This study aims to investigate the acute effects of fatigue in the tibialis anterior and gastrocnemius muscles on joint position sense, balance, pain threshold, and muscle elasticity in healthy young adults. Muscle fatigue is known to directly affect physical performance and motor function; therefore, understanding its influence on neuromuscular parameters such as proprioception, postural control, and tissue properties is of great importance. By examining how these specific muscle groups respond to fatigue, the study seeks to clarify the role of fatigue in altering joint position sense, balance performance, pain perception, and muscle mechanical properties. The findings of this research are expected to provide new insights that may contribute to the development of more effective strategies in physical therapy and rehabilitation practices.

Study Aim and Rationale This study aims to investigate the acute effects of fatigue in the tibialis anterior and gastrocnemius muscles on joint position sense, dynamic balance, muscle elasticity, and pain threshold in healthy young adults. Muscle fatigue is known to directly impact motor function and physical performance; therefore, understanding its influence on neuromuscular parameters such as proprioception, postural control, and biomechanical properties is crucial for developing effective clinical strategies. By evaluating how these specific muscle groups respond to fatigue, the study seeks to generate evidence that can guide physiotherapy and rehabilitation practices.

Methodology A total of 35 healthy adults aged 18-24 who meet the inclusion criteria will participate. All procedures will be conducted at the Physiotherapy and Rehabilitation Laboratory of Istanbul Beykent University. Dominant lower limbs, determined by the foot used in a spontaneous ball-kicking task, will be used for testing. Participants will first undergo a tibialis anterior fatigue protocol, followed one week later by a gastrocnemius fatigue protocol. Each participant will be assessed at four time points: before the fatigue protocol, immediately after, 24 hours later, and 48 hours later.

Participants will be instructed to refrain from high-intensity exercise, stretching, massage, hot/cold applications, and medications for 48 hours prior to testing. These instructions will be provided both verbally and in writing.

Fatigue will be induced through repeated isometric contractions (30 seconds contraction, 15 seconds rest). For tibialis anterior, participants will lie supine and perform dorsiflexion against resistance at a fixed 100° ankle angle using a customized apparatus. For gastrocnemius, participants will lie prone with feet hanging off the table and perform plantarflexion against a wall. Fatigue threshold will be defined as a score ≥7 on the Modified Borg Scale.

Assessments will include:

Joint Position Sense (JPS): Measured using active angle reproduction at 10° dorsiflexion and 20° plantarflexion. The DrGoniometer app will record angular error, and absolute error values will be computed.

Dynamic Balance: Assessed with the Modified Star Excursion Balance Test (SEBT) in anterior, posteromedial, and posterolateral directions. Reach distances will be normalized to limb length, and composite scores will be calculated.

Muscle Elasticity: Evaluated using the MyotonPro device in multiscan mode with standardized settings. Measurements will be taken from the most prominent points of the tibialis anterior, medial gastrocnemius, and lateral gastrocnemius.

Pain Threshold: Measured using a pressure algometer device.

Fatigue Metrics: Captured with the ForceDecks dual force plate system (VALD Performance). The system will record jump height, concentric peak velocity, eccentric duration, braking phase duration, peak power, and RSI-mod index.

All assessments are non-invasive and conducted under standardized conditions. Rest periods will be provided between tests to prevent cumulative fatigue from affecting results.

Sample Size and Randomization Sample size was calculated using G*Power (α=0.05, power=95%) based on stiffness values of the medial gastrocnemius reported by Lall (2022). The required sample size was 29; to compensate for potential dropouts, 35 participants will be included. Simple randomization will be conducted from a list of 200 eligible individuals using randomizer.org.

Inclusion Criteria:

• Aged between 18 and 24 years
• Body Mass Index (BMI) between 18.5 and 24.9
• Low to moderate physical activity level
• No history of neurological or orthopedic disorders
• Full range of motion in the ankle joint
• Voluntary participation and informed consent provided

Exclusion Criteria:

• Does not meet the inclusion criteria
• History of trauma or surgery in the lower extremities
• Regular participation in exercise or being a professional athlete

This study does not plan to share individual participant data (IPD) due to ethical considerations and institutional policies regarding participant confidentiality.