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.


CONCLUSIONS
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.
Arts et Métiers ParisTech, LBM, 151 bd de l'Hôpital, 75013 Paris, France. maria.vinti@neuf.fr
Maria
M
Vinti
Annabelle
A
Couillandre
Jérôme
J
Hausselle
Nicolas
N
Bayle
Aldo
A
Primerano
Andrea
A
Merlo
Emilie
E
Hutin
Jean-Michel
JM
Gracies
D000328:Adult; D000368:Aged; D000842:Ankle / Q000502:physiology*; D000843:Ankle Joint / Q000503:physiopathology*; D001696:Biomechanical Phenomena / Q000502:physiology; D005260:Female; D006801:Humans; D007719:Knee Joint / Q000502:physiology; D008297:Male; D008875:Middle Aged; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000503:physiopathology*; D010291:Paresis / Q000503:physiopathology*; D019415:Torque