Muscle fatigue is a multifactorial physiological phenomenon characterized by a reduction in the ability of a muscle to generate force during sustained or repetitive contractions. This cross-sectional observational study aimed to investigate the relationships among muscle thickness, surface electromyography (sEMG) median frequency, accelerometer-derived mechanical vibration parameters, and isometric force characteristics in healthy male soccer players. Thirty participants aged 18-30 years underwent ultrasonographic muscle thickness assessment, synchronized sEMG and accelerometer recordings, and isometric strength testing at different knee flexion angles (90°, 60°, and 30°). The study evaluated the interaction between morphological, electrical, and mechanical indicators of muscle fatigue to support the development of multidimensional fatigue monitoring approaches.

Muscle fatigue is commonly assessed using surface electromyography (sEMG), particularly through frequency-domain parameters such as median frequency. However, fatigue-related changes are not limited to electrical activity and may also involve alterations in muscle mechanical behavior and morphological characteristics. Accelerometer-derived signals provide information regarding muscle vibrations and mechanical oscillations, while ultrasonographic measurements of muscle thickness provide information about muscle morphology and force-generating capacity.

The purpose of this study was to examine the relationships among muscle thickness, sEMG median frequency, accelerometer-derived mechanical vibration parameters, and isometric force variables during maximal voluntary isometric contractions performed at different knee flexion angles. Healthy male soccer players participated in a laboratory-based assessment protocol including ultrasonography, synchronized sEMG recordings, triaxial accelerometry, and isokinetic dynamometry.

Muscle thickness of the vastus lateralis obliquus (VLO), vastus medialis obliquus (VMO), and rectus femoris obliquus (RFO) muscles was measured using ultrasonography. Participants subsequently performed maximal voluntary isometric contractions at 90°, 60°, and 30° knee flexion angles. Surface electromyography, accelerometer signals, and force-related variables were recorded simultaneously. Median frequency values, acceleration-based root mean square (RMS) parameters, peak torque, mean torque, and time-to-peak torque variables were analyzed.

The study aimed to provide a multidimensional framework for muscle fatigue assessment by integrating morphological, electrical, and mechanical indicators and to explore their associations in healthy athletic populations.

Inclusion Criteria:

• Healthy male soccer players aged 18-30 years
• No lower extremity injury within the previous six months
• No condition limiting lower extremity exercise performance
• No caffeine or stimulant consumption on the testing day
• Non-smoker
• Provided written informed consent

Exclusion Criteria:

• Female athletes
• Serious injury to the dominant leg within the previous year
• Intense exercise within the previous 24 hours
• Use of pain relievers or muscle relaxants on the testing day
• Inability to complete testing procedures

Only male participants were eligible for inclusion in this study. The study was designed to evaluate muscle fatigue, muscle thickness, electromyographic, and accelerometer-derived mechanical parameters in a homogeneous sample of healthy male soccer players aged 18 to 30 years.

Healthy male soccer players aged 18 to 30 years who were actively participating in regular football training and competition. Participants were recruited from football teams and underwent assessments of muscle thickness, surface electromyography (sEMG), accelerometer-derived mechanical vibration parameters, and isometric strength during laboratory-based testing sessions. Only individuals meeting all eligibility criteria and providing informed consent were included in the study.

Individual participant data will not be publicly shared. The study was conducted as part of a doctoral thesis, and no formal data-sharing plan has been established. Aggregate study results are reported in publications, while access to underlying data may be considered by the corresponding author on a reasonable request basis and in accordance with applicable ethical and institutional requirements.