A data archive including processed spiking data, raw EMG datasets, and video data during locomotion behavior from six mice

During locomotion, the coordinated activity of dozens of muscles shapes
the kinematic features of each stride, including systematic changes in
limb movement across walking speed. Motor units, each of which consists of
a single motor neuron and the muscle fibers it innervates, contribute to
the total activation of each muscle through their recruitment and firing
rate when active. However, it remains unknown how the nervous system
controls locomotor speed by changing the firing of individual motor units.
To address this, we combined quantitative behavioral analysis of mouse
locomotion with single motor unit recordings from the lateral and long
heads of the triceps brachii, which drive monoarticular extension of the
elbow and biarticular movements of the elbow and shoulder, respectively.
In contrast to prior studies employing bulk EMG to examine muscle
activity, our recordings revealed the diversity of spike patterning across
motor units as well as systematic differences in motor unit activity
across muscles and locomotor speeds. First, motor unit activity differed
significantly across the lateral and long heads, suggesting differential
control of these two closely apposed elbow extensor muscles. Second, we
found that individual units were recruited probabilistically (during only
a subset of strides), showing that the highly repeatable bulk EMG signals
observed across strides in fact reflect stochastically varying subsets of
individual motor units. Finally, although recruitment probability and
firing rate both increased at faster walking speeds, increases in
recruitment were proportionally larger than rate changes, and recruitment
of individual units accompanied changes in limb kinematics. Together,
these results reveal how the firing of individual motor units varies
systematically across muscles and walking speeds to produce flexible
locomotor behavior.