You can also try to do chair crunches

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That way each week, your muscles will  Formula 41 Extreme grow bigger and become dense and strong. You can also do some other exercises like dumbbell or weight ball, Leg press and Leg extensions, Dead Lift, Squat, Cable Pull Down. with wrist-weights on your hand to build more resistance.Cardio exercises do not help directly in building strong muscles. Cardio exercises increase your overall stamina so you can do strength training for longer time and build muscles in shorter period of time. Finally take extra proteins for building and repairing muscle tissue. Now you will get strength, power and big strong muscles  http://www.strongmenmuscle.com/formula-41-extreme/