Isokinetic Exercise

By Robert Donatelli, PhD, PT

James Perrine first introduced the concept of Isokinetic exercise to the physical therapy profession in the late 1960’s. Traditional weight lifting exercises are performed at variable speeds at a fixed resistance or weight. Isokinetic exercises are performed at a dynamic preset fixed speed (ranging from 1 degree per second to approximately 1000 degrees per second) with resistance that is accommodating throughout the range of motion (ROM). Accommodating resistance means that Isokinetic exercise is the only way to dynamically load a muscle to its maximum capability throughout every point in the ROM. Therefore, the resistance varies to exactly match the force applied by the athlete at every point in the ROM.

Isokinetics can be performed as an open kinetic chain exercise or a closed kinetic change exercise. An open kinetic chain exercise has limited functional carry over. Open kinetic chain (OKC) exercise is considered to be an activity in which the distal component of the limb (leg) is not fixed or weight bearing but free in space. Closed kinetic chain (CKC) exercise is considered to an activity in which the distal fixed end of the limb may be either stationary or moveable. For example, a CKC exercise in which the distal end (the foot) is stationary is a squat exercise in which the foot is fixed on the ground, as apposed to moving on a leg press.

Isokinetic machines have the ability to objectively measure muscle strength. The Isokinetic device is attached to a computer, that assesses the torque output of the muscles being testing. The torque output is converted to a foot-pound measurement. Isokinetic testing allows for a variety of testing protocols ranging from strength, strength ratios between two muscle groups, power, and endurance. For example, the ratio between the external and internal rotators in an overhead-throwing athlete should be 70%. In other words the external rotators (muscles behind the shoulder) should be 70% of the strength of the internal rotators (muscles in front of the shoulder, pectoralis major is one). The 70% ratio between the shoulder rotators is critical for the enhancement of performance and prevention of injuries. It is important to note that the only way to document weaknesses in muscle groups is through performance of isolated OKC testing. Furthermore, specific muscle weakness at the injury site can be identified only by isolated OKC testing.

Despite the fact that Isokinetic OKC exercise has been labeled as a non-functional exercise as compared to CKC, research has demonstrated the some significant improvements in functional activities after Isokinetic training. For example, Dr. Mont et al, published a study in the American Journal of Sports Medicine 1994, where he had three groups of tennis players that exercised on an Isokinetic device for 6 weeks. One group of tennis players exercised three times per week and performed only one exercise for strengthening of the shoulder rotators, while the other group of tennis player did not participate in any exercise program except playing tennis. The serve velocity in both groups of tennis players was recorded before and after the training with a radar gun. In addition, strength testing of the shoulder rotators was performed on the Isokinetic device before and after the training. A tennis professional was asked to select the tennis players for the study that were all at the same skill level. After 6 weeks the results indicated that compared to a control group, the Isokinetic training group showed a significant increase in their muscle strength of the shoulder rotators and a significant increase in their serve velocity. The average increase in serve velocity in the Isokinetic training group was 10-11 miles per hour. The above study confirms that OKC Isokinetic exercise testing and strengthening can be an important part of the rehabilitation and performance enhancement program for athletes.

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