Isotonic exercise causes a change in muscle length and is commonly performed to strengthen and stabilize the knee joint. The most effective isotonic exercise will focus on building the muscles that cause the knee joint to flex and extend: the quadriceps and the hamstrings. However, knee function is directly related to the ankle, hip and their surrounding muscles, which should also be strengthened to reach optimal knee stability.
Using Machines
The quad extension and leg curl machines will isolate the quads and hamstrings, respectively. The quad extension machine requires being in a seated position while contracting the four quad muscles so the ankles press against the padded stopper, lifting the weight and "extending" the knee joint. Similarly, the leg curl machine may require you to sit upright or lie face down while contracting the hamstrings against the padded stoppers touching the back of the ankles, lifting the weight and "flexing" the knee joint.
For best results with the machine, perform two to three sets of eight to 12 repetitions at 50 percent to 80 percent of your maximum effort for that exercise.
Free Weights
If you don't have access to machines, other exercises utilizing free weights can be performed that will work both the quads and hamstrings. James Hesson, author of "Weight Training For Life" recommends dead lifts and normal or variations of squats, such as front squats or dumbbell squats. Not only do these exercises emphasize the quads and hamstrings, but they also strengthen large and small stabilizer muscles in the upper thigh, buttocks, lower back, and lower leg.
Perform two to three sets of eight to 12 repetitions
Body Weight Exercises
Some isotonic exercises that work well, especially for beginners, use only body weight and can easily be performed at home. Performing leg lifts from four different positions--lying face up, face down, and on your right and left side--will effectively strengthen the quads and hamstrings as well as the inner thigh muscles and buttocks. Other exercises such as lunges and step-ups also are recommended for knee strength, according to the American College of Sports medicine. As strength improvements occur, you may consider adding more weight with dumbbells or by resistance bands.
Because the body weight exercises generally require less of your maximum effort, increase the repetitions to 15 to 20 in two to three sets. Perform your strengthening routine two to three days per week.
References
- "Essentials of Strength Training and Conditioning"; The National Strength and Conditioning Association; 2008
- "Weight Training for Life"; James L. Hesson; 2007
- "ACSM's Resource Manual for Guidelines for Exercise Testing and Prescription"; American College of Sports Medicine; 2006
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