The intervention will be applied to only the stable ankle of the athletes with unilateral chronic ankle instability.

The intervention will be applied to only the unstable ankle of the athletes with unilateral chronic ankle instability.

No intervention will be applied.

The structured balance training protocol consists of 7 exercises including balance and hop stabilization exercises for athletes with chronic ankle instability.

The Prokin PK 252 will be used for proprioceptive-stabilometric assessment of the lower extremities, incorporating static and dynamic evaluations. The sensorimotor assessment will include the monopodal controlled-load proprioceptive control test and its variant with total load. Participants will perform movements with one foot on the platform, with angular displacements converted into electrical impulses for data acquisition. Static balance will be assessed on a stable platform for 30 seconds per ankle under eyes-open and eyes-closed conditions. Dynamic balance will be measured on an unstable platform under the same conditions. Proprioception will be evaluated on an unstable platform for 90 seconds, guiding the platform through horizontal, diagonal, and circular patterns as per the test program.

Transcranial Magnetic Stimulation (TMS), a non-invasive neurophysiological technique, will be used to assess motor cortical function. Corticomotor excitability will be evaluated using dual-cone coil TMS to examine adaptations following cross-training after unilateral training. TMS will induce corticospinal volleys of direct (D) and indirect (I) waves.

The resting motor threshold (rMT), a biomarker of upper motor neuron function, will be recorded. EMG recordings will assess corticomotor excitability, with Ag/AgCl electrodes placed on the tibialis anterior, peroneus longus, and gastrocnemius medialis muscles, following SENIAM guidelines.

Participants will be seated in a TMS chair, and the hotspot in the left primary motor cortex will be identified. The MEP amplitude will be assessed at 120% rMT with 10 repetitions, and latency will be recorded. A 15-minute rest will be observed between muscle tests.

The H-reflex will be measured to assess motor neuron pool excitability within the spinal loop, aiming to explain the mechanism underlying bilateral adaptations. As a valuable tool for evaluating neurological function in athletes, the H-reflex will provide insights into neurophysiological changes.

EMG signals will be pre-amplified and bandpass filtered between 30-300 Hz and recorded from the peroneus longus, tibialis anterior, and gastrocnemius medialis muscles. The tibial nerve will be stimulated using single 1 ms square-wave pulses applied over the popliteal fossa.

The test sequence will be randomized between stable and unstable ankles, ensuring balanced measurements. H-reflex excitability will be assessed for both ankles before and after the protocol to determine neuromuscular adaptations.

Cumberland Ankle Instability Tool (CAIT) containing a 9-item 30-point scale is a valid and reliable questionnaire for distinguishing and measuring the severity of functional instability (FI). Clinically, CAIT is an effective tool for evaluating the severity of FI, monitoring progress, and measuring treatment outcomes. In researches, the CAIT provides the identification, objective definition, and comparison of more homogenous subject groups. Participants scoring 25 or higher are less likely to have FI, while participants scoring 25 or lower are more likely to have FI.