Nonlinear viscous properties of stretch and unloading reflexes in the human wrist were examined using constant-velocity ramp stretches and releases in the range between 5 and 500 mm/s. Subjects were asked to oppose an initial flexor preloaded and were instructed not to intervene voluntarily when the changes in position were applied. Electromyographic (EMG) activity and net force exerted by the wrist were measured. Although subjects were instructed not to intervene to the applied stretches, even well-practiced subjects sometimes showed unintended triggered reactions, which character could be assisting or resisting. A trial comparison method was used to detect and eliminate responses contaminated by unintended reactions. Ramp stretches further loaded the preloaded flexor muscles. Responses of EMG and force increased steeply initially but after about 1-cm displacement, the slope of these responses decreased to a lower value and remained constant during the remainder of the 5-cm ramp. For higher stretch velocities, the magnitudes and slopes of the responses of EMG and force increased but less than proportionally with ramp velocity. Except for the initial transient, EMG in the loaded flexor muscles and force responses could be described by a product relationship between a linear position-related term and a low fractional power of velocity, after a correction was made for delays in the reflex arc. Mean value of the exponent in the power function of velocity was 0.3 for EMG and 0.17 for force. For higher preloads, incremental responses of force to constant-velocity stretches, plotted as a function of wrist position, shifted to higher values and the slope of increase of force with position became somewhat steeper. This upward shift of the force trace reflects a change of apparent threshold of the stretch reflex. Ramp releases shortened and unloaded the preloaded flexor muscles and stretched the initially inactive extensor muscles. Flexor EMG activity declined progressively with a time course that was independent of velocity. Extensor EMG response depended on preload. At high preloads, there was no activity except for some bursting at the highest velocities. At low preloads, EMG activity was initially absent but started part was through the ramp. The increase of activity was somewhat greater for higher ramp velocities. Force responses to shortening ramps depended on preload. At high preloads, force responses superimposed at all of the low velocities but fell to slightly lower forces at the higher velocities. At low preloads, force traces again superimposed for low velocities and at high velocities only during the initial part of the response. The velocity sensitivity of EMG responses may reflect the dynamic character of the responses of muscle spindles for which a similar velocity dependence has been shown earlier. The velocity-dependent responses of force represent a nonlinear viscosity that may perform an important function in damping limb movements when variable loads are involved.
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