Comparison of Three Gold-Standards to Measure Ground Contact

Comparison of Three Gold-Standards to Measure Ground Contact Time in Runners
Ammann R, Wyss T
Swiss Federal Institute of Sport Magglingen SFISM, Switzerland
Introduction:
Time, heart rate, distance covered and speed are the most often monitored parameters to quantify an
athlete’s running training today (Fleming, Young, Dixon, & Carré, 2010). However, an even more
fundamental and systematical monitoring of training might be of further benefit for the training
regulation. For example, ground contact time (GCT) is a paramount parameter in running. Several
authors have demonstrated, that the shorter the GCT, the faster the runner (Bushnell & Hunter, 2007;
Hasegawa, Yamauchi, & Kraemer, 2007; Weyand, Sternlight, Bellizzi, & Wright, 2000). Even at an equal
speed, a more successful athlete has a shorter GCT than a less successful athlete. Hasegawa et al.
(2007) showed a significant correlation between the GCT and the final result in a half marathon race.
Shorter GCT was associated with a better end ranking. Hence, a reduction in GCT of only milliseconds,
while maintaining the same ground reaction force, can lead to significant improvements in
performance. In the literature, GCT is mainly assessed in laboratory settings by means of force plates
(FP), optical timing systems or optical motion capture systems (Bushnell & Hunter, 2007; Debaere,
Jonkers, & Delecluse, 2013; Girard, Millet, Slawinski, Racinais, & Micallef, 2013). Yet, it is not clear to
what extend these three methods agree on GCT measures. For athletes, coaches and researchers it is
of great importance to know what method can be best applied to assess GCT. Thus, the purpose of the
present study was to compare three gold-standards, most cited in the literature, to determine GCT in
runners.
Methods:
One well trained male athlete (25.8 years of age, 182 cm, 67 kg), familiarized with the procedure used
in this study, volunteered for this research. The participant run several times a distance of 30 m at
different self-selected velocities (4.71 – 6.71 m/s) on an indoor track. After 25 m, a .90 x .90 m FP
(Quattro Jump, Kistler, Winterthur, Switzerland), with 500 Hz sampling rate, was placed. Contact time
on the FP was defined in two ways, according to varying indications in the literature (Castagna et al.,
2013; Girard, et al., 2013; Greene et al., 2010): firstly, as the time at which the force level was ≥ 5 N,
secondly, ≥ 10 N (subsequently labeled as FP5N and FP10N, respectively). Two Optojump (OJ;
Optojump Next, Microgate, Bolzano, Italy) bars, sampling at 1000 Hz, were located on top of the FP,
one bar on each side of the FP’s edge. The OJ bars communicate continuously by optical light-emitting
diodes (LEDs), whereby one bar sends and the other receives the LED signals. For instance, when a
foot is placed between these bars, the LED communication is interrupted and this time window is
defined as GCT of one step. Further, at surface level with the FP and OJ, a high speed video camera
(VC; Red Epic Mysterium-X, Red Digital Cinema Camera Company, Lake Forest, California), with 350 Hz
sampling rate and full HD (1920 x 1080), was installed. To ensure best possible lighting for the VC,
additional headlights spotlighted the abovementioned setup. The evaluation of the GCT of every
videotaped step was executed by visual inspection by two independent experts. For this purpose the
software Adobe Premiere Pro CC (Adobe Systems Incorporated, San Jose, California) was used. The
two experts stated no difficulties in defining initial foot contact and toe off, additionally, they both got
the same results on GCT of every step. This setup allowed to capture one single step each trial by all
three instruments at the same time. After testing for normal distribution, GCT data of eight valid steps
were presented in Bland-Altman plots. To examine differences between the instruments, analysis of
variance (ANOVA) with Tukey HSD post-hoc, correlation coefficients and root mean square errors
(RMSE) were conducted. Data derived from the VC was regarded as the true value.
Results:
The VC, OJ, FP5N and FP10N recorded an average GCT of 164 (±0.02) ms, 162 (±0.02) ms, 160 (±0.02)
ms and 158 (±0.02) ms, respectively. Compared to GCT recorded by VC, the use of OJ, FP5N and
FP10N, respectively, underestimated GCT by 1.16 %, 2.67 % and 3.91 %. However, these differences
were not significant (p = .998, p = .971 and p = .919). Correlations between data recorded by each of
the instruments; OJ, FP5N, FP10N and the VC measures of GCT were r = .998, r = .994, r = .994 and
RMSE were 2.26 ms, 4.81 ms, 6.68 ms, respectively (Figure 1).
6. Jahrestagung der SGS in Fribourg/Freiburg
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a)
b)
c)
Figure 1. Bland-Altman plots of GCT measured by high speed video camera (VC) and a) Optojump (OJ), b) force plate ≥ 5 N
treshold (FP5N), c) force plate ≥ 10 N treshold (FP10N). The dotted lines represent the mean and ± 1.96 SD of the difference between
the VC and the values of the other instruments; the solid lines represent the origin.
Discussion / Conclusion:
All instruments assessed GCT within 4 % variation among each other. Hence, comparability among
studies using one of these three gold-standards to measure GCT seems to be good. In the present
study, OJ showed best accuracy concerning GCT when compared to the high speed VC regarded as the
true value. The results revealed, that the 5 N threshold to define ground contact using a FP was closer
to the data recorded by VC than the use of a 10 N threshold. The FP can assess only one step each
trial. This extends to the VC, which as well can only capture one step at a time and furthermore entails
high costs in filming and evaluating. Whereas, the OJ can record continuous steps over several meters,
due to the single 1 m bars, which can arbitrarily be linked together. In conclusion, the present study
showed, that all three gold-standards (VC, OJ and FP) seem to be precise in the detection of GCT.
However, OJ claimed best feasibility among the three systems. Therefore, the application of the OJ
system is recommended to collect accurate data of athlete’s GCT during training sessions and as goldstandard to validate new measurement devices.
References:
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