Ageing is accompanied by a cognitive decline altering gait and increasing the risk of falling. Physical activity could reduce this cognitive decline by an improvement in the oxygen supply to the brain. At the moment, the effect of physical activity on brain oxygenation during walking in elderly has never been described. Moreover, the increase in cognitive request during a dual task could lead to an increase in the risk of falling. In this context, the investigators of Orleans's hospital want to examine the effect of the physical activity level of elderly on their brain oxygenation during walking and during a dual task.

Moreover, Parkinson' patients could display an impairment of gait and cognitive function. Attentional task and dual task situation are particularly impaired in this pathology. This is why the investigators of Orleans's hospital want to also examine cerebral activation of these patients during walking and during a dual task.

The hemodynamic cerebral activation will be studied using the near infrared spectroscopy at the level of the prefrontal cortex. This non-invasive technic allows the evaluation of the regional oxyhemoglobin and deoxyhemoglobin variations induced by cerebral activation.

The main aim of the study is to determine the effect of the physical activity level on variations of the pre-frontal oxygenation during walking and during a dual task in elderly. The other aims are to study the effects of age, gender and the Parkinson' disease on variations of the pre-frontal oxygenation during walking and during a dual task in elderly.

For that, 160 participants (females and males) will be included and 8 groups will be constituted (woman and man non active young people, woman and man non active old people, woman and man active old people, woman and man patients with Parkinson's disease).

Every participants will performed a set of 6 tasks of 2 minutes. Each task will be separated by a 2 minutes resting period. The tasks will be a cognitive task, a control cognitive task, walking at 3 kilometers/hour, walking while doing the cognitive task, walking at the preferred speed, walking at the preferred speed while doing the cognitive task.

During the set, prefrontal oxygenation will be measured using the near infrared spectroscopy.

Inclusion Criteria:

• men and women non active young people and men and women non active old people : practice less than one hour of physical activity per week
• men and women active old people: practice at least 3 hours of physical activity per week

Non-Inclusion Criteria :

For all subjects:

• Resting electrocardiographic abnormality, Systolic blood pressure >16
• Heart disease, recent and unusual shortness of breath or needing several pillows to sleep at night, effort/rest chest pain, palpitations with effort, history of discomfort with effort
• Respiratory failure
• Disabling rheumatological pathology for walking
• Neuro-progressive disease or sequelae
• Perceptual disorders
• Medication (neuroleptics, psychotropic treatment modifying alertness) potentially having an impact on walking
• Thickness of the skin fold at the vast lateral area greater than 15 mm
• Inability of the volunteer to perform the Random Generation of Numbers task for at least 1 min or frequency of enunciation numbers during the Random Generation of Numbers task > 4/s
• Refusal to sign informed consent

For the young sedentary group (woman and man) and the senior group (woman and man, sedentary and active):

-Subjective health > 3 out of 5

For the senior group (woman and man, sedentary and active):

• Mini-Mental State Evaluation (MMSE) score < 24
• Geriatric Depression Scale (GDS) score > 10

For the Parkinson group (woman and man):

• Presence of a freezing or clinically embarrassing festination (walking disorder)
• Score of Hoehn and Yahr ≥III

Exclusion Criteria:

-Inability of the volunteer to walk on the treadmill at a speed of 3 km/h without assistance (stand at the helm or ask for help)