Collapsing weakness, or "give-way" weakness, suggesting intermittent voluntary effort, is often taken as a sign of functional weakness. Although sometimes labeled as "volitional weakness", it is not clear that such weakness is in any conscious sense willed, and it is therefore probably better to use a noncommittal term, such as "apparent weakness." Such collapsing weakness has also been recorded following acute brain lesions, such as stroke.
Gould R, Miller BL, Goldberg MA, Benson DF. The validity of hysterical signs and symptoms. Journal of Nervous and Mental Disease1986; 174: 593-597
Stone J, Zeman A, Sharpe M. Functional weakness and sensory disturbance. Journal of Neurology, Neurosurgery and Psychiatry 2002; 73: 241-245