To lose weight, eat a well-balanced diet, consuming fewer calories than you expend, and follow an exercise program. Aim for two or three days a week of strength training and a minimum of 30 minutes a day of cardiovascular exercise at least five days a week. While losing weight will reduce the stress you place on your knees, doing too much too fast may aggravate existing conditions. Over the long term, exercise reduces keen pain due to osteoarthritis. Before starting a weight-loss program, consult a health-care professional concerning the activities that are recommended or discouraged for your specific knee conditions.

Pain

A key to successful weight loss is avoiding injury. Never ignore pain. Unless you are working out under the close and immediate supervision of a medical professional, avoid any activities that cause pain to your knee joint during exercise. Especially as you begin an exercise program, you may experience minor swelling and soreness in your knee the day after you work out. Applying an ice pack to your knee for 20 minutes after your workout and again before you go to bed will reduce inflammation. If you still experience post-workout pain or swelling despite ice treatment, cut back or modify your program. Your physiotherapist may be able to suggest braces or therapeutic modalities that will help manage pain and inflammation.

Cardiovascular Exercises

Aerobic or cardiovascular exercises can burn 200 to 1,000 calories per hour depending on intensity. Non-weight-bearing aerobic exercises such as swimming, water aerobics, bicycling, canoeing, kayaking and stationary rowing burn calories without placing stress on your knee joints. Many people with bad knees can also engage in low-impact aerobic exercises such as walking, snowshoeing, cross-country skiing on level terrain, low-impact aerobics classes, Pilates, tai chi and indoor cross-country ski and elliptical trainer machines. Appropriate footwear minimizes the stress on your knees.

Resistance Training

Resistance or strength training done two to three times a week using free weights, weight machines or resistance bands is important for weight loss and knee rehabilitation. Stronger leg muscles help stabilize your knee joint, something especially important if you have damaged knee cartilage or ligaments. Resistance exercise helps weight loss in two ways -- you burn calories as you do resistance exercises and your muscles are metabolically active. Increasing the ratio of muscles to fat in your body makes you burn more calories even at rest. Because muscles are denser and firmer than fat, a program of resistance exercises will help you lose inches even if your weight remains the same. Strength training can reduce knee arthritis and lessen your risk of osteoporosis.

Exercises to Avoid

Avoid high-impact exercises such as running long distances on hard surfaces, high-impact aerobics classes, skiing or snowboarding on moguls or anything that involves jumping. Activities that involve lateral or twisting forces on the knee and contact sports are also problematic. Such activities include football, basketball, rugby, hockey, soccer, ballet, tennis and most extreme sports. Stair-climbing machines are not recommended for people with injuries to menisci or anterior cruciate ligaments. Modify or avoid any exercises that require bending your knees more than 90 degrees.

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