Objective testing plays an important role in rehabilitation and performance. Whether we are assessing strength with a dynamometer, evaluating jump performance on force plates, or monitoring an athlete during the return-to-sport process, objective data can help guide clinical decisions.

But that data is only useful if we can compare it consistently over time.

A testing device may be highly accurate, but if the testing setup changes from one assessment to the next, the result may change too. That leaves us with an important question:

Did the athlete actually change, or did the way we tested them change?

Knee extension strength testing is a good example. Joint angle, body position, stabilization, dynamometer placement, pretension, instructions, and athlete effort can all influence the result.

Testing knee extension strength at 60 degrees of knee flexion is not necessarily measuring the exact same strength capacity as testing at 90 degrees. Changing the joint angle changes muscle length, leverage, and the mechanical conditions under which the quadriceps produce force.

The athlete may be the same. The test is not.

Small Changes in Setup Can Change the Number

This is one of the challenges of objective testing.

An athlete may produce more force during a reassessment, and at first glance, that could look like an improvement in strength. But what if the athlete was positioned differently? What if the dynamometer was placed in a different location? What if they were stabilized differently?

Some of the change in the number may be related to the testing setup rather than a true change in strength.

For example, imagine an athlete produces 450 N during an initial assessment and 500 N four weeks later. We want to be able to interpret that increase with confidence.

To do that, the conditions surrounding the test need to remain as consistent as possible.

If both the athlete’s physical capacity and the testing conditions are changing, it becomes much harder to determine what caused the difference.

Joint Angle Matters

Knee extension testing shows how something as simple as joint position can change what we measure.

Knee extension strength can be assessed at different joint angles, including 60 degrees and 90 degrees of knee flexion, depending on the purpose of the assessment. Changing the knee angle changes quadriceps muscle length and the mechanical advantage of the knee extensor mechanism.

Because of that, a strength value collected at one angle should not automatically be compared with a value collected at another.

Neither position is necessarily wrong. They are simply different tests.

The same principle applies throughout the body.

Hip strength testing can change depending on hip position. Hamstring strength can change depending on hip and knee angle. Isometric testing can change based on joint position. Jump metrics can change depending on countermovement strategy, arm position, or starting position.

When those parameters change, the result can change too.

Stabilization Matters

Stabilization is another important part of the testing setup.

During a maximal knee extension contraction, the athlete is not only using the quadriceps. The rest of the body needs to provide a stable platform from which force can be produced.

If the pelvis, trunk, or thigh is allowed to move, the athlete may compensate or reposition during the test. That can change the force recorded by the device.

Stabilization is not just about making a test look cleaner. It helps define the test itself.

If an athlete is heavily stabilized during the initial assessment but minimally stabilized during the reassessment, we cannot be as confident that we are measuring the same thing in the same way.

Equipment Placement Matters

Where the dynamometer is placed can also change the result.

Moving the dynamometer farther down the leg changes the lever arm between the knee joint and the point where force is measured.

The same athlete producing the same knee extension torque can generate a different measured force depending on where the sensor is positioned.

That is why a general instruction such as “place the dynamometer above the ankle” may not be precise enough when tracking changes over weeks or months.

Equipment placement should be measurable, documented, and reproducible.

Instructions Matter Too

Testing becomes even more sensitive when we look beyond peak force.

Metrics such as rate of force development depend heavily on what happens during very small windows of time. Pretension, countermovement, instructions, and how quickly the athlete initiates the contraction can all affect the result.

An athlete who gradually builds into a maximal contraction may eventually reach the same peak force as an athlete who contracts explosively. Their force-time curves, however, can look very different.

If we are measuring rate of force development, telling an athlete to “push as hard as possible” may not produce the same result as telling them to “push as hard and as fast as possible.”

The instructions are part of the test.

Standardize What You Can Control

Testing conditions will never be perfectly identical.

An athlete may come into one session more fatigued, sore, motivated, rested, or ready than they were during the previous assessment. Those factors can be difficult to completely control.

That makes it even more important to standardize the parts of the test that we can control.

Whenever possible, testing protocols should keep the following consistent:

• Joint position

• Body position

• Stabilization

• Equipment placement

• Warm-up

• Pretension

• Instructions and verbal cues

• Number of trials

• Rest periods

• Testing order

These details should also be documented clearly enough that the test can be reproduced, whether it is performed by the same clinician or another clinician.

The Goal Is Repeatability

The goal of objective testing is not simply to produce a number. It is to produce a number that can be meaningfully compared with future results.

That is what makes testing useful for monitoring rehabilitation, evaluating training adaptations, and informing return-to-sport decisions.

When testing parameters remain consistent, we can be more confident that a change in the result reflects a change in the athlete. When the setup changes, it becomes harder to know how much of the difference came from the athlete and how much came from the test itself.

At Petroski Physio, this is why the details of objective testing matter. The technology is valuable, but the quality of the information depends on how consistently the test is performed.

Good testing is not just about using good technology. It is about creating a repeatable process that allows that technology to provide meaningful information.

Looking for a more objective approach to rehab and performance? Schedule an evaluation with Petroski Physio to learn how testing can help guide your rehabilitation and return-to-sport process.