Coordinated modulation of locomotor muscle synergies constructs straight-ahead and curvilinear walking in humans.

We describe the muscle synergies accompanying
steering of walking along curved trajectories, in order
to analyze the simultaneous control of progression
and balance-threatening emerging forces. For this purpose,
we bilaterally recorded in ten subjects the electromyograms
(EMGs) of a representative sample of leg
and trunk muscles (n=16) during continuous walking
along one straight and two curved trajectories at natural
speed. Curvilinear locomotion involved a graded, limbdependent
modulation of amplitude and timing of
activity of the muscles of the legs and trunk. The turnrelated
modulation of the motor pattern was highly
coordinated amongst muscles and body sides. For all
muscles, linear relationships were detected between the
spatial and temporal features of muscle EMG activity.
The largest modulation of EMG was observed in gastrocnemius
medialis and lateralis muscles, which showed
opposite changes in timing and amplitude during curvewalking.
Moreover, amplitude and timing characteristics
of muscle activities were significantly correlated with the
spatial and temporal gait adaptations that are associated
with curvilinear locomotion. The present results reveal
that fine-modulation of the muscle synergies underlying
straight-ahead locomotion is enough to generate the
adequate propulsive forces to steer walking and maintain
balance. These findings suggest that the turn-related
command operates by modulation of the phase relationships
between the tightly coupled neuronal assemblies
that drive motor neuron activity during walking.
This would produce the invariant templates for locomotion
kinematics that are at the base of human navigation
in space.