Work Your core

Plank

The plank works your entire core and upper- and lower-body muscles. Lie down on your stomach. Lift your body off the floor with your forearms (elbows at 90° degrees) and your toes. Keep your body in a straight position (without arching your back) and hold for 30 seconds to one minute. Lift one foot in the air for added difficulty.

Pushup plank

This exercise is the same as the plank, except that you are in a pushup position. The pushup plank works the core, chest and biceps. This is a great exercise to end your workout with; it will fully fatigue almost every muscle in your upper body.

V-sit hold

This targets your abdominal  muscles and improves your balance. Lie on your back and bend at the waist as you extend your legs and arms into the air to form a “V.” Hold this position for as long as you can.

Twisting crunch

This is one of the most effective crunch workouts, as it hits all of your stomach muscle fibers at once. Assume a standard crunch position, raise your torso to a 45° angle, and then twist from side to side. For an advanced movement, extend your legs and pretend to peddle a bicycle while you continue to twist. 

Strong core muscles around your trunk and pelvis make it easier for you to do most physical activities. If you have a weak core, you're more likely to have poor posture and suffer lower back pain and muscle injury. Building core muscles can improve your balance and stability, tone your abs and help you reach your fitness goals.

Abdominal Crunches

MayoClinic.com describes abdominal crunches as a classic core exercise. It recommends doing crunches by lying on the floor with your feet against a wall and your knees and hips bent at a 90-degree angle. Tighten your abs, and then raise your head and shoulders off the floor. If you prefer, you can do crunches on a fitness ball by sitting on the ball with your feet resting on the floor. Cross your arms over your chest, tighten your abs and lean back until you feel those abdominal muscles working. Hold for three deep breaths and return to your starting position.

Push-Ups

Push-ups are another of those classic core-conditioning exercises. If you find it too hard to do a traditional push-up, MayoClinic.com suggests that you do push-ups on your knees or standing against a wall. All three varieties will help strengthen your core.

Push-Away Isometrics

The Diet Channel recommends doing some push-away isometrics to build your core. Begin by lying on your back. Bend your knees and place your feet flat on the floor. Pull your navel toward your spine as you tilt your pubic bone toward your navel. Holding that position, keep your knees bent as you lift both legs. Place a hand on each knee and push forward with your hands as your knees try to resist the force. Hold for a count of at least six before returning to the starting position. Work up to six or more repetitions.

Bridges

Bridges are another good exercise to work on those core muscles. Begin by lying on your back with your knees bent. Tighten your abdominal muscles and raise your hips off the floor until they are aligned with your knees and shoulders. Hold for three deep breaths before returning to the starting position. MayoClinic.com notes that you should keep your back in a neutral position -- don't arch it or push it into the floor.

Leg Abdominal Press

Single- and double-leg abdominal presses will give your core a workout too. For both presses, begin by lying on your back with your knees bent. As with the bridges, MayoClinic.com warns that you should keep your back in a neutral position. Tighten your abs and avoid tilting your hips. Raise your right leg off the floor until your knee and hip are bent at a 90-degree angle. Place your right hand on that knee and try to push it down. Use your abs to resist the pressure. Hold for three deep breaths, return to the starting position and repeat on your left side. For a harder workout, lift both legs at the same time.