Metaphorically, whether running long distances might slow down sprinters can be compared to entering the same car in an endurance race and a drag race. The two types of racing require different engine setups and gearing. Long-distance running and sprinting require a different type of human engine and gearing as well. Any determination regarding a sprinters' speed and long-distance running must consider the nature of each type of movement.
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Anyone watching an Olympic marathon race might notice that the majority of the contestants sport similar physiques. Overall, distance runners have slender legs and hips, angular arms, and modest upper body definition. Ask trainers and distance runners what's required in the way of conditioning and you'll receive similar answer from each. Conditioning involves running long distances at a specified pace, and the distances become greater as endurance improves.
Competitive sprinting took a big turn in the late 1980s when trainers realized upper body strength contributes to speed. In the 20 years prior to this discovery, record times in the 100-meter sprint lingered around 10 seconds. Within five years of implementing intensive upper body weight training, overall times in the event dropped to the range of 9.85 seconds. The record has been bettered several times since and currently stands at 9.58 seconds. If you watched the 2008 Summer Olympics, you may have noticed world record holder and sprinter Usain Bolt sports a muscular upper body and arms. The level of competition is such that Bolt must dedicate to weight training and sprinting to remain competitive. But he could elect to maintain cardiovascular conditioning by running long distances at a relaxed pace.
Many sprinters and athletes that play speed positions, such as wide receiver and the outfield in baseball, enjoy the solitude of long distance running. In some localities, you are likely to see athletes with defined upper bodies jogging along bike paths or on beaches at any time of the year. In addition, you might encounter slender distance runners in similar localities, but it's rare to see the two types running together.
Sprinters have a different mindset than distance runners. Sprinters start in the blocks and expend their physical resources over a short distance. As a sprinter, it's possible to maintain your competitive edge by limiting the amount of distance running so that it doesn't interfere with training. If competitive long distance running did not affect speed, you might see world-class sprinters entered in both types of events.