Exercise is undeniably a good thing ... but it's also possible to have too much of a good thing. Let's start with yoga, a low-impact workout practiced by an estimated 20 million Americans. In yoga studios across the country, uninitiated students are put through the standard paces of downward-facing dog and basic inversions like headstands. But some top yogis argue that even basic yoga positions can cause serious injury to people with existing health problems like back or joint issues [source: Broad].
And what about those marathon runners, the very model of physical fitness and endurance? A number of recent studies have shown that extreme endurance training can actually damage the heart. The prolonged cardiovascular stress of running a marathon can cause problems like arrhythmia (irregular heartbeat), calcification and even scarring. According to the research, there's a limit to how much the heart can be pushed before it sustains damage. The culprit appears to be inflammation of the heart tissue during prolonged endurance training [source: Collier Cool].
Another inconvenient truth of exercise: It's not a great way to lose weight. Major changes in diet — avoiding carbohydrates, sugars and starchy foods — will do much more to slim your waistline than walking briskly on the treadmill for 30 minutes a day [source: Bowden]. While an hour of vigorous daily exercise has proven effective for maintaining weight loss, exercise alone isn't the most efficient way to shed unwanted pounds.