Comparisons between systems to measure contact and flight times in elite race walking

Race walking is an Olympic event dictated by a rule that states that no visible loss of contact with the ground should occur and that the leg must be straightened from first contact with the ground until the ‘vertical upright position’ (IAAF Rule 230.2). The measurement of flight times during race walking is therefore of great interest to coaches, athletes and judges. The aim of the study was to compare different methodologies used to measure contact and flight time in race walking. Ten male race walkers (stature: 1.78 m (± 0.05), mass: 64.4 kg (± 4.9)) and seven female race walkers (stature: 1.68 m (± 0.10), mass: 56.7 kg (± 11.0)) participated. Fourteen of the athletes had competed at the 2016 Olympic Games or 2017 World Championships. The men race walked down an indoor track at 11, 12, 13, 14 and 15 km/h (measured using timing gates and in a randomised order), whereas the women’s trials were at 10, 11, 12, 13 and 14 km/h. Contact and flight times were measured for the midsection of each trial using three adjacent 900 x 600 mm Kistler force plates (1000 Hz), 5 x 1 m strips of an OptoJump Next system (1000 Hz) and a Fastec high-speed camera (500 Hz). Results from the OptoJump Next system were extracted using five settings based on the number of LEDs that needed activating (contact begins after_contact ends when), and were annotated as 0_0, 1_1, 2_2, 3_3 and 4_4. The force plate values were considered the criterion values and measurements were assessed for reliability using Intraclass Correlation Coefficients (ICC) and 95% limits of agreement (LOA: bias ± random error). For flight time, the ICCs between the force plate and OptoJump Next were 0.846 for the 0_0 condition (LOA: .011 ± .014 s), 0.901 (1_1) (LOA: .008 ± .014 s), 0.983 (2_2) (LOA: .000 ± .008 s), 0.844 (3_3) (LOA: –.011 ± .014 s), and 0.563 (4_4) (LOA: –.023 ± .018 s). The ICC between the force plate and the high-speed video for flight time was 0.975 (LOA: –.003 ± .008 s). For contact time, the ICCs between the force plate and OptoJump Next were 0.967 for the 0_0 condition (LOA: –.011 ± .011 s), 0.982 (1_1) (LOA: –.008 ± .010 s), 0.995 (2_2) (LOA: .000 ± .010 s), 0.960 (3_3) (LOA: .011 ± .016 s), and 0.874 (4_4) (LOA: .024 ± .015 s). The ICC between the force plate and the high-speed video condition for contact time was 0.991 (LOA: .004 ± .010 s). The OptoJump Next system provided results similar to those of the gold standard force plates, with the 2_2 setting the most reliable. Users of the OptoJump Next system should therefore note that adjusting the settings of the device (from 0_0, the most likely default setting) might be necessary to achieve the most accurate results. The high-speed video recordings also provided very good reliability although the time-consuming nature of video analysis means the OptoJump Next system is better suited to providing immediate results.