Isokinetic training involves muscular contractions -- or exercises -- performed at a constant rate of speed. This sort of training requires specifically-modified training equipment to limit the rate at which you perform an exercise. While generally only performed using the concentric, or shortening phase, eccentric training is sometimes done isokinetically. Consult a health care professional before beginning any exercise program.
Isokinetic Training
Isokinetic training requires highly-specialized equipment not available at most gyms. These devices allow you to train through something approaching a normal range of motion, but have adjustable settings to control your speed and power output. By adjusting the settings, you attempt to ensure that the resistance is spread out evenly over the entire exercise. An example would be when squatting, as you get closer to the top when standing up, your leverage improves and the lift becomes easier. Using an isokinetic squat machine, the resistance increases as you get closer to locking out your squat, keeping your speed constant. For these devices to work properly, you must attempt to move the weight as fast as possible and work hard against the limits of the machine.
Concentric Training
Concentric training is the shortening phase of a lift, often called the positive aspect. Examples include rising out of the bottom of a squat, pressing the bar up when benching and standing up with a deadlift. You cannot use as much weight during the concentric portion of the lift, but you can generate more force. A 2007 study published in the "Journal of Sport Rehabilitation" showed that high-speed isokinetic training of your quadriceps improved the activation of your hamstrings. One of the functions of your hamstrings is to stabilize your knee joint when your quadriceps, or the muscles on the front of your thigh, are active. The greater the activation of your hamstrings, the more they help protect your knee joint.
Eccentric Training
Eccentric training, or the lengthening phase, allows you to handle more weight than the concentric phase. Eccentric training using isokinetic machines can be difficult to perform, as you are working with gravity, and the devices must be adjusted specifically to compensate for the ease with which you lower the weight. This also limits the involvement of your nervous system, resulting in lower overall muscle activation than concentric isokinetic exercise. A 2010 study published in the "Annals of Physical and Rehabilitation Medicine," showed that variable speed training using eccentric actions produced greater gains in strength than isokinetic actions.
Limitations of Isokinetic Training
Isokinetic training also forces you into a specific plane of movement. When using any machine, you do not line up your joints the same way you would when performing a free-weight exercise. This can affect movement patterns and performance. An example of this would be squatting or performing jump squats on an isokinetic resistance device. Your hips do not rotate to the same degree, resulting in altered lift mechanics, as the bar is moving straight up and down. This can lead to multiple issues when performing isokinetic exercises exclusively, so use them sparingly, and only to teach yourself to increase power.