  Over the past two decades, there has been a radical transformation in our
ability to extract useful biological signals from the surface electromyogram
(EMG). Advances in EMG electrode design and signal processing techniques have
resulted in an extraordinary capacity to identify motor unit spike trains from
the surface of a muscle. These EMG grid, or high-density surface EMG (HDsEMG),
recordings now provide accurate depictions of as many as 20-30 motor unit spike
trains simultaneously during isometric contractions, even at high forces. Such
multi-unit recordings often display an unexpected feature known as onion skin
behavior, in which multiple motor unit spike trains show essentially parallel
and organized increases in discharge rate with increases in voluntary force,
such that the earliest recruited units reach the highest discharge rates, while
higher threshold units display more modest rate increases. This sequence
results in an orderly pattern of discharge resembling the layers of an onion,
in which discharge rate trajectories stay largely parallel and rarely cross.
Our objective in this review is to explain why this pattern of discharge rates
is unexpected, why it does not accurately reflect our current understanding of
motoneuron electrophysiology, and why it may potentially lead to unpredicted
disruption in muscle force generation. This review is aimed at the practicing
clinician, or the clinician scientist. More advanced descriptions of potential
electrophysiological mechanisms associated with onion skin characteristics
targeting the research scientist will be provided as reference material.
