Functional Coupling of Motor Units Is Modulated During Walking in Human Subjects

Abstract

Time- and frequency-domain analysis of the coupling between pairs of electromyograms (EMG) recorded from leg muscles was investigated during walking in healthy human subjects. For two independent surface EMG signals from the tibialis anterior (TA) muscle, coupling estimated from coherence measurements was observed at frequencies </=50 Hz, with identifiable peaks occurring in two frequency bands ranging approximately from 8 to 15 and 15 to 20 Hz. The coherence between TA recordings was greatest toward the end of swing, reduced in early swing, and largely absent in midswing. In time-domain estimates constructed from paired TA EMG recordings, a short-lasting central peak indicative of motor-unit synchronization was observed. This feature of motor-unit coupling was also reduced in mid swing. In paired recordings made among triceps surae, quadriceps, and hamstring muscles, a similar pattern of correlation to that for paired TA recordings was observed. However, no significant coupling was observed in recordings for which one EMG recording was made from an ankle flexor/extensor muscle and the other from a knee extensor/flexor muscle. These results demonstrate that for TA a modulation exists in the functional coupling of motor units recruited during swing. The data also indicate that human motoneurons belonging to different muscles are only weakly coupled during walking. This absence of widespread short-term synchronization between the activities of muscles of the leg may provide a basis for the highly adaptive nature of human gait patterns.

Changes in Cortically Related Intermuscular Coherence Accompanying Improvements in Locomotor Skills in Incomplete Spinal Cord Injury

“…The presence of high-frequency coherence in moderately strong subjects fits with previous studies demonstrating that EEG/EMG coherence, which can be disrupted by TMS to the motor cortex (Hasan and Nielsen 2004), occurs at this higher-frequency band (Brown and Marsden 2001). In contrast, noninjured control subjects exhibited little to no high-frequency coherence between antagonist muscles during walking as reported previously (Halliday et al 2003;Hansen et al 2001). The lack of coherence between antagonist muscles in noninjured controls does not necessarily indicate that there is little to no descending drive to muscles of the leg during intact walking.…”

“…Rather, it is likely that during intact walking there is very little common descending drive to antagonist muscle pairs. Leg muscles appear to be activated rather independently during intact walking because higher-frequency coherence is only present between different motor units of a given muscle or between synergistic muscles (Halliday et al 2003;Hansen et al 2001). However, results from this study show that after chronic and incomplete spinal cord injury, the strength of common descending connections to antagonist muscle pairs appears to be increased to generate functional but not necessarily "normal," walking.…”

Neural Control of Unloaded Leg Posture and of Leg Swing in Stick Insect, Cockroach, and Mouse Differs from That in Larger Animals

“…The stick insect, mouse, and cat data differed markedly from the human and horse data, with the smaller animals having swing motor neuron activity that continued throughout swing and the larger ones having swing activity only at swing beginning. Swing motor neuron activity also continues throughout the entirety of swing in guineafowl (Gatesy, 1999), chick (Jacobson and Hollyday, 1982), and newt (Székely et al, 1969) legs and small (ankle) limb segments in humans (Prochazka et al, 1989;Halliday et al, 2003;Cappellini et al, 2006).…”

Usefulness of intermuscular coherence and cumulant analysis in the diagnosis of postural tremor

“…Plots were classified as 1) a broad positive peak around zero, indicating tremor burst activity that was more in-phase and synchronous, 2) a broad negative peak around zero, indicating tremor burst activity that was more out-of-phase and alternating, and 3) a narrow central peak close to zero, indicating short-term synchronization with tremor bursts consistent with a common presynaptic drive (Sears and Stagg 1976;Hansen et al 2001;Halliday et al 2003)see figure 2 for typical examples). We chose cumulant density In some data sets, cumulant analysis did not reveal any significant correlation structure between EMG bursts.…”