Sit-ups once ruled as the way to tighter abs and a slimmer waistline, while "planks" were merely flooring. Now plank exercises, in which you assume a position and hold it, are the gold standard for working your core, while classic sit-ups and crunches have fallen out of favor. Why the shift? Get your copy of Core Exercises: 5 workouts to tighten your abs, strengthen your back, and improve balance Want to bring more power to athletic pursuits? Build up your balance and stability? Or are you simply hoping to make everyday acts like bending, turning, and reaching easier? A strong, flexible core underpins all these goals. Core muscles need to be strong, yet flexible, and core fitness, like that found in the Special Health Report Core Exercises: 5 workouts to tighten your abs, strengthen your back, and improve balance, should be part of every exercise program. One reason is that sit-ups are hard on your back — they push your curved spine against the floor and work your hip flexors, the muscles that run from the thighs to the lumbar vertebrae in the lower back. When the hip flexors are too strong or too tight, they tug on the lower spine, which can create lower back discomfort. Second, plank exercises recruit a better balance of muscles on the front, sides, and back of the body during exercise than do sit-ups, which target just a few muscles. (Your core goes far beyond your abdominal muscles.) Finally, activities of daily living, as well as sports and recreational activities, call on your muscles to work together, not in isolation. Sit-ups or crunches strengthen just a few muscle groups. Through dynamic patterns of movement, a good core workout like plank exercises helps strengthen the entire set of core muscles you use every day. For more on the benefits of strengthening your core, buy Core Exercises, a Special Health Report from Harvard Medical School. Share this story:

The right stuff: These simple items can help you strengthen your core Image: iStock You needn't spend a cent on fancy equipment to get a good core workout. Many core exercises rely on your body weight alone. But with the help of some simple equipment, you can diversify and ramp up your workouts. The following items can help you put a new twist on your core exercises. Most of them can already be found around your house or are available at low cost from a sporting goods store. Chair. Choose a sturdy chair that won't tip over easily. A plain wooden dining chair without arms or heavy padding works well. Mat. Use a nonslip, well-padded mat. Yoga mats are readily available. A thick carpet or towels will do in a pinch. Yoga strap. This is a non-elastic cotton or nylon strap of six feet or longer that helps you position your body properly during certain stretches, or while doing the easier variation of a stretch. Choose a strap with a D-ring or buckle fastener on one end. This allows you to put a loop around your foot or leg and then grasp the other end of the strap. Medicine balls. Similar in size to a soccer ball or basketball, medicine balls come in different weights. Some have a handle on top. A 4-pound to 6-pound medicine ball is a good start for most people. Bosu. A Bosu Balance Trainer is essentially half a stability ball mounted on a heavy rubber platform that holds the ball firmly in place. For more ways to challenge your core muscles, purchase Core Exercises, a Special Health Report from Harvard Medical School.