Lower body reaction testing using ultrasonic motion capture

This paper presents a lower body reaction test that utilizes a new portable ultra-sound based motion capture system (MobiFit) combined with a synchronized visual stimulus. This novel system was tested first for criterion validity and agreement against a gold standard laboratory based optical motion capture system (CODA). It was subsequently tested in the field during Gaelic football (GAA) team gym sessions with 35 subjects to demonstrate its utility and versatility. The lower body reaction test itself is novel in that it can be applied to a gross motor task. During testing, participants had sensors attached to their lower limbs and trunk. The speed of movement for each sensor was monitored at 500Hz using the Mobifit motion capture system, and reaction time was measured as the elapsed time from the appearance of a green indicator on the screen to a sensor reaching a set threshold velocity as the participant raised the corresponding leg. Pearson's correlation coefficient tested criterion validity against the CODA system and Intra class correlation coefficients and Bland-Altman plots assessed agreement of velocity measures obtained from the MobiFit and CODA systems. Results indicate that the MobiFit system is an accurate device to assess lower body reaction time and has advantage over standard laboratory measures in terms of portability and ease of set-up.