Isokinetic Strength Test: What It Is, How It's Done, and When to Use It

The isokinetic strength test is the "gold standard" for assessing muscle strength in sports physiotherapy and rehabilitation, especially after knee injuries. Unlike field tests (which depend on the assessor's skill and external factors), an isokinetic dynamometer controls the speed of movement and measures the torque (rotational force) produced at every point in the range of motion.

This test requires specialized equipment (not something you'll find in every gym or clinic), but understanding what it measures and how to interpret it is useful for any professional working with patients who undergo this type of assessment.

What does this test measure?

It measures the maximum torque (peak torque) a muscle group can produce at a controlled, constant angular velocity (for example, 60°/second or 180°/second), during a specific joint movement (knee extension/flexion, shoulder, etc.). From there, several parameters can be calculated:

• Peak torque (maximum force in N·m)
• Agonist/antagonist ratio (for example, the hamstring/quadriceps ratio, important for injury prevention)
• Side-to-side deficit (comparison between the injured and healthy leg)

What you'll need

• An isokinetic dynamometer (specialized equipment, such as Biodex or Cybex)
• Staff trained in operating the equipment and interpreting the data

Testing protocol

1. The subject is positioned in the dynamometer, with the joint being assessed properly secured (for example, the knee, with the machine's axis of rotation aligned with the joint axis)
2. A specific warm-up is performed with submaximal repetitions
3. Sets of maximal repetitions are performed at different angular velocities (typically low speeds like 60°/s for strength, and higher speeds like 180-300°/s for power/endurance)
4. The equipment's software records the torque produced on each repetition and automatically calculates peak torque, ratios, and side-to-side deficits
5. The protocol is repeated on the contralateral leg/arm for comparison

Common mistakes

• Poor joint stabilization, which introduces compensation from other muscle groups
• Not performing an adequate warm-up before the maximal sets
• Comparing results between different equipment or protocols without accounting for the fact that values aren't always directly comparable

How to interpret the results

Parameter	Approximate reference value
Hamstring/quadriceps ratio (60°/s)	0.6-0.8 (healthy knee)
Limb Symmetry Index (LSI) deficit	< 10-15% is generally considered acceptable for return to sport
Deficit > 15-20%	Indicates more work is needed before progressing to high-demand efforts

These values are general reference points and should always be interpreted within the patient's specific clinical context, the type of injury, and the stage of the rehabilitation process.

How it relates to other tests

The isokinetic test is often combined with more accessible field tests for a comprehensive assessment:

• Single leg bridge test — an accessible field test that complements isokinetic assessment
• Isometric quadriceps strength test — a more accessible alternative when an isokinetic dynamometer isn't available

Track and monitor progress

While the isokinetic test requires specialized equipment that isn't available at every facility, its results (peak torque, ratios, side-to-side deficits) are key data points worth keeping centralized alongside the rest of the athlete's assessment. With Movalytics you can log these results as part of the athlete's history, alongside other field tests, to get a complete picture of their progress through rehabilitation.

Frequently asked questions

Is an isokinetic test required for return to sport? It's not always accessible, but when it is, it provides extremely valuable information (especially ratios and side-to-side deficits) that complements field tests. What does a low hamstring/quadriceps ratio mean? A low ratio can indicate relative hamstring weakness compared to the quadriceps, a factor that some studies have linked to a higher risk of ACL and hamstring injuries. What speeds are typically tested? Several velocities are usually tested: low speeds (60°/s) to assess maximal strength, and higher speeds (180-300°/s) to assess muscular endurance and power.