Equipment Advances to Improve the lab-based Measurement of a BMX Sprint Start Authors: Jensen MJ1, 2, Sporer B1, 3, Wooles AL4, Klimstra MD1 Affiliation: 1Exercise Science, University of Victoria, Victoria, Canada; 2 Canadian Sport Institute Pacific, Victoria, Canada; 3 ReSync Consulting, Vancouver, Canada; 4 Cycling Canada, Vancouver, Canada
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he SRM PowerMeterTM is usually referred to as the ‘Gold Standard’ when it comes to measuring the power output of a cyclist and has been shown to be accurate to within manufacture’s specifications (±0.5% Science Model). However, a major limitation with the SRM PowerMeter TM is it does not measure instantaneous angular velocity of the crank throughout the pedal cycle, but rather only averages over one pedal revolution. The sport of BMX demands the generation of high power from stand-still, requiring the athlete to produce power at a high rate via large changes in angular velocity during just the first few (<6) pedal revolutions (0-240rpm). Thus, averaging angular velocity over one revolution does not provide the temporal resolution required to assess very small, but meaningful biomechanical metrics linked to performance. Further, there is a need to improve and expand on the metrics available for BMX sprint analysis.
Purpose:
Results:
To collect angular velocity for every 5° segment during a pedal revolution, along with torque data. This will 1) Create a more accurate pedal power profile to determine peak power for any part of the pedal revolution and 2) Create the opportunity to define new performance metrics.
The SRM PowerControlTM readings were on average 7.7% (SD: ±3.2%) lower than the custom device. The collection rate of the PowerControl (0.1s vs 0.5s) had no effect on improving accuracy, 8.1% and 7.3% respectively. The gearing of the SRM ergometer had an effect on the accuracy of the power reading, with gear 1 being 11.1% lower and gear 4 being 4.1% lower. Gear 1 has the least resistance and therefore the greatest increase in angular velocity (rpm). The new metrics that were defined were, Time to Kink (time to travel 5.4m), Time to 20m, 360 Peak Watts (Time, Location), Displacement, Start Angle, Peak Cadence, Absolute peak Watts (5° segment), Individual revolution wattage, and pedal smoothness. Along with these metrics, we were able to accurately plot the power output for each 5° segment.
Methods: A custom SRM sensor add-on was developed to measure angular velocity and torque every 5° of a pedal rotation. Data was collected from the top male BMX riders in Canada. Subjects performed standing starts in a lab (mimicking the track starts) on a modified SRM ergometer. In total, 63 starts consisting of 4-6 pedal revolutions were analyzed, with 51 being used to compare to SRM PowerControlTM data. Trials were performed in varying gears, ranging from 1 to 4.
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HP SIRCuit Winter 2016
Matt Jensen holds a BSc in Kinesiology/Biology and is currently pursuing a doctorate in Biomechanics at the University of Victoria. Matt is an ongoing consultant for performance analysis and technology innovation with CSI-Pacific and B2ten working with cycling based disciplines. Matt is a former world-class rower and is an assistant coach and physiologist for the UVic Men’s rowing team.
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