calisthenics, or bodyweight exercises, can add variety to a traditional strength training routine that uses free weights and resistance machines. In addition to modifying exercises to fit your fitness level, performing them quickly with just 15 to 30 seconds of rest between sets can increase the intensity of the exercise, resulting in an intense workout. (See Reference 1) Challenging Push-ups Push-ups primarily work the chest and also involve the shoulder, triceps and core. Standard push-ups are performed by bending and extending your elbows 90 degrees while face down in a plank position during which you align your body from head to toe. While hardcore variations of this exercise may include handstand push-ups against a wall or push-ups with your feet elevated on a platform, even heavy repetitions of easier push-ups can be challenging. These may include push-ups on your knees or with your hands on an elevated surface. Depending on the intensity, perform 10 to 25 repetitions and two to three sets. (See References 1, 2 and 3) Effective Pull-upsAs if it doesn't get more challenging...