The Ankle Inversion Strength Test measures the maximum isometric force produced when the client pushes the inside of the foot against the Anker. It provides an objective measure of ankle invertor strength and is commonly used for baseline testing, side-to-side comparison and monitoring changes over time.

The assessment primarily evaluates the tibialis posterior and tibialis anterior, with contributions from the flexor hallucis longus and flexor digitorum longus depending on foot position. These muscles play an important role in foot stability, medial arch support, walking, running and change-of-direction activities.

When used with Measurz, the Anker records peak force and can also calculate additional metrics including force relative to body weight, impulse, torque (when the lever arm is entered), rate of force development, time to peak and fatigue index.

The assessment measures strength only and should always be interpreted alongside symptoms, movement assessment and functional performance.

What Is the Ankle Inversion Strength Test?

The Ankle Inversion Strength Test is an isometric assessment where the client pushes the inside of the foot against the Anker without producing visible ankle movement.

Using the Anker's four independent load cells allows the assessment to be performed in a fixed, repeatable position, reducing variability between testing sessions when the same setup is maintained.

Step-by-Step Protocol

1. Prepare the client

Explain that the assessment measures how strongly they can push the inside of their foot against the Anker.

Record any:

• ankle pain
• swelling
• stiffness
• recent injury
• previous surgery
• neurological symptoms
• fatigue

Complete one or two submaximal practice contractions before maximal testing.

2. Position the client

Seat the client with:

• hips flexed approximately 90°
• knees flexed approximately 90°
• pelvis against the backrest
• trunk upright
• feet facing forwards

Use the same seated position during every reassessment.

3. Position the foot

Position the testing foot so that:

• the heel is centred on the footplate
• the second metatarsal aligns with the centre of the platform
• the ankle begins in a neutral position
• the tibia remains vertical

Position the medial load cell against the medial border of the first metatarsal head. Avoid contact with the medial malleolus or the toes.

Record the contact point so it can be reproduced during future testing.

4. Stabilise the client

Prevent movement of the pelvis, trunk, hip and knee throughout the assessment.

Only the ankle should generate force into the load cell.

5. Testing instructions

Use consistent verbal instructions.

"Push the inside of your foot into the pad."

"Build the pressure smoothly."

"Push as hard as you can."

"Hold."

"Keep breathing."

Repeat the same cueing during every reassessment.

6. Record the assessment

Use:

• 1–2 practice trials
• 2–3 maximal trials
• 3–5 second contractions
• 30–60 seconds rest between trials

Record either:

• the highest trial, or
• the average of the recorded trials

Use the same scoring method for future testing.

7. Repeat the trial if

• the heel lifts
• the foot rotates
• the knee moves
• the pelvis shifts
• the trunk leans
• the load cell loses contact
• pain limits maximal effort
• the client starts before instructed

Why It Is Used

The assessment may be useful for:

• baseline ankle strength assessment
• left versus right comparison
• monitoring changes following reduced loading
• lower-limb strength profiling
• athlete performance assessment
• objective progress tracking
• monitoring response to exercise
• client education

The assessment should support clinical reasoning and should not be used as a stand-alone diagnostic or clearance assessment.

What It Measures

The primary outcome is peak isometric ankle inversion force.

When analysed in Measurz, additional metrics may include:

• Peak force
• Force relative to body weight
• Impulse
• Torque
• Rate of force development
• Time to peak
• Fatigue index

The assessment does not directly measure:

• ankle joint mobility
• ligament integrity
• tendon pathology
• balance
• gait quality
• readiness for sport or work

Understanding the Result, Reference Values and What to Look For

Higher force values generally indicate greater ankle inversion strength in the tested position.

Lower values may be influenced by:

• pain
• fatigue
• apprehension
• reduced effort
• previous injury
• poor familiarisation
• inconsistent positioning

When interpreting results:

• Compare with the client's previous assessment.
• Compare left and right limbs.
• Consider symptoms during testing.
• Assess movement quality and compensations.
• Relate the findings to the client's functional goals.

Published Anker-specific normative values are currently unavailable.

Research using handheld and fixed dynamometry has demonstrated that ankle inversion strength can be measured reliably when testing procedures are standardised. However, reported values differ between studies due to differences in positioning, stabilisation and equipment. For this reason, comparing results with the client's own baseline and the opposite limb is generally more meaningful than comparing absolute force values with published studies.

Healthy individuals commonly demonstrate less than a 10% side-to-side difference in ankle strength. Larger asymmetries should be interpreted alongside symptoms, previous injury and functional performance rather than used as an isolated finding.

Assessing Different Client Populations

Youth

Use additional familiarisation trials and interpret results relative to age, growth and activity level.

Adults

Useful for baseline strength assessment and monitoring progress over time.

Older adults

Interpret alongside walking ability, balance and daily function.

Athletes

Useful as part of a comprehensive lower-limb strength profile for running, jumping, cutting and landing activities.

Clients with persistent symptoms

Interpret results alongside pain, confidence, movement quality and functional performance.

Common Errors and Limitations

Common errors include:

• inconsistent foot placement
• heel lifting
• excessive toe gripping
• trunk movement
• pelvis shifting
• incorrect load cell placement
• inconsistent verbal instructions
• allowing ankle plantarflexion during the contraction

Limitations include:

• results are specific to the testing setup
• pain may reduce force production
• muscle strength alone does not determine functional performance
• published Anker-specific normative values are currently limited

Practical Applications

The assessment may be useful for:

• establishing a baseline
• monitoring strength changes over time
• comparing left and right limbs
• athlete profiling
• lower-limb performance assessment
• objective reporting using Measurz
• educating clients about measurable progress

FAQs

What does the Ankle Inversion Strength Test measure?

It measures maximal isometric ankle inversion strength.

Which metric should be used routinely?

Peak force is the primary outcome measure for routine testing.

Can this assessment diagnose ligament injuries?

No. It measures force production only and should be interpreted alongside other assessment findings.

Should both ankles be tested?

Yes. Bilateral testing allows meaningful side-to-side comparison.

Why is consistent positioning important?

Small changes in foot position, ankle angle or load cell contact can significantly influence force measurements, making consistent positioning essential for accurate retesting.

Key Takeaways

• Measures maximal isometric ankle inversion strength.
• Primarily assesses the tibialis posterior and tibialis anterior.
• Peak force is the primary outcome measure.
• Measurz can provide additional force-time metrics.
• Consistent positioning improves repeatability.
• Compare results with previous assessments and the opposite limb rather than relying solely on published reference values.

References

Alfuth, M., & Hahm, M. M. (2016). Reliability, comparability, and validity of foot inversion and eversion strength measurements using a hand-held dynamometer. International Journal of Sports Physical Therapy, 11(1), 72–84.

Mentiplay, B. F., Perraton, L. G., Bower, K. J., Adair, B., Pua, Y. H., Williams, G. P., McGaw, R., & Clark, R. A. (2015). Assessment of lower limb muscle strength and power using hand-held and fixed dynamometry: A reliability and validity study. PLOS ONE, 10(10), e0140822.

Martins, J., da Silva, J. R., da Silva, M. R. B., & Bevilaqua-Grossi, D. (2017). Validation of ankle strength measurements by means of a hand-held dynamometer in adult healthy subjects. Rehabilitation Research and Practice, 2017, 5426031.