Intrinsic and reflexive contributions to endpoint stiffness during motor learning of unstable force fields

Intrinsic and reflexive contributions to endpoint stiffness during motor learning of unstable force fields

Rosenbrand, D.M.

Van der Helm, F.C.T. (mentor)
Schouten, A.C. (mentor)

Mechanical, Maritime and Materials Engineering

BioMechanical Engineering

BM

2011-04-11

Endpoint stiffness is known to increase in the direction of instability during learning to accurately execute unstable motor tasks. Reflexes might be present in the time interval used for endpoint stiffness calculations suggesting a possible influence of reflexes on endpoint stiffness measurements. In addition, changes of reflexes during motor learning are still unknown. The purpose of this research was to investigate the separate contributions of intrinsic and reflexive stiffness to the observed change in endpoint stiffness during learning to move the hand in unstable force fields. A divergent (unstable) force field (DF) was applied with a two degrees of freedom manipulator (ARMANDA). Subjects performed 100 point-to-point arm movements in a null field and 300 point-to-point arm movements in the divergent force field holding the manipulator with their right hand. In random (catch) trials a minimum jerk position perturbation was applied in the middle of the movement. Force and EMG responses to the perturbation were used to examine endpoint stiffness and reflexes. Endpoint stiffness is defined as the force response to the imposed perturbation (in the interval 160-200 ms after perturbation onset) divided by the position displacement. Unperturbed trials were analyzed to investigate the decrease of errors (deviations from the straight path between start and target) and changes of co-contraction with motor learning. We found that errors decreased in the first 150 movements in the DF and leveled off from then on. Intrinsic, reflexive and endpoint stiffness were rapidly increased before the 35th DF trial. No significant changes of the stiffness parameters were found after this first learning period. An additional investigation of reflex response timing showed variability in the reflex timing between subjects. Reflexes were seen to influence the endpoint stiffness measurements for some subjects. In case of other subjects reflexes were not present in the time interval used for endpoint stiffness calculation and therefore no reflexive contribution to endpoint stiffness was assumed. In conclusion, our results showed a rapid increase of all stiffness parameters during the early phase of learning and suggest the involvement of other (unknown) mechanisms in the later learning phase. Because of the found variations in reflex response timing, we recommend to always include reflex analysis during endpoint stiffness measurements.

motor learning
endpoint stiffness
reflexes
co-contraction
unstable
force fields

http://resolver.tudelft.nl/uuid:1eb29cd1-2a1d-43c9-923c-d2e2dfd00e88

2011-05-02

Student theses

master thesis

(c) 2011 Rosenbrand, D.M.