In reaching to a target, stability near the target may be more critical for success than stability far from the target. Consequently, we postulated that high instability near the start would evoke less compensation than high instability near the target. Three stability conditions were implemented using a robot manipulandum: neutral stability everywhere (null field); high instability along the first half of the trajectory decreasing as the target was approached (start unstable); and instability increasing along the first half of the trajectory and remaining high as the target was approached (end unstable). Under the start unstable condition, the stiffness of the arm in the region of highest instability was significantly less than under the end unstable condition. Furthermore, the stability of the system (manipulandum plus arm) was much lower under the start unstable condition than under the null field condition whereas it was similar under the end unstable and null field conditions.
Theodore E. Milner, Emily J. Lai and Antony J. Hodgson
Michael J. Grey, Charles W. Pierce, Theodore E. Milner and Thomas Sinkjaer
The modulation and strength of the human soleus short latency stretch reflex was investigated by mechanically perturbing the ankle during an unconstrained pedaling task. Eight subjects pedaled at 60 rpm against a preload of 10 Nm. A torque pulse was applied to the crank at various positions during the crank cycle, producing ankle dorsiflexion perturbations of similar trajectory. The stretch reflex was greatest during the power phase of the crank cycle and was decreased to the level of background EMG during recovery. Matched perturbations were induced under static conditions at the same crank angle and background soleus EMG as recorded during the power phase of active pedaling. The magnitude of the stretch reflex during the dynamic condition was not statistically different from that during the static condition throughout the power phase of the movement. The results of this study indicate that the stretch reflex is not depressed during active cycling as has been shown with the H-reflex. This lack of depression may reflect a decreased susceptibility of the stretch reflex to inhibition, possibly originating from presynaptic mechanisms.