Single-motor-unit discharge characteristics in human lumbar multifidus muscle

The underlying neurophysiology of postural control of the lower back in humans is poorly understood. We have characterized motor unit (MU) discharge activity in the deep lumbar multifidus (LM) muscle in nine healthy subjects (20-40 yr, 3 females). Bilateral fine wire electrodes were implanted at L4 spinal level using ultrasound guidance. EMG was recorded during spontaneous sitting and standing and during voluntary force production. Individual MUs were analyzed with regard to instantaneous discharge rate, interspike interval variability, alternation of activity between MUs, and cross correlation between concurrently active MUs quantified by the common drive coefficient (CDC). Significant effects of sitting vs. standing were seen on median discharge rate and interspike interval variability. Median discharge rate in 71 units was 5.4 and 6.9 pulses/s during spontaneous sitting and standing and 7.4 pulses/s during voluntary force production. Several MUs fired doublets. CDC analysis of 87 MU pairs showed a significantly higher common drive in spontaneous than in voluntary activity and significant differences between unilateral and bilateral pairs, although not when spontaneously active in standing. In spite of common drive, MUs were recruited from inactivity to tonic discharge lasting for several minutes without changes in discharge rate in already active MUs, and several instances were documented where activity was rotated between MUs. We argue that this behavior is indicative of self-sustained discharge in LM motoneurons, establishing intrinsic motoneuron properties as a central mechanism for postural control of deep back muscles.