Isokinetic exercises are performed using specialist machines commonly found in sports science labs, rehabilitation centers and doctors' offices. The speed of movement is limited and controlled so momentum is eliminated and tension remains on the muscle being trained at all times. While costly and not readily available, isokinetic machines offer a number of benefits to users.
Isokinetic machines provide an effective way to overload your muscles. Each machine has a specific purpose, be it working your quadriceps or knee extensor muscles or your rectus abdominis or abs. Because the resistance offered by these machines is minutely adjustable, it is possible to customize the exercise according to the exercisers' precise needs. In contrast, regular resistance machines are often adjustable only in jumps of 10 or 20 pounds. This makes isokinetic machines especially suitable for rehabilitation where the user may be very weak and unable to make large increases in resistance.
When you lift a barbell, dumbbell or use a regular resistance-training machine, you have to get the weight moving from a dead stop. This is called overcoming the moment of inertia. Getting the weight moving can be very demanding and can lead to injury or make existing injuries worse. With isokinetic machines there is no such moment of inertia to overcome, which makes these machines safer than traditional forms of strength training. Isokinetic exercises also produce little in the way of post-exercise muscle soreness.
With isokinetic machines, while the speed of movement is constant, the weight is not. In fact, the weight changes according to how hard you push or pull against the machine. The harder you pull, the greater the load, whereas the weaker your efforts, the lighter the load becomes. In more traditional forms of strength training, exercises are at their hardest when the weight is furthest from the fulcrum and the level is longest -- this is not the case with isokinetic machines. The resistance adjusts though the entire range of motion to ensure the speed of movement remains constant and the working muscles remain under tension throughout the exercise.
To test your strength, you could simply load up a barbell with as much weight and try to lift it. If the lift is successful, you put more on the bar and try again. The aim of this procedure is to find the maximum amount of weight you can lift for a single repetition -- your 1RM, or one repetition maximum. 1RM testing is fraught with danger and not especially accurate or appropriate for all exercisers. Isokinetic machines can be used to test strength far more accurately and without the risk associated with 1RM testing.