Intermuscular coherence reflects functional coordination

Abstract

Coherence analysis has the ability to identify the presence of common descending drive shared by motor unit pools and reveals its spectral properties. However, the link between spectral properties of shared neural drive and functional interactions among muscles remains unclear. We assessed shared neural drive between muscles of the thumb and index finger while participants executed two mechanically distinct precision pinch tasks, each requiring distinct functional coordination among muscles. We found that shared neural drive was systematically reduced or enhanced at specific frequencies of interest (~10 and ~40 Hz). While amplitude correlations between surface EMG signals also exhibited changes across tasks, only their coherence has strong physiological underpinnings indicative of neural binding. Our results support the use of intermuscular coherence as a tool to detect when coactivated muscles are members of a functional group or synergy of neural origin. Furthermore, our results demonstrate the advantages of considering neural binding at 10, ~20, and >30 Hz, as indicators of task-dependent neural coordination strategies. It is often unclear whether correlated activity among muscles reflects their neural binding or simply reflects the constraints defining the task. Using the fact that high-frequency coherence between EMG signals (>6 Hz) is thought to reflect shared neural drive, we demonstrate that coherence analysis can reveal the neural origin of distinct muscle coordination patterns required by different tasks.

The human central nervous system transmits common synaptic inputs to distinct motor neuron pools during non‐synergistic digit actions

“…Global EMG. EMG signals were averaged across all channels (64 per muscle), low-pass filtered at 250 Hz (fourth-order Butterworth), full-wave rectified and normalized to unit variance (Z-score) (Laine & Valero-Cuevas, 2017). The similarity between FDI and thenar normalized EMGs was assessed by pooled coherence analysis (Amjad et al 1997).…”

“…Moreover, we report that cross-talk between myoelectrical activity of two hand muscles significantly biases coherence estimation. Previous research has focused on the extraction of intermuscular coordination by coherence analysis of the global EMG activity Farmer et al 2007;Laine & Valero-Cuevas, 2017). These studies report a significant level of coherence at cortical frequencies.…”

“…Moreover, it has been hypothesized that correlated muscle activity should be associated with correlated spiking activity of the respective motor neuron pools (De Luca & Erim, 2002;Winges & Santello, 2004;Hockensmith et al 2005;Winges et al 2008;Bizzi & Cheung, 2013;Laine et al 2015). However, previous studies have assessed the amount of neural connectivity across synergistic muscles (Winges & Santello, 2004;Winges et al 2006;Laine & Valero-Cuevas, 2017). A remaining question is whether and the extent to which correlated activity in intramuscular or surface EMG signals might be a byproduct of biomechanical constraints.…”

“…Lastly, we note that previous studies have estimated neural connectivity mainly using interference surface EMG signals. These studies have reported relatively high values of intermuscular coherence when using global EMG during concurrent activation of hand muscles in human and non-human primates Farmer et al 2007;Laine & Valero-Cuevas, 2017). A potential issue with these findings is that global EMG may bias coherence estimation due to cross-talk between EMG signals.…”

Non‐synergistic synergies of muscle activation: an apparent oxymoron

“…Thus, correlated inputs to multiple MUs denote divergent projections or lateral connections in the neural pathways innervating spinal motor neurons. Intermuscular coherence between thumb and index finger muscles has previously been reported during various synergistic tasks (Laine & Valero-Cuevas 2017). However, Del Vecchio and colleagues are the first to report correlated inputs to motor neurons of these two sets of muscles during a task not belonging to the common behavioural repertoire.…”