ABSTRACT
The muscle stiffness of spasticity, often with associated spasms, is an extremely common symptom seen in many neurological conditions, including head or spinal cord injury, stroke, cerebral palsy and multiple sclerosis. In people with spasticity, stretch reflexes can be elicited at low-velocity stretches meaning that an assessment of the passive, nonneural component requires even lower-velocity stretches (e.g. 5°/second). The clasp knife phenomenon is a clinical sign that has been described as one of the characteristics of hypertonia secondary to spasticity. It refers to the sudden reduction in tone that occurs when moving a limb with spasticity. The degree of asymmetry was correlated with the degree of spasticity. It was speculated that following a stroke there may be an asymmetrical vestibular drive to motorneuron pools, with a higher input going to the spastic–paretic side that keeps the motor neurons closer to the threshold for eliciting an action potential.
