Ankle vs. Hip vs. Combined Strengthening in Chronic Ankle Instability (SHAC)

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.