Force variability is mostly not motor noise: Theoretical implications for motor control
Fig 1
Schematic representation of our new model of a motor unit pool.
The model consists of three modules. Module 1 converts synaptic input, Ueff, into spike trains of individual motor units. Module 2 turns spike trains into motor unit activation, A, through three-stage process shown below. Stage 1 simulates calcium kinetics driven by action potentials (R). The calcium kinetics is described using five states, [s], [cs], [c], [f] and [cf] with associated rate constants (k1, k2, k3 and k4) between those states. Stage 2 converts [cf] into the intermediate activation, , through a non-linear filter, which describes cooperativity and saturation of calcium binding and cross-bridge formation. Stage 3 introduces an additional first-order dynamics to generate motor unit activation, A, from
. Module 3 describes the contraction dynamics between muscle and a series elastic element and generates tendon force, Fse, as the output. The detail descriptions of each module are given in the text.