Isokinetic exercise is a form of dynamic exercise in which the velocity of muscle shortening or lengthening and the angular limb velocity is predetermined and held constant by a rate-limiting device known as an isokinetic dynamometer. The term isokinetic refers to movement that occurs at an equal (constant) velocity. Unlike DCER exercise where a specific weight (amount of resistance) is selected and superimposed on the contracting muscle, in isokinetic resistance training the velocity of limb movement, not the load, is manipulated. The force encountered by the muscle depends on the extent of force applied to the equipment.

Isokinetic exercise is also called accommodating resistance exercise. Theoretically, if an individual is putting forth a maximum effort during each repetition of exercise, the contracting muscle produces variable but maximum force output, consistent with the muscle’s variable tension-generating capabilities at all portions in the range of movement, not at only one small portion of the range as occurs with DCER training. Although early advocates of isokinetic training suggested it was superior to resistance training with free weights or weight-pulley systems, this claim has not been well supported by evidence. Today, use of isokinetic training is regarded as one of many tools that can be integrated into the later stages of rehabilitation.

Characteristics of Isokinetic Training

A brief overview of the key characteristics of isokinetic exercise are addressed in this section. For more detailed information on isokinetic testing and training, a number of resources are available.

Constant velocity. Fundamental to the concept of isokinetic exercise is that the velocity of muscle shortening or lengthening is preset and controlled by the unit and remains constant throughout the ROM.

Range and selection of training velocities. Isokinetic training affords a wide range of exercise velocities in rehabilitation from very slow to fast velocities. Current dynamometers manipulate the speed of limb movement from 0°/sec up to 500°/sec.Ttraining velocities are classified as slow, medium, and fast. This range theoretically provides a mechanism by which a patient can prepare for the demands of functional activities that occur at a range of velocities of limb movement.

Selection of training velocities should be as specific as possible to the demands of the anticipated functional tasks. The faster training velocities appear to be similar to or approaching the velocities of limb movements inherent in some functional motor skills such as walking or lifting. For example, the average angular velocity of the lower extremity during walking has been calculated at 230° to 240°/sec. Nevertheless, the velocity of limb movements during many functional activities far exceeds the fastest training velocities available.

The training velocities selected may also be based on the mode of exercise (concentric or eccentric) to be performed. The range of training velocities advocated for concentric exercise is substantially greater than for eccentric training.

Reciprocal versus isolated muscle training. Use of reciprocal training of agonist and antagonist muscles emphasizing quick reversals of motion is possible on an isokinetic dynamometer. For example, the training parameter can be set so the patient performs concentric contraction of the quadriceps followed by concentric contraction of the hamstrings. An alternative approach is to target the same muscle in a concentric mode, followed by an eccentric mode, thus strengthening only one muscle group at a time. Both of these approaches have merit.

Specificity of training. Isokinetic training for the most part is speed-specific, with only limited evidence of significant overflow from one training speed to another. Evidence of mode-specificity (concentric vs. eccentric) with isokinetic exercise is less clear. Because isokinetic exercise tends to be speed-specific, patients typically

Compressive forces on joints. During concentric exercise, as force output decreases, the compressive forces across the moving joint are less at faster angular velocities than at slow velocities.

Accommodation to fatigue. Because the resistance encountered is directly proportional to the force applied to the resistance arm of the isokinetic unit, as the contracting muscle fatigues, a patient is still able to perform additional repetitions even though the force output of the muscle temporarily diminishes.

Accommodation to a painful arc. If a patient experiences transient pain at some portion of the arc of motion during isokinetic exercise, this form of training accommodates for the painful arc. The patient simply pushes less vigorously against the resistance arm to move without pain through that portion of the range. If a patient needs to stop a resisted motion because of sudden onset of pain, the resistance is eliminated as soon as the patient stops pushing against the torque arm of the dynamometer.

Training Effects and Carryover to Function

Numerous studies have shown that isokinetic training is effective for improving one or more of the parameters of muscle performance. In contrast, only a limited number of studies have investigated the relationship between isokinetic training and improvement in the performance of functional skills. Two such studies indicated that the use of high-velocity concentric and eccentric isokinetic training was associated with enhanced performance (increased velocity of serving in tennis and throwing a ball).

Several factors inherent in the design of most types of isokinetic equipment may limit the extent to which isokinetic training carries over to improvements in functional performance. Although isokinetic training affords a spectrum of velocities for training, the velocity of limb movement during many daily living and sport-related activities far exceeds the maximum velocity settings available on isokinetic equipment. In addition, limb movements during most functional tasks occur at multiple velocities, not at a constant velocity, depending on the conditions of the task.

Furthermore, isokinetic exercise usually isolates a single muscle or opposite muscle groups, involves movement of a single joint, is uniplanar, and does not involve weight bearing. Although isolation of a single muscle can be beneficial in remediating strength deficits in specific muscle groups, most functional activities require contractions of multiple muscle groups and movement of multiple joints in several planes of motion, some in weight-bearing positions. However, it is important to note that some of these limitations can be addressed by adapting the setup of the equipment to allow multiaxis movements in diagonal planes or multijoint resisted movements with the addition of an attachment for closed-chain training.

Special Considerations for Isokinetic Training

From a pragmatic perspective, one limitation of isokinetic exercise is that a patient can incorporate this form of exercise into a rehabilitation program only by going to a facility where the equipment is available. In addition, a patient must be given assistance to set up the equipment and often requires supervision during exercise. These considerations contribute to high costs for the patient enrolled in a long-term rehabilitation program.

The setups recommended in the product manuals often must be altered to ensure that the exercise occurs in a position that is safe for a particular joint. For example, even though a manufacturer may describe a 90°/90° position of the shoulder and elbow for strengthening the shoulder rotators, exercising with the arm at the side may be a safer, more comfortable position.

Initiation and Progression of Isokinetic Training During Rehabilitation

Isokinetic training is begun in the later stages of rehabilitation, when active motion through the full (or partial) ROM is pain-free.

• Initially to keep resistance low, submaximal isokinetic exercise is implemented before isokinetic exercise with maximal effort.
• Short-arc movements are used before full-arc motions, when necessary, to avoid movement in an unstable or painful portion of the range. This is accomplished by a mechanical range-limiting device or with a computerized dynamometer.
• Slow to medium training velocities (60°-180°/sec) are incorporated into the exercise program before progressing to faster velocities.
• Maximal concentric contractions at various velocities are performed before introducing eccentric isokinetic exercises for the following reasons.
• Concentric isokinetic exercise is easier to learn and is fully under the control of the patient.
• During eccentric isokinetic exercise the velocity of movement of the resistance arm is robotically controlled by the dynamometer, not the patient.

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