SJ at 20% and 40% 1RM loads (p > .05) and different values at the 30% 1RM load (p < .01). Mean differences (20, 30, & 40%, respectively) for the CMJ were 9.8N, 15.1N, 8.1N and for the SJ were -1.5N, 15.4N, 5.9N (Table 1). For relatively light countermovement and static jump squats, dynamometer placement on both ends of barbell may result in differential peak force output. It appears that accelerometer placement during jump squats should either be as close as possible to the center of the bar or a squat machine with a horizontally-fixed bar should be used. Although not tested herein, it is likely that linear position and linear velocity transducers would have the identical constraints. As contiguous accelerometer placement was not tried herein, it is unclear what, if any, contribution the dynamometers made to measurement error.