The muscle-reflex mechanisms of primate limbs are studied and modeled so that robotic controls may benefit from the findings. An extensive body of experimental evidence indicates that velocity-dependent force responses of the neuromuscular system have a nonlinear damping effect proportional to a fractional power of velocity. This highly nonlinear viscosity may help limbs adapt to different loads and bring movements to graceful terminations. To explore the characteristics of this nonlinear damping property, a theoretical study using the phase-plane approach is presented. The effects of different loads, damping constants, and stiffnesses are analyzed and simulated. From the results of this phase-plane analysis, a muscle-reflex model is developed and proposed for robotic compliance control.