Startling stimuli increase maximal motor unit discharge rate and rate of force development in humans

Abstract

Maximal rate of force development in adult humans is determined by the maximal motor unit discharge rate, however the origin of the underlying synaptic inputs remains unclear. Here, we tested a hypothesis that the maximal motor unit discharge rate will increase in response to a startling cue, a stimulus that purportedly activates the pontomedullary reticular formation neurons that make mono- and disynaptic connections to motoneurons via fast-conducting axons. Twenty-two men were required to produce isometric knee extensor forces "as fast and as hard" as possible from rest to 75% of maximal voluntary force, in response to visual (VC), visual-auditory (VAC; 80 dB), or visual-startling cue (VSC; 110 dB). Motoneuron activity was estimated via decomposition of high-density surface electromyogram recordings over the vastus lateralis and medialis muscles. Reaction time was significantly shorter in response to VSC compared to VAC and VC. The VSC further elicited faster neuromechanical responses including a greater number of discharges per motor unit per second and greater maximal rate of force development, with no differences between VAC and VC. We provide evidence, for the first time, that the synaptic input to motoneurons increases in response to a startling cue, suggesting a contribution of subcortical pathways to maximal motoneuron output in humans.

Identifying the role of the reticulospinal tract for strength and motor recovery: A scoping review of nonhuman and human studies

“…The findings in two experimental animals and eight human studies Anzak, Tan, Pogosyan, Djamshidian, et al, 2011;Colomer-Poveda et al, 2023;Fernandez-Del-Olmo et al, 2014;Glover & Baker, 2020;Glover & Baker, 2022;Li et al, 2017;Maitland & Baker, 2021;Škarabot et al, 2022;Xu et al, 2017) suggest that RST activity is important for the expression of strength or maximum force production. For example, four studies on healthy adults Colomer-Poveda et al, 2023;Fernandez-Del-Olmo et al, 2014;Škarabot et al, 2022) revealed that startle stimuli, which activates the RST, resulted in a shorter reaction time, increased motor discharge per motor unit per second (maximum motor output), increased rate of force development and greater force production, when compared to the visual acoustic and visual only stimuli. Moreover, a cross-sectional study on healthy humans (Maitland & Baker, 2021) showed that older adults with better strength had greater RST connectivity than weaker, older adults.…”

Motor Unit Discharge Characteristics and Conduction Velocity of the Vastii Muscles in Long-Term Resistance-Trained Men

“…Participants were seated in a strength-testing chair with knee and hip flexed at 115° and 126° (180° = full extension), and tightly fastened with straps across pelvis and shoulders to prevent extraneous movement. The participants’ dominant leg was strapped (35-mm wide reinforced canvas webbing strap) to a metal brace positioned at ~15% of tibial length (measured between lateral malleolus, i.e., 0%, and knee joint center), in series with the strain gauge that was positioned posterior and perpendicular to the tibia (for graphical representation of the setup, please see the sketch and image in (24) and (25), respectively). The strength-testing chair settings in the present study were selected to maximize the validity of testing knee extensor forces: the selected knee joint angle has been previously shown to correspond to the knee joint angle at which maximal knee extensor forces are produced (26), and the custom-made strength-testing chair used in this study has been shown to minimize joint angle changes during maximal efforts (≤4° compared with up to 20° changes with some commercial dynamometers (27)).…”

Neuromechanics of The Rate of Force Development

“…The underlying factors that determine this divergent behavior are unknown, but this could be attributed to distinct neural modules transmitted to motor neurons by brainstem or cortical pathways. There is evidence that an auditory startling cue might increase the discharge rate of a subset motor neurons that show a relatively low initial discharge rate during rapid contractions (21), suggesting that different supraspinal inputs may mediate the neural modules observed in these two clusters. This indicates that the input may be a suprathreshold for some, but not all, motor neurons.…”

“…Each dot represents one subject. The ankle-dorsiflexion force data (13), the elbow flexion(20), and knee extensors(21) was analyzed with the same methods and collected in two different laboratories (University of Rome "Foro Italico" and Loughborough University, the references(13,20,21) contains the full details of the anthropometrical characteristics of the subjects and their physical activity levels).…”