Answer:

The treadmill is definitely one of the better pieces of cardio equipment out there. It has actually been shown to burn the most calories out of all the different pieces of cardiovascular training equipment.

As far as replacements for the treadmill go, I have the following recommendations:

• Stationary Bike - The stationary bike is fairly easy on the knees but the only drawback is you won't be burning as many calories as the treadmill.
• Elliptical Trainer - The elliptical trainer is effective as long as it doesn't hurt your knee injury.
• Kickboxing or "cardio" martial arts classes. - I would definitely stay away from these types of classes. They contain a lot of pivoting movements (such as when you plant your foot then twist your body around to throw punch) that can actually cause injury in healthy knees if you're not careful. With an injured knee, you would be taking a huge risk.
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• Swimming - Swimming is great cardio exercise, just be careful with your kicking. DO NOT do the breaststroke as the kick places all the stress on the knees (especially the inner side). There are floats you can squeeze between your legs to do arms-only swimming.
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• Weight Training - Weight training will help your weight loss efforts immensely and I would recommend it wholeheartedly.