Analysing The Impact of Foot Arch Height on Leg Muscle Tone and Balance in Young Adults.

Foot posture abnormalities due to high or low arches, such as pes planus (flatfoot) and pes cavus (high-arched foot), significantly impact daily activities, particularly among young adults engaged in prolonged standing, walking, or physical activities. Pes planus is commonly associated with ligamentous laxity, muscle weakness, or genetic predisposition, leading to diminished medial, lateral, and transverse arches, resulting in excessive pronation, instability, and compromised shock absorption. This has been linked to knee pain and altered lower extremity alignment, and individuals with pes planus often experience foot pain, fatigue, and reduced physical performance.

Conversely, pes cavus is characterized by an exaggerated medial and lateral arch and diminished transverse arch, often linked to neuromuscular disorders, tight plantar fascia, or congenital factors. This condition leads to reduced shock absorption and uneven weight distribution, increasing the risk of recurrent ankle sprains, calluses, and metatarsalgia. Individuals with pes cavus frequently encounter discomfort during weight-bearing tasks and difficulties maintaining balance, which can hinder participation in physical and social activities.

Pes planus is linked to excessive pronation, while pes cavus is associated with excessive supination and rigidity, limiting adaptive responses to uneven surfaces. Both conditions have been associated with deficits in static and dynamic balance due to altered foot and lower limb alignment, leading to postural instability and increased sway during quiet standing and movement tasks. These balance deficits highlight the need for comprehensive assessments to identify individuals at greater risk of mobility impairments.

Lower extremity muscle tone plays a critical role in postural stability and overall functional movement, contributing to joint stabilization, shock absorption, and postural control during both static and dynamic activities. Individuals with lower muscle tone and stiffness have been shown to exhibit greater postural sway, indicating reduced postural control. Myotonometry provides reliable, repeatable, and non-invasive measurement of muscle tone, stiffness, and elasticity. Muscle strength and power also play a crucial role in foot function, balance, and postural stability by providing the necessary force for efficient movement and weight-bearing. Deficiencies in muscle power can lead to compensatory movement patterns that increase the risk of falls and injuries, particularly in individuals with foot posture abnormalities.

Prior research has examined dynamic balance, static balance, and muscle tone across different foot arch types separately, but no study has integrated foot posture, static and dynamic balance, muscle tone, and muscle strength within a single framework. This study addresses that gap by evaluating these factors together to provide a more comprehensive understanding of how foot posture abnormalities influence postural stability across genders.

Aim of Study

This study aims to examine how different foot arch types (pes planus, pes cavus, and normal arch) affect lower extremity muscle tone, muscle strength, and static and dynamic balance, in order to better understand their impact on postural stability and movement efficiency. The primary goals are to examine the relationship between foot posture abnormalities and muscle tone, balance performance, and strength in young adults; to identify differences in these factors based on gender; and to determine the functional impact of foot posture on stability, movement efficiency, and injury risk.

Hypotheses

The null hypothesis states that foot posture (pes planus, pes cavus, and normal arch) has no significant effect on static and dynamic balance performance, lower limb muscle tone, or muscle strength. The alternative hypothesis states that foot posture significantly affects static and dynamic balance performance, lower limb muscle tone, and muscle strength, with variations in arch structure influencing postural stability and muscle function, and that this effect differs between male and female participants.

Overview of Procedures

Participants will be classified into one of three foot arch groups using the Arch Height Index and confirmed with the Foot Posture Index-6. Each participant will complete a sociodemographic questionnaire and anthropometric measurements, followed by static balance assessment (Single Leg Stance), dynamic balance assessment (Star Excursion Balance Test and Single-Leg Hop for Distance), muscle tone assessment using a myometer, and muscle strength assessment using a handheld dynamometer. Assessments will be conducted in a standardized order for all participants to minimize order-related bias. Detailed eligibility thresholds and outcome measure definitions are provided in the corresponding registration modules.

• Arch Height Index (Diagnostic test): The goal of this assessment is to objectively classify foot posture as low-arched, high-arched, or normal using the Arch Height Index (AHI). Participants will sit with hips and knees at 90°, feet lightly weight-bearing (\~10%), and measurements will be taken with a caliper and paper. AHI is calculated as arch height at 50% of total foot length divided by truncated foot length. Feet with a ratio ≥ 0.356 are classified as high-arched, ≤ 0.275 as low-arched, and 0.276-0.355 as normal. Both feet will be assessed, focusing on the medial longitudinal arch. AHI demonstrates excellent inter-rater and test-retest reliability (ICC = 0.98-1.00).
• Foot Posture Index (Diagnostic test): The participant will be asked to stand still with arms along the side, looking forward and their feet will be assessed according to the index components while being observed by the physiotherapist. Scores between 0 and+5 indicate normal feet; +6 to+9 indicate pronated feet; ≥ +10 indicate highly pronated feet;-1 to-4 indicate supinated feet;-5 to-12 indicate highly supinated feet. The goal of this assessment is to help classifying the participants' foot arch. High inter-rater and test-retest reliability for FPI scoring (ICC values of 0.923 and 0.931).
• Single Leg Stance (Diagnostic test): Static balance will be assessed using the Single Leg Stance (SLS) test. Participants will stand barefoot on one leg with arms along the sides and the non-stance foot lifted clear of the ground. Timing begins when the foot is lifted and ends when balance is lost, defined as placement of the lifted foot, movement of the stance foot, or grasping for support. Each leg will be assessed for a maximum of 30 seconds over three trials, and the mean value will be used for analysis. Longer durations indicate better static postural control. The test will be performed under eyes-open conditions. The SLS test has demonstrated good to excellent test-retest reliability in healthy adults, with ICC values ranging from 0.79 to 0.95, and has shown acceptable reliability and predictive validity for balance assessment in clinical settings.
• Star Excrusion Blance Test (SEBT) (Diagnostic test): Participants will perform the Star Excursion Balance Test (SEBT) barefoot, standing on one leg at the center of a star-shaped layout with 8 directions: ANT, AM, M, PM, P, PL, L, and AL, each at 45° angles. With hands on hips to reduce arm compensation, they will reach in each direction using the opposite leg without shifting weight or losing balance. Reach distance will be marked and trials repeated if form is compromised. Three trials per leg will be completed with 2-minute rests to reduce fatigue. Reach distances will be normalized to leg length (ASIS to medial or lateral malleolus) to ensure fair comparison. The SEBT assesses dynamic balance and has shown good reliability (ICC = 0.75-0.9).
• Single-Leg Hop for Distance (Diagnostic test): The participant will be asked to assume a single-leg stance behind a marked starting line and will be instructed to hop forward as far as possible using a single explosive movement. They must land on the same leg while maintaining stability for at least 2-3 seconds. If the participant loses balance, touches the ground with the other foot, or falls forward, the attempt is not counted. Each participant will perform 3 trials per leg, with a rest period of 30-60 seconds between trials. The longest valid hop distance is recorded for each leg to calculate Leg Symmetry Index. The goal of single-leg hop for distance is to assess the dynamic balance, and it has excellent ICC values for test-retest reliability exceeded 0.86.
• Myometer Device (Diagnostic test): Muscle tone, stiffness, and elasticity will be measured using the MyotonPRO device, a non-invasive tool for evaluating muscle biomechanical properties. Participants will assume standardized positions, and measurements will be taken from the broadest cross-sectional area of the following muscles: M. Peroneus Longus, M. Tibialis Anterior, M. Gastrocnemius, Quadriceps (Vastus Medialis, Rectus Femoris, Vastus Lateralis), and Hamstrings (Biceps Femoris, Semitendinosus). The probe will be placed perpendicular to the skin. Five parameters will be recorded: frequency, stiffness, decrement, mechanical stress relaxation time, and creep. The method demonstrates high intra-rater (ICC = 0.63-0.99) and inter-rater (ICC = 0.63-0.97) reliability for lower limb muscles.
• Dynamometer (Diagnostic test): Muscle strength will be assessed using a handheld dynamometer (HOGGAN MICROFET2), following the manual muscle testing protocols outlined by Kendall. Target muscles include: M. Peroneus Longus, M. Tibialis Anterior, M. Gastrocnemius, Quadriceps, Hamstrings, M. Extensor Hallucis Longus, and Toe Flexors (M. Flexor Hallucis Brevis and M. Flexor Digitorum Brevis). Participants will perform isolated muscle actions against the device at standardized joint angles. Each test will be repeated for 3 trials, with average values recorded. This method effectively detects muscle imbalances and strength changes, and shows high reliability (ICCs often > 0.9), strong validity versus isokinetic dynamometers, and an MCIT of \~0.58-17.2 N.
• Height (Other): Standing height was measured using a flexible metal measure to the nearest 0.1 cm
• Weight (Other): Body weight was measured using a calibrated digital scale to the nearest 0.1 kg.
• Body Mass Index (Other): Body mass index (BMI) was calculated as weight (kg) divided by height squared (m²) .

Primary outcomes

• Lower Extremity Muscle Tone [At baseline (single session)]: Muscle tone of the lower extremity will be measured using the MyotonPRO device, which provides non-invasive assessment of muscle biomechanical properties including tone, stiffness, and elasticity. Target muscles include the quadriceps, hamstrings, tibialis anterior, gastrocnemius, and peroneus longus.
• Lower Extremity Muscle Strength [At baseline (single session)]: Muscle strength will be assessed using a handheld dynamometer (MicroFET2) following Kendall's manual muscle testing positions. Participants will perform isolated maximal effort contractions for key lower limb muscles including the quadriceps, hamstrings, gastrocnemius, tibialis anterior, and toe flexors. The average value across 3 trials will be recorded in KgF.
• Single Leg Stance (Static Balance) [At baseline (single session)]: The Single Leg Stance (SLS) test assesses static balance by timing how long a person can stand on one leg (in seconds) without support or losing position. Arms are kept by the sides or on the hips, the non-stance leg is lifted, and the trial ends when the lifted foot touches down, the stance foot moves, or support is needed. It is quick, low-cost, and widely used in clinical trials to quantify balance and fall risk.
• Star Excursion Balance Test (Dynamic Balance) [At baseline (single session)]: The Star Excursion Balance Test is a dynamic balance assessment in where participant stands on one leg at the center of an 8-direction star and reaches as far as possible (measured in centimeters) along each line with the opposite foot while maintaining control and not shifting weight onto the reaching leg. Reach distances are usually normalized to leg length and averaged over multiple trials.
• Height [At baseline (single session)]: Standing height was measured using a flexible metal measure to the nearest 0.1cm
• Weight [At baseline (single session)]: Standing weight was measured using a calibrated digital scale to the nearest 0.1 kg
• Body Mass Index (BMI) [At baseline (single session)]: Weight in kilograms and height in meters will be combined to report BMI in kg/m\^2.

Secondary outcomes

• Effect of Gender on the outcomes [At baseline (single session)]: Differences in muscle tone, strength (KgF), static balance (s), and dynamic balance (cm) will be compared between male and female participants across all foot posture groups.
• Leg Symmetry Index [At baseline (single session)]: The symmetry of dynamic balance between dominant and non-dominant legs will be evaluated using the Single-Leg Hop for Distance to determine leg functional imbalances.

Sex: All, ages: 18 Years to 25 Years, accepts healthy volunteers

Inclusion Criteria:

* Aged 18-25 years old-
* Not diagnosed with other medical conditions, fractures, or deformities
* No prior foot or lower limb surgeries affecting musculoskeletal health
* Wears sport shoes majority of the time

Exclusion Criteria:

* Neurological Disorders: Conditions affecting balance or muscle tone
* Balance Disorders like vestibular or inner ear conditions
* Women that wears high heels majority of the time
* Men that wear stiff and hard material shoes majority of the time

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• Prof. Dr. Habibe Serap İNAL, Doctor of Physical Therapy, Principal investigator, Istinye University, Faculty of Health Sciences, Department of Physiotherapy and Rehabilitation