If you're running, hotter or colder weather can affect how fast you run. The same holds true for swimming, even when you swim in a temperature controlled environment at the gym. How warm or cold the water is can affect your swimming performance.
Your Body and Cool Water
When you jump into a pool that's cold, your blood vessels dilate so warm blood can warm you up, then close to keep internal organs from getting cold before opening up again. When temperatures are too freezing, that's when cool blood flows to the point where it can no longer keep you warm, causing hypothermia.
Your Body and Warm Water
When you swim in warmer water, you are swimming in excess heat. Your body is not trying to warm itself up, but rather cool itself down. This forces your body to generate more energy to cool the body. As a result, you end up overexerting yourself as you swim, which can even cause your body to overheat.
Cold Water Versus Warm Water
In a 1993 study done by "The Journal of Sports Medicine and Physical Fitness," the water temperature had a direct effect on performance effort, average swimming speed and heart rate. Overall, warmer water increased swimming speeds, but the study notes that performance efforts were "accompanied by greater metabolic and cardiovascular loads." Warmer water makes your body work harder, but even if you swim at faster speeds, the cooler water temperature is healthier for your body.
Open Water Swimming Temperatures
For indoor events, the water temperature can be measured beforehand. The Federation Internationale de Natation (FINA) dictates that temperatures must be between 25 and 28 degrees Celsius, or 77 and 82.4 degrees Fahrenheit. But open waters vary depending on the weather. Events can be canceled if the water temperature is higher than 84 degrees.
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