Chronic ankle instability (CAI) is a common condition that can develop after a lateral ankle sprain. Up to 40% of people who sprain their ankle experience a recurrence within a year, and as many as 30% develop persistent symptoms such as ankle "giving way," weakness, or instability. These symptoms can limit daily activities, sports participation, and increase the risk of future sprains. Muscle weakness in both the ankle and hip has been identified as an important factor contributing to CAI. Strengthening these muscles may therefore help improve stability and function.

The SHAC study is a randomized controlled trial designed to compare three different strengthening programs in people with chronic ankle instability: (1) an ankle-focused strengthening program, (2) a hip-focused strengthening program, and (3) a combined ankle-hip strengthening program. The goal is to determine which approach leads to the greatest improvement in functional ability, muscle strength, balance, and ankle mobility.

A total of 45 participants aged 18 to 35 years with documented chronic ankle instability will be recruited from local sports clubs and the University of Liège. All participants must have a history of at least one ankle sprain that occurred more than 12 months before entering the study, report episodes of ankle instability in the previous six months, and meet specific questionnaire criteria (CAIT < 24 and FAAM Sport < 80%). People with recent injuries, previous lower-limb surgery, fractures requiring realignment, neuromuscular conditions, or current pain or swelling will not be able to participate.

After the first assessment (T1), participants will be randomly assigned to one of the three groups. Each intervention lasts four weeks and includes three supervised sessions per week. All strengthening exercises are isometric, performed using a handheld dynamometer to measure force accurately. The ankle program includes plantarflexion, dorsiflexion, inversion, and eversion exercises. The hip program includes clamshells, hip external rotation, abduction, and extension. The combined group alternates between hip and ankle exercises. A warm-up and standardized familiarization are performed before each session.

All participants will attend two additional assessment sessions: one immediately after the four-week program (T2) and one at a later follow-up time point (T3). During each visit, they will complete functional questionnaires, report their sports participation, and undergo several strength and performance tests. These include hip and ankle muscle strength measurements using a dynamometer, the Heel Rise Test, the modified Y-Balance Test, the Side Hop Test, and a dorsiflexion mobility test.

The study aims to identify whether one of the strengthening approaches leads to better functional outcomes for individuals with chronic ankle instability. Understanding these effects may help clinicians design more effective rehabilitation programs for people recovering from ankle sprains and for those experiencing ongoing instability.

Participant data will be kept confidential and stored securely in anonymized form for 20 years. Participation is voluntary, and individuals may withdraw at any time without consequences. Results will be presented anonymously in scientific publications and conferences.

Chronic ankle instability (CAI) is a multifactorial condition resulting from a previous lateral ankle sprain and characterized by repeated episodes of ankle "giving way," recurrent sprains, and persistent functional impairment. While the condition is common among active individuals, its underlying mechanisms remain complex. According to the contemporary model proposed by Hertel and Corbett, CAI results from a combination of pathomechanical, sensory-perceptual, and motor-behavior deficits. Among these components, weakness of muscles surrounding both the ankle and the hip has been consistently identified as a contributing factor to impaired neuromuscular control and reduced ability to maintain postural stability during functional activities. Several studies have reported deficits in both ankle and proximal hip strength in individuals with CAI, suggesting that rehabilitation strategies should not limit their focus to the ankle alone.

This clinical study seeks to deepen the understanding of how targeted strengthening influences functional recovery in CAI. Specifically, the trial compares the effects of three distinct strengthening interventions: an ankle-specific isometric training program, a hip-specific isometric training program, and a combined ankle-hip training program. The choice to integrate hip musculature into rehabilitation is grounded in evidence demonstrating that proximal weakness, particularly in abductors and external rotators, can alter lower-limb kinematics and increase susceptibility to recurrent ankle injury. Conversely, strengthening these muscles may enhance dynamic alignment and reduce excessive inversion moments during weight-bearing tasks.

Rationale for the Intervention Isometric exercise was selected as the primary strengthening modality. Compared with isotonic or plyometric training, isometric contractions provide a high-intensity stimulus with minimal joint motion and limited mechanical stress. This is particularly relevant for individuals recovering from instability episodes, as it reduces the likelihood of symptom exacerbation. Isometric training has also been shown to produce rapid neuromuscular adaptations in untrained individuals, even over short intervention periods. By normalizing intensity to each participant's maximal voluntary contraction (MVC), the protocol ensures individualized and progressive overload across sessions without altering the structure of the training program.

Since CAI is partly characterized by impaired motor control, an intervention capable of enhancing maximal force production, reducing neural inhibition, and improving joint stiffness may contribute to better stability. The comparison between isolated ankle strengthening, isolated hip strengthening, and their combination will help clarify whether proximal strengthening alone can meaningfully influence CAI-related impairments, whether local strengthening remains essential, or whether a combined approach provides synergistic effects.

Study Design and Methodological Considerations This randomized controlled trial is conducted at the University of Liège and includes three participant visits in addition to the training sessions. The assessments encompass functional, strength-based, and mobility-related measures relevant to CAI. All testing procedures follow standardized protocols to ensure high reproducibility.

Randomization is performed after the baseline assessment to avoid influencing participants' expectations or performance during initial testing. Each group completes 12 training sessions over four weeks (three per week). Sessions are structured to include a brief warm-up, movement familiarization at submaximal intensity, and a series of high-intensity isometric contractions. All force measurements during training are collected using a handheld dynamometer, providing objective data on individual progress and allowing researchers to track variations in force output across sessions.

Assessment Procedures Participants undergo comprehensive baseline, post-intervention, and follow-up evaluations. Strength assessments include hip abductors and external rotators measured in distraction using a VALD dynamometer. This measurement technique ensures consistent joint positioning and minimizes compensation. Ankle strength is assessed through handheld dynamometry in four primary movement directions: plantarflexion, dorsiflexion, inversion, and eversion. Because both proximal and distal deficits may influence movement control, these measurements are critical for evaluating whether strength gains differ based on intervention type.

Intrinsic foot muscle endurance is evaluated using the Heel Rise Test, a functional endurance task widely used in lower-limb rehabilitation research. Dynamic postural control is assessed via the modified Star Excursion Balance Test (Y-Balance Test), normalized to limb length to allow inter-participant comparison. Reactive agility and lateral movement control are evaluated using the Side Hop Test. Ankle mobility is assessed through the Weight-Bearing Lunge Test, a reliable measure of dorsiflexion range of motion.

Collectively, these performance tests capture key domains involved in CAI-strength, balance, mobility, and functional endurance-allowing the analysis to examine global changes across motor-behavior deficits.

Intervention Programs The ankle strengthening program consists of four isometric exercises targeting plantarflexion, dorsiflexion, inversion, and eversion. These muscle groups are directly implicated in maintaining ankle joint stability and controlling rear-foot positioning during dynamic activities.

The hip strengthening program includes clamshells at 60° of hip flexion, hip external rotation, hip abduction, and hip extension. These exercises engage the gluteus medius, gluteus maximus, and deep external rotators-muscles essential for controlling femoral positioning and mitigating excessive internal rotation or adduction during single-leg tasks.

The combined program alternates exercises from both regions, ensuring equal exposure to distal and proximal strengthening stimuli. Because CAI encompasses both local impairments and broader kinetic-chain dysfunctions, this hybrid approach may offer a more comprehensive therapeutic stimulus.

Each strengthening session includes three sets of each targeted movement, with six maximal 5-second isometric repetitions per set. The intensity target is 100% MVC, with short recovery periods between contractions. This structure ensures a high neuromuscular demand within a time-efficient session design.

Data Management and Analysis All participant data, including force outputs and functional performance results, are anonymized and stored securely. The dataset is analyzed using mixed factorial ANOVA to examine group effects, time effects, and Group × Time interactions. This statistical model is particularly suited for longitudinal interventional studies, as it evaluates whether improvements over time differ across treatment groups. Post-hoc testing is conducted when significant interactions arise, using Bonferroni adjustments to control for multiple comparisons.

This analytical approach helps determine whether the interventions produce distinct recovery trajectories and clarifies which program is most effective for improving various CAI-related outcomes. Because prior research often relied on unifocal strengthening strategies, this study may highlight the relative importance of targeting hip musculature versus ankle musculature or both.

Ethical Considerations All participants provide informed consent prior to enrollment and are informed of their right to withdraw at any time. Data are handled according to GDPR requirements and stored for 20 years in anonymized form. Only aggregated, non-identifiable results will be published.

Expected Impact The study has the potential to inform best-practice rehabilitation strategies for chronic ankle instability. Should the combined strengthening program demonstrate superior outcomes, it would support integrating proximal strengthening as a standard component of CAI rehabilitation. Conversely, if isolated hip or ankle strengthening proves equally effective, clinicians may adapt treatment plans based on patient tolerance, preference, or resource availability. Beyond clinical implications, the study contributes to the understanding of how neuromuscular training influences the diverse deficits encompassed by the CAI model.

Inclusion Criteria:

• Have a history of at least one major ankle sprain that occurred at least 12 months prior to enrolment in the study, was associated with inflammatory symptoms (pain, swelling, etc), created at least one interrupted day of desired physical activity.
• Have had at least two episodes of feeling like an "unstable" ankle in the six months preceding the study.
• The Cumberland Ankle Instability Tool (CAIT) with a score of less than 24.
• The Foot and Ankle Ability Measure (FAAM) Sport scale with a score below 80%.
• Between 18 and 35 years old
• Must be able to understand the study procedures and provide written informed consent

Exclusion Criteria:

• Have had a recent injury of less than three months prior to enrolment in the study.
• A history of surgery on the musculoskeletal structures of the lower limbs.
• Have a history of fracture of one of the lower limbs requiring realignment.
• Being diagnosed with a neuromuscular disease.
• The appearance of swelling or pain in the lower limb during the experiment.