Coupling between single muscle spindle afferent and EMG in human wrist extensor muscles: physiological evidence of skeletofusimotor (beta) innervation

Objectives: This study was conducted to find physiological evidence of skeletofusimotor innervation in man. Methods: Discharges of 38 single muscle spindle afferents from m. extensor carpi radialis were recorded from 9 subjects during steady isometric contractions of wrist extension. Correlation between these afferents and rectified surface EMG was investigated by estimating cumulant density function. Results: In the cumulant density estimate between spindle afferent and EMG, a positive EMG peak was obtained prior to afferent firing between −30 and −10 ms in 15 afferents (39%). Conclusions: The present finding indicates coupling between spindle afferents and extrafusal activity and suggests the widespread skeletofusimotor innervation in human muscle spindle.