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Skeletal muscles vibrate under sustained contraction, and generate low frequency side band clutter in the doppler signal. Both shivering in the hand of the operator and muscle vibrations in the patient itself give rise to the clutter. Clutter rejection filters are commonly used to remove the low frequency components, but the doppler signal from low velocity blood flow is then also lost. This paper describes a model for the pulsed wave (PW) doppler signal from vibrating muscles, reviews a model for the PW doppler signal from moving blood, and by comparing these models presents a theoretical minimum for detectable blood velocity in small vessels, being typically 6.4 mm/s for 6 MHz doppler. The limit has a nonlinear relation to the ultrasound frequency. The model also shows that the radial component of the muscle vibrations can be estimated from the phase of the doppler signal.