It is unclear why humans typically swing their arms during gait. To date, the debate on how to arm swing comes about (i.e. whether it is caused by accelerations of the shoulder girdle or muscular activity) is still going on. There needs to be consensus on whether the arm swing is actively controlled or merely passive and on why humans swing their arms during walking (i.e. what the purpose of arm swing is, if any). Suggested reasons include minimising energy consumption, optimising stability, and optimising neural control. Pathologies such as hemiplegia after stroke, Parkinson's disease, Cerebral Palsy, Spinal Cord Injury, and Multiple Sclerosis may directly affect arm swing during gait. Emerging evidence indicates that including arm movements in gait rehabilitation may be beneficial in restoring interlimb coordination and decreasing energy expenditure.

This project hypothesises that the arms swing, at least at low and intermediate walking speeds, reflects the body's Center of Mass (CoM) accelerations. Arm swing may thus depend mainly upon the system's intrinsic mechanical properties (e.g., gravity and inertia). In this perspective, the CoM is seen as moving relative to the upper limbs rather than the other way around. The contribution of major lower limb joints, in terms of power injected into the body motion, will be simultaneously explored.

The study aims to investigate the mechanism and functions of arm swinging during walking on a force treadmill. To simulate asymmetric walking, healthy subjects will be asked to walk with a toes-up orthosis to induce claudication and asymmetry in ankle power. In this way, it will be possible to highlight the correlation among arm swinging, ankle power, and the acceleration of the CoM in a 3D framework. In addition, subjects affected by unilateral motor impairments will be asked to walk on the force treadmill to test the experimental model and highlight significant differences in the kinematic parameters of the upper limbs.

The question of whether arm swing is actively controlled or merely passive and the relationship between arm swinging and the total mechanical energy of the CoM will be faced.

Asymmetric oscillations of the upper limb will be related to dynamic asymmetries of the COM motion, and of the motion of lower limbs. In addition, cause-effect relationships will be hypothesized. Finally, the dynamic correlates of upper limb oscillations will make the clinical observation an interpretable clinical sign applicable to rehabilitation medicine.

Results from the present study will also foster the identification of practical rehabilitation exercises on gait asymmetries in many human nervous diseases.

Inclusion Criteria:

• presence of claudication (spatiotemporal asymmetry between subsequent steps), at visual inspection;
• unilateral motor impairments of one lower limb as a consequence of various pathologic conditions, such as (not not limited to): poststroke hemiparesis (ischemic or hemorrhagic), Parkinson's disease, multiple sclerosis, unilateral amputation with prosthetic correction, surgical orthopedic interventions;
• ability to walk for at least 100 meters without support; prostheses or orthoses admitted.
• ability to wittingly sign the informed consent form

Exclusion Criteria:

• drug therapy underway up to three months before recruitment, with impact on balance and gait;
• systemic pathologies or other sensory or neurological pathologies with impact on balance and gait;
• Mini Mental State (MMSE) score < 24/30;
• alterations in the passive mobility of upper limbs;
• painful syndrome which could alter the locomotion;
• pregnancy

The subjects will be recruited in the proponent Unit on an outpatient basis, from a physiatric referral or from a local inpatient rehabilitation unit.