Welcome Focus Dynamic Stiffness Stretch Reflex Contributions System Identification Contact Information

Welcome to REKLAB Focus The general focus of REKLAB (for the initials of its founder and director, Dr. R.E. Kearney), is the exploration of the role of peripheral neuromuscular mechanisms in control of posture and movement. Dynamic Stiffness Dynamic stiffness is the relation between the position and the torque acting about a joint. Stiffness defines the resistance to external perturbations that occur before voluntary muscle intervention. This is important in the maintenance of posture. It is also essential to understanding voluntary movement by defining the properties of muscle dynamics that the central nervous system must control. Knowledge of joint stiffness is an integral part of understanding normal control of posture and movement, as well as its disorders. Spasticity, rigidity, hypertonia and “stiffness” are clinical symptoms involving changes in joint dynamics that characterize neuromuscular diseases such as multiple sclerosis. The REKLAB findings will also have implications in the fields of robotics, tele-operations, human-machine interfaces and biomechanics. Stretch Reflex Contributions Dynamic joint stiffness has two components: Intrinsic stiffness, generated by passive joint properties (such as visco-elastic and inertial) in combination with the mechanics of active muscle. Reflex stiffness, resulting from changes in muscle activation mediated by the stretch reflex. Although the intrinsic and reflex contributions are often treated as independent and adding linearly, they are most likely to be complex and nonlinear. The stretch reflex, in terms of EMG activity, has been studied in a wide variety of muscles under quasi-static, postural conditions. However, the mechanical consequences of reflex activity, in terms of force or torque, are less well understood. The complex, nonlinear interactions between intrinsic and reflex mechanisms make it difficult to dissociate their individual contributions. System Identification Since position and torque are the only important signals that can be readily observed and manipulated, the characteristics of the various subsystems (e.g. dissociation of intrinsic and reflex contributions) are difficult to determine independently. Hence, measuring joint dynamics can be considered a classic problem in system identification: obtain a model of the system (joint dynamics) through the analysis of the relation between inputs (position or torque) and outputs (torque or position). REKLAB has successfully developed a parallel-cascade identification method capable of separating the intrinsic and reflex components using experimental data. This powerful tool has opened a new window of opportunity to explore the role of stretch reflexes from a mechanical standpoint. (see Research Areas) Contact Information For further information contact the BioMedical Engineering Department