Muscles, paretics and spastics, of cerebral palsy children have very early, for growth (three at five aged), every alteration of theirs properties. Gastrocnemius muscle, often reaches on the cerebral palsy, characterised by a modification of the structure (volume decrease) and the elasticity (increase of the passive stiffness). Every alterations of the muscular property have impact clinic and functional important. The many treatments of orthopaedic targeting (lengthening of cast, injections of botulinic toxin), repeated to a multiple recovery during the growth influences the muscular characteristics. There is a real interest to measure the muscular property development during children's growth.

Elastography is a technical assessment of characteristic of the soft tissues. In this area, the method " Supersonic Shear Imaging® (SSI) " provide an important technological change measuring, swiftly and no invasive, a elasticity modulus reflecting the property visco-elastic of the muscle. A lot of ex vivo study led in isolated muscle and in vivo in healthy adults demonstrated the validity of the method SSI on the evaluation of the passive stiffness from a number of muscle group whose the gastrocnemius muscle. The main objective of this study must be checked the intersession reproducibility of the method SSI on the evaluation of the passive stiffness at the medial bundle of the gastrocnemius muscle therefore their passive lengthening in cerebral palsy children. The secondary objective is to test the correlation level of the relation elasticity modulus/ joint angle and couple force-angle obtained throughout the passive lengthening at the medial bundle of the gastrocnemius muscle in cerebral palsy children.

Inclusion Criteria:

• Parents affiliated or entitled to a social security;
• Children diagnosed spastic cerebral paralyzed, with a clinical picture of hemiplegia infantile cerebral;
• Children walking GMFCS (Gross Motor Functional Classification System) between I and III;
• Children with spasticity level in the paralysing gastrocnemius muscle greater than or equal X1, VII, on the scale of Tardieu and 2 on the Ashworth scale.
• Children aged 5 to 12 years.
• Children can understand and follow the simple instructions in the examination process.
• Consent signed by the holder of parental authority.

Exclusion Criteria:

• Children with concomitant muscle disease (myopathy ...).
• Children who received another treatment to target relaxant (baclofen, ... muscle relaxant) which would have begun in the months preceding the assessment.
• Children who underwent neuro-orthopedic surgery at the where paretic members.
• Children who received an injection of botulinic toxin for less than 15 days.
• Children who have benefited from extensions casts for less than 7 days.
• Children who received a therapeutic target for orthopedic (botulinum toxin, plaster of extensions, installation of equipment of lower limb) between the two sessions evaluations.
• Children who received a physiotherapy session the day before evaluation elastography and ultrasound.