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Go outside and play. Warm-up before exercising. Use the weight of your body to build muscle anywhere. Do push-ups. Do sit-ups with a ball and a friend. Organize “crazy races” to build different muscles. Spell out your alphabet with your legs. Stretch when you are done. Know that you should not lift weights until after puberty.
Running, biking, swimming, playing sports, and walking around the woods all build muscles naturally, and this is often the safest, most fun way for kids to build muscles. Grab a group of friends and play a game of basketball, start a scavenger hunt, jump in the pool, and wrestle in the backyard – just like many adults “cross-train” to build muscle with different activities, kids can secretly build muscles while playing. Just because kids are flexible and full of energy doesn’t mean they can skip the warm-up. Do 5-10 minutes of light aerobic activity, like walking, jogging, or jumping rope, before working out to get your muscles loose and your blood flowing. Kids should not just take an adult’s workout plan and scale it down. Not only can this be dangerous, kids have an energy level and natural flexibility that allows them to do a variety of exercises without needed weights. Perhaps more importantly, these exercises are easily turned into games or small competitions, making them much more fun to get through then a trip to the gym.  Traverse monkey bars to do modified "pull-ups," or offer to push your friends on the swings to build arm muscle. Hops, skips, and lunges all use your body weight to train your leg muscles. Climbing, whether at a rock wall on the playground, is a great workout for your arm and leg muscles. One of the best exercises for upper body muscles is still one of the simplest. Lie on the ground with your hands and toes touching the floor. Push your entire body up with both hands until your elbows are barely bent, then lower yourself slowly down towards the ground. When you are about 6 inches from the floor, push up again and repeat. Try to get 10 in a row, then rest for 1-2 minutes and try again.  Keep your butt down, even with your shoulders. Keep your back straight. Your hands should be shoulder-width apart. However, the further apart they get the more you work out your chest muscles. The closer together your hands are the more you’ll work your arm muscles. Sit across your friend with your knees bent and your toes facing each other. One of you should have a ball in your hands. At the same time, bend up from your stomach so that you are looking each other in the eyes and pass the ball. Only your feet and butt should still be touching the ground. Keep doing sit-ups and passing the ball until one of you is too tired to continue.  Keep your feet on the ground the entire time, and try to keep your shoulders lined up with your partner's. Focus on using the muscles around your stomach, your abs, to pull you up each time. There are tons of fun variations to a normal race that activate certain muscles and encourage kids to exercise without knowing it. Try making a relay race that switches between the following exercises to encourage great upper body strength.   Bear crawl: With your hands and feet on the ground, stick your butt high in the air and run forward on all fours. You’ll be surprised how quickly you get tired – many football and rugby teams still do this for strength conditioning.   Crab walk: Sit on the floor with your knees bent and your feet and hands on the ground. Lift your butt and walk forward, backward, or side to side to exercise your arms, abs, and thighs.  Burpee: Jump forward with both feet. When you land, get down and do one push-up. Then get up quickly and leap forward again.  Moon lunges: While they look slow, these are great hip and leg building exercises. Take the biggest step forward you can with your right foot, then lower your left knee and your butt slowly towards the ground. Stand up and repeat with your left leg. This helps to work your core and leg muscles. This exercise isn’t for the faint of heart, but it is a fun way to work some thinking and competition into your exercises. Lay on your back. Lift both legs together and point your toes down so that they form a long, straight pencil. From here, spell out the alphabet with your legs. How far can you get?  It is often easiest to keep your hands underneath your butt for balance. Make a “Work-out Spelling Bee” by challenging friends to spell words with your legs. Not only does spelling count, but you have to get through the word as well. Cooling down helps you muscles relax, making them more effective the next time you need them. Do some light stretching when you are finished to feel great the next day. Make sure you take time to rest. Your body needs time recover after a workout, so don’t exercise the same muscles two days in a row. Trying to lift difficult weights and “get huge” is not only impossible for young kids, it is unhealthy. Your muscles, tendons (which attach muscles to bones) and ligaments (which attach bones to other bones) are not fully developed, and could tear under the stress. Be patient and wait on weights until you are a teenager.  Small weights, ranging from 1-5lbs, can be safely substituted for resistance bands in younger children. Body weight exercises are much better for younger kids. You can still build muscle without injuring yourself.