McArdle’s sign is the combination of reduced lower limb strength, increased lower limb stiffness and impaired mobility following neck flexion. The difference may best be appreciated by comparing leg strength (e.g., hip flexion) with the neck fully extended and fully flexed.
The sign was initially described in multiple sclerosis but may occur in other myelopathies affecting the cord at any point between the foramen magnum and the lower thoracic region. The mechanism is presumed to be stretch-induced conduction block, due to demyelinated plaques or other pathologies, in the corticospinal tracts. McArdle’s sign may be envisaged as the motor equivalent of Lhermitte’s sign.
McArdle MJ. McArdle’s sign in multiple sclerosis. Journal of Neurology, Neurosurgery and Psychiatry 1988; 51: 1110
O’Neill JH, Mills KR, Murray NMF. McArdle’s sign in multiple sclerosis. Journal of Neurology, Neurosurgery and Psychiatry 1987; 50: 1691-1693