A biomechanical analysis of the BMX SX gate start.

  • Josephine Grigg

Student thesis: Doctoral Thesis


Bicycle Motor Cross (BMX) Supercross (SX) racing is a relatively new sport with little formal research to support it. A fast start is critical to race performance as an advantage in the first few seconds allows the athlete to select the optimal line into the first jump. Research suggests that the first athlete to land the first jump is the most likely to win the race. Given this known association, a considerable amount of training time is devoted to practising the start action and training the related muscle groups. A key performance outcome for the gate start is the kink time, that is the time split from the start gate to the change in gradient on the SX start ramp at ~ 3 m. Little is known about the mechanics of the optimal start action. This thesis presents five studies that provide insight into the determinant phases and kinematics of the BMX SX start action and investigates whether race start reaction time (RT) can be improved with training. A key aim of this thesis was to provide pragmatic research for coaches and athletes on means to optimise the BMX SX gate start action. As such, feasibility and ecological validity of all studies were directed to maintain a coach/athlete centric focus. This program of research was conducted in collaboration with the Cycling Australia BMX unit.

In the first study, the BMX SX start action was divided into distinct phases. The temporal invariability of the phases within, and between, five BMX SX World Class (WC) athletes was examined. WC athletes were considered to be those who had achieved a podium finish in Union Cycliste Internationale (UCI) competition during the year of testing, whereas Elite athletes had a UCI ranking but no podium finish at UCI international level. The study demonstrated that the phase most likely to relate to performance for this cohort was the weight transfer of the second crank. Using the phases defined in Study 1, Study 2 was undertaken to examine the differences in absolute and relative phase duration between WC and Elite athletes, and male and female athletes. The results of the second study identified that the WC athletes had faster second crank weight transfer times than the Elite athletes, and that the male athletes had a faster first crank, second crank weight transfer and power stroke time, and greater temporal variation than the female athletes. Findings from both studies identified that the reaction time (RT) phase may account for ~ 7% of the total gate start action.
The third study was an intervention study with the aim of reducing the race start RT. The intervention consisted of a two-week training intervention program (14 sessions) following which the difference in RTs between the intervention group (n = 4) and a control group (n = 5) were compared with the pre-intervention measures. Whilst the RT on the training device was shown to improve for the intervention group (but not the control group), this did not transfer to a clear improvement in race start RT on the ramp or the kink time (i.e. performance outcome).

The final two studies focused on the athlete kinematics of the gate start action. The results of Study 4 showed that the markerless motion capture method was valid to within 2˚ and had an intra-tester reliability within 6˚ across five joint angles (ankle, knee, hip, elbow, shoulder) and two segment angles (head and torso). The aim of the final study (Study 5) was to use kinematics to describe a ‘fast’ gate start for 14 WC and Elite athletes. The validated markerless motion capture method as described in Study 4 was used to maintain ecological validity (n = 14, 5 trials each). Three key set (i.e. starting) positions were identified; the upright, back and angled. Three key hub trajectory shapes were also identified: hairpin, up and over, and half circle. The set position was linked to performance with the back set position being favoured by the faster athletes. The back set position was most likely to result in the hairpin hub trajectory, which was also used by the fastest athletes. Thus a ‘fast’ gate start action was characterised by the back set position and moved through a hairpin hub trajectory for this cohort.

The conclusion of the thesis is that the set position is critical to the execution of the BMX SX gate start action. The back set position is most likely to result in a fast gate start as it enables the body to most efficiently execute the second crank weight transfer phase which was shown to relate to gate start performance.
Date of Award12 Oct 2019
Original languageEnglish
SupervisorJustin Keogh (Supervisor), Rob Orr (Supervisor) & Eric Haakonssen (Supervisor)

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