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The Journal of Neurophysiology Vol. 81 No. 5 May 1999, pp. 2451-2463
Copyright ©1999 by the American Physiological Society
1Department of Physiological Science and 2Brain Research Institute, University of California, Los Angeles, California 90095-1527; 3Institute of Biomedical Problems, Moscow 123007, Russia; and 4Oregon Regional Primate Research Center, Beaverton, Oregon 97006
Recktenwald, Matthew R.,
John A. Hodgson,
Roland R. Roy,
Sergey Riazanski,
Gary E. McCall,
Inessa Kozlovskaya,
David
A. Washburn,
John W. Fanton, and
V.
Reggie Edgerton.
Effects of Spaceflight on Rhesus Quadrupedal Locomotion After
Return to 1G. J. Neurophysiol. 81: 2451-2463, 1999.
Effects of spaceflight on Rhesus quadrupedal locomotion after return to
1G. Locomotor performance, activation patterns of the soleus
(Sol), medial gastrocnemius (MG), vastus lateralis (VL), and tibialis
anterior (TA) and MG tendon force during quadrupedal stepping were
studied in adult Rhesus before and after 14 days of either spaceflight
(n = 2) or flight simulation at 1G (n = 3). Flight
simulation involved duplication of the spaceflight conditions and
experimental protocol in a 1G environment. Postflight, but not
postsimulation, electromyographic (EMG) recordings revealed clonus-like
activity in all muscles. Compared with preflight, the cycle period and
burst durations of the primary extensors (Sol, MG, and VL) tended to
decrease postflight. These decreases were associated with shorter
steps. The flexor (TA) EMG burst duration postflight was similar to
preflight, whereas the burst amplitude was elevated. Consequently, the
Sol:TA and MG:TA EMG amplitude ratios were lower following flight,
reflecting a "flexor bias." Together, these alterations in mean
EMG amplitudes reflect differential adaptations in motor-unit
recruitment patterns of flexors and extensors as well as fast and slow
motor pools. Shorter cycle period and burst durations persisted
throughout the 20-day postflight testing period, whereas mean EMG
returned to preflight levels by 17 days postflight. Compared with
presimulation, the simulation group showed slight increases in the
cycle period and burst durations of all muscles. Mean EMG amplitude
decreased in the Sol, increased in the MG and VL, and was unchanged in
the TA. Thus adaptations observed postsimulation were
different from those observed postflight, indicating that there was a
response unique to the microgravity environment, i.e., the modulations in the nervous system controlling locomotion cannot merely be attributed to restriction of movement but appear to be the result of
changes in the interpretation of load-related proprioceptive feedback
to the nervous system. Peak MG tendon force amplitudes were
approximately two times greater post- compared with preflight or
presimulation. Adaptations in tendon force and EMG amplitude ratios
indicate that the nervous system undergoes a reorganization of the
recruitment patterns biased toward an increased recruitment of fast
versus slow motor units and flexor versus extensor muscles. Combined,
these data indicate that some details of the control of motor pools
during locomotion are dependent on the persistence of Earth's
gravitational environment.
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