Relation Between Cortical Activation and Graded Force Level During Robot-assistance Walking in Healthy People : A Functional Near-infrared Spectroscopy Neuroimaging Study.

An important goal of motor systems neuroscience is to characterize how neural activity in the brain mediates movement parameters such as force, velocity, frequency of movement or movement direction. The neural codage of force has been studied in animal and human with TEP , fMRI , functional near-infrared spectroscopy (fNIRS), EEG or magnetic stimulation.

Electrophysiological studies in nonhuman primates demonstrated a correlation between neuronal discharge rates in multiple regions of the contralateral motor cortex and exerted force amplitude .

In humans, functional magnetic resonance imaging (fMRI) studies confirmed the relation between increasing neuronal activation and increasing amplitude of force in the contralateral primary motor/somatosensory (M1/S1) cortices, supplementary motor area (SMA), and premotor cortex. The ipsilateral motor cortex could also contribute to the force codage. There are also evidences that the basal ganglia-thalamo-cortical loop participates to the regulation of force control. The internal portion of the globus pallidus (GPi) and subthalamic nucleus (STN) had a positive increase in percent signal change with increasing force, and the ventral thalamic regions were also implied in the the same way. More recently, studies with fNIRS confirmed the relationship between force level and cerebral activation in contralateral and ipsilateral hemisphere.

Most of the human studies concerned isometric static tasks. Only few studies considered dynamic movements. As a rule, the processing of repetitive transient force changes requires more metabolic activity than the generation and control of a static force. There is a correlation between the force level and cortical changes within the neuronal network in contralateral M1 and anterior cerebellum.

Most of the studies concern the upper limb. To our knowledge only to studied concern the lower limb in an isometric force level. One concerns the neural correlates of quadriceps torque control in chronic obstructive pulmonary disease patients and the other one concerns isometric contractions with the ankle dorsiflexor in healthy controls.