You can get pecs -- pectoralis muscles, in your chest -- from pushups but not from situps. Pushups and situps use your body weight as resistance to build muscle mass and strength. They can be part of a strength-training program that targets all your major muscle groups.
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The term “pecs” refers to the pectoralis major and minor muscles, which make up the bulk of your chest. Everyone has pectoral muscles. They rotate and flex your shoulder as well as bring one arm across to the opposite side of your body. Developing your pecs through strength training makes them bigger and stronger. Pushups can be a part of an effective pec workout routine to develop strength in these muscles, because doing a pushup requires a muscle contraction from your pecs of 95 to 105 percent of maximum.
Classic situps do not build your pecs because they do not target your chest muscles as a primary or secondary muscle. Situps target your abdominal muscles to help you get a flatter tummy. Common situp variations, including reverse situps, bicycle crunches and fitness ball crunches, do not target your pecs, either.
Pushups help you get larger and stronger pecs. Kneel down and place your palms on the floor, just wider than shoulder-width apart. Extend both your legs behind you with your knees straight. Your toes are flexed and resting on the floor. To assume the starting position, align your body so it forms a 45-degree angle from the floor with a straight line from your heels to the top of your head. Bend your elbows out to the side up to 90 degrees and return to the starting position to complete one repetition. Work your muscles to fatigue during every pushup workout. Add reps over time and work your pecs on non-consecutive days at least three days a week.
Variations on the classic pushup will add spice to your plan to get pecs. You can use a bar about thigh-height at the gym or even your own desk at work to perform pushups. You can also try the wide-grip pushup; where your hands are placed at least six inches further from your shoulders. Use a bench or even your bed to perform elevated feet pushups.