Tonic stretch reflex threshold as a measure of spasticity: implications for clinical practice.

Lance's definition of spasticity states: (1) the excitability of the tonic stretch reflex threshold is the conceptual unit of measure of spasticity; (2) because spasticity is a velocity-dependent phenomenon, different velocities of stretch should be used to evaluate spasticity; and (3) measures should provide insight into the role of spasticity in disordered motor control. This article reviews studies of clinical and laboratory-based methods to evaluate spasticity. There is a lack of consensus regarding the conceptual unit that best captures spasticity. Several biomechanical variables and parameters such as stretch reflex gain, stretch reflex threshold, and/or suprathreshold phenomena are often measured alone or in combination without a unifying conceptual framework. Most studies do not establish links between spasticity and other motor deficits and adhere only partly to Lance's definition. A promising alternative to current measures is offered by the lambda model that uses the tonic stretch reflex threshold as the descriptor of spasticity. The lambda model also provides a framework in which spasticity and the presence of disorders in muscle activation can be described and explained. We introduce a portable device to measure spasticity, based on the measurement of the tonic stretch reflex threshold, which is more closely related to Lance's definition.