Let's move to the upper body. Push-ups strengthen the chest, shoulders, triceps, and core muscles. Starting on all fours, place hands a bit wider than your shoulders. Place toes on the floor, creating a smooth line from shoulders to feet. Keeping core muscles engaged, lower and lift your body by bending and straightening your elbows. Too hard? Place knees on the floor instead of toes. To boost the workout, add an exercise ball under the hips, knees, or feet. Do 10 times.