Distinct effects of visual gain manipulation on force variability and smoothness during constant isometric contractions

Increasing the visual gain of the force output during constant isometric contractions reduces the amplitude of force fluctuations in young adults. However, these findings are based on metrics of force variability, such as the standard deviation (SDF) or coefficient of variation (CVF) in force, ignoring force smoothness. Here, we examined the effects of increasing the gain of visual feedback on force variability and force smoothness during constant isometric contractions. Fourteen young adults (20.1 ± 1.4 yr; 10 F) performed ankle dorsiflexion at 10% maximum for 40 s with low-gain (LG; 0.1°) and high-gain (HG; 5.0°) visual feedback. We quantified force variability (SDF and CVF), force smoothness [d<i>F</i>/d<i>t</i>; first time-derivative of force (YANK)], and power spectral density of the force and tibialis anterior (TA) muscle activity [electromyogram (EMG)]. Participants exhibited lower SDF and CVF (lower variability), but greater YANK (lower smoothness) during the HG condition. The reduction in SDF with HG was associated with a reduction in the power of 0-0.5 Hz force oscillations (<i>R</i>² = 0.82) and a reduction in the power of 1.5-2 Hz TA EMG oscillations (<i>R</i>² = 0.29). The increase in YANK with HG was associated with an increase in the power of 7-8 Hz force oscillations (<i>R</i>² = 0.82) and an increase in the power of 35-60 Hz TA EMG oscillations (<i>R</i>² = 0.25). These findings suggest that force variability and force smoothness are distinct concepts, reflecting separate physiological processes influenced by visual gain manipulation.<b>NEW & NOTEWORTHY</b> These findings suggest that force variability and force smoothness are distinct motor control features influenced differently by visual gain manipulation. Although variability reflects fluctuations in force amplitude, smoothness captures the consistency of force transitions. The dissociation between these measures indicates that they are governed by separate physiological mechanisms. This distinction has important implications for understanding sensorimotor integration and designing targeted interventions to improve motor performance in tasks requiring precise and sustained force control.