A simulation study of the effects of activation-dependent muscle stiffness on proprioceptive feedback and short-latency reflex

A biologically-inspired model of the Soleus neuromuscular system was used to study the influences of musculotendon mechanical properties on the proprioceptive feedback and on the short-latency reflex. The model structure comprised: i) stochastic descending drive to a motoneuron pool; ii) a Hill-type muscle model; and iii) a muscle spindle model with its Ia sensory afferents establishing a feedback loop with the motoneurons. H-reflex and short-latency stretch reflex were simulated considering their possible dependence on musculotendon mechanical properties and on the muscle activation level. The main results showed an intrinsic relationship between the Ia response to muscle stretch and the relative stiffness between muscle and tendon. In addition, fusimotor activation and synaptic gain could modulate the short-latency reflex, indicating that these mechanisms may help to maintain reflexes functionally active during different motor tasks. These preliminary results match some experimental data and provide insights on the intricate relationship between muscular mechanics and neuronal activity. Moreover, the adopted model of the closed-loop neuromuscular system has shown its potential as a tool for studies of complex mechanisms involved in motor control.