INTRODUCTION: Instep wearable resistances at running trainings(1) change mechanical parameters modifying muscle activity(2), ground plantar support and run biomechanics; changes could reduce lower limb`s impairment risk produced by abnormal stepping during the race(3). Instep weights show positive effects on the race, due to the influence of the instep specific placement without decreasing inter-muscular coordination nor altering running technique(2)(4)(5). Objective: to analyze if instep weights use during warm-up for 50-meter running series could significantly modify race times in sprinters and hurdlers and evaluate genders differences. METHODS: 30 semi-professional sprinters ran 6 consecutive 50-meter series at maximum speed (15 minutes resting between series) with or without instep weights, randomized into three groups: no-weight, Ascending (50, 100, 150, 200g load cadence) or Descending (200, 150, 100, 50g load cadence). 1st and 6th series were run without weight. Weight devices: Power Instep(R). Total and partial times and speed were collected as well as gender and anthropometric variables. Variables were statistically analyzed and compared. This study was approved by Universitat Autònoma de Barcelona Research Ethics Committee (CEEAH-UAB, number 4987). RESULTS: Instep weights were always safe and well tolerated. Total race time between 1st and 6th series increased in all groups. In women, all groups` race times were longer in the 6th than in the 1st race. Instead, men in Ascending group decreased their race time an average of 667ms (SD=19.03) in the 6th race compared to the 1st (p=0.48), while Descending and no-weight worsened their running time 490ms (SD=13.76). CONCLUSION: Instep load was safe and well tolerated (50 to 200g). Improvements obtained in Ascending group men in the last series (without weight) were statistically non-significant due to the small sample size, but magnitudes observed could be considered important in some elite sports contexts: using increasing instep loads in men during 50-meter series at maximum speed in training or competition warm-ups could slightly increase final race speed a few hundredths of a second. Different muscle stimulation(2) and adjustments to positive biomechanical changes(2) during warm-up with ascending loads could slightly modify race parameters to a small but significant speed increase. Conclusion: Instep weights are safe and well tolerated as a warm-up technique in short distance runners. Upward weight cadence instep load in men during warm-up could positively modify 50-meter race time in sprinters and hurdlers, but larger studies are needed; if confirmed these results could be important in a professional sports context where hundredths of a second separate success from failure.
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