We propose to carry out a single center, prospective, interventional trial to assess the natural evolution of lower limb muscle structure and muscle stiffness of hemiparetic patients following brain damage. Patients will be assessed two weeks, one month, three months and six months after their brain lesion.

Secondary objectives:

To compare the muscle stiffness and structure of patients who will develop muscle overactivity to patients who won't develop muscle overactivity.

To compare the muscle structure and muscle stiffness of the spastic lower limb muscles and the non-spastic lower limb muscles in the affected limb.

To compare the muscle structure and muscle stiffness of the affected and the non-affected lower limb.

To study the relationship between impairment and function by applying a comprehensive and original method to assess all aspects from muscle structure and mechanics to patient's functional capacities.

Inclusion Criteria:

• Female or male ≥ 18 years and ≤ 80 years at screening
• Hemiparesis secondary to a first stroke or traumatic brain injury
• < 1 month post-stroke/brain injury Subjects must meet all of the inclusion criteria in order to be eligible to participate in the study.

Exclusion Criteria:

• Bilateral limb paresis
• Major cognitive impairments limiting the functional evaluation
• Neurological or orthopedic affection interfering with the paretic limbs function
• Underlying neuromuscular disease (e.g. myasthenia gravis, Lambert-Eaton syndrome, amyotrophic lateral sclerosis)
• Contraindication to perform MRI Subjects meeting any of the exclusion criteria at baseline will be excluded from study participation.