Strength Testing -- Fixed Dynamometry
Isometric Knee Strength Testing: A Cost-Effective Solution Using a Crane Scale
As we discussed in the podcast Electrical Stimulation & Quad Strength Testing after ACL Reconstruction, you know I believe in the principle: what gets measured gets managed. If we aren't getting objective numbers on our patients' strength, we are essentially guessing at their recovery. And guessing isn't good enough for return-to-sport decisions.
I recently discussed why it is so critical to test strength in that podcast episode, but I also know the reality of clinical practice. High-end isokinetic dynamometers are the gold standard, but they come with a massive price tag and take up valuable floor space. Even handheld dynamometers can be pricey for some clinics.
That is why I love this alternative. It is a reliable, valid, and significantly cheaper way to get the objective data you need: Isometric Knee Strength Testing using a Crane Scale.
The Strength Testing Using Crane Scale
Here is what you need:
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Crane Scale: This is the key component. I prefer one that holds the "peak" value so you don't have to stare at the screen while cueing the patient. I recommend this Crane Scale
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Ankle Strap: To attach the scale to the patient's leg. I use this ankle strap from Amazon
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Fixed Strap: To connect the scale to the table. This is the fixed strap I use
Key Setup Points:
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Ensure the strap is attached to a stable base (like the frame of a treatment table).
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Create a straight line of pull from the anchor point to the ankle.
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Eliminate any slack in the system before the patient begins kicking.
The Procedure: Step-by-Step Protocol
Standardization is everything. If you test at 60 degrees of flexion today and 90 degrees next month, your data is useless. Whatever setup you choose, you must replicate it exactly every single time.
For this protocol, we standardize at 60 degrees of knee flexion.
1. Patient Positioning
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Knee Angle: Set the knee to 60 degrees of flexion. Ensure there is full tension on the strap at this angle.
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Hip Position: Keep the hip neutral. Watch out for the patient abducting or shifting their hips to generate more force.
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Hands: Have the patient place their fists on top of their thighs. This prevents them from grabbing the table for leverage.
2. The Warm-Up (Built-In)
Before going for a max effort, we want the patient to get a feel for the resistance. We build the warm-up directly into the testing flow:
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Rep 1: 25% Effort (Check for pain or discomfort).
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Rep 2: 50–75% Effort
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Rep 3: 75–100% Effort
3. The Test
Once they are warm, it’s time for the maximal effort trials. Give them a few seconds of rest between reps, then instruct them to kick as hard as they can.
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Cueing: Use loud, encouraging vocal cues
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Duration: Have them hold the maximal contraction for at least 3-5 seconds to ensure they hit their peak.
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Record and Repeat: Check the peak value on the crane scale. Repeat the max effort trials until you see the score plateau or drop off. If they get stronger on the second rep (which often happens), keep going until they max out.
Why This Matters
Whether you are testing an ACL reconstruction patient at 3 months, 6 months, or a year post-op, you need apples-to-apples comparisons. By using a fixed strap and a digital scale, you remove the variable of the tester's strength (which is a major flaw of hand held dynamometry).
This method gives you a hard number. You can track it, show the patient their progress, and know exactly when they are ready for the next phase of rehab.
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