Stretch-sensitive paresis and effort perception in hemiparesis

Area of Science:

• Neuroscience
• Motor Control
• Rehabilitation Science

Background:

• Spastic paresis involves inappropriate muscle co-contraction during voluntary movement.
• The effect of antagonist muscle stretch on agonist recruitment and effort perception in spastic paresis is not well understood.

Purpose of the Study:

• To investigate how antagonist muscle stretch affects agonist recruitment and effort perception in individuals with hemiparesis.
• To determine if these effects differ between healthy individuals and those with hemiparesis.

Main Methods:

• Quantified voluntary activation of ankle dorsiflexors, effort perception, and plantar flexor co-contraction during graded dorsiflexion efforts.
• Measured Root Mean Square EMG and Agonist Recruitment/Co-contraction Indices (ARI/CCI) in tibialis anterior, soleus, and medial gastrocnemius.
• Subjects performed isometric dorsiflexion efforts with varying gastrocnemius muscle lengths.

Main Results:

• In hemiparetic subjects, gastrocnemius stretch significantly reduced tibialis anterior voluntary activation (up to 98%) and increased perceived effort.
• These changes were not observed in healthy subjects.
• Effort perception in hemiparetic individuals depended on both agonist recruitment and antagonist co-contraction, unlike in healthy subjects.

Conclusions:

• Antagonist muscle stretch impairs agonist motoneuron recruitment in hemiparesis, a phenomenon termed 'stretch-sensitive paresis'.
• Spastic co-contraction contributes to increased effort perception in hemiparesis.
• These findings suggest that targeting spastic co-contraction may be beneficial for improving motor function and reducing perceived effort in individuals with hemiparesis.