The dual task (walking and cognitive task at the same time) is increasingly used in geriatrics in protocols for detecting people who are falling and/or for highlighting cognitive disorders. However, frail elderly people tend to get tired quickly and a lack of awareness of the effects of fatigue on dual-task performance could alter the diagnosis.

There are two types of fatigue: mental fatigue and peripheral muscle fatigue. Each participant will do both types of fatigue over 2 appointments spaced by 7 days so that the subject can recover from induced fatigue. The sequence of fatigue tasks will be randomized.

At the first appointment (day 0):

1. Pre-fatigue assessment :

   • The subject must walk on 10m with round trip for 1min.
   • the subject must do the arithmetic count of 3 in 3
   • Then comes the evaluation in double task (walking and counting at once for 1min). The subject must walk on 10m with round trip for 1min by doing the arithmetic count of 3 in 3 at the same time.

2. mental fatigue task or muscle fatigue task (randomized)

3. post-fatigue assessment (the same as pre-fatigue assessment)

7 resting days

Second appointment (day 7)

1. Pre-fatigue assessment :

   • The subject must walk on 10m with round trip for 1min.
   • the subject must do the arithmetic count of 3 in 3
   • Then comes the evaluation in double task (walking and counting at once for 1min). The subject must walk on 10m with round trip for 1min by doing the arithmetic count of 3 in 3 at the same time.

2. mental fatigue task or muscle fatigue task (randomized)

3. post-fatigue assessment (the same as pre-fatigue assessment)

Young group

Inclusion Criteria:

• 20 to 40 years old

Exclusion Criteria:

• psychiatric disorders
• Severe hearing or vision loss
• Motor or sensory disability that may interfere with test completion

Old group

Inclusion Criteria:

• 65 to 90 years old
• less than 3 Fried's criteria of fragility
• walking ability without technical assistance (at least 10m).

Exclusion Criteria:

• psychiatric disorders
• Severe hearing or vision loss
• Motor or sensory disability that may interfere with test completion

Old frail group

Inclusion Criteria:

• 65 to 90 years old
• 3 or more Fried's criteria of fragility
• walking ability without technical assistance (at least 10m).

Exclusion Criteria:

• psychiatric disorders
• Severe hearing or vision loss
• Motor or sensory disability that may interfere with test completion