A comparison of 2 optical timing systems designed to measure flight time and contact time during jumping and hopping

This study was designed to investigate the interchangeability of 2 commercial optical timing systems for measuring flight time and contact time during jumping and hopping. Seventy-three physical education students (33 men and 40 women) participated in this study. They were instructed to perform 3 jump protocols (squat jump, countermovement jump, and countermovement jump free arms) and a hopping test (10 seconds with straight legs at a frequency of 2 Hz). Flight time and contact time were measured with 2 optical timing systems (Optojump, Microgate, Italia and IR-mat, Ergotest, Sweden), consisting of 2 bars placed opposite to each other. Both systems trigger a timer with a precision of 1 millisecond each time the infrared light is interrupted by the feet. Jump height was given by the systems, whereas leg stiffness was computed from contact time and flight time. Flight time was higher when measured with the IR-mat (bias +/- 95% LOA [limits of agreement] = 5 +/- 14 ms, p < 0.001). This difference was trivial (effect size <0.2) and clinically meaningless. The high correlation between sets of data (r = 0.99) together with narrow 95% LOA (3%) support the interchangeability of both systems to measure flight time. Similar results were found with contact time (bias +/- 95% LOA = 8 +/- 23 ms, p < 0.001, effect size <0.2 and r = 0.99), with the exception that it was the Optojump that provided the higher values. These trivial but significant differences between both systems had minor impact on jumping height (bias +/- 95% LOA = 0.6 +/- 1.7 cm, p < 0.001), effect size <0.2 and r = 0.99), and stiffness (bias +/- 95% LOA = 0.8 +/- 1.4 N.m.kg, p < 0.001, effect size <0.2 and r = 0.98). We concluded that both systems can be used interchangeably.