If you're looking for abdominal exercises that don't require working out in a gym, isometric ab exercises are an option -- you can do these exercises anywhere. Isometric exercises involve contracting your muscles without any joint movement. You'll find that your abdominal muscles respond best when you hold or do these isometric exercises for a length of time.
The plank is the most well-known isometric ab exercise and is used in both yoga and Pilates workouts. To perform the plank, kneel on all fours and rest your forearms on the floor. Walk your legs back so that your weight is supported on your forearms and forefeet only. Make sure your heels, hips and shoulders form a straight line -- hold this position for the desired duration. Do not allow your hips to drop toward the floor, as this places undue strain on your lower back.
Side planks target your abs and core muscles. Lie on your side with your legs straight -- rest on your elbow on the floor directly underneath your shoulder. Lift your hips off the floor so that your weight is supported on your arm and sides of your feet only. Hold this position for the desired duration. Make this exercise more challenging by elevating your feet on an exercise bench or step.
The waiter's walk is an effective isometric abdominal exercise that also improves shoulder joint stability. Stand with your feet together and a dumbbell in one hand. Curl the weight to shoulder level and press it above your head. Tighten your abdominal muscles and contract your upper back and shoulder muscles to support the weight. With your arm extended, walk around your training area while focusing on keeping your spine perfectly vertical. On completion, change arms and repeat.
The Pallof press is an isometric ab exercise that uses weights to strengthen your entire midsection. This anti-rotation exercise is used by therapists to help strengthen the muscles responsible for spine stability. To perform this exercise, stand sideways on a chest-high pulley cable and grasp the handle in both hands. Hold your hands close to your chest and step away from the pulley to tension your arms. From this position, press your arms out in front of you at shoulder level -- extend your arms to lengthen your levers and increase the degree of rotation you must resist. Draw your arms back into your chest to rest and then repeat. Perform an identical number of repetitions on the opposite side.
Avoid holding your breath when performing isometric exercises -- holding your breath can significantly raise your blood pressure. Start out by holding the contraction or doing the exercise for shorts periods of time -- increase the length time as you become more accustom to the exercise. If you're new to isometrics, take a class or work with a personal trainer to learn how to do these exercises properly.