Estimating descending activation patterns from EMG in fast and slow movements using a model of the stretch reflex

Due to spinal reflex loops, descending activation from the brain is not the only source of muscle activation that ultimately generates movement. This study directly estimates descending activation patterns from measured patterns of muscle activation (electromyographic; EMG) during human arm movements. A simple model of the spinal stretch reflex is calibrated in a postural unloading task and then used to estimate descending activation patterns from muscle EMG patterns and kinematics during voluntary arm motion performed at different speeds. We observed three key features of the estimated descending activation patterns: <i>1</i>) Within about the first 15% of movement duration, descending and muscle activations are temporally aligned. Thereafter, they diverge and develop qualitatively different temporal profiles. <i>2</i>) The time course of descending activation is monotonic for slow movements, nonmonotonic for fast movements. <i>3</i>) Varying model parameters such as the spinal reflex gain or the level of cocontraction do not qualitatively change the temporal pattern of estimated descending activation. Our findings highlight the substantial contribution of spinal reflex loops to movement generation, while at the same time providing evidence that the brain must generate qualitatively different descending activation patterns for movements that vary in their mechanical dynamics.<b>NEW & NOTEWORTHY</b> We propose a new method that directly estimates descending activation from measured electromyographic (EMG) signals and arm kinematics by inverting a model of the spinal stretch reflex, without the need for muscle models or an arm dynamics model. This approach identifies key features of the time structure of descending activation as movement speed is varied, while also revealing the significant contribution of the spinal stretch reflex to movement generation.