It has been documented that concurrently active motor units fire under the control of a common drive. That is, the firing rates of motor units are cross-correlated with near-zero time lag.. This degree of cross-correlation has been found to vary among muscles, and among contractions performed at different force levels in the same muscle. This study provides an explanation indicating that motor units recruited during a contraction cause an increase in the variation (standard deviation) and a decrease in the degree (amplitude) of the cross-correlation of the firing rates. The degree of cross-correlation is lower in muscles having greater spindle density. This effect appears to be mediated by the proprioceptive feedback from the spindles and possibly the Golgi tendon organs. Muscle spindles in particular respond to the mechanical excitation of the non-fused muscle fibers and provide a discordant excitation to the homonymous motoneurons, resulting in a decrease in the cross-correlation of the firing rates of motor units. The implication of this work is that the decreased cross-correlation of the firing rates in some muscles is not necessarily an indication of a decreased common drive from the Central Nervous System, but rather an inhibitory influence of the proprioceptive feedback from the Peripheral Nervous System. This explanation is useful for understanding various manifestations of the common drive reported in the literature.