Influence of effort intensity and gastrocnemius stretch on co-contraction and torque production in the healthy and paretic ankle.

OBJECTIVE: Spastic co-contraction is a misdirected supraspinal command in spastic paresis. We quantified the influence of effort and gastrocnemius stretch on plantar flexor co-contraction and torque during dorsiflexion efforts in hemiparetic and healthy subjects.
METHODS: Eighteen healthy and 18 hemiparetic subjects produced "light", "medium" and "maximal" isometric dorsi- and plantar flexion efforts in two gastrocnemius positions, stretched (knee extended) and slack (knee flexed), ankle at 90°. Measuring ankle torque and soleus and medial gastrocnemius surface EMG, we calculated the co-contraction index (CCI) as the ratio of the EMG root mean square (RMS) from the muscle acting as antagonist over its RMS when acting as agonist in a maximal effort, in each knee position.
RESULTS: Co-contraction was abnormally high in hemiparetic subjects at all effort levels, e.g. for soleus in the knee extended position (CCI(SO) 0.37±0.08 in hemiparesis vs 0.18±0.02 in healthy subjects, p<0.05). In hemiparetic subjects knee extended, dorsiflexion torque, (i) was reversed or canceled in 26% trials; and (ii) correlated negatively with plantar flexor CCI. SIGNIFICANCE: Major dynamometric impact of co-contraction with stretched position of the cocontracting muscle may justify muscle length modifications (e.g. through aggressive stretch programs) to improve function in spastic paresis.