The goal of this observational study is to investigate whether ankle evertor muscle fatigue impairs force perception and alters neuromuscular activation patterns during submaximal isometric contractions, and whether these effects differ between individuals with chronic ankle instability (CAI) and individuals without CAI.

Participants will first be familiarized with the research protocol for approximately 20 minutes. After a short break, the experimental protocol with data collection will begin and last between 45 and 60 minutes. Measurements will be performed on the side where the participant reports symptoms of chronic ankle instability. In cases of bilateral CAI, the leg with the lower CAIT score will be tested.

Participants will be tested on an isokinetic dynamometer (Humac Norm, CSMi, USA) in a semi-recumbent position with the hip and knee flexed to ensure comfort and alignment. The trunk and tested leg will be secured with straps to minimize accessory movement. After initial maximal voluntary isometric contraction (MVIC) of ankle evertors, participants will perform force-matching tasks at 25% and 50% of MVIC. Each trial will consist of 5 seconds with visual feedback followed by 5 seconds without feedback, repeated three times per intensity. After completing these tasks, participants will undergo a fatigue protocol for the ankle evertors. After the fatigue protocol, participants will repeat the force-matching tasks at both 25% and 50% MVIC.

The muscle fatigue protocol consists of consecutive isotonic contractions of ankle eversion, performed through the full range of motion at 30% of MVIC torque until visible fatigue. Fatigue will be defined as a clear inability to perform full range of eversion, i.e. the range of motion fell below 10% of the initial value despite evident effort by the participant. The test will be paced using a metronome, with a 1-second concentric and a 1-second eccentric phase to ensure consistent movement velocity.

Inclusion criteria for the healthy control group (no CAI):

• Age between 18 and 45 years,
• No history of injuries or surgical procedures to the lower limbs,
• No peripheral or central neurological impairments.

Inclusion criteria for the chronic ankle instability (CAI) group:

• Age between 18 and 45 years,

• No history of major surgical procedures on the lower limbs

• No peripheral or central neurological impairments,

• Diagnosed chronic ankle instability:

  • First ankle sprain occurred at least one year prior to testing,
  • At least three months since the most recent ankle sprain,
  • Subjective feeling of ankle instability - CAIT (Cumberland Ankle Instability Tool) score < 24.

Exclusion criteria for the CAI group:

- Other pathologies of the ankle joint.

Participants will be recruited by convenience sampling from the general population and clinical settings.

All collected IPD in anonymized form.

No limit.

All IPD will be shared upon reasoned request for the purpose of further scientific analysis (i.e. systematic reviews, meta-analyzes, etc.). A proposal describing the planned analyzes must be submitted in writing to the principal investigator, Dr. Alan Kacin, by email to alan.kacin@zf.uni-lj.si.