In quasi-sinusoidal (0.5-3.0 Hz) voluntary muscle contractions, we studied the 6-10 Hz motor unit (MU) firing synchrony and muscle force oscillation with emphasis on their neural substrate and relation to rhythmical motor control. Our analyses were performed on data from 121 contractions of a finger muscle in 24 human subjects. They demonstrate that coherent 6-10 Hz components of MU discharges coexist with carrier components and coherent modulation components underlying the voluntary force variations. The 6-10 Hz synchrony has the frequency of the tremor synchrony in steady contractions, and is also widespread and in-phase. Its strength ranges from very small to very large (MU/MU coherence >0.50) among contractions; and it is not related to the contraction parameters, in accord with the notion of a distinct 6-10 Hz synaptic input to the MUs. Unlike the coherent MU modulations and the voluntary force variations, the in-phase 6-10 Hz MU components are suppressed or even eliminated during ischemia, while the respective force component is drastically reduced. These findings agree with the widely assumed supraspinal origin of the MU modulations, but they also strongly suggest a key role for muscle spindle feedback in the generation of the 6-10 Hz synaptic input. They therefore provide important information for the study of generators of the 6-10 Hz rhythm which subserves the postulated rhythmical control and is manifested as force and movement components. Moreover, they argue for a participation of oscillating spinal stretch reflex loops in the rhythm generation, possibly in interaction with supraspinal oscillators.