Muscle Fiber Recruitment Characteristics in Trained Older Adults: An EMG Frequency Analysis During Voluntary Contraction.

Background: Age-related declines in skeletal muscle quality and quantity contribute to frailty and sarcopenia, leading to physical dysfunction in older adults. In particular, the selective atrophy of fast-twitch fibers is closely linked to reduced lower limb strength and increased fall risk. This study aimed to evaluate the effects of habitual body weight resistance training on muscle fiber recruitment patterns during maximal voluntary contraction (MVC) and squatting movements using spectral characteristics of surface electromyography (EMG). Methods: Fifty healthy community-dwelling older adults (28 males, 22 females; aged 64-84 years) were categorized into exercise and non-exercise groups based on training habits. Surface EMG signals were recorded from the rectus femoris muscle during MVC, normal squat, half-squat, and squat movements. Power spectral density (PSD) was analyzed using Welch's method and divided into frequency bands: type I (20-60 Hz), type IIa(1) (60-115 Hz), type IIa(2) (115-170 Hz), and type IIx (170-350 Hz). Results: While statistical significance was limited, the exercise group tended to show higher ratio PSDs in type IIa and IIx bands, and lower PSDs in the type I band during MVC. During half-squats, the non-exercise group exhibited higher contributions above 60 Hz, suggesting inefficient fast-twitch fiber recruitment. Conclusions: Habitual body weight resistance training may influence muscle fiber recruitment patterns in older adults. EMG spectral analysis offers a non-invasive means to detect age- and training-related neuromuscular adaptations.