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If you are getting serious about swimming then you should be warming up outside the pool (no more than 30 mins is necessary) before you go swimming. Do hamstring stretches, prone holds, sprinting, push ups, sit-ups, burpees, and streamline burpees (where you have to do a burpee but end in a tight streamline). If you can't make it to the pool you can still improve your strokes and build muscle. Practicing flutter kicks can be a great exercise for your core. Lay on your back and position your hands together underneath your rear. Then elevate your legs lightly and begin kicking your legs alternately. Try doing this for about thirty seconds, rest, and repeat. Planks are effective body weight exercises that strengthen your upper and lower body, as well as your shoulders, arms, and glutes. This is a great out of water exercise. Practice these steps to perfect your planks:  Position yourself like you are about to do a push-up. Ground your arms slightly wider than your shoulders length. Use your toes to hold your feet and squeeze your glutes (your butt), to stabilize your body. Align your head to be in line with your back. Focus looking down at a single spot on the floor. Hold this position for about twenty seconds. You want to make sure that your legs aren't holding any of the pressure. Practice this at intervals that are comfortable to you. You don't need to hit the gym every time you need to workout. Set yourself up for a routine of about twenty minutes. Try a few of these in your exercise:  10-15 reps of push-up 20-30 reps of crunches 5-10 reps of pull ups 10-15 reps of goblet squats Rest for a minute and repeat Your core is the most important group of muscles that helps you do just about everything. Swimming relies heavily on the strength of your core. Practice a few routines like these:  The bird dog position. Get on your hands and knees, and hold your spine as flat as possible. Then extend your left arm and right leg out away from your body. Do not raise your limbs over your spine, but rather keep them on level with your back. Hold this position for three-four seconds and then switch limbs. V-Sits. Start in a seated position and raise your legs to a 45 degree angle. Extend your arms out to your knees and hold this position for 10-30 seconds. Scissor crunches. Lie on your back and extend your legs flat on the ground. Keep your hands down by your side. Raise your right leg straight up towards the ceiling and lift your left leg about three to four inches off the ground. Take your left arm and extend it to meet your right foot up by the ceiling. Hold this pose for about 10-30 seconds and then switch limbs. Keeping up your cardiovascular system when you don't have a chance to hit the pool will help you stay in shape. Soccer is a great sport that will challenge your lungs and muscles. It also takes a good deal of hand eye coordination that parallels to syncing into your breathe and stroke.
Warm up before you get in the pool. Strengthen your kicks. Work on your planks. Perform weightless exercises. Strengthen your core. Practice other sports outside the pool.