Effects of Spaceflight on Rhesus Quadrupedal Locomotion After Return to 1G

Abstract

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.

Kinematic and EMG Determinants in Quadrupedal Locomotion of a Non-Human Primate (Rhesus)

“…7 and 8). Similar observations have been reported in the Rhesus (Recktenwald et al 1999) as well as in rats (de Leon et al 1994;Roy et al 1991b), cats (English 1984;Pierotti et al 1989), and humans (Nilsson et al 1985). This coordinated, speed-related modulation of HL and FL muscle activity may arise from similar mechanisms in Rhesus and in cats, i.e., velocity-dependent tuning of spinal circuits via the brain stem tonic commands and the phasic afferent input signaling hip extension (Grillner and Rossignol 1978) and loading of the leg (Duysens and Pearson 1980).…”

“…There are relatively few data on the kinematic and electromyographic (EMG) determinants for non-human primate locomotion (Mori et al 1996;Recktenwald et al 1999). For example, little is known about the pattern of hindlimb (HL) and forelimb (FL) muscle activation during walking or about how the recruitment of these motor neuron pools is modulated with respect to the speed of locomotion.…”

Metabolic Modulation of Muscle Fiber Properties Unrelated to Mechanical Stimuli

“…The total mean integrated EMG activity per hour (IEMG/h) was calculated as IEMG times the number of bursts [25]. Computer software was used to detect and display the start and end of each EMG burst based on a given threshold level above the baseline noise for each channel [26] and averaged over 10 repetitions of each behavior.…”

Plasticity of interneuronal networks of the functionally isolated human spinal cord

“…These results suggest that load related afferent input interacts with the local networks to modulate the timing of their activation and consequently determine the efferent pattern generated. In addition, neurologically intact humans and non-human primates that return from space exhibit clonus in their lower limbs [96] suggesting that the loss of loading, not the loss of supraspinal input is the prominent mechanism of the development of these oscillatory patterns. These results suggest that clonus is generated by internuncial oscillatory networks with inhibitory and excitatory connections that are modulated by afferent input and are capable of interlimb coordination.…”