The investigators will use 3D freehand ultrasound to investigate changes in muscle morphology of the paretic m. rectus femoris and m. gastrocnemius medialis during the first 3 months after stroke. Additionally, 2D ultrasound and surface EMG will be synchronized to examine muscle activation and morphological changes resulting from this activation. Clinical tests will be conducted on the same test occasions to objectify lower limb impairment and gait recovery. As such, the investigators will explore the relationship between the muscle specific changes and functional recovery.

During this longitudinal study, patients who suffer their first-ever stroke will be included within the first week after their stroke (if the inclusion criteria are met). Thereafter, each participant will be assessed at four pre-determined time-points: week 1, 5, 8 and 12 after stroke.

On each of the four time-points, the investigators will assess muscle specific parameters of the affected m. rectus femoris (RF) and m. gastrocnemius medialis (GM), together with clinical tests to assess lower limb impairment (Fugl-Meyer Assessment for the lower limb and the Tardieu scale) and gait parameters (10-meter walk test). Specifically, muscle specific parameters will be investigated using 3D freehand ultrasound (3DfUS) and combined dynamic 2D US and surface electromyography (sEMG). Such assessments will help us characterize the electromechanical properties of these muscles, their time-related trajectories and the relationship with the recovery of gait and lower limb function after stroke. At the end of the investigation period, after 3 months, patients who can walk independently will perform a 3D gait analysis. As such, the relationship between changes in muscle specific parameters and the eventual gait quality at the end of the investigation period will be investigated.

Inclusion Criteria:

• Adults (≥ 18 years old)
• Diagnosed with a first-ever stroke (as defined by WHO)
• Stroke onset ≤ 1 week (± 3 days)
• Able to provide written or verbal informed consent at admission
• Presence of gait problems as a consequence of the stroke (FAC≤4)

Exclusion Criteria:

• Presence of other neurological or orthopedic problems present prior to, or not caused as a direct consequence of, the stroke leading to impaired gait.
• Modified Ranking scale pre-stroke > 1/6, meaning presence of slight disability before the stroke.
• Presence of severe comorbidities (e.g. osteoporosis, cardiovascular instability or chronic obstructive pulmonary disease).
• Cerebellar or bilateral stroke.
• Presence of severe deficits in communication, memory or understanding precluding informed consent.

Persons who suffer their first-ever stroke (ischaemic or hemorrhagic) will be investigated during the subacute phase after the stroke (i.e. first 3 months).